Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on June 14, 2020, 09:27:33 am
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Hi,
I've been looking at the 6G3 schematic, and I'm not clear on the purpose of the cap I've circled in the attached image. Given my limited knowledge, it looks like it's letting part of the signal bypass the pots and the second gain triode. If so, why? If not, what is it doing?
Thanks!
David
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It shunts some hi freq signal to ground thru the B+ filter caps. This makes the normal channel darker than the bright channel.
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Thank you! If I was to build a single channel version of this amp, I guess I could add a switch to take it in or out of the circuit. That seems easier than moving the guitar cable to another jack.
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Thank you! If I was to build a single channel version of this amp, I guess I could add a switch to take it in or out of the circuit. That seems easier than moving the guitar cable to another jack.
If you build a single-channel version, you might just try the amp with & without the cap. My guess is you'll pick one and stick with it.
Until a few months ago, I had a '62 and '63 6G3 at the same time. I mostly played in the Bright channel (no-cap).
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If you build a single-channel version ...
Thanks HotBluePlates. I'm thinking about it. (My schematic uses the bright channel, and I probably attached sluckey's trem-o-nator in the wrong place.)
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I probably attached sluckey's trem-o-nator in the wrong place.
Yes, you did. Well, it's actually connected to a good point but there must be another cap between C12 and the LTP-PI circuit. Speed pot is wired wrong. And the optocoupler does not use a phototransistor. It uses a LDR.
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This type of tremolo works by applying a varying load on the 12AX7 causing a varying change in gain. In order to do this, the intensity pot must be connected to the plate via a coupling capacitor. Connecting the intensity pot to the 15K/100K junction barely changes the load on the 12AX7 and it is in the load range that doesn't have very much gain change with load change.
The output impedance is around 14K as drawn. It is going to take an LDR that is capable of a very low resistance to work against this low output impedance.
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Thanks sluckey and 2deaf.
I couldn't find an LTSpice model for a VTL5C1 so I just stuck something in there that looked kind of similar to me. :icon_biggrin: :dontknow:
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Some suggestions
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Thanks 66Strat. I think I fixed the things you pointed out.
Until I can find or teach myself how to model a VTL5C1 my LTSpice schematic won't run. It just freezes up. No big deal.
One thing I did discover, there's no way I'll get the same plate voltages on the first 12ATX7 shown on the original schematic using those plate load resistors.
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One thing I did discover, there's no way I'll get the same plate voltages on the first 12ATX7 shown on the original schematic using those plate load resistors.
That doesn't matter. The preamp will operate just about the same on a wide range of voltages.
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You're overthinking things. Voltages in Fender schematics and layouts rarely match real life conditions. The plate resistors in the schematic are key in voicing and setting the gain structure of the amp. The 220K ohm resistor in combination with the 1 meg volume pot sets the gain of the first triode. The 15K ohm and 100K ohm resistors are a distributed load that sets the gain stage of the second triode.
As you only plan for a single channel amp, the resistance of the first triode cathode resistor should be double to maintain the same bias and the by pass capacitor should be halved to maintain the same frequency response of the stock circuit. However, this would be an area to experiment with to voice the amp to your taste. The 10uf capacitor in combination with the 3K ohm resistor has a 3db down turnover frequency of 5 hz compared to the stock 25/25 combination of 4 hz. Bypass capacitors of 4.7 uf and 2.2 uf in combination with the 3K ohm resistor would have turnover frequencies of 11 hz and 24 hz respectively.
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And another thing to be aware of related to voicing: Whatever speaker you pick, it will probably be darker than the original Oxford.
When I bought one of my 6G3 amps, the prior owner had "saved" the Oxford 12K5 and installed a Celestion alnico Cream speaker. He wanted to know if I wanted to buy the amp with the new speaker, and my answer was "No!" I wanted to hear the amp as it originally was.
I pretty much stopped building amps the past 8 years or so and went down a vintage rabbit-hole, mostly coming to find that while voltages matter (especially the imperfections of some power transformers), that speakers matter a whole lot more.
Another finding was that sometimes if you "fix the flaws" in some amp designs (or make the "more versatile") you can wind up squashing the amp's unique identity.
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Wow! Thanks for all the input and guidance. Much appreciated.
I should state up front that I've never played through a 6G3. And, I think the examples on YouTube are not a good source because who knows what's been modded, or how the recording was done.
The truth is, I've been casting around for a second amp project and this one looked "not too complicated," yet pretty cool. Also, given the tweeds, browns, blondes and blacks that I have played through or heard others use, I've always liked the brown and blonde tolex era amps best. Not sure how empirical vs subjective that preference actually is.
HotBluePlates, I've felt that way about speakers for a long time. I plan to buy a 12" cabinet for this amp so I'll have the option of using a great speaker.
Having said all that I did choose a 58yr old design after all, so I would like it to sound as close - within practical reason - to the original configuration. I'm thinking now that I should build it as the two channel original. It's actually the same number of tubes either way.
The AC4 I just built sounds great. I love it. sluckey and others helped me a great deal in making that project a success. Their help made it possible. However, over the weekend I went back in and changed the .1u coupling cap à la AC15 to the .047u shown on the schematic. Man, the sparkle really came out. Noticeably brighter, but not shrill. I guess chime is the word Vox people use. So, there's an example of the original parts working better for me, at any rate.
So, back to the original schematic it is!
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So, back to the original schematic it is!
Hoffman has an eyelet board for $22.
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So, back to the original schematic it is!
Hoffman has an eyelet board for $22.
Super! Thanks. His pricing and shipping are legit. Too tired to struggle with LTSpice modelling tonight. I'll get your Trem-O-Nator added to my drawing tomorrow. I have no knowledge of tremolo workings, but I like the idea of not wobbling the bias voltage, for some reason. BTW, I was trying to follow the rat's nest of a tremolo circuit in the 5G13 Vibrasonic. Looks like that was only used about once from what I can tell. Is that true pitch vibrato?
Added the other channel. FWIW, and I agree, I get too absorbed in LTSpice, there's only 10V difference between the first and second preamp tubes now - 160 vs 170. That brackets the 165 on the schematic!
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So, back to the original schematic it is!
Hoffman has an eyelet board for $22.
I have no knowledge of tremolo workings, but I like the idea of not wobbling the bias voltage, for some reason.
Why not? There is nothing wrong with the original tremolo circuit.
I've actually played through several vintage 6G3 amps. The first belonged to a buddy of mine that I played with in 1968 when I was a freshman in high school. I have played through more than one since that time. :wink: The only thing lacking in those amps was the Normal "Mud" channel. It certainly wasn't the tremolo.
I like Slukey's idea of using Hoffman's circuit board as a starting point. I would loose the .003 uf cap shunt across the Normal channel plate resistor to clean up the "Mud" channel. And, I would change the Bright channel coupling cap to .002 uf.
Modded to add Waylon just because. :icon_biggrin:
https://www.youtube.com/watch?v=PO4CGuh9xR4 (https://www.youtube.com/watch?v=PO4CGuh9xR4)
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Modded to add Waylon just because. :icon_biggrin:
Nice. I had to go listen to some Don Rich after that. That guy on the Electric XII with the mop cut and sideburns looks like he would be more at home in the Turtles than in Waylon's band. :icon_biggrin:
Good point about the Fender tremolo. I think I get stuck in the cathode biased amp mentality. This would be the first fixed bias amp build for me (only my third tube amp build ever). So, would there be any need or real advantage to adding a bias pot here?
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My best friend growing up was a huge Don Rich fan. I enjoy Buck and Don, especially after consuming a few adult beverages. There's nuthin more funner than drinkin beer and playin Buck Owens songs. :icon_biggrin: Gotta put flatwounds on that tele to capture that early Don Rich tone.
https://www.youtube.com/watch?v=UkNsqdGm0wo (https://www.youtube.com/watch?v=UkNsqdGm0wo)
It's not essential to have a bias pot, but there certainly would be no harm in adding one to the circuit.
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On the bias pot issue - I have built both a 6G2 and a 6G3 - I put a bias pot in both and am glad I did - from my post in the 6G2 build thread (including a Monty Python reference):
My plate B+ is about 316, which is right on spec, but I found that with the standard -35V bias supply I could not set the power tube bias where I wanted (+/- 24ma). The highest I could get it was 13ma - pretty darn cold, so I changed the trim resistor from 27K to 22K and got a good range on the bias pot, and -28V.
Interesting to note that Rob Robinette's site states (for the 6G3 which uses the same trem circuit) that "Bias is non-adjustable because adjusting the bias will change the intensity of the tremolo which is the major weakness of bias wiggle tremolo." But I would counter that adding adjustable bias allows you to dial in the sweet spot. I found that if you bias the tubes too hot, the tremolo starts to make a flutter noise like the wing beat of an unladen African Swallow. But setting to about 60-70% of max range gets great drive and nice tremolo. So I suspect Fender biased these cool intentionally to avoid issues with the range of tubes that might be installed.
I suppose that an amp manufacturer wants to make sure that the amp will function ok with any set of power tubes - but we have the ability (ok, need) to tinker, eh? So once you decide on your power tubes, you can dial in the trim resister and set the bias to your own ears.
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Great info bmccowan. Thanks!
As everyone here has probably always known, the big expense of amp building - which I'm now realizing - is in the tx's, chassis, tubes, cabinet and speaker(s). In other words, if I'm going to commit to investing $800 or more in an amp I need to be sure this is the amp I really want.
Another option - and, I'm leaning this way - is to not build this as a combo (but, just as a chassis) and buy a 12" speaker cabinet that could also be used with my AC4, etc. I'll never be using more than one amp at a time.
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I built mine as a head for that reason and for the weight issue.
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I built mine as a head for that reason and for the weight issue.
Nice! :thumbsup:
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That looks great bmccowan!
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Thanks - I bought the chassis and plates from Marsh. I built the cab from pine - I enjoy the woodwork, but hate gluing up Tolex.
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Just a thought. If you're searching for a new project, why not build a ReVibe. You could use this with your AC4 as well as any other amp that you may build or acquire in the future. I'm not much of a reverb fan, but I always preferred the 6G15 reverb unit to Fender amps with on-board reverb, and the ReVibe is an iteration of the Fender harmonic tremolo that arguably is one of the best trems ever.
https://el34world.com/Hoffman/files/Hoffman_ReVibe.pdf (https://el34world.com/Hoffman/files/Hoffman_ReVibe.pdf)
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Just a thought. If you're searching for a new project, why not build a ReVibe. You could use this with your AC4 as well as any other amp that you may build or acquire in the future. I'm not much of a reverb fan, but I always preferred the 6G15 reverb unit to Fender amps with on-board reverb, and the ReVibe is an iteration of the Fender harmonic tremolo that arguably is one of the best trems ever.
https://el34world.com/Hoffman/files/Hoffman_ReVibe.pdf (https://el34world.com/Hoffman/files/Hoffman_ReVibe.pdf)
Now that's an interesting idea! I've been looking at that harmonic tremolo circuit in the "6-series" amp schematics, and I've heard it's really good. It would be a challenging project, too. I'm going to study up on that. I could definitely get a lot of use out of one. Thanks!
edited to add: I could build that, and buy a speaker cabinet for less than a full-blown combo project, too.
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Thinking it over, I'm going to build the 6G3. Mojotone has a chassis/faceplate combo that's reasonably priced. That saves a lot of drilling and filing by hand. I'll use the Hoffman board. I can buy a head cabinet later, after it's built.
But, I'm finding these old Fender layouts difficult to follow in places.
In the detail below, the line with "+270V" beside it is clearly connecting B+ to the two plates of the first 12AX7 via the 220k load resistors. So, I guess the X's inside the boxes must indicate that those two points are connected under the board, since the other X leads to the second 12AX7 plate via the 15k and 100k resistors. The little circle with the dotted line makes no real sense, unless it's trying to imply that the connection is made under the board. I assume that the grounded line with "Black" beside it simply means that B+ gets grounded inside the chassis, as opposed to being grounded at the doghouse?
It's not a big deal, I can follow the schematic to see where things are connected. I need to Google photos of actual chassis wiring. Then, the odd symbols in the Fender layout will make sense. I just need a visual reference to verify things.
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OK, this photo clears up the drawing on the layout and my assumption about the black grounding wire...
But, why run the yellow wire through the hole and connect it to B+ under the board? What's the reason for that?
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But, why
1st thought, safer;
as the "B+" wire bounces to the beat n saws into the eCap it's riding on
keeps it close to chassis to possibly help with PS noise
looks cooler, but that's cosmetic :icon_biggrin:
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Thanks shooter.
I also see that if soldered on top it would create a junction of five connections in one place.
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Thinking it over, I'm going to build the 6G3. Mojotone has a chassis/faceplate combo that's reasonably priced. That saves a lot of drilling and filing by hand. I'll use the Hoffman board. I can buy a head cabinet later, after it's built.
But, I'm finding these old Fender layouts difficult to follow in places.
In the detail below, the line with "+270V" beside it is clearly connecting B+ to the two plates of the first 12AX7 via the 220k load resistors. So, I guess the X's inside the boxes must indicate that those two points are connected under the board, since the other X leads to the second 12AX7 plate via the 15k and 100k resistors. The little circle with the dotted line makes no real sense, unless it's trying to imply that the connection is made under the board. I assume that the grounded line with "Black" beside it simply means that B+ gets grounded inside the chassis, as opposed to being grounded at the doghouse?
It's not a big deal, I can follow the schematic to see where things are connected. I need to Google photos of actual chassis wiring. Then, the odd symbols in the Fender layout will make sense. I just need a visual reference to verify things.
Good plan. :thumbsup:
IMO, your biggest challenge will be finding a power transformer that will give you a target B+ voltage 370V DC with a GZ34 rectifier, that will also fit the chassis. IMO, the transformer that will get you closest voltage wise is the Hammond 272DX, but it will not fit the Mojo chassis.
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The Hammond 290BX is the best option IMO that will fit the chassis, but the B+ voltage will come in around 400V DC with a 5U4 rectifier. That wouldn't be a bad option, but you will need an adjustable bias pot to optimize the bias.
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The Hammond 290BX is the best option IMO that will fit the chassis, but the B+ voltage will come in around 400V DC with a 5U4 rectifier. That wouldn't be a bad option, but you will need an adjustable bias pot to optimize the bias.
Yeah, that's the one Hammond lists as a drop-in replacement for the 125P2A.
Classictone has one with multiple secondary windings. This datasheet shows 375V with a 5U4GB:
http://www.classictone.net/40-18028.pdf
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That 375V is at a 120 milliamp current draw. The voltage at 120 ma would be pretty much full tilt boogy for the 6G3. It will be around 400 volts at idle. The 600 volt taps will get you closer.
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Thanks.
So, my math skills are nonexistent. Using Tubenit's current draw pdf I get the following range for tube current draw in the 6G3:
Preamp 12AX7 - 1-2ma * 2 = 2-4ma
PI 12AX7 - 5-10ma * 1 = 5-10ma
6V6 - 70ma/pair = 70ma
Totals = 77ma - 84ma
So, how do I calculate the PT's voltage drop with loads of 77ma to 84ma with 120V wall AC?
Then, I guess I would multiply that figure by the GZ34's voltage increase factor of 1.3 to get plate voltage?
I'd like to be able to calculate rough estimates when shopping for transformers.
Thanks,
David
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Unfortunately Classictone doesn't give much information to work from. It would be nice if they provided no-load voltages or winding resistance specifications to go along with the voltage at the current rating. That is one of the reasons why I am not a fan of Classictone power transformers. Another is that there are inconsistencies/errors in the information that they do provide. Take for example the voltages that they provided. They show a 5U4GB dropping more voltage than a 5Y3GT at the same current load.
Using the voltage at current rating that they provide you can however back into an estimate for the idle voltage using graphic measurement of the rectifier tube operating characteristics graph. No math is needed. With 330 volts AC applied to the plates a 5AR4/GZ34 will provide about 385 volts DC at 120 ma. If we draw a line parallel with the AC voltage lines on the chart it leads us back to about 410 volts DC at 75 ma. These tube data charts are useful in selecting a transformer. For example pick a desired idle voltage at a desired current and mark that point on the chart. You can then estimate the amount of rectifier sag by drawing a line from the idle current point to the maximum signal current point on the graph.
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That's really cool 66Strat!
So, I can see that if I want about 375V DC output from the rectifier at 75ma ( 355@120ma) I would need just under 300V AC input to the rectifier.
Using the Classic Tone datasheet, that would require the R/W wires and a 5Y3GT giving 354VDC at 120ma.
Thanks for that lesson. :icon_biggrin: Very useful knowledge.
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You can't rely on the Classictone numbers. The 5Y3 numbers are way off. The numbers are more in line with what one would expect from a 5U4. The red/white (600 volt) wires with either a 5AR4 or a 5U4GB will get you closer to your target voltage and will not sag like a 5Y3.
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OK, got it. Thanks.
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The 5AR4 will come in around 380 volts at 75ma. A 5U4 will come in around 365 volts at 75 ma. Either would work well.
Verbiage added:
These numbers reflect how one would expect the rectifiers to perform. But we still have no idea as to the DC resistance of the transformer. The transformer AC voltage at the specified load of 120 ma. (300 volts) will be higher at the 75 ma. load. For that reason, I would opt for the 5U4GB.
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Sounds good. Thanks for calculating that! I'd rather be on the low side anyway.
I'll also add that the PT doesn't have to fit the chassis cutout. I'm not planning on making a physical replica. Hopefully, a sonic replica, but who knows what these things sounded like 60yrs ago.
So, something like this might work, too:
https://www.edcorusa.com/xpwr014
I used Edcors in my EL34 stereo amp. They are affordable and of very good quality. But, they are built to order and take a while to get.
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Sounds good. Thanks for calculating that! I'd rather be on the low side anyway.
I'll also add that the PT doesn't have to fit the chassis cutout. I'm not planning on making a physical replica. Hopefully, a sonic replica, but who knows what these things sounded like 60yrs ago.
So, something like this might work, too:
https://www.edcorusa.com/xpwr014
I used Edcors in my EL34 stereo amp. They are affordable and of very good quality. But, they are built to order and take a while to get.
Edcor does make nice transformers. If it doesn't need to fit the chassis, the Hammond 272DX would be a good option that is readily available from several online retailers for about $80.
The 6G3 sounded real nice 52 years ago. :icon_biggrin:
https://www.hammfg.com/files/parts/pdf/272DX.pdf (https://www.hammfg.com/files/parts/pdf/272DX.pdf)
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Cool. Would you still recommend a 5U4 with the 272DX?
I was in the first grade 52yrs ago. But, I was watching American Bandstand with my older sister! :icon_biggrin:
There were these brothers in the neighborhood who had a - what seemed back then like a huge - Kustom amp with the vinyl glitter upholstery. Though not a tube amp, it must have been the first guitar amp I ever saw. They would put it in the driveway and blast the neighbors.
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I would use either a 5U4GB with the Hammond 272DX or a 5AR4 with the Hammond 270EX.
Attached are the 5AR4/GZ34 operating characteristic graph with an approximation of the Hammond 270EX AC voltage supply superimposed and the 5U4GB operating characteristic graph with an approximation of the Hammond 272DX voltage supply superimposed. For either tube and transformer combination, I would use a lower estimate for 6V6 tube current for the tremolo to operate properly (about 70% of maximum plate dissipation). To find an estimate of the DC voltage under load, extend a vertical line from estimated current under load to it's intersection with the curved blue line. Extend a horizontal line to the left from this intersection to find the DC voltage output on the graph.
I have also attached a copy of the 6G3 Schematic for reference. In real life, one would expect the DC voltage to be considerably higher than 375V DC with 333V AC under load applied to the plates. The schematic power supply numbers are more reflective of what would be expected with a 5Y3 rectifier rather than a 5AR4/GZ34. In comparison, the AB763 Deluxe schematic reflects 420 V DC under load with 325 V AC applied to the plates of a GZ34.
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_ab763_schem.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_ab763_schem.pdf)
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Thanks again 66Strat; I'll need to take some time to read through that and study the graphs.
...I would use a lower estimate for 6V6 tube current for the tremolo to operate properly (about 70% of maximum plate dissipation).
Not sure what this means. Are saying to alter the bias voltage (by using a bias pot) to lower the plate dissipation to 70%? If I used sluckey's trem-o-nator would that alleviate the need for that?
In the mean time, here's a good quality schematic that I cleaned up in Photoshop.
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Unfortunately Classictone doesn't give much information to work from. It would be nice if they provided no-load voltages or winding resistance specifications to go along with the voltage at the current rating. That is one of the reasons why I am not a fan of Classictone power transformers. Another is that there are inconsistencies/errors in the information that they do provide. Take for example the voltages that they provided. They show a 5U4GB dropping more voltage than a 5Y3GT at the same current load.
66, I have used quite a few Classictone transformers - good quality at good prices. But I have been annoyed by the exact same thing. Its a real head scratcher that they publish so much misinformation. You are prompting me to send them an email.
And dwinston - my experience with that amp is if the dissipation climbs much above 70%, the trem start to produce a bit of a thump. Nothing wrong with 70% though. Its a good sounding trem - as is Sluckey's by all accounts. I'm going to try his soon in a blackface concert chassis I picked up.
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If I used sluckey's trem-o-nator
don't believe it works in your configuration, the ToN wants to wiggle the signal not bias as your version does.
here's how I stuffed it into my last build, compare schematics so you have an understanding the difference.
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Thanks again 66Strat; I'll need to take some time to read through that and study the graphs.
...I would use a lower estimate for 6V6 tube current for the tremolo to operate properly (about 70% of maximum plate dissipation).
Not sure what this means. Are saying to alter the bias voltage (by using a bias pot) to lower the plate dissipation to 70%? If I used sluckey's trem-o-nator would that alleviate the need for that?
In the mean time, here's a good quality schematic that I cleaned up in Photoshop.
No. It appears that the 35 ma number was referenced from the 6V6GT data sheet. The 35 ma figure is valid for only one set of AB1 operating points. At a plate voltage of 285 volts DC, with 35 ma of plate current, plate dissipation is 9.975 watts or 71.25% of 14 watts at this operating point. As plate voltage is increased, plate current needs to be decreased, to remain in class AB1, and for the tremolo to operate properly. The intent of my post was to illustrate the voltage sag associated with transformer and rectifier IR losses and to provide you with some predictive data so that you could select your own operating points. A bias pot, while not a necessity, would be a good idea to help dial the amp in. I would also add 1 ohm 1% cathode resistors to each 6V6 cathode to measure tube current.
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don't believe it works in your configuration, the ToN wants to wiggle the signal not bias as your version does.
Thanks shooter. But, that's what I was trying to say: if I use a trem that doesn't wobble the bias voltage would that alleviate the problem of pushing the 6V6's above 70%. Or, am I just completely confused about all of this?
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66Strat, this is all starting to go way over my head. I'll need to back up and review what I do understand.
I've only built cathode biased amps, never a fixed bias type. So, I really don't know what it means to "estimate a lower tube current." I understand that I divide plate voltage (285V) by the plate current (.035A) to get dissipation in watts, but I don't know how to manipulate plate current in this type of amp.
So, are you saying that a plate voltage of 285V is too high, or is it the target I should aim for? And, should this be calculated with a signal or at idle? In other words, are you saying that the tremolo in the 6G3 will act weird if the PT supplies more than 285V to the plates unless I change the plate current?
Sorry, if I'm missing something obvious. I feel like I have to learn a whole new set of things with every new project.
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Using an online bias calculator I got 29.5ma for 70% of 6V6 dissipation. What I've never done and don't know how to do is achieve 29.5ma of plate current. I have no idea which components to change to get to this figure.
Or, do I need to lower the plate voltage to do this. 240V plate voltage and 35ma would hit the 70% target.
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66Strat, this is all starting to go way over my head. I'll need to back up and review what I do understand.
I've only built cathode biased amps, never a fixed bias type. So, I really don't know what it means to "estimate a lower tube current." I understand that I divide plate voltage (285V) by the plate current (.035A) to get dissipation in watts, but I don't know how to manipulate plate current in this type of amp.
So, are you saying that a plate voltage of 285V is too high, or is it the target I should aim for? And, should this be calculated with a signal or at idle? In other words, are you saying that the tremolo in the 6G3 will act weird if the PT supplies more than 285V to the plates unless I change the plate current?
Sorry, if I'm missing something obvious. I feel like I have to learn a whole new set of things with every new project.
To calculate plate dissipation, you multiply plate current times plate voltage. (285 x .035 = 9.975 watts) In a fixed bias amplifier, to adjust plate current, you adjust the negative bias supply voltage by either substituting resistors in the voltage divider of the bias supply circuit or implement a bias voltage pot.
IF one chose to operate at a plate voltage of 365 volts (375 - 10 volts output transformer IR loss), 35 ma would result in 12.775 watts plate dissipation (91.25% of design maximum of 14 watts). This is on the hot side of things for class AB1 operation and too hot for the tremolo to work properly IMO. Seventy percent of the Design Maximum plate dissipation of 14 watts or 9.8 watts would be a better target. The 9.8 watts plate dissipation divided by the plate voltage of 365 equals 26.85 milliamps of plate current, plus 2 milliamps for screen current equals 28.85 ma current for each 6V6 tube. To measure the current, measure the voltage across the 1 ohm cathode resistor of each 6V6 tube with the volt ohm meter set to millivolts. Each millivolt of cathode voltage equates to 1 milliamp of tube current. For 70% plate dissipation at 365 plate volts, the cathode voltage reading would be 28.85 millivolts across the 1 ohm resistor. To adjust the bias current adjust the negative bias voltage until you reach the 28.85 millivolts across the cathode resistor.
Keep in mind that the plate voltage will move some as you manipulate the bias voltage. This is where the rectifier graphs that I previously posted come into play as a starting point to estimate what the expected DC voltage will be at an estimated current. Use the charts as a starting point to establish the operating conditions and then fine tune the circuit by adjusting the bias voltage.
The starting points would be about 360 DC supply volts at 65 milliamps with the 270EX transformer and a 5AR4 rectifier or about 400 DC supply volts at 60 milliamps with the 272DX transformer and a 5U4 rectifier. Back out 10 volts or so to account for the IR loss in the output transformer and then adjust the negative bias voltage until the cathode current reads 30 ma for the 270EX 5AR4 combination or 27 ma for the 272DX 5U4GB combination.
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this is all starting to go way over my head. I'll need to back up and review what I do understand
dwinstonwood, in my opinion you can make this as simple or as complicated as you want. If you are wanting to build a 6G3 (and I think you still are) - great choice. There are confirmed schematics and scores of folks without the textbook type knowledge have built that amp and many have added adjustable bias and have kept the bias wiggle type tremolo. I always get lost if I jump into details before I understand basics. You say that you are missing a basic understanding of how fixed bias amps work. I suggest that you read Rob Robinette's pieces, and/or others' on that subject. And then you can dive as deep as you want into charts, calculations, modelling, etc. I am not taking away from anything that 66 is stating - he knows a lot of technical details that I do not, but I sympathize with your :w2:
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Thanks bmccowan. I have confidence that I can learn the basics of this stuff, but like you said, it's confusing diving into the middle of it. I actually did read Robinette's info last night and it cleared things up a good bit.
Thanks 66Strat. I was tired and frustrated last night and typed a bunch of errors. I meant to say 365V and to multiply ma by plate v to get dissipation. I was also mistakenly looking at 6V6's (12w max), not 6V6GT's (14w max).
So, I now understand the basic workings of biasing the amp. I can grasp how making the grid voltage more or less negative in relation to the cathode will adjust the idle current.
What I am starting to glimpse here is that finding a currently available PT is going to be a tightrope act involving the best rectifier choice and the correct negative bias voltage on the grid. So, this might mean disregarding the numbers on the Fender schematic to some extent in order to hit the 70%, 10 watt dissipation target.
Having said all that, I'm fine with running the 6V6GT's at 60%, or even less - that is, creating more leeway as opposed to trying to hit 70% on the nose. A PT/rectifier combo resulting in above or below 375 plate voltage if fine with me, if it works.
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Thanks bmccowan. I have confidence that I can learn the basics of this stuff, but like you said, it's confusing diving into the middle of it. I actually did read Robinette's info last night and it cleared things up a good bit.
Thanks 66Strat. I was tired and frustrated last night and typed a bunch of errors. I meant to say 365V and to multiply ma by plate v to get dissipation. I was also mistakenly looking at 6V6's (12w max), not 6V6GT's (14w max).
So, I now understand the basic workings of biasing the amp. I can grasp how making the grid voltage more or less negative in relation to the cathode will adjust the idle current.
What I am starting to glimpse here is that finding a currently available PT is going to be a tightrope act involving the best rectifier choice and the correct negative bias voltage on the grid. So, this might mean disregarding the numbers on the Fender schematic to some extent in order to hit the 70%, 10 watt dissipation target.
Having said all that, I'm fine with running the 6V6GT's at 60%, or even less - that is, creating more leeway as opposed to trying to hit 70% on the nose. A PT/rectifier combo resulting in above or below 375 plate voltage if fine with me, if it works.
Bingo!
There are some numbers on the Fender schematic that can be used to back in to an operating point. Others appear to be artifacts left over from a different transformer that Fender may have experimented with prior to production. The voltage under load numbers are very similar to 6G9 power transformer voltages.
This reminds me of a motorcycle safety course that I took some years back. The instructor taught us to avoid target fixation, and to look through the curve at where we wanted to go. On a bike, you go where you look. If you look at the ditch, that's where you wind up. Don't fixate on achieving the 375 V DC number. Any DC plate voltage between 350 and 400 will work well in the circuit. You just have to adjust the bias according to what you have.
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This reminds me of a motorcycle safety course that I took some years back. The instructor taught us to avoid target fixation, and to look through the curve at where we wanted to go. On a bike, you go where you look. If you look at the ditch, that's where you wind up. Don't fixate on achieving the 375 V DC number. Any DC plate voltage between 350 and 400 will work well in the circuit. You just have to adjust the bias according to what you have.
Spot on advice - I too ride motorcycles and got the same advice. When I took up mountain biking - it was the same. A friend of mine races and teaches pro rally off road driving - same advice. Look where you want to go.
Have fun building the amp!
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avoid target fixation
works with rally cars also :icon_biggrin:
I call it "the line", it's the brain doing complex math without numbers, an hopefully without trees n ditches and...... :laugh:
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I got the exact same advice, too! But for snow skiing. Same principle. When you stop looking at the tips of the skis and start looking at the path ahead you stop wiping out. :icon_biggrin:
66Strat, thanks for the starting point calculations. The 270EX/5AR4 combo looks good to me.
The one blindspot for me is how you arrived at 65ma? Why not 60ma and 370V, for example. You mentioned "28.85 ma current for each 6V6 tube." What am I missing here?
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This reminds me of a motorcycle safety course that I took some years back. The instructor taught us to avoid target fixation, and to look through the curve at where we wanted to go. On a bike, you go where you look. If you look at the ditch, that's where you wind up. Don't fixate on achieving the 375 V DC number. Any DC plate voltage between 350 and 400 will work well in the circuit. You just have to adjust the bias according to what you have.
Spot on advice - I too ride motorcycles and got the same advice. When I took up mountain biking - it was the same. A friend of mine races and teaches pro rally off road driving - same advice. Look where you want to go.
Have fun building the amp!
One of my bucket list items is to go to the salt flats during speed week. I would love to ride my bike as fast as I can go on the flats. When I was a sophomore in high school a couple of friends and I rode our bikes down to Myrtle Beach. My parents went to Disney World on vacation and I stayed home. They asked what my plans were. I said that that I might go camping, and I did. My friends and I camped on the beach amongst some dunes next to a large family campground. We used the showers and toilet facilities at the campground. There was a beach club that we went to at night. The Friends of Distinction and Junior Walker & the All Stars were appearing at the time. Both of these songs were on the radio at the time.
https://www.youtube.com/watch?v=-UHsjvPOZ3c (https://www.youtube.com/watch?v=-UHsjvPOZ3c)
https://www.youtube.com/watch?v=ub72eylahJg (https://www.youtube.com/watch?v=ub72eylahJg)
That was one of the best vacations I ever had.
I meant to ask earlier, what transformers/voltages did you use/get in your 6G3?
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I got the exact same advice, too! But for snow skiing. Same principle. When you stop looking at the tips of the skis and start looking at the path ahead you stop wiping out. :icon_biggrin:
66Strat, thanks for the starting point calculations. The 270EX/5AR4 combo looks good to me.
The one blindspot for me is how you arrived at 65ma? Why not 60ma and 370V, for example. You mentioned "28.85 ma current for each 6V6 tube." What am I missing here?
If you look at the 6G3 schematic, it shows a plate voltage of 365 volts. Divide the target 9.8 watts plate dissipation by the plate voltage and you arrive at a plate current of 26.85 ma, adding 2 ma for screen current gets you to 28.85 ma. of tube current for each 6V6. Current for two 6V6 tubes is 57.7 ma add in 6 ma of 12AX7 tube current (1 ma for each triode) gets you to 63.7 ma. I just round up to 65 ma. The intersection of the 65 ma line with the blue AC line gets you to the DC supply voltage of 360 volts. Subtracting 10 volts to account for the output transformer IR losses gets you to an anticipated plate voltage of 350 V DC. Subtracting 10 ma (4 ma for the screens + 6 ma for the AX7s) from 65 ma gets you to 55 ma of plate current for two 6V6 tubes (27.5 ma each tube). Plate dissipation at this operating point would come in at 9.625 watts or 68.75% of the 14 watt maximum plate dissipation limit.
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Perfect explanation! I forgot about the 12AX7 current. And the 68.75% range sounds great.
Well, it looks like you solved the difficult part of updating this amp 66Strat.
I could save some money by buying a chassis and punching it myself. I could use part of those savings to buy this and then save more money down the road:
https://store.triodestore.com/punchkit.html
That way, I can use an upright PT without the large hole in the custom 6G3 chassis.
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adding the 1 ohm cathode resistors makes all these #'s easier, lots easier!
I would love to ride my bike as fast as I can go on the flats
:laugh:
Once I dialed in driving my Evo, took it to TN, found a hair-pin twisty straight up, 180 turn, twisting turning down. hit ~115 up, running gears down fast for the apex and there was a TN State Trooper just parked! he just pointed down with his finger, I nodded thanks, as I past, he said, "watch out for the 3rd turn". Sure enough, I read it wrong, but his warning saved me from a 200' short-cut :icon_biggrin:
commissioned an artist friend to draw this up for me :headbang:
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Perfect explanation! I forgot about the 12AX7 current. And the 68.75% range sounds great.
Well, it looks like you solved the difficult part of updating this amp 66Strat.
I could save some money by buying a chassis and punching it myself. I could use part of those savings to buy this and then save more money down the road:
https://store.triodestore.com/punchkit.html
That way, I can use an upright PT without the large hole in the custom 6G3 chassis.
Keep in mind that I am not advocating this transformer/rectifier combination and that the numbers represent best guesstimates to get in the ball park as a starting point for this combination.
You might check this guy out. He will make a chassis to your specs, including all punch-outs and holes. It might be worth your while to sketch out the chassis dimensions and layout for tubes and control holes on his order form and email it to him for a quote. He also does faceplates. I haven't used him, but his products look promising.
http://www.juicyamps.com/chassis.html (http://www.juicyamps.com/chassis.html)
adding the 1 ohm cathode resistors makes all these #'s easier, lots easier!
I would love to ride my bike as fast as I can go on the flats
:laugh:
Once I dialed in driving my Evo, took it to TN, found a hair-pin twisty straight up, 180 turn, twisting turning down. hit ~115 up, running gears down fast for the apex and there was a TN State Trooper just parked! he just pointed down with his finger, I nodded thanks, as I past, he said, "watch out for the 3rd turn". Sure enough, I read it wrong, but his warning saved me from a 200' short-cut :icon_biggrin:
commissioned an artist friend to draw this up for me :headbang:
Tail of the Dragon?
ISP passed me while i was doing 110 on I-65. He shook his head and gave me the same slow down motion. :icon_biggrin:
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I meant to ask earlier, what transformers/voltages did you use/get in your 6G3?
It was a ClassicTone PT - and I remember being confused by the spec sheets :icon_biggrin: I do not recall which model, but I'll look when I get home, as I'm away for the weekend. I think it was one of the ones with 2 secondary taps and I used the lower voltage red/white taps. But can't trust the memory. I was thinking I built it in 2017, but I found the chassis order and it was 2013!
Now old and foolish, I ride a Moto Guzzi and am pretty happy staying under 80mph these days. She'll do more, but the top end starts sounding like a washing machine full of nuts and bolts.
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bmccowan, yes, please post the specs for your PT when you get a chance.
I'm looking into Edcor again. I got great service from them and a good quality product. Here's two that might work.
XPWR224: 540V(270-0-270)@125mA, 6.3V(3.15-0-3.15)@3A, & 5V@2A
https://www.edcorusa.com/xpwr224
and,
XPWR163: 550V(275-0-275)@150mA, 6.3V(3.15-0-3.15)@5A & 5V@3A
https://www.edcorusa.com/xpwr163
I will attempt to plot 66Strat's graphs tonight with these two.
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I been using Edcor >10yrs, nice stuff
while you're learn'n curves, keep you rec tube 5V Amps in mind, you listed a 2 and 3A
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Thanks shooter. That's another spec I don't know how to figure out: not sure how to use those amperage numbers or what they mean exactly.
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OK, another PT option! Sorry for posting so many.
Any reason why the Hammond Harvard 290HAX replacement wouldn't work? It's 275-0-275 @ 100ma, has a 50v bias tap, and 6.3v center tap:
https://www.hammfg.com/files/parts/pdf/290HAX.pdf
Thanks
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OK, another PT option! Sorry for posting so many.
Any reason why the Hammond Harvard 290HAX replacement wouldn't work? It's 275-0-275 @ 100ma, has a 50v bias tap, and 6.3v center tap:
https://www.hammfg.com/files/parts/pdf/290HAX.pdf
Thanks
It could work. The heater and rectifier currents are OK. The secondary current rating is marginal. As a result, the B+ supply voltage will sag more under load. But there is nothing inherently wrong with that, or anything there to cause the Earth to spin backwards, or cause the magnetic poles to reverse. It's all a mater of personal taste. I would encourage you to plot the dynamic load lines of any transformer that you consider. It is a helpful learning experience.
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Thanks 66Strat. Will do!
Comparing the Harvard 5F10 to the Vibrolux 5E11 (and, 5F11), they are remarkably similar circuits in component values and stated voltages. I'll bet Fender used the same PT for these amps, and probably others, too, maybe even the 6G3. All three of these are 6V6GT amps with plate voltages of 365V or less. The 5F10 and 5F11 used 5Y3GT's and a different PI than the 6G3.
Years ago I really wanted a Harvard because I read that Cropper used one in the Stax studio. From what I can see, the Tweed Vibrolux is a Harvard with tremolo. Now, that's looking interesting...
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As long as you're investigating other tremolo amps, check out the 5E9-A Tweed Tremolux. This amp mates the preamp of a
5D3 5E3 Deluxe with the phase inverter and power amp topology of a 5B3 Deluxe. The tremolo wiggles the bias of the phase inverter.
https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_5e9a.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_5e9a.pdf)
https://www.youtube.com/watch?v=-Rmqaqe0y7k (https://www.youtube.com/watch?v=-Rmqaqe0y7k)
https://www.youtube.com/watch?v=1cxoi8P2XSI (https://www.youtube.com/watch?v=1cxoi8P2XSI)
Edited to change 5D3 to 5E3
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That's another spec I don't know how to figure out
66Strat was using a 5U4 example, that tube needs a 5V winding at 3A, a 5Y3 requires a 5volt winding at 2A. that data is on the tube datasheets
SS rectifiers don't need an extra winding
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Cool. But, I'm going to try and stay the course with the 6G3. In that first video, near the end, sounded like X's Billy Zoom playing Hungry Wolf. Maybe I need a Sparkle Jet, too.
Thanks shooter. There're a heck of a lot of details to keep up with, aren't there? :think1:
I tried to graph the Hammond 290HAX for the 5AR4. A bit lower plate voltage I think.
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So, let's say 9.8/355=27.6ma + 2ma screen current = 29.6 x 2 = 59.2 + 6ma 12AX7 current = 65.2ma. I tried to draw the graph at 65ma and 275V.
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There're a heck of a lot of details to keep up with, aren't there?
:laugh:
that's why humans invented computers :icon_biggrin:
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Cool. But, I'm going to try and stay the course with the 6G3. In that first video, near the end, sounded like X's Billy Zoom playing Hungry Wolf. Maybe I need a Sparkle Jet, too.
Thanks shooter. There're a heck of a lot of details to keep up with, aren't there? :think1:
I tried to graph the Hammond 290HAX for the 5AR4. A bit lower plate voltage I think.
Good estimate. :thumbsup:
The transformer is rated 275V at 100 ma. The no load voltage is 296.4 volts per plate. The transformer AC voltage will sag down from 296.4 volts at 0 ohms current draw to 275 volts at 100 ma. With a 65 ma. load the AC voltage will be a little higher than 275 volts ~282 volts. Not enough difference to matter, just mentioned to show the effect transformer IR losses have. The lower the VAC current rating, the more the transformer voltage will sag under load.
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Thank's for teaching me how to graph all of this! It's a great thing that this can be done visually on a graph. I need to be able to "see" things to really understand them.
I've heard the amplifier term "sag" for years and only thought of it as a tone factor that some might be able to hear from the speaker. But now I can see what it actually is, why it happens, and how it affects the operation of the power tubes.
And, 282V - ironically - graphs out closer to the 365 plate voltage on the schematic. :laugh: Makes me think Fender might have been using the same PT when originally designing the 6G3, 5F10, and 5E11...
The Harvard 290HAX is a bit more expensive, but it's a much better match than the 290BX that Hammond reccomends for the 6G3. The 290BX seems more like an AB763 PT.
Well, I've burnt up a few brain resistors trudging through all this, but I'm a lot farther along than I was. Much appreciated 66Strat.
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Thank's for teaching me how to graph all of this! It's a great thing that this can be done visually on a graph. I need to be able to "see" things to really understand them.
I've heard the amplifier term "sag" for years and only thought of it as a tone factor that some might be able to hear from the speaker. But now I can see what it actually is, why it happens, and how it affects the operation of the power tubes.
And, 282V - ironically - graphs out closer to the 365 plate voltage on the schematic. :laugh: Makes me think Fender might have been using the same PT when originally designing the 6G3, 5F10, and 5E11...
The Harvard 290HAX is a bit more expensive, but it's a much better match than the 290BX that Hammond reccomends for the 6G3. The 290BX seems more like an AB763 PT.
Well, I've burnt up a few brain resistors trudging through all this, but I'm a lot farther along than I was. Much appreciated 66Strat.
Very possible. As I mentioned before, the 375 volt number looked like an artifact from a previous iteration. It's also close to the voltage that would be expected with a 5Y3 rectifier under load at 333 VAC.
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One manufacturer I didn't really look into is Heyboer. They have no specs on their website, and since I wasn't sure what specs I needed I didn't bother to email them. But, I'm considering the PT issue resolved. I'll order a 290HAX.
Also, I might just go ahead and order a custom Hoffman turret board instead of the eyelet version. I already have experience soldering up turrets, and I know what I'll do differently/better this time. Like following these instructions more closely:
https://el34world.com/Hoffman/instructions.htm
This drawing has the same dimensions as the eyelet board and follows the Fender layout as close as I get it.
Still a lot of drawing left to do...
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Home again. That Hammond should do fine.
It was a ClassicTone PT - and I remember being confused by the spec sheets :icon_biggrin: I do not recall which model, but I'll look when I get home, as I'm away for the weekend. I think it was one of the ones with 2 secondary taps and I used the lower voltage red/white taps. But can't trust the memory. I was thinking I built it in 2017, but I found the chassis order and it was 2013!
As is typical my memory was wrong. The PT is a Weber 25130 which is 540V@150ma on the red/white lower voltage secondaries. I have just a tick under 350 on the plates using a GZ-34. I have not used Weber transformers for quite a while, not because they have failed me, but I just prefer to buy US made components. I did use a classic tone OT. If you are going to go the adjustable bias route, the Weber Lenora has a nice schematic and layout of that arrangement for a 6G3 circuit. I had to monkey with the value of the trim resister as the stock value didn't allow me to get to the sweet spot.
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Thanks bmccowan! I grabbed the Lenora schematic and layout from the Weber site. The designer said he played with some of the power supply voltage divider resistor values to get the plate voltages on his schematic. It will be interesting to see where I land with the Harvard PT, and whether or not I should adjust anything from the original schematic. But, the bias pot will definitely go in the amp! Much appreciated.
I need to start buying parts (I put together a BoM in a Google spreadsheet), because I keep getting tempted to make modifications. For, example: grafting on a 6GXX-like preamp with treble and bass, and/or getting in way over my head by swapping in the harmonic vibrato circuit. Or, building a 6G11 Vibrolux with 6V6GT's, and so on... :icon_biggrin:
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need to start buying parts
:icon_biggrin:
buy them by 10's on the commons, makes the next build cheaper
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:icon_biggrin:
buy them by 10's on the commons, makes the next build cheaper
You don't even have to say it. I paid more on shipping fees with the AC4 than I did for everything on the turret board. :sad2:
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I grabbed the Lenora schematic and layout from the Weber site. The designer said he played with some of the power supply voltage divider resistor values to get the plate voltages on his schematic. It will be interesting to see where I land with the Harvard PT, and whether or not I should adjust anything from the original schematic.
There is also a downloadable BOM spreadsheet for all of the Weber amps. Of course it has Weber part numbers but also the generic description. It can be edited so comes in handy as a shortcut for a BOM. I'm sure there are other BOMs out there too.
IMO, don't sweat hitting the exact B+ numbers too much. I sometimes play this amp with a 5R4 (not 5AR4) which adds sag dropping the B+ close to 40V. Amp still sounds great but somewhat different; you get nice grind at low volumes; kind of an oldtime blues vibe.
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Thanks bmccowan. Yeah, I'm not focusing too much on the schematic plate numbers anymore. 66Strat and you were finally able to get me to see that staying near or under 70% dissipation is the important target to hit.
One question I do have concerns resistors. I only selected 3 watt resistors for plate load, the B+ power filter section, and cathode bias locations. Are 1/2 watts fine for everywhere else, or is there another place where I need higher wattage rated parts?
Thanks
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Well this is controversial - resistor sniffing and capacitor sniffing. So I will tell you what I do - but there are many other opinions.
I rarely use carbon comp, unless I'm repairing a valuable old amp.
For most locations I use either 1/2 watt metal film - or 1 watt carbon film. This is partly due to Sluckey's warnings about the lower quality of 1/2 watt carbon films.
For plate load I now use Dale military spec metal films - Tube Depot and the big houses like Mouser have them - the W rating is confusing as the military rates these differently. But they are low noise (not that I can really tell, but what the Hell, they have values printed on them instead of color bands!)
For power rail - I think 3 & 2 watt are fine for amps in this category, but I usually use a 5 or 10W ceramic in the first dropping position.
There is not need to go any higher than 1/2 watt for other resistors including preamp cathode, plate load. Some go higher stating that higher rating resisters create less noise. Well, my ears are hammered, so I can't hear any difference.
Now - on to the signal cap sniffing - just kidding.
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OK, I hope I can explain this question in the correct way:
On the original schematic the bias voltage was tapped from a 333V AC secondary lead. The bias supply circuit used a 100K dropping resistor to get to -26V. See detail image.
The PT I intend to buy has a dedicated 50V AC secondary bias voltage supply. I modeled this in LTSpice. In order to achieve the -26V bias voltage I had to change the 100K resistor to a 3.9K resistor. This gave me a negative voltage very close to -26V. See detail image.
My question is: Have I successfully estimated a good dropping resistor value to start with given the 290HAX's 50V AC secondary bias supply?
Thanks!
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I don't model, but a few things.
You are right that you do not want a 100K resistor. That is being use to knock down the B+ to a useful range for bias voltage. I believe I used a 10K for that resistor. But you plan to use a bias trim pot, as I did, eh? If that's right, take a look at that Lenora schematic as its a 6g3 with a 40-50v PT bias tap and a bias trim pot. I recall playing with the resistor values to get the trim pot in a useful range. Keep in mind that it can vary with the pair of power tubes you use. Others can help you with the math and modelling - I tend to see what others did and then; trial>error>error>success. Both approaches reach the same destination.
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Thanks bmccowan. I don't put too much faith in LTSpice because I don't know it well enough to know if I'm using it right. :icon_biggrin: I did see the 10K in the Lenora schematic and I started with that, and ended up with the odd 3.9K. Like you, I will more than likely have to try a few resistor values until it's right. But, all of this is a ways down the road.
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> I don't put too much faith in LTSpice because I don't know it well enough to know if I'm using it right.
So what is "SINE 50 120"?
50V 120Hz? 50Hz 120V?
In my spice, the simmed V is *peak*. Transformers are marked in RMS. So if 50V transformer, my spice wants "70.7V".
120Hz wall-power is unlikely. Yes, a FULL-wave working on 60Hz makes 120Hz, but this is a halfwave; anyway you are specifying the "source" not the "result".
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> I don't put too much faith in LTSpice because I don't know it well enough to know if I'm using it right.
So what is "SINE 50 120"?
50V 120Hz? 50Hz 120V?
In my spice, the simmed V is *peak*. Transformers are marked in RMS. So if 50V transformer, my spice wants "70.7V".
120Hz wall-power is unlikely. Yes, a FULL-wave working on 60Hz makes 120Hz, but this is a halfwave; anyway you are specifying the "source" not the "result".
Once again PRR, you catch my mistakes! Thanks :worthy1:
I completely forgot about the 1.414 multiplier. I had learned about that when I first started using LTSpice, and then I immediately proceeded to un-learn it.
Correcting the AC voltage to 70.7 peak and going back to a 10K resistor results in...
about -25VDC.
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Hoffman has a nice write up on adding a bias pot if you choose to do so. I think that a bias pot combined with 1 ohm 1% cathode resistors to provide tube current measuring points would make bias adjustment a lot easier.
https://el34world.com/charts/Biascircuits.htm (https://el34world.com/charts/Biascircuits.htm)
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Thanks 66Strat. Yep, the 1ohm resistor method is definitely something I'll do. My hands aren't steady enough, and my nerves not brave enough to measure across the OPT primaries, or, whatever the other method entails.
I'll read the Hoffman guide.
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BTW, it's apparently OK to mod 6G3's:
https://reverb.com/item/8002544-dumble-modded-fender-brown-deluxe-1962?utm_source=google&utm_medium=cpc&utm_campaign=6481783183&merchant_id=240050081&utm_content=813512089571&gclid=EAIaIQobChMIks3Kvs6d6gIViq_ICh1dWQ6BEAQYASABEgInPvD_BwE
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I'm not sure how this will actually work out in real life, but in my model I need a 10K bias pot (which Hoffman sells), and the resistor values in the image below.
Doing this LTSpice stuff at least helps me see how the circuit works and how the components interact with each other, so it's worth my time.
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Howard Dumble modded 6G3 - $22,100. :laugh: But the seller is accepting offers. Maybe he'd knock the 100 bucks off?
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:icon_biggrin:
push it;
tell the computer to predict the voltage range -23 to -31 kinda thing
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:think1: It's not a sin to tape off that bias tap and use the HT winding just like the original amp.
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... 66Strat, thanks for the starting point calculations. The 270EX/5AR4 combo looks good to me.
The one blindspot for me is how you arrived at 65ma? Why not 60ma and 370V, for example. You mentioned "28.85 ma current for each 6V6 tube." What am I missing here?
If you look at the 6G3 schematic, it shows a plate voltage of 365 volts. ...
Just as with the advice to not put too much faith in LTSpice, don't put too much faith in the voltages listed on the 6G3 Deluxe schematic (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_6g3.pdf).
I've got an original 1962 Deluxe with its original 125P17A power transformer (Fender used 3 different PTs in this amp over time), and was told by other 6G3 owners that my amp's PT delivered lower B+ voltages than the 125P2A.
- My amp lands on 6.3vac for the heater winding at a hair over 117vac on the primary.
- When fed 117vac, I have 411vdc at the 1st filter cap, 405vdc on the 6V6 plates and 402vdc on the screens when using a GZ34 and drawing 24.7mA (~71% of 14 watts). Bias was at -33vdc.
- The high voltage winding output is 326.5v - 0 - 326.5v in this configuration.
- Preamp supply & plate voltages throughout are ~30v higher than the schematic says.
I used to also own a 1963 6G3 Deluxe and the 1st filter cap B+ was 440vdc, with 433vdc at the 6V6 plate, 432vdc screen, and bias voltage at -32.5vdc. Don't have the 6V6 plate current in my notes for this amp, but the power transformer was the third type as used in the Deluxe Reverb amps that followed the next year.
So don't get too hung up on the exact voltages listed in the schematic, and do add a bias pot to the bias supply. It just makes everything easier.
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Thanks for all the input everyone. I'm putting in at least one order tomorrow, so pics of real amp parts - instead of pc simulations - will appear soon.
sluckey, the thought of not using the 50V tap had crossed my mind. It's an alternative, for sure.
bmccowan, well, it does say "+$1 Shipping," so that could be considered a bargain.
shooter, I'm working on it! :icon_biggrin:
Thanks HotBluePlates. Whatever my plate voltages end up at will be fine. I might be mistaken about this, but from what I understand higher plate voltages can provide more clean headroom. Wasn't there a Music Man 130, or something, that had like 700V on the plates for that reason? Leo Fender's ultimate clean amp?
Since this is for home use I'd probably be happier with plate voltages nearer 365, or less. That's really the main reason I settled on the 290HAX PT (275-0-275) instead of a Deluxe Reverb type. Your 326.5v - 0 - 326.5v secondary windings seem closer to the latter. So, correct me if I'm misunderstanding the effects of higher plate voltages. But, the bias pot and the 1 ohm resistors will definitely be added.
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:icon_biggrin:
push it;
tell the computer to predict the voltage range -23 to -31 kinda thing
I took that as a challenge. And, you were pretty darn close to the voltage range I got. :icon_biggrin:
(and, notice I read Hoffman's bias guide and used a 27k resistor and a 47uF cap)
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per HBP;
Bias was at -33vdc.
now that you're a wizzz, see what R values you'll need to get it ~~~ -28 to -36 give or take a few :icon_biggrin:
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Thank (whatever God you want) that tube amps are a mix of art and science. I can do the art part and fake the science part. Yes, you will be happy at any voltage within shouting distance of 350.
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I just put in a pretty big order (for me, anyway) at Hoffman Amps. No turning back now. I did think to buy eyelet boards for the filter caps and bias circuit, along with the main board. I almost overlooked those.
AES has a 4th sale, so I'll save a bit on TX's and tubes. Mojotone has the chassis with front and back plates. I need to sell some stuff on eBay to justify a cabinet purchase. I'm still leaning towards a head. But, obviously, the amp can be built and tested without that.
Thank (whatever God you want) that tube amps are a mix of art and science...
bmccowan, fwiw, I'm pretty sure I failed every math class I ever took, and I know I never took a science class. :icon_biggrin: I'm what they call an ABD art historian.
shooter, I was going to use Ohm's law, not the software, to solve that riddle (which bias adjustment resistor value to use for -33vdc). But, I hit a wall trying to figure out how to understand the resistances in the bias circuit. I can calculate current and voltage drops when the resistors are all in series, but this looks confusing to me because the 27k goes to ground, so I gave up. :w2: Maybe a 6k8 instead of 10k? The Harvard schematic shows that value...
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but this looks confusing to me
:laugh:
that's why I build cathode biased amps :icon_biggrin:
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but this looks confusing to me
:laugh:
that's why I build cathode biased amps :icon_biggrin:
Yep, I spent a few hours trying to figure out if it's even possible - or, necessary - to draw output tube load lines for a fixed bias amp, and totally gave up. :BangHead:
The parts, transformers, and chassis (with face plates) have been ordered. I know very little about tubes, so I need to do some research on which ones I'll want to buy. But, the Tung-Sol 6V6GT's look interesting.
bmccowan, did you make that head cabinet yourself? It doesn't look like anyone stocks a 6G3 head cabinet; I'd probably have to order something custom made. The 6G3's chassis length is unique. Thanks.
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I rarely calculate bias voltage resistor values. I find it more than adequate to just know if the negative bias voltage will increase or decrease if I change this or that resistor. Knowing this and having a full stock of one watt and half watt resistors gets me the proper bias voltage every time, usually within 5 minutes.
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bmccowan, did you make that head cabinet yourself? It doesn't look like anyone stocks a 6G3 head cabinet; I'd probably have to order something custom made. The 6G3's chassis length is unique. Thanks.
Yes, I made it myself. I've been woodworking for quite a few years and have a full shop - so it was not difficult. You just have to careful to provide just enough clearance for the Tolex in the opening for the chassis. Finger joints are easy if you have a jig, a bitch if you don't. The Tolexing is not that difficult either, but I hate gluing that stuff on. Anyways, I think any of the companies that make that combo cabinet would make the head for you. Mather cabs look really nice. And I doubt Mojo makes their own cabs, so whoever does would likely make a head version of their cab to fit that chassis. There are also a few Ebay sellers who make nice bare pine cabs, if you want to do the covering. There are some really good tutorials on Youtube for gluing up the tolex or other covering.
On power tubes - I have a stash of NOS 6V6s, but I put JJs in my 6G3 and think they sound pretty darn good.
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Thanks bmccowan. I actually wrote to Peter Mather today. We're in the same state, close enough to drive to, in fact! He's made 6G3 head cabinets for Jim Nickelson of Lil'Dawg in NV, whom I'm also communicating with, so I'm sure he can build what I want. BTW, Guitar Cabinets Direct and Mojotone are both in Burgaw, NC, and they offer the same cabinet products, so I'm sure they connected, maybe?
For the second half of the 80's I was a cabinetmaker in Philly. I made custom furniture. Currently, I don't have a shop or tools. I really miss woodworking, but I don't have the room, living in a townhouse in the city. Anyway, I would definitely build my own cabinets if I had the tools and the space.
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Well you have the skills for sure. If I did not have a woodworking shop at home I would look into one of those shared shops (well post Covid I suppose) as I would really miss working with wood. At least amp building can be done in a more confined space. A friend of mine did a complete rebuild of a Harley Knucklehead in his living room - he's divorced. That story is sure to induce a Shooter story. :laugh:
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induce a Shooter story. :laugh:
:offtopic1:
I'm in the home stretch, modernizing a 1930's barn foundation, my woodworking skills are rough :icon_biggrin:
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per HBP;
Bias was at -33vdc.
now that you're a wizzz, see what R values you'll need to get it ~~~ -28 to -36 give or take a few :icon_biggrin:
... shooter, I was going to use Ohm's law, not the software, to solve that riddle (which bias adjustment resistor value to use for -33vdc). ...
You can't get a good answer using Ohm's Law for the bias supply (as drawn in the 6G3 schematic), because rectification is a non-linear process and doesn't adhere to Ohm's Law.
You could use Ohm's Law if you remove the 22kΩ resistor shown across the bias filter cap in the schematic, assume some voltage to be present at the schematic, and then replace the 22kΩ resistor with a rheostat+resistor. However, as Sluckey said it's often easier to figure this out with a handful of resistors than with a pencil or software.
... I spent a few hours trying to figure out if it's even possible - or, necessary - to draw output tube load lines for a fixed bias amp, and totally gave up. :BangHead: ...
No need to bother with that. You're not trying to design an amp, but rather build an amp to an existing design. Your main challenge will be figuring out if the transformers & speaker used approximate the original amp's transformers & speaker, cause everything else is pretty well spelled out (use the same resistor values, but don't get too hung up on whether the amp voltage match the schematic voltages, as those don't seem to match the real amps anyway).
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Implement Hoffman's better bias circuit.
https://el34world.com/charts/Biascircuits.htm (https://el34world.com/charts/Biascircuits.htm)
You will need to play with resistor values for the circled resistor to get the adjustment range that you want. Once implemented, biasing the amp will be much easier anytime you change output tubes.
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Thanks HotBluePlates. I've always been told I overthink things. :icon_biggrin: If I encounter something that's not clear, it's in my nature to try and figure it out. Thanks for keeping me focused.
66Strat, I've been working on adding the Hoffman bias circuit to my layout. I've also been watching videos on measuring and adjusting fixed bias amps.
From what I understand, measuring across the 1 Ohm cathode resistors - that I will add - and measuring from pin 3 of the 6V6GT's to ground will give me the info I will need to calculate plate dissipation.
If it's not in the 60% to 70% ballpark, and I can't get it there with the bias pot, I will start changing the resistor you circled in red.
I'm guessing one needs to work quickly when doing this in case the negative grid (bias) voltage is way off.
I will measure the 290HAX's secondary bias tap, confirm that all resistor values are within tolerance, and quadruple check all wiring and solder joints before that first "roll-of-the-dice" start up.
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I'm guessing one needs to work quickly when doing this in case the negative grid (bias) voltage is way off.
The smart thing to do is set the bias pot for maximum negative voltage at pin 5 of the output tubes. DO THIS BEFORE YOU EVEN PLUG IN ANY TUBES!
I will measure the 290HAX's secondary bias tap, confirm that all resistor values are within tolerance, and quadruple check all wiring and solder joints before that first "roll-of-the-dice" start up.
I'm pretty sure that if you use the bias tap on that PT you will need to change that 10K to about 470Ω. Get the bias voltage right BEFORE YOU EVEN PLUG IN ANY TUBES!
Your head must be in overload from all this preliminary info. I hope you will remember some of this when/if you ever build this amp. :icon_biggrin:
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Thanks sluckey. That's something that I think I can see and understand.
When I'm ready to take bias measurements I will:
1.) With amp off, rectifier in, but power tubes not installed, connect my test leads to pin 5 and ground. Power up and and rotate the bias pot until I have the most negative voltage. Check other tube socket, too.
2.) With amp off, rectifier in, but power tubes not installed, connect test leads to pin 3 and ground. Power up and measure plate voltage. Repeat for other tube socket.
But! Here's the blind spot I can't seem to see around:
If I know the grid negative voltage and the plate voltage, but not the voltage drop across the 1 Ohm cathode resistor, which I can't measure with the tube out, how do I calculate the current that will flow through the tubes? So, how will I calculate plate dissipation?
That's the catch that might be obvious, but I can't see it!
Thanks.
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1.) With amp off, rectifier in, but power tubes not installed, connect my test leads to pin 5 and ground. Power up and and rotate the bias pot until I have the most negative voltage. Check other tube socket, too.
HELLO. Are you listening? What did I say above? I even used all caps and colored it red.
If I know the grid negative voltage and the plate voltage, but not the voltage drop across the 1 Ohm cathode resistor, which I can't measure with the tube out, how do I calculate the current that will flow through the tubes? So, how will I calculate plate dissipation?
Don't worry about dissipation until you have the amp working and have heard a guitar played through it. Setting the bias is a fine tuning adjustment that comes later. Just be sure that you have at least -35V on pins 5 of the output tubes. But if you connect that PT bias tap to that 10K on the bias board you will not even be close to -35V or greater. Last time I'm gonna say this... Start with a 470Ω on that bias board.
This bias is such a simple circuit but it is so critical. Several expensive components rely on it to be correct and stable. It's OK to muck around with the bias circuit, but it's dangerous to muck about when you don't understand what you're doing. That's why early on I suggested to just build it according to the original schematic.
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Sometimes it takes larger mallet.
Sorry, due to a serious brain lapse I pictured the bias supply exiting the rectifier. I know it comes straight off of the PT secondary, but confusion took over.
Thank sluckey. I'll use a 470Ω resistor.
...when/if you ever build this amp. :icon_biggrin:
All three of my orders have shipped. I should have them by the weekend!
Hey, thanks y'all for showing such a genuine interest in my project. I know I've made it a challenge for you. :laugh:
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can't recall if I passed this on, worth reading while you're twitchin for the UPS truck :icon_biggrin:
http://www.aikenamps.com/index.php/what-is-biasing
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Thanks shooter. That was a clearly written read.
sluckey, I'm not on this forum to question or challenge your - or anyone else's - knowledge and instructions. Sorry if I've come across that way. I'm a rank beginner and you have the expertise and experience. And, you're generous enough to share it. I appreciate it.
I'm building the bias circuit with the 470 Ohm resistor, using the 50V AC bias tap on the PT secondary.
I don't understand it, but, like I said, I'm not questioning it. At this point it's all very confusing and I'm just trying to see how it works. I'm sure it will make a lot more sense when the amp is built and I'm actually measuring real-world voltages.
If I need at least -35V on the grids (pins 5) that's what I'll make happen. I have no idea why the schematic says -26V and I guess it doesn't matter; but, anyway, that's why I had chosen the 10K resistor. I'm not going to think about it anymore, other than that -35V seems safer for the tubes (when they're installed).
[What's so weird and confusing about this is this strange coincidence: with 375V on the plates the tubes would need 26ma of current flowing through them to be at 9.8 watt of dissipation. That number 26 could be what's mixiing me up.]
Enough for one day! I'm fried.
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I don't feel challenged at all. But if you don't understand what you're being told it's always good to ask questions. We all learn by doing so. You've received a lot of good info in this thread, but a lot of it is not directly aimed at building this 6G6, and a lot of it is way to deep for a beginner to firmly grasp. It's easy to become overwhelmed and begin to wonder how it all ties together. My approach is usually aimed at the specific topic at hand, like this bias tap/resistor value thing. You can read all the theory about load lines, different bias circuits, setting bias, etc., but in the end you will need to put a 470Ω resistor to the bias tap on that PT for this 6G3 project. Or you can use a 100K connected to the high voltage winding just like the original. Quick and direct to the point. Just git'er done! Just my style. No need to write pages and pages about it.
I'm building the bias circuit with the 470 Ohm resistor, using the 50V AC bias tap on the PT secondary. I don't understand it, but, like I said, I'm not questioning it.
I didn't just make up that 470Ω number. I copied Fender. Most all Fender amps from that era that used a 50V bias tap (and most did) used a 470Ω 1W. Take a look at this next generation Deluxe...
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_aa763.pdf
If I need at least -35V on the grids (pins 5) that's what I'll make happen. I have no idea why the schematic says -26V and I guess it doesn't matter; but, anyway, that's why I had chosen the 10K resistor. I'm not going to think about it anymore, other than that -35V seems safer for the tubes (when they're installed).
-35V is not the magic number. You may very well end up with -26V for your final bias setting, although I bet it will be more like -30 to -36. For this amp, I would like to make the bias range adjustable from -25 to about -40. Somewhere in that range will be your happy bias voltage. Make that range happen and you will be good to go when it comes time to actually adjust the bias for your desired plate dissipation. But for now, just set the pot for max negative voltage on pin 5 of the output tubes and you will not likely have any red plate surprises when you turn the power on.
[What's so weird and confusing about this is this strange coincidence: with 375V on the plates the tubes would need 26ma of current flowing through them to be at 9.8 watt of dissipation. That number 26 could be what's mixiing me up.]
Do not confuse -26V on the grid with 26mA. The voltages listed on Fender schematics are + 20%! And your 375V plate and 26mA current are just napkin doodles. Nice to talk about but just wait. It will be interesting to see what the REAL numbers are.
BTW, which PT did you order?
This has been a wordy post. My next post will be short and aimed at the specific question. :icon_biggrin:
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Cool, thanks sluckey. :icon_biggrin: That breakthrough moment of clarity when the puzzle pieces fit together and make sense is around the corner, I'm sure.
I bought the Hammond 290HAX:
https://www.hammfg.com/files/parts/pdf/290HAX.pdf
When I chose to build the 6G3 it was based on a deliberate and conscious decision not to build an AA763 Deluxe.
Most TX manufactures I looked into sell the same PT for the 6G3 and the AB763. The difference between 365V and 415V plate voltages seemed substantial enough to suggest different PT's were used in the original amps.
I'm just ready to start building. It's going to sound like an amp that I built. What that sound is...
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A new amp arrived! This one has an orange theme. :icon_biggrin:
The chassis is hardcore. It's more rugged than a truck bumper.
Still waiting on the TX's.
I ordered the parts before I knew I needed the 470 Ohm resistors, but two 1K 3W that measured low twisted together in parallel give me 490 Ohms 6W, so that worked out. A good omen?
This time I'm following the Hoffman sequence, wiring up first, and soldering the parts onto the board near the end. With the AC4 I did some things out of order and it made the build more difficult.
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This time I'm following the Hoffman sequence, wiring up first, and soldering the parts onto the board near the end.
That works fine for a turret board but is a terrible idea for an eyelet board. Which type board do you have?
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Nice to see you getting started. I have two suggestions:
I ordered the parts before I knew I needed the 470 Ohm resistors, but two 1K 3W that measured low twisted together in parallel give me 490 Ohms 6W, so that worked out. A good omen?
Yes, that'll work. But you are building a nice new amp. And those resistors go right on tube sockets. So I suggest you order those resistors. You'll have them in plenty of time and you'll do it right the first time.
Second suggestion - listen to Sluckey about the eyelet board (and anything else) there is a reason for the acronym WWSD.
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Thanks bmccowan. I also realized I overlooked ordering the 820 Ohm resistor in the LTPPI circuit. I ordered several 82K PI plate resistors, and thought I had everything. And, I need two 500pF caps for the pots. So, I'm going to have to do another order, anyway! :sad2:
Believe me, I take sluckey's instructions very seriously. It goes back to the discussion above about not getting too focused. I stepped away for a couple of days in order to reboot my brain and clear its cache, too many processes running at once. I'm enjoying every post of this thread.
Yesterday, Uncle Doug posted a new video on a '63 6G3. I enjoyed watching that, and I got to actually see a real chassis layout and wiring. It's difficult comparing Fender's old "shorthand" layout drawing conventions/practices to whatever photos of real amps I can find on the web. DiyLC, they're not. :icon_biggrin:
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Well, this might be a major design mistake, but I'd like to install the filter cap board inside the chassis. It keeps it closer to the PT and power tubes with short wire runs (especially to ground).
I measured everything and drew it to scale, so there's plenty of room.
I also did my best to not cross power wires and signal wires on the same side of the board. Where they do cross on the board, they're on opposite sides.
There may be serious electrical issues with my layout, but it seems more straightforward with fewer power wires running back and forth to a doghouse. :think1: At least I understand where each connection is made.
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Nice looking layout. I didn't check the main board for errors but the control panel has many booboos. Better take a close look.
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Thanks! Will do. These "digital dry runs" are really useful for me.
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now all you have to do is increase your ready spares so you can quit spending beer money on shipping :icon_biggrin:
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now all you have to do is increase your ready spares so you can quit spending beer money on shipping :icon_biggrin:
shooter, there's a serious danger with stocking parts. If all I need to build that "next amp" are the TX's and a chassis, what's to stop me from doing it? :laugh:
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what's to stop me from doing it?
So you are single
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what's to stop me
:laugh:
here's my addicts list
eventually you get old and slow down :icon_biggrin:
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what's to stop me
:laugh:
here's my addicts list
eventually you get old and slow down :icon_biggrin:
What is a 369er? Is that similar to a 3:59er? :icon_biggrin:
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what's to stop me from doing it?
So you are single
there're +'s & -'s
Ability to fill the house with soldering flux smoke falling in the first column.
shooter, that's quite a list. I'll hopefully run out of money and/or space before friends need to stage an intervention.
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that's quite a list
:laugh:
most were "college" lab projects, til I learn't enough to become half-ass-k
369'r got thumbs up from the guitar guys
3SE 6AU6 dissipating ~~ 9W in heat ~~ 4W audio :icon_biggrin:
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Happy 4th everyone.
So, I'm trying to get started with installing components into the chassis. But, before I get too far along I want to be certain that I understand the grounding of the pots and input jacks - before I attach any wires.
I've spent a few hours looking at Fender layouts and photos online, but there are still some points of confusion. It seems like all the layouts I can find leave out one or two key bits of info that leaves me scratching my head.
Here's the best I can do on my own. If it's wrong, where am I screwing up? Thanks!
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Buss looks much better. I suggest these changes...
1. Break the buss so the INT and Speed pots are NOT on the preamp buss. INT and Speed will ground at the filter caps. Be sure to connect the filter caps and the PT HT center tap to chassis near the PT. Many people just use one of the PT bolts but I prefer a dedicated screw. OK to connect the filament CT to this same screw.
2. Put a jumper between the ground lugs of the input jacks as shown and also connect the jacks to the preamp buss as you already have it. Then connect the ground buss to chassis using a dedicated screw near the input jacks.
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Awesome. Much appreciated sluckey!
I followed the original schematic for the speaker jacks and the tremolo jack. I've also uploaded the Mojotone wiring which is different. Neither indicate grounding the jacks to the chassis bolt. Maybe this is one of those things that's supposed to be so obvious that it's just not drawn? :dontknow:
Should I run a ground wire from the speaker jack to the chassis bolt? I did this on the AC4 build.
Thanks again!
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I don't like to rely on the input jack body to provide the ground. Leo did. Hoffman does. But I've fixed several amps that had problems due to loose nuts on the input jacks. Providing a dedicated ground bolt totally eliminates this problem.
Same idea applies to the speaker jacks. Since your amp has a NFB loop from the speaker jack, the NFB circuit relies on the OT secondary to have a good ground. Run a wire from the speaker jacks to the power ground to provide a reliable ground connection for the NFB. Leo didn't. Hoffman did.
Bookmark this page...
https://el34world.com/charts/grounds.htm
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Bookmark this page...
https://el34world.com/charts/grounds.htm
:icon_biggrin: That page, along with a few others have been on my bookmark toolbar for a couple of weeks now.
FWIW, I did see Hoffman's speaker jack ground wire, and had added it at one point. But, when I saw that Fender, Mojotone, and Weber's Lenora didn't specify it I guess I was compelled to removed it.
However, your explanation of why it adds an extra degree of reliability (a jack being a mechanical part that will work loose) makes it clear why to add it.
Thanks for the explanation. The things you're sharing are "good practice" concepts that I'll carry over into future builds.
On another note, the 290HAX PT is a good fit in the chassis. I had to elongate the mounting holes about 1/8" but it's a solid, rigid mount.
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Take a look at your B+ nodes - I think you have a mistake feeding V2.
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Take a look at your B+ nodes - I think you have a mistake feeding V2.
Thanks, bmccowan. I'm sure you're right. I was trying to follow the schematic attached. It looks like the tremolo plate of V2 is fed from the first node, before the 1K resistor, and the post-attenuation gain half of the tube is fed by the last node along with V1.
I'll trace the path on the layout and post that, too.
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And, here's the way I interpreted the layout:
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Thanks, bmccowan. I'm sure you're right.
Actually, I'm sure I'm wrong - and I've built one of these amps! I jumped too quickly - sorry.
Or as Emily Latella used to say; never mind.
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Believe me, I had to stare at that zigzag path to the first plate of V2 for a long time before I finally saw it! In an earlier post I referred to "Fender's old "shorthand" layout drawing conventions/practices." That's why I needed to spend the time drawing a DiyLC version, so I could trace out every connection myself.
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...Just be sure that you have at least -35V on pins 5 of the output tubes.
I just read this Valve Wizard page, and now my thick head finally has a grasp on this.
http://www.valvewizard.co.uk/bias.html
The mu factor for the 6V6GT is 10. So, if my screen voltage is somewhere between 350V and 375V I'll need at least -35V on the grid to reach cut-off.
Exactly what you told me sluckey! :laugh: Hey, I wasn't doubting you, I'm just one of those obnoxious people who always has to asks, "why?"
I've started soldering up the control panel and have the tx's and most of hardware installed in the chassis.
I'll update with photos when I hit a wall.
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Progress. Eyelet board is next.
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Neat work! Have you soldered the ground buss to the back of the pots before? I know it works and is a common method, but I gave that up after seeing other methods. A couple of weeks ago I opened up an amp that I built 6 years ago and found that 3 of my solder joints on pot backs had come loose. I suppose I did not use enough heat, or didn't clean the backs of the pot well enough.
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Thanks bmccowan!
I took some extra care soldering the ground buss. I sanded the backs of the pots, cleaned them with lacquer thinner, put an extra-wide tip on the Hakko and set the temperature to 800. :icon_biggrin: The solder joints seem really solid.
This was the first time I'd done this. I'm sure control panels are routine for everyone here. But, I really had to plan and think it out. I can't imagine trying to assemble it inside the chassis, especially the way it angles inward.
I had overlooked buying those Alpha pot spacer/washers that Hoffman sells. But, I found a bag full of propeller shaft spacers from my other hobby (r/c planes) that were a perfect fit. So, the pots are perfectly centered in the holes.
Anyway, I can go over it and make sure every cap and resistor is firmly soldered to the buss wire.
I just finished drilling the necessary holes in the eyelet boards. I'll populate the main board tonight. I made paper templates for drilling the chassis.
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I took some extra care soldering the ground buss. I sanded the backs of the pots, cleaned them with lacquer thinner, put an extra-wide tip on the Hakko and set the temperature to 800.
You are taking the care that I did not, so I don't think you'll suffer the fate I did. 6 years ago I did not know this Forum and its been a huge help. Before that my source of info were Gerald Weber books - some good info and a lot of fantasy.
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bmccowan, building a tube amp has been a goal of mine for 35yrs. It was never something I could have done on my own. I read some books, but didn't have the confidence, or the basic understanding to even start.
You're right, this forum is the reason I'm on my second guitar amp. If you're self-taught that's a remarkable achievement. I also have an old Gerald Weber book of schematics that I've held onto through four moves; I was determined to do this one day!
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... were Gerald Weber books - some good info and a lot of fantasy.
I had to un-learn a lot of things I read in his books.
Tubebooks.org (http://tubebooks.org/technical_books_online.htm) is a very good resource.
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Thanks for the link HotBluePlates. I'm looking at the dept. of Army & Air Force book first.
I soldered the components onto the boards. Taking a lunch break and I'll start planning out the wires I need to attach. :icon_biggrin: There's one missing resistor (15K) on the main board; if I only overlooked one part with this project that's a major improvement over my AC4 build.
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If you're self-taught
Mostly - I got started about 25 years ago pretty much by accident, and not being able to resist good deals.
First one - rummaging through a huge secondhand store that was getting ready to close for good, I spotted a 300b Fisher power amp with 4-EL34s in nice shape. The owner saw me looking at it and told me I could take it for $25. if and only if I also took the matching preamp and a serviceman's tube caddy full of tubes - all for $25!
I was busy building a house so I put them all in the basement.
A month later I spot a Fender Princeton Reverb at a flea market. The guy selling it said it did not work so he wanted $60. I recorded his driver license info in case in turned out to be stolen. It was clear, so it joined the others in the basement. A couple years later I walked past the amps and thought I'd give it a shot. I quickly found a broken connection in the Princeton; bought a soldering iron and some solder and fixed it. It worked but sounded like mud. So I read up a bit, bought a multi-meter, caps and resistors and after getting the safety stuff down - dove in. This Forum helps me continue to get better at the craft. I admire the work that many do here, and I will never have the patience or skill to make the beautifully aligned circuitry that some craft. But I make and fix some damn nice sounding amps. and that's good enough for me.
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I soldered the components onto the boards. Taking a lunch break and I'll start planning out the wires I need to attach.
Next time put your interconnecting wires into the eyelets and bend them over the rim BEFORE you solder the components. You'll probably know why I say that before you finish. :icon_biggrin:
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Next time put your interconnecting wires into the eyelets and bend them over the rim BEFORE you solder the components. You'll probably know why I say that before you finish. :icon_biggrin:
Yep, I can already see how that's a better way! :embarrassed: Well, at least I installed all of the component leads in the eyelets before soldering.
The only saving grace is that I'm using Hoffman's solid, cloth-covered wire. I can either melt the solder in the eyelets and push the wire in, or, bend a small hook and loop it around a component lead, which I guess, must have been my plan. :icon_biggrin:
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loop it around a component lead
:laugh:
yep
I even add them with turrets, just makes it easier, AND, when there's no more loose wires, it's guitar time :icon_biggrin:
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bend a small hook and loop it around a component lead, which I guess, must have been my plan.
Yuck! Why would you do such a nice looking board and then crap on it?
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I'm with Sluckey. I build with turrets, but when I repair amps with eyelet boards, I melt the solder, poke in the wire and bend it over before the solder sets. When done I go back over each connection. With the newer wimpy component leads, there is a lot of room in the eyelets - not so much with components from the 50s.
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OK! :laugh:
I will heat up the eyelets, and push the solid wire in. It will actually be pretty easy to do. Plus, as you say, bmccowan, I totally intend to go over every joint and make sure it's well soldered.
Fortunately, there's only one wire connection point on the entire board with more than two component leads. All the rest still have plenty of free space inside the eyelets.
But, I do appreciate being called out on taking shortcuts sluckey. Don't ask me why I'd invest all this time and money in a project and still look for the quick&dirty way. :dontknow:
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I melt the solder, poke in the wire and bend it over before the solder sets
Not a recommended way to "re-flow" solder. It gets brittle, cracks easier. clean it out start with new solder.
IF you can get 2 - 3 wraps around the "imbedded" leads and a light touch solder, you get a pretty good looking "wire wrap" look, too much solder looks like Billy-Bobs '56 dashboard wires :icon_biggrin:
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shortcut
Actually to my mind inserting all the wires and all the component leads in the eyelets prior to soldering is a shortcut - but a good short cut. Solder connections only once. It also allows you to more easily put a slight bend in the wire for a bit of mechanical connection. I leave all the leads exiting the board extra long so that when I trim them to fit, I have a usable length of wire left over, not a bunch of tiny scraps. But that's just the thrifty Scot in me trying to avoid waste.
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Not a recommended way to "re-flow" solder
I understand, and if the solder does not seems to be reflowing right I do remove it with a solder sucker and start fresh. I assume that with fresh solder in the eyelets of this amp, it would reflow fine. But my assumption could be wrong. :dontknow: Oddly I always remove the old solder from tube sockets and terminal strips, clean them good and add a bit of flux. So I guess I'm inconsistent with the eyelets.
But I do not wrap wire 2 or 3 times as it is such a pain to work on an amp where a previous tech did that.
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And here is Doug's piece on that process from "servicing Fender amps":
Heat up the positive side first and push the lead down into the hot solder. I do all the caps positive ends first and then push any shared negative wires into the hole at one time.
I was worried that I'd been screwing up, so I looked for some info - at least I have some expert company!
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While cutting lengths of wire, it suddenly occurred to me that I should place everything in the chassis and see - for the first time - how things are going to fit (or, not).
It's going to be tight. The bias board can't go left because the pilot light fixture is there. The filter cap board can't really go anywhere other than exactly where it is. The main board shouldn't go to the right because of the mounting bolt hole (although, I plan to use Rivnuts on the flange, so that hole might not get used). That leaves the bias pot. It's in the only available place it can be, almost... I'm not installing the ground switch on the back panel, so maybe that would be a clever location for it? It's a pretty long wire run back to the Intensity pot. [Although, after watching the Uncle Doug 6G3 video I really wanted to use that spot for a NF on/off switch.]
It's that slanted control panel that eats up valuable room. But, it's a Fender.
Thoughts?
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Put the cap board on the outside of the chassis like Leo did.
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Thanks. I went over my layout a bit more, and the one in my photo is going to work fine. I did price an external cap pan, and with shipping that's close to $30 I can put toward tubes, cabinet, or speaker.
I also read up on Hoffman's installation method, but my chassis is too shallow to fit the caps under the pots (see his photo below).
It looks like some older Marshalls, etc., used multi-section caps to take up less space in the area near the PT.
I'm staying the course and focusing on finishing. :icon_biggrin:
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I still vote for putting the cap board on the outside. You don't heve to have a cap pan. Here's one alternative that I used on my TDR...
http://sluckeyamps.com/tdr/tdr_04.jpg
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You may regret squeezing everything so tight. If you don't want to buy a doghouse or build Sluckey's thing of beauty. Two options I can think of:
That cap board has a lot of free space you could squeeze the caps together, trim off a set of eyelets and add an eyelet on the positive side - the neg side can share.
Or do similar but buy an F&T 2x16 cap for the first two filters.
Or my choice - we can't go to bars or restaurants these days, use the money saved to buy that doghouse.
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Man, you guys are tough.
Well, I went ahead and drilled the chassis for my Hoffman filter cap board. I may still need to trim the long sides a hair; I won't know until the I have the pan.
The pan hole centers on my chassis are 5.75" x 3"so I can use this one:
https://www.tubesandmore.com/products/capacitor-cover-fender-reissue-vibroverb
I'm buying the tubes from them anyway, so I might save a bit on shipping.
I've already drilled the chassis for the main and bias boards. They're staying where they are; I'm not drilling anymore holes! :icon_biggrin: Also, I've already installed the bias pot, and it's not moving, either.
But, if I go with the external filter cap setup one of you here is going to have to either explain the very confusing blue area on the original schematic (see attachment), or sketch a simpler, better wiring scenario for me. :icon_biggrin: I can't stare at that corner of the board any longer.
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do you have some rubber grommets for those through chassis holes?
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I can't stare at that corner of the board any longer.
stare at the schematic, start with the tube pins indicated in the layout, you'll "see" the fog lift :icon_biggrin:
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Stare at this...
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And maybe this too
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do you have some rubber grommets for those through chassis holes?
Yes, you can see where I've already installed them for the OPT wire holes, I just haven't put them in yet for the external cap setup.
I can't stare at that corner of the board any longer.
stare at the schematic, start with the tube pins indicated in the layout, you'll "see" the fog lift :icon_biggrin:
I understand how to wire the amp. I had it wired correctly with the filter cap board inside the chassis. It's the weird area inside the blue rectangle in my attachment that I can't decipher! :w2:
Everything should start with the wire from pin 8 of the GZ34. It should go straight to the positive terminal of the first filter cap. The red wire from the OPT should also connect there. The tremolo plate - second plate of V2 - is fed from that node.
The second node - positive terminal of the second filter cap - supplies pins four of the 6V6's.
The third node - positive terminal of the third filter cap - supplies pins 1 & 6 of v3 - the PI tube.
And, the fourth node - positive terminal of the fourth filter cap - supplies V1's plates, and the first plate of V2.
All that I completely understand. It's the area inside the blue rectangle that makes no sense to me.
So, I'm just going to ignore that "doodling" on the original schematic and wire the board as I described above. :icon_biggrin:
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Oops, sluckey, you and bmccowan both posted while I was typing my last post.
sluckey, your diagram clears it all up!
Thanks. I like the idea of running the yellow wires straight to the tubes.
And, yes, all of my boards will be sitting on 1/4" nylon standoffs.
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That cap board has two ground wires...
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That cap board has two ground wires...
Excellent, I see it now. So does mine!
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Pushing the wires through the eyelets proved easy enough. I'll use RG174 for the two input runs (I had enough on hand).
Still a couple more wires to add.
I'm running the tremolo plate wire straight to the filter cap board instead of bouncing off the corner of the main board and then back again - any reason not to do this?
I'll follow sluckey's advice and run the yellow wire straight from the filter cap board to pins 4 of the 6V6's.
I guess I'll go ahead and run pin 8 of the GZ34 and the OPT CT to the main board first, and then hop them over to the filter cap board as shown on the schematic.
I'm pretty drained, so this is enough for right now.
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I would run the two input shielded cables under the board straight to V1 tube socket. No sane reason to thread those cables through the board. Make it easy on yourself. Install those two cables ***BEFORE*** you even put the board in the chassis.
http://sluckeyamps.com/supro/s-11.jpg
http://sluckeyamps.com/6v6plexi/P-6V6_04.jpg
http://sluckeyamps.com/VAC15/15.jpg
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That sounds like a plan.
I'd like to try wiring a ground buss like you did on those amps. Looks really functional and neat.
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That ground buss method works great. When I saw Sluckey's method I immediately switched to it. I have yet to switch to his heater wiring method but I trust that he does not build noisy amps. But I'm also sure the twisted pair police are on his trail. Heater wiring is a pain especially on 9 pin sockets - anything to make it more enjoyable.
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bmccowan, I'm doing the "Fender down from above" heater wiring. Yep, I'm a bit concerned about the 9-pin sockets because I'm using Hoffman's 18ga green cloth wire. This stuff seems thick and heavy for those tiny socket eyelets. But, it will make for very rigid wiring.
Does anyone ever learn every technical improvement and then finally build the perfect amp? :icon_biggrin:
Speaking of Hoffman, I followed his instructions for prepping the RG174 wire:
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make sure n ohm shield to center, just in case :laugh:
and to "perfect", I like to think professional, or at least top drawer amateur, that walkin on water gets tough some days :icon_biggrin:
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Perfect can be the enemy of good. And the enemy of finishing an amp so you can play your guitar!
I use that heavy 18 for the octal sockets and then go down a size for preamp tubes (octal or 9 pin) I think others here do the same. There are some tricks for getting that 18 gauge doubled up into 9 pin sockets - but not worth the trouble IMO. You should see the wimpy heater wire that Hammond used on their organs, and a B3 is about the most glorious sound in the world.
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make sure n ohm shield to center, just in case :laugh:
and to "perfect", I like to think professional, or at least top drawer amateur, that walkin on water gets tough some days :icon_biggrin:
Off range on my meter! :icon_biggrin:
What I was really getting at is that there's always a new (and, sometimes better) technique to building amps, it seems. I find myself in that situation where I'm thinking, "cool, I'll use that on my next amp project." So, I'm already thinking there's going to be a "next project." :sad2: I think I was warned.
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there's always a new
sometimes called "best practices"
Ideally you're always "finding the new..", otherwise you're repeating the past :m17
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I would never try to poke two 18AWG wires into the lugs on a nine pin socket. 22AWG only for me.
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I would never try to poke two 18AWG wires into the lugs on a nine pin socket. 22AWG only for me.
I’ve already gathered up some solid 22ga wire. :icon_biggrin:
When I bought that green 18ga wire I didn’t know how heavy it would be. Just trying to bend it seems like it would snap off a socket lug.
Tubes and cap pan are ordered!
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One final thought before cashing in...
I wonder if those power connections on the upper-left corner of the board were put there by Fender as a place for the amp assemblers to check voltages during final checkout (after the cap pans were attached). I guess it also serves as a place to anchor long runs of wire, too. :dontknow:
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I've always wondered what that white stuff in bird poop is. :think1: :dontknow:
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> what that white stuff in bird poop is
You don't want to know.
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I've always wondered what that white stuff in bird poop is. :think1: :dontknow:
it's bird pee - uric acid in paste form. we had a west african gray parrot, had many Qs about that and other things.
bird took too many chunks of skin. funny as all get-out when you were getting hammered or baked with him around. i miss that bird, he was dad's pet.
back on topic: any pics of the work done so far on this amp?
--pete
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back on topic
Not so fast. African Grays are a riot. Smart birds. Good friends of mine had one. Sidney talked up a storm and was a wiseguy. They also had a cat and Sidney would waddle over to the door and repeat "wanna go out" a few times. The cat would come to the door and Sid would waddle away laughing loudly. He also remembered derogatory names he heard individuals called, and when one particular band member arrived for a cookout or for practice he'd always shout out "Hi Dipshit." He loved to entertain.
On topic - he did post a couple pics of the board and chassis.
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back on topic: any pics of the work done so far on this amp?
--pete
Between full-time work and other commitments it's taking longer than I hoped.
And, I overlook things like not inserting the wires into the eyelets before soldering up the components.
So, redoing things wastes a lot of time. :icon_biggrin:
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As for wasting time...
Another complete beginner mistake led to over an hour of fierce fighting with the inputs. I attached the buss wire too low on the pots and it got in the way of the 68K grid stoppers. I had to remove the resistors and rewire them. :BangHead:
It's ugly, I know (and, maybe I could have put the resistors directly on the tube sockets), but they're well insulated with no bare wires to rub against the chassis or ground buss.
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More progress. Just about ready to mount the main board.
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Its fine - we've all installed things that turned out to be in the way of something else. Law of unintended consequences. A lot of folks mount those grid stoppers right on the tube sockets - my ears hear no difference.
And if I can offer some advice - do not worry about being slow - enjoy the process. When you finish it, and have it sounding great, you will immediately start thinking about your next amp project anyways. Its like when you return from a great vacation trip and immediately start searching the Web for your next one. We all do that: don't we?
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Thanks bmccowan!
I do enjoy projects more when I take my time. So much of this is new stuff that I like to finish a step and then look at for errors and then plan how and what to tackle next.
So, let me ask this. What gauge wire should I use for grounding? I haven't soldered up the ground wires to the chassis bolt yet because I wanted to get advice on that first.
Thanks.
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I like 20AWG for power grounds and 22AWG for preamp grounds. Anything bigger that that red/yellow PT wire is overkill.
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Thanks sluckey! I'm thinking stranded, not solid, since it's less likely to break under stress.
I'll replace those wires tomorrow, solder up the two ground bolts (one near the PT for power and another near the Normal input jacks for preamp), and get the main board in and connected up.
Then, I can power up without tubes and find out what happens!
BTW, I have a black car, and all I know is that white stuff is hard to wash off.
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OK, I'm trying to solve things myself instead of asking. :icon_biggrin:
My 1 Ohm cathode resistors are 1/2 watt. Let's say I measured a voltage drop of .05V. Converting that to .05 amps times the voltage equals .0025 watts. So, 1/2 watt resistors are plenty big for this application, yes?
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yes
:icon_biggrin:
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Thanks shooter!
Here's my solution to installing the 1 Ohm resistors. Since pin 1 is not used - the JJ 6V6S's I bought don't even have a pin 1 - I used it to tie off the resistor.
Now, I can simply connect my test clip to the bare wire between pins 1 and 8 to take my readings.
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I hate soldering a wire to a dangling end of a resistor. I like resistors to be firmly supported on each end. I would have mounted the resistor ***BETWEEN*** pins 1 and 8. Like this...
(http://sluckeyamps.com/sunn/mod_1.jpg)
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I hate soldering a wire to a dangling end of a resistor. I like resistors to be firmly supported on each end. I would have mounted the resistor ***BETWEEN*** pins 1 and 8. Like this...
I like that!
I did notice that if the wire moves the the resistor moves, too. Over time it could get weak and snap off at pin 1.
Thanks!
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Going back and doing this over is like building three amps. :icon_biggrin:
The bright side is it's practice and the methods stick for future builds.
But, installing the main board and wiring it up is not something I want to do multiple times.
Is there anything wrong with my preamp wiring layout?
Thanks.
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what ground does the wire I indicated "feed"?
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shooter, Fender has it indicated as a grounded wire.
It looks like part of the NF circuit.
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that's the PI main ground for the "long tail"
I typically ground the PI with the PA circuit, not the pre circuit
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See, glad I asked! :icon_biggrin:
Could I just run it into the doghouse hole and ground it at the negative end of the PA node one? It would be a much shorter wire that way.
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short wires are good
I'd ground it with the cap I indicated
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Oh...
I actually have that cap grounded with the 4th cap.
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Oh...
I actually have that cap grounded with the 4th cap.
And that's the way fender did it. I'd leave it just as your layout drawing shows.
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I pointed it out not as a problem, but a potential one :laugh:
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Thanks. It's just as easy to ground it here:
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wire it per Fender, should be fine. IF there is a noise hum issue with PI, it's easy to change
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Thanks.
I found a photo of an original 6G3.
It's hard to tell, but it looks like the ground wire is attached to the underside of the board and is then soldered to the long brass sheet that other preamp grounds are attached to.
(see red arrow)
I'll start with the wiring in post #208 first.
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I reached a saturation point.
Going outside to get air.
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I just need to solder up the power cord.
I'll go over every connection tonight, and once more in the morning, and then I guess it's dim bulb time.
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Looking pretty good. A question. It appears that you have a white wire running from the fuse to the switch. When dealing with the house current side of the amp I think it best to stick to norms. White is neutral. Hot should run from fuse to switch to PT. Neutral should go directly to the other PT lead. Good sketch of that in both Doug and Sluckey's pieces. If I'm seeing the photo wrong, sorry, eyes are old.
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bmccowan, yes, you're right. I intend to wire it the way you're saying, I just used a spare piece of white wire I had. But, to avoid confusion I'll change it to a black wire! :icon_biggrin:
In looking at sluckey's layouts I see he wires his amps: line in > fuse > power switch > PT.
I've seen others put the fuse after the power switch, but I'll stick with his sequence.
Thanks! I'm almost at the end.
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In looking at sluckey's layouts I see he wires his amps: line in > fuse > power switch > PT.
I've seen others put the fuse after the power switch, but I'll stick with his sequence.
Yup - that way you have fuse protection any time it's plugged in.
I'm hoping it fires right up with no issues. But if you have some - they should be easy to find as you have been methodical.
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I'm hoping it fires right up with no issues. But if you have some - they should be easy to find as you have been methodical.
Ha! I was just in there soldering up the power cord and noticed that I attached the V1 plate wire to pin 9, not pin 1. Fixed it.
It would not have fired right up. :laugh:
Here's the proper power cord wiring. Thanks again for calling that out.
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noticed that I attached the V1 plate wire to pin 9, not pin 1
I have done things like that so often it's not funny. Just recently I confused pins 1 & 8 on a 6V6. That did not work out so well!
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Just recently I confused pins 1 & 9 on a 6V6. That did not work out so well!
:huh:
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pin 8?
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Yup - typo I corrected - fat fingers and good wine!
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Hoffman:
"Double check all your work:
Stop now and look at this post on my forum on how to double check all your work, this is very important
How to double check your work the easy way
Do not continue until you have double checked everything using my method above
Every time someone has an issue with the amp not working it is always because they skipped a step or did not double check their work thoroughly.
Hoffman law says this.
The amp would be working if it was wired properly and all the parts were installed and working correctly."
A voice is saying, "Aw, you can skip all that! You took your time and worked carefully."
Nope. I"m following the rules. :icon_biggrin:
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I've started measuring voltages.
With no tubes installed:
Pilot light glows.
40 watt dim bulb just barely glowing.
No smoke. No pops. No sparks.
Looking into chassis from the top (open side). Control panel facing away.
Right 6V6 tube socket grid - bias pot gives range of -61.4v to -45.8v
Left 6V6 tube socket grid - bias pot gives range of -61.5v to -45.8v
This is with a 470 Ohm bias resistor, 10K pot, and 27K safety resistor on the pot.
With no tubes - including no rectifier - no voltage on any of the six preamp tube plates and no voltage on the power tube plates or screen grids.
What should I do next?
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next?
got filament volts were they belong?
IF so;
put rectifier only in and verify the PS makes VDC
IF so
probe the plate pins and make sure the DC is there;
IF so;
add preamp tubes, set your bias to max neg, measure vdc at plates and cathodes.
IF good-ish;
did you install 1 ohmers?
IF so, set up a meter to read across one, install PA tubes and get a quick read on the one ohmer's, do the math and evaluate
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I'm also measuring 6.33V AC on pins 4 & 5 of the first 12AX7.
Unfortunately, my tubes haven't arrived yet! AES is not the fasted shipper. :icon_biggrin:
Anyway, I suppose - with guidance - I can tweak the bias resistors to get the negative voltage range I need for the JJ 6V6S's.
Thanks.
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you have the 1 ohmers install wrong. Put the ground wire on pin one.
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you have the 1 ohmers install wrong. Put the ground wire on pin one.
Thank you punkykatt!!! Dang! I would have gotten different measurements and probably never figured out why.
Thank you!
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Unfortunately, my tubes haven't arrived yet! AES is not the fasted shipper. :icon_biggrin:
I take that back. They arrived today! Less than a week is pretty fast.
But, I found another big mistake in my work. I need to redo the heater wiring. I did it all wrong. It took me a long time to find photos of how to do it. I'm also out of solder, but that is supposed to also arrive today.
You either get frustrated and shelve it, or, you try to look at it as gaining experience. Perspective...
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I just looked at your pics to see the heater wiring. You connect pins 4&5 but not 9 on the preamp tubes? You probably found the solution, but good heater wiring instructions here:
http://www.valvewizard.co.uk/heater.html (http://www.valvewizard.co.uk/heater.html)
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Yep! That's apparently series for a 12.6v secondary. :BangHead:
I just got it fixed. I'm measuring 6.35V AC down the line.
OK, so I have the tubes. I guess the next thing to do is to install the rectifier and check for plate and screen voltages? This is the part where I get apprehensive. I'll be charging the filter caps.
I'm going to take a breather and eat. I just spent an hour twiddling with those tiny socket pins to get the heater wiring right. I want to approach this with a calm and alert state of mind. :icon_biggrin:
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I had some free time to take more measurements.
With ONLY the rectifier in:
Right 6V6S Plate Pin 3 = 325.1VDC
Right 6V6S Screen Pin 4 = 324.8VDC
Left 6V6S Plate Pin 3 = 325.2VDC
Left 6V6S Screen Pin 4 = 324.7VDC
Should I measure all of the 12AX7 plates, too, at this point - with no tubes installed?
Also, since I have the cap pan screwed on and the caps are under the chassis, anyway, I've been draining the filter caps at the point in the image. Is that a good, reliable place to drain them? I'm being ultra safety conscious!
Thanks.
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the reason I poke the no tube plates is to "catch" a wiring error befor it puts 300vdc on the grid, instead of the plate.
your drain hole is good, what are you using?
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Thanks!
I made a discharge tool. I might could have used a smaller resistor, it takes about 3 minutes to drain the caps down to zero. Plus, I might add a clip instead of the probe, but I don't want to forget it's connected! The probe is sort of idiot-proof in that regard. Plus, it makes me stand there and watch the meter.
OK, I measured all six of the 12AX7 socket plate pins:
V1 - pin 1 = 315.0VDC
pin 6 = 315.4VDC
V2 - pin 1 = 319.1VDC
pin 6 = 318.8VDC
V3 - pin 1 = 322.2VDC
pin 6 = 321.7VDC
Just to consolidate numbers. Here are the grid negative voltage ranges. I have the pot set to maximum negative voltage.
Right 6V6 tube socket grid - bias pot gives range of -61.4v to -45.8v
Left 6V6 tube socket grid - bias pot gives range of -61.5v to -45.8v
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one like I used in my career days :icon_biggrin:
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:icon_biggrin: I like the C-clamp ground. It shouldn't accidentally come loose! How many volts were you dealing with?
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Just to consolidate numbers. Here are the grid negative voltage ranges. I have the pot set to maximum negative voltage.
Right 6V6 tube socket grid - bias pot gives range of -61.4v to -45.8v
Left 6V6 tube socket grid - bias pot gives range of -61.5v to -45.8v
Too high for 6V6s. Shoot for a range of about -20 to -40. Change that 470Ω bias range resistor on the bias board to 1K and see if that gets you there. If not, experiment with that resistor value to give those 6V6s what they need. Fender claimed -26v on the original but since your B+ is gonna be lower your bias voltage will probably be lower too.
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Thank you sluckey!
I bought a range of resistors from 470, 1K, 4.7K, 8.2K, to 10K. I'll get the bias pot to cover that -40-ish to -20-ish range next.
I might wire up some clip leads so I don't have to solder in resistors between each test (thinking out loud).
But, so far, no shorts, smoke or sparks!
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Those B+ voltages look low. Were the voltage readings taken with the current limiter light bulb in circuit? What are the AC secondary voltage readings, and how do they compare to the transformer's published specifications?
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reply #68 has 270-0-275, tube rec?, if so I'd think 325vdc would be close, loaded 290?
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Those B+ voltages look low. Were the voltage readings taken with the current limiter light bulb in circuit? What are the AC secondary voltage readings, and how do they compare to the transformer's published specifications?
Thanks.
Yes, the dim bulb tester was being used. Now that I know there are no shorts I can measure again without that in series.
But, I did intentionally go with a PT with lower secondary voltages than the AB763 Deluxe Reverb PT that Hammond suggests for this amp.
I'll post voltages without the dim bulb.
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Thanks for reminding me about the dim bulb tester 66Strat.
No power tubes: Plate on the right 6V6 socket now measures 417VDC, and the left socket is 418VDC.
With tubes installed I might actually be near my target voltage of 375, or somewhere near the schematic voltage of 365. I don't know how much the 6V6S's will drop the voltage to be honest.
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That looks good for no-load. :thumbsup:
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cool
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sluckey, I started with a 10K bias resistor and thought I'd work back down from there.
With no tubes at all - no rectifier, that wouldn't make a difference, would it? - the pot gives a range of -33.97v to -25.31v.
What do you think?
I have 8.2k's. but no 12k's. I do have 3 watt 15k's.
Also, I can change the safety resistor on the pot from 27K to 22K - LTSpice shows that will make the negative voltage about 2V more positive, probably making my range around -31.7v to -23v.
FWIW, the JJ 6V6S's I bought have the same max plate diss. rating as as 6V6GT's - 14 watts.
Thanks!
[edited to correct voltages]
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Try it and see.
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Thanks!
Tomorrow. Long day staring at databases & spreadsheets and the eye's are tired.
I edited the numbers in post #252:
With the 22K pot resitor it should be around -31.7v to -23v.
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New negative bias voltage measurements:
No rectifier installed.
10K bias resistor and 22K pot safety resistor: -31.6v to -22.3v range. The middle of that range is -26.95v.
I have the pot rotated to give the maximum negative voltage of -31.6v
Should I install ALL of the tubes, hook up a speaker, and measure across the 1 Ohm cathode-to-ground resistors first?
Or, should I measure the power tube plate voltages first?
I'm sorry for being dumb about this. I've never set the bias on a PP amp before.
I want to have some idea of what I should be looking for, so I did the math for a range of plate voltages and 9.8 watts of plate dissipation (70%):
355VDC and 27.6ma
365VDC and 26.8ma
375VDC and 26.1ma
385VDC and 25.5ma
395VDC and 24.8ma
Thanks again for all the help! I'm almost there.
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Plug the amp straight to the wall with speaker connected. Measure plate voltage and cathode voltage. Calculate. Adjust bias as needed and repeat.
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That had my heart racing. I didn't know what was going to happen. :icon_biggrin:
Plate voltages: 357VDC for both tubes. That's within 2% of the 365 on the schematic.
So, with the bias pot turned to maximum negative voltage (-31.6v) I measure 25.6mv and 25.2mv across the 1 Ohm resistors.
That's an average of 9.07 watts of plate dissipation, or about 65%-ish.
For 9.8 watts of plate dissipation I want 27.45ma @ 357vdc. Although, I understand the plate voltage will change a bit as I adjust the bias.
One thing I noticed is that there is no noise at all. No hum, hiss or buzzing. But, that's with all knobs at zero and no guitar plugged in.
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I think my current measurements are wrong. I think the tubes might be in cut off.
I'm getting no sound through the speaker from my guitar.
What do I check first?
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Power amp good. It's guitar time.
Check speaker jack wiring.
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Does this look ok?
Thanks.
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It's wired correctly. Ain't lovin' the solder joints.
Monitor voltage on the 1Ω resistors while turning the bias pot. Do the voltages change?
Double check wiring with the schematic and layout. Confirm they all agree. Measure and post voltages on all tube pins. Post hi-rez pics.
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I might have figured it out. :think1:
I'm measuring 1.4 Ohms from pin 8 to pin 1 on the power tube sockets. I need to buy some more 1 Ohm resistors and solder them better.
If I measure 25.4mv that's only 18ma.
I would need to measure 38mv to have 27ma. I think... is that math right?
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Those 1Ω resistors are not why you have no sound from the speakers.
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Yes, the voltage changes as I rotate the bias pot.
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V1 Pin:
1 - 160.6v
2 - 0v
3 - 1.632v
6 - 160.3v
7 - 0v
8 - 1.632v
V2 Pin:
1 - 171.2v
2 - 0v
3 - 1.415v
6 - wild fluctuations and speaker static/scratching
7 - 0v
8 - wild fluctuations and speaker static/scratching
V3 Pin:
1 - 329.7v and speaker static/scratching
2 - 0v
3 - 5.15v
6 - 328.9v and speaker static/scratching
7 - 0v
8 - 5.15v
V4 Pin:
3 - 350.5v
4 - 349.5v
5 - -28.3v and speaker static/scratching
V5 Pin:
3 - 349.5v
4 - 349v
5 - -28.3v and speaker static/scratching
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6 - wild fluctuations and speaker static/scratching
is that the trem tube? If so you need to ground the trem FS to turn off
static scratch sounds like bad solder
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Thanks shooter. I wondered about the trem. So, I can just take a male RCA plug and connect shield to pin and plug it into the footswitch jack?
The static only happens when my multimeter test lead rubs on the socket pins. My hand might have been shaking a bit. :icon_biggrin: There's no noise when the test lead is making firm contact with the pins.
V3 plate voltages seem high, but I don't know.
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Using a regular DMM, resistance readings below a few ohms must be regarded as ‘indication only’.
Try imagining that you’re using an analog meter.
Accurate measurements require a dedicated, specialist ’low ohm meter’.
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V3 plate voltages seem high, but I don't know.
is V3 the PI?
here's the fender numbers, does look like you have something in the PI wrong (IF V3 is PI). double check the cathode config and values
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V3 pins 2, 3, 7, 8 are way wrong. Probably the cause of pins 1,6 being high. Start looking there. Check pin 3,8 resistance to chassis, meter probe directly on socket and chassis. What have you?
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Long Post...
Thanks shooter.
OK, it's working. Tremolo, tone knobs, and all.
I lifted out the board and removed all but one wire from underneath. I transferred them to the top of the board.
I didn't find any errors in my components or connection points.
But, I think I know what was going on. That upper left corner that was under my skin from the beginning was the problem, I'm pretty sure. When I pushed the wires down into the eyelets, some of the wires went in too far, and I think something in that corner was shorting to the chassis. I removed the whole cluster (f...) and rewired it in a simpler, more logical way (see photo).
Now, the anodes of V3 - yes, shooter, that's the PI - measure: pin 1 - 224.5v and pin 6 - 226.8v. Or, the other way around, my mind is spinning. Anyway, that's spot on with the schematic which says 225v and 230v!
There is work to do. It is dead quiet with no guitar plugged in. I get some hum with the guitar. It is completely controlled by the volume knob. It sounds just like single coil pickup noise to me.
Also, I don't have the plastic knobs attached and I hear a slight sound when I touch the metal pot shafts. That could be me and the carpet, I don't know. Also, the normal volume pot is a bit scratchy.
Power tube plates measure 359.7vdc for the right and 360.3vdc for the left.
Power tube cathode voltage readings are 24.8mv for right tube and 23.7mv for the left one. I can juice it up to 26ma if I want.
So, it's a working amp. It's a loud amp. I still need help smoothing out some noise issues, but some of that might be my soldering.
I have all you all to thank. Especially sluckey who put up with my questions, etc.
I need a break. Then, I'll tackle what's left to do.
Thanks!
David
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V3 pins 2, 3, 7, 8 are way wrong. Probably the cause of pins 1,6 being high. Start looking there. Check pin 3,8 resistance to chassis, meter probe directly on socket and chassis. What have you?
sluckey, I was writing while you posted that.
I will measure 3 and 8 resistance to chassis as you state. Probably later tonight. I just got off the amp build rollercoaster. But, something I did knocked that anode voltage down from 329v to 225v! :icon_biggrin:
BTW, the Tremolo is great! It is strong and has a wide range.
The 6V6S's are running at about 8.75 watts dissipation. That's like 62.5%. I'll see if I like that before messing with the bias.
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...Check pin 3,8 resistance to chassis, meter probe directly on socket and chassis. What have you?
Wow, for one thing the pin 8 wire had broken loose. Probably from when I was rewiring the heaters. :dontknow:
Resistance from V3 pin 3 to ground and pin 8 to ground are both 9.12K ohms. Is that good or bad?
V3 cathode voltage measures 18.72v. The schematic indicates 20v, so that's fine, yes?
I guess I'm messing with the bias because it's a new trick. :icon_biggrin: I adjusted the current up to 26ma (same as schematic). Plates are at 362v (365 on schematic) and dissipation is 9.4 watts, or 67%.
I'm glad I went with the Hammond 290HAX Harvard PT. All of the voltages I've measured have been very close to the schematic's numbers. I know that doesn't necessarily mean a lot in the real world, but I at least feel like I succeeded in something.
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Resistance from V3 pin 3 to ground and pin 8 to ground are both 9.12K ohms
add 6800+820+1500 :icon_biggrin:
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Resistance from V3 pin 3 to ground and pin 8 to ground are both 9.12K ohms
add 6800+820+1500 :icon_biggrin:
Well, I'll be! :l2: :worthy1:
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Have you tried it with a guitar?
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Glad to see you have it up and working. Reason to celebrate.
Wow, for one thing the pin 8 wire had broken loose.
I had noticed in some of your photos that some of the solder connections looked a bit dodgy. You may want to read up a bit on technique and go over connections before you consider it a wrap.
BTW you said that a few connections may have grounded under the eyelet board. I use turrets, but does you eyelet board have a blank board under it. From the Fenders I've worked on that is pretty standard and the blank board prevents unintended grounding. Maybe they are not all done that way? :dontknow:
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Have you tried it with a guitar?
Yes! It sounds really good. The Tremolo works great and it's loud. The tone controls have a wide effect; the normal channel does not seem dark at all. But, I'm testing with a cheap 8" speaker sitting on its magnet on the bench. So I'm not getting the best results and that is probably making it sound brighter than it will in a cabinet. :icon_biggrin: Tomorrow I'll run it through the AC4 cabinet and speaker.
I'm spreading out the spending on this project. Now that I know it works I'm buying a combo cabinet and a 12" speaker (that's a rabbit hole in itself). I think the single coil-type noise will abate when it's in a solid cabinet with shielding under the top. Right now, it's open to every electrical thing in the room. But, it's pretty quiet.
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Glad to see you have it up and working. Reason to celebrate.
Wow, for one thing the pin 8 wire had broken loose.
I had noticed in some of your photos that some of the solder connections looked a bit dodgy. You may want to read up a bit on technique and go over connections before you consider it a wrap.
BTW you said that a few connections may have grounded under the eyelet board. I use turrets, but does you eyelet board have a blank board under it. From the Fenders I've worked on that is pretty standard and the blank board prevents unintended grounding. Maybe they are not all done that way? :dontknow:
After using a turret board I found the eyelet board more of a challenge. It's hard to seal up those things without solder running down. And, yes, I intend to go over the entire amp and touch up the joints.
I used a Hoffman board - very thick and rigid - and mounted it on 1/4" nylon standoffs. But, a second piece of insulation might have been a good idea.
Thanks for all of your great advice and observations bmccowan!
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buying a combo cabinet
since your heads in the hole, leave it a head unit and get a 2 X 12 :icon_biggrin:
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I used a Hoffman board - very thick and rigid - and mounted it on 1/4" nylon standoffs
That sounds good and different and better than the fender eyelet boards which seem to be some kind of cardboard soaked in some magical waxy concoction. You won't need a blank board under that board. After a while those fender boards look like the rolling hills of Oklahoma.
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since your heads in the hole, leave it a head unit and get a 2 X 12 :icon_biggrin:
Great, then I can multiply exponentially the number of options by mixing two different speakers! It could take me months to decide. :sad2:
This thread has 2230 views. So, I guess a number of people have seen me stumble through all my trials and errors. But, it's a great collection of a lot of expert advice and technical knowledge from you all. :thumbsup:
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unless you added your voltage readings to the schematic already, you're NOT done :icon_biggrin:
play 1st though :laugh:
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unless you added your voltage readings to the schematic already, you're NOT done :icon_biggrin:
Good point! And, I definitely intend to share my schematic, layout and BoM.
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I guess I'm messing with the bias because it's a new trick. :icon_biggrin: I adjusted the current up to 26ma (same as schematic).
Get yer terms right. There is no mention of "current of 26ma" on the schematic. There is mention of -26V of negative bias voltage. That -26V bias will determine how much current flows through the tube, so the two numbers are related but not the same. It's pure coincidence that there is -26V on the schematic and you set your tube current to be 26mA.
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But, it's a great collection of a lot of expert advice and technical knowledge from you all.
And that's what makes this board - and the internet for that matter - so great. Sometimes you need to weed out the chaff, but the advice has often been a savior for my projects and no need to wait for a book to be published. Need advice? It is less that 5 minutes away! What a world we are experiencing!
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Get yer terms right. There is no mention of "current of 26ma" on the schematic. There is mention of -26V of negative bias voltage. That -26V bias will determine how much current flows through the tube, so the two numbers are related but not the same. It's pure coincidence that there is -26V on the schematic and you set your tube current to be 26mA.
Sorry! That's like the third time I've mixed those two numbers up. It's hard for me to keep straight because of that weird coincidence.
But, if I understand it correctly, it's getting the current right that really matters, yes? I mean, the negative grid voltage - the thing on the schematic and the thing I adjust with the bias pot - is what regulates the current flowing through the tube at idle. So, the current is what I'm ultimately setting when I bias the tubes, as I understand it, right?
Since I set the bias by measuring across the 1 Ohm resistor and adjusted the bias pot until I read 26mv, I guess I should also measure the negative voltage from pin 5 to ground to see what negative voltage I ended up with. But, whether that turns out to be -26v or -30v on the grid doesn't really matter does it?; it's getting 26ma of current at idle that does, yes?
That's how I grasp the concept of biasing the amp. I just seem to keep getting those two "26's" mixed up. :think1:
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I've been noodling for an hour, or so. I'm playing it through my AC4 Celestion speaker.
It really does sound great. Very full sound using the Normal channel. The Tremolo is crazy good. It has excellent speed and Intensity ranges.
Using the center position on my guitar - which is wired like a humbucker - it is honestly dead quiet. I hear no hum, or hiss at all. Thanks sluckey and Hoffman for your grounding wisdom.
But, in single coil settings I get some buzz depending on how I turn the guitar. I can make it go away if I turn the guitar away. (I was sitting about two feet in front of the amp, facing it.) I think a lot of that will diminish when the amp is in a cabinet and I'm standing farther away. But, I don't think I need to fiddle with wiring or grounding. It's great like it is.
After an hour the PT is warm, but I can keep my hand on it; it's not hot.
I have the 6V6S's set at 26.4ma and 25.7ma. I'm guessing that's pretty close between tubes (I'm guessing because I don't know what's normal matching is).
Here's a photo of the rig. Next thing is to order a cabinet and choose a 12" speaker. This amp deserves a good investment.
And, yes, I built the whole thing on that little table in my bedroom. :icon_biggrin: I don't have a workshop where I currently live. But, I did have a window to open to let out the soldering smoke! :laugh:
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You set a goal, asked a lot of questions, learned a lot, took a lot of ribbing, stayed the course, and ended up with a good looking amp! You should be proud! :worthy1:
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Thank you sluckey.
This has been an awesome project and every minute I've spent on this forum has been great - even the reprimands. :laugh:
This tube amp stuff is mysterious and deadly for beginners like me. And, that's a lot of the allure. And, owning a handmade vintage-design amp that I could never afford to buy is totally unbelievable.
I hope there's somebody else here who wants to build an amp and saw the support and direction I got and decides to dive in. :thumbsup: ...and, buy your parts from Hoffman!
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Well done! Your attitude is perfect. I've repaired/rebuilt about 60 amps and built maybe 30 from scratch or from organ or PA chassis, and I still learn stuff every week from this forum. It's the best. I love the North Face motto: "Never Stop Exploring" It applies to the stuff we all do. Congratulations on finishing your amp!
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Thanks bmccowan! There were one or two points when I was almost ready to just box it all up. Your encouragement helped me stick to it. :thumbsup:
I'm still finishing up my layout, schematic and BoM, but here are the last measurements I took. It's weird, but they're all within about 5% of the schematic (except the negative grid voltage). If that's what one wants. I'm sure this design sounds great with the BF AB763 DR PT and its higher voltages; most new ones are probably built that way. But, as I said, I'm glad I went with the Hammond 290HAX Harvard PT.
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Its good to be in the voltage range intended for the design IMO, but not go nuts trying to hit some ideal figure. My 6G3 is in the same range, as you know. With the AB763 amps, Fender was trying to get loud/clean, and succeeded. That sound is great too, but different. When using adjustable bias on this model, the bias setting affects the tremolo. So for me, once I arrived at the resistor values that allowed the bias pot to be useful, I adjusted the bias with the tremolo on until it sounded good to me. Get it too hot and it sounds like the choppers in Apocolypse Now.
I wish I had documented my previous builds like you are doing. Now if someone asks a question, or if I want to repeat something in a new build, I need to open the darn thing up! I just started a new project and will document as i think there will be twists and turns. It's an all octal 6V6 take on a Matchless Clubman circuit with iron from a Hammond AO-43. Sluckey's website has really helped me build amps from retired Hammond equipment.
Enjoy the amp!
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When using adjustable bias on this model, the bias setting affects the tremolo. So for me, once I arrived at the resistor values that allowed the bias pot to be useful, I adjusted the bias with the tremolo on until it sounded good to me. Get it too hot and it sounds like the choppers in Apocolypse Now.
That's interesting. I understood how the DC bias voltage passes through the Intensity pot along with the Tremolo signal en route to the power tube grids, but I didn't really connect the dots to think about how the bias voltage affects the Tremolo. Way back in this thread someone - probably you - stated that too hot of a bias would mess with the Tremolo in a bad way. As it is, I think my Tremolo sounds great, that is, it sounds the way I think I want it to.
It's an all octal 6V6 take on a Matchless Clubman circuit with iron from a Hammond AO-43.
Well, please document the build! It will be an opportunity for me to learn a bunch of stuff. :icon_biggrin:
I'd like to build something next with slightly uncommon tubes... maybe a cathode biased 7591A amp with Reverb.
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Well, please document the build! It will be an opportunity for me to learn a bunch of stuff.
Probably me too. I'm headed out for a long weekend, but when I get back I'll pick that project back up - I've populated the board and laid out the locations of iron and tube sockets so far.
There are good threads here for using unusual tubes. My project will use 2 6SL7s and a 6SH7 or 6SJ7 in the preamp. fairly unusual today, but pretty common years back.
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I've ordered the cabinet from Peter Mather. He's just down I-64 from me. I went with the combo. His cabinets look really nice.
I think I've also decided on a WGS G12C/S. It's based a the vintage Jensen, but the "/S" version has a softer top end. Buying a speaker is tough, but I haven't heard a bad recording of this one, and I haven't read a bad review.
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Well, a lot more playing and a bit more tweaking.
I went back to the 27K resistor (from 22K) on the bias pot. This allowed me to lower the bias current a little more. I now have it set to about 22ma and 23ma. This is cooler - closer to 60% - but, it really sounds better to my ears. It may not be for everyone.
Also, yesterday I had a chance to crank it a bit. I noticed that when using the Tremolo, if I muted the strings I could hear a faint thumping. Not surprisingly, I came across a fix proposed by sluckey. I wired in a 1N4007 on the Intensity pot and the amp is now totally quiet with the guitar muted, even with the Intensity pot up full.
So, my question is this: is the thumping due to a more negative grid voltage? In order to bias my amp at around 23ma I need about -31V on the grids. Could this be why Fender called for -26V on the schematic?
Anyway, I knew there would be a fine-tuning stage involved in this build. So far it hasn't been frustrating, just methodical.
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why Fender called for
I suspect fender did what you did, adjusted for current, then measured bias voltage and wrote it on the schematic.
NONE of the systems I every worked on were = for bias, just "within excepted range"
think of it like an "indicator" that something might be amiss if it's way off, 5 is not way of
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Thanks shooter. And, I guess some tubes have a wider range of useable/practical bias current settings than others. The 6V6 seems to have a somewhat narrow range of about 7ma - 8ma from being too hot to too cold. At least with my plate voltages.
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Wow!
I owe a debt of such enormity to you all, that I could never repay it.
Thank you Doug for this great forum and parts store; sluckey for your patience and expertise; bmccowan for your moral support; shooter for keeping it stress-free, 66Strat, pdf64, and everyone else who contributed/participated in this project!
I'm sitting here with a dead quiet, perfectly functioning 6G3. And, it sounds unreal. I went with the Warehouse G12C/S and it's perfect (I didn't want a super bright sound). It took about six weeks to get the cabinet made, but it was worth the wait. I can highly recommend Mather Cabinets.
I just finished building a BYOC Classic OD kit, but I just want to play through it straight for awhile. It sounds too good.
Thank you everyone!
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I've learned a few more things here on the forum (thanks in particular sluckey) since I built this amp. I thought I'd look back inside this one for any questionable soldering, or any wire runs, etc. that could be improved. There were some places where I rushed a bit.
One area where I make mistakes is with grounding. So, my question is, should the tremolo circuit be grounded with the preamp and input, or at the power ground? Or, does it matter? I've circled the ground wire in question below.
Thanks again for all the help!
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Look at the schematic and determine which B+ node filter cap supplies the plate of the trem oscillator. Now connect the trem circuit ground to the same place that node cap is grounded.
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So, the answer is usually there if I just look in the right place. :icon_biggrin:
Seriously, your answer is better than the one I asked for sluckey. Now I can see where the ground should connect. :thumbsup: Thanks again.