Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on March 29, 2022, 07:27:54 pm
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Hi all, I'm brainstorming for a new amp project. I want a change from building things in Fender BF-style chassis. I've been looking at sluckey's Dual amps, and I like his board layout style, along with using a Hoffman Stout chassis (my Musing 40 was built that way).
So... since I started with this amp hobby I've wanted to build a 6G6-B. Would it be ridiculous to build one modelled on sluckey's Dual amp concept? It would be the Bass and Normal channels with a three-way switch for either, or both channels (each channel has an even number of triodes, so that would work, right?). I'd only need to drill one hole for the Presence pot, and use one tube socket adapter plate for the PI tube. At any rate, it would be a compact amp. I would probably go with 6V6's since I already have a PT for them.
Thanks for any and all comments and criticism. :icon_biggrin:
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Sounds like a nice spring project. However, you can't jumper the channels because the Bass channel has three inverting stages, and the Normal channel has two inverting stages.
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In addition to what sluckey said, getting rid of oscillation is trickier in a 6G6B because of the way B+ is supplied to the pre-amp channels, and because the signals between Ch1 output and the Ch2 output are 'DC-coupled' to each other. I built a 6G6B about 4 years ago and found I had to add an extra 0.022uF coupling cap before each 470k mixing resistor to get the problem to go away. Your experience might be different.
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Thanks tubeswell and sluckey!
sluckey, I missed that inversion issue. I guess I thought that because the Bass channel has four triodes, and the Normal has two that they would end up the same where they join. :dontknow:
tubeswell, I didn't even notice that. Yes, now I see that coupling caps should be added before each 470K mixing resistor.
Also, it is a bit weird how the B+ string is connected. The Bass channel is fed off of "B" and the Normal channel off of "C." But, the Bass channel actually has lower plate voltages than the Normal channel, even though "C" is stepped down with a 27K dropping resistor! Seems backwards.
I would make four nodes: A for OPT > choke > B for screens > C for the PI > and add a D for both preamps.
Thank for the input! here's the schematic:
https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6b.pdf
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The bass channel has a 220k plate resistor and the regular channel has a 100k plate resistor. I suspect that this could be the reason the voltages are not as you anticipated.
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Also, there are four triodes on the “B” and only two on the “C” node.
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It would seem to me that as long as both channels have an even number of triodes, the inversion should end up the same. If one channel had an even number of triodes, and the other had an odd number, that would cause an out of phase problem where the come together for the PI.
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If you use a 0.022 decoupling cap before the 470k resistor on the output of each of the channels, you shouldn't need the .005 cap shown on the input to the PI tube.
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The bass channel has a 220k plate resistor and the regular channel has a 100k plate resistor. I suspect that this could be the reason the voltages are not as you anticipated.
Thanks brewdude!
The preamp plate load resistors are something I want to keep original. The Bass channel uses one 220K and three 100K's, and the Normal channel has one of each. I'll also want to keep the plate voltages "sort of close" to the schematic. But, I should be able to do that without using the 428V screen node (B) and forcing it down to 150V for the Bass channel. Plus, the grounding method I've settled on is easier to implement if I isolate the nodes for different parts of the amp, i.e., not using a power amp node for the preamp. I'll try to work something out in PSUD2.
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If you use a 0.022 decoupling cap before the 470k resistor on the output of each of the channels, you shouldn't need the .005 cap shown on the input to the PI tube.
Thanks AmberB. Maybe I should use two .005's instead of the .022's, and then leave out the one you're referring to.
Does it matter if the coupling caps are before or after the mixing resistors?
And, yes, I was understanding the inverting the same way! Two or four should end up with the same non-inverted signal, yes?
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It would seem to me that as long as both channels have an even number of triodes, the inversion should end up the same. If one channel had an even number of triodes, and the other had an odd number, that would cause an out of phase problem where the come together for the PI.
Except that one of the stages in the bass channel is a cathode follower, so the signals will be out of phase if jumpered.
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It would seem to me that as long as both channels have an even number of triodes, the inversion should end up the same. If one channel had an even number of triodes, and the other had an odd number, that would cause an out of phase problem where the come together for the PI.
Except that one of the stages in the bass channel is a cathode follower, so the signals will be out of phase if jumpered.
Ahhhhhhhhh!
And that's all fine. I'm really interested in just having both channels to switch between. It looks like a lot of folks built this amp as a single channel. Some love the Normal, and others rave about the gain of the Bass channel. With a sluckey-style Dual config I can have both in a small chassis.
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I'm really interested in just having both channels to switch between. It looks like a lot of folks built this amp as a single channel. Some love the Normal, and others rave about the gain of the Bass channel. With a sluckey-style Dual config I can have both in a small chassis.
The Bass channel is the best channel for guitar on this amp YMMV - check out this demo from about 2:50 on this clip
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I'm really interested in just having both channels to switch between. It looks like a lot of folks built this amp as a single channel. Some love the Normal, and others rave about the gain of the Bass channel. With a sluckey-style Dual config I can have both in a small chassis.
OK, but keep in mind that you can only select one or the other (Nor or Bass) but not both. You would need a common DPDT (ON-ON) switch as seen in my Dual Lite.
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I have a Bassman 50 that I've rewired multiple times. Currently ch 2 is the normal 6G6-B channel - and I really like that channel. I tried the 6G6 bass channel and then decided I wanted something that diverged more and tried several different pentode circuits, but found that the tone stack loaded that pentode preamp down too much, and/or I'm just not focused on the right issue. So i took a break from that amp and ch 1 is up for grabs again. So I think you have cool project idea and I'll be watching to see how you make out.
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Thanks for that vid tubeswell.
Thanks sluckey. I have a mini DPDT that I bought for my Hoffman 6V6 Plexi w/Hot Switch build, but didn't use (hot switch is on all the time). So, that's one less part to buy.
bmccowan, thank you for that info and your interest. If I can pull this off it will be cool to have both channels.
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If you're looking at the 6G6B schematic, there is no cathode follower in that preamp. The 5F6A has a cathode follower to drive the tone controls, but the 6G6 does not.
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...the 6G6B schematic, there is no cathode follower in that preamp. ....
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If you're looking at the 6G6B schematic, there is no cathode follower in that preamp. The 5F6A has a cathode follower to drive the tone controls, but the 6G6 does not.
6G6, 6G6-A, 6G6-B, and 6G6-C all have a cathode follower in the bass preamp. So, which schematic are you looking at?
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Not that I want to do this, but is there some kind of "little solid state thing," like a transistor, that will shift a signal's phase 180 degrees?
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Yes, thousands of them.
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Yes, thousands of them.
Haha, well, I figured as much. Though, I know less about solid state that I do about tube amps. :icon_biggrin:
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Haha, well, I figured as much. Though, I know less about solid state that I do about tube amps
They just took the stuff that was in tubes, shrunk it, and stuffed it in little aluminum cans. Now they are taking that stuff and stuffing it in little nano-cans.
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some kind of "little solid state thing," like a transistor, that will shift a signal's phase 180 degrees?
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OK, I see that I was looking at the lines on the bass preamp wrong. Once pointed out to me, I see the cathode follower.
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Thanks for that schematic PRR; I've saved it in my files for future reference.
I think every time I post a new project here I change directions and pursue something different. So, this thread is no exception.
I've redrawn my Musing 40 for 6V6's with some changes. The original wasted triode is now a Cathode Follower before the tone stack. Instead of fixed bias, I stole the cathode bias from the 5E3. I dumped the NFB. I thought I'd try the valve Wizard's "low-noise" grid stopper (it was recently mentioned in another thread). I don't know yet what values the B+ dropping resistors should be. I'll use PSUD2 to get in the ballpark.
I'm sure there are errors galore in this schematic.
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V1: when driving a cathode follower, the 1.5k bias sets the plate too high. The cathode follower distorts way early. Some call this "toan". But will it be reliable in the first stage where signals are low? For reference, Fender liked 820 Ohms in this plan.
But you are driving a tone-stack with 100k first resistor. You don't need a cathode follower for that, Fender only did it for the 56k 5F6a tone stack.
Output stage: 0.1uFd grid caps is hi-fi, when you never clip. We clip guitar. Fat caps here take a long time to recover. Plagiarize many well-known plans and you will see 0.5u, 0.2u, even smaller when there is NFB around the power stage.
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V1: when driving a cathode follower, the 1.5k bias sets the plate too high. The cathode follower distorts way early. Some call this "toan". But will it be reliable in the first stage where signals are low? For reference, Fender liked 820 Ohms in this plan.
But you are driving a tone-stack with 100k first resistor. You don't need a cathode follower for that, Fender only did it for the 56k 5F6a tone stack.
Output stage: 0.1uFd grid caps is hi-fi, when you never clip. We clip guitar. Fat caps here take a long time to recover. Plagiarize many well-known plans and you will see 0.5u, 0.2u, even smaller when there is NFB around the power stage.
Thanks PRR. I obviously don't know enough to know that I don't need the CF. :icon_biggrin:
OK, smaller output stage caps makes sense. An easy thing to change.
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As far as the extra triode, I think I'll just not try to use it. The TW Express doesn't.
What I want to do is read up more on the effects of different cathode resistors/bias points and different plate loads. I'm convinced that one of the reasons the 6G3 sounds so good is because the input triodes have 220K plate resistors and share a 1.5K cathode resistor, which is sort of like having 3k (or, 2.7K) on each one, I think. Also, there's the whole thing of bypassed/un-bypassed gain stages. I've never experimented with that, either. I've read the theory, but haven't actually listened to the differences.
This is a pretty simple amp, and it would be "easy" to experiment with just those three things to see what I like.
Thanks
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> sounds so good is because the input triodes have 220K plate resistors and share a 1.5K cathode resistor, which is sort of like having 3k (or, 2.7K) on each
There's no magic numbers. Ratios are more important. 220k and 3k is not a lot different than 100k and 1.5k. Though you also need to compare the 100k/200k to the load resistance.
Books may not tell you as much as an hour with a hot iron and a box of resistors.
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...Books may not tell you as much as an hour with a hot iron and a box of resistors.
I think that will definitely prove to be true.
I did find a table of gain figures for different resistors and voltages on another site. Here's a portion of the data that the person gathered and posted:
Plate B+ 180 volts
Resistor Cathode Gain
100k 1.8k 40
100k 2k 47
100k 2.2k 52
220k 3k 53
Thanks
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> I did find a table of gain figures
That does not even look right. Larger cathode resistor, less current, makes more gain?? Then let's reduce current to zero, save lots of power!!(?)
Also, does this include any load? With any likely load, increasing the plate resistor can only go so far.
Also I wonder what "Plate B+ 180 volts" means. Is the plate forced to be 180V? (How?) Or is that the power supply, plate naturally sitting lower?
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Again, PRR, I don't really know what I posted. :dontknow:
Somehow I'm going to have to gain a "bigger picture" perspective of not just a common-cathode triode, but how it relates with the preceding guitar pickup and cable on one side, a tonestack on the other, and a certain DC voltage too. I realize I can't look at just one isolated component.
So, swapping real resistors and hearing the results will be the best way.
That's actually what I did with my AC4. After about five or six changes to EF86 resistors I finally hit on what sounds "right" to me. I may have broken several datasheet rules, but it sounds the way I want it to. :icon_biggrin:
Thanks
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found this chart, came in handy when I was BB'n circuits
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Thanks shooter!
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I'm playing around with PSUD2, and I'm wondering if it's a bad idea to add another RC filter before the choke. The purpose would be to lower the screen voltage by about 15 to 20 volts, or so.
Thanks!
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You don't actually need C2 for anything. There's as much filtering as almost any commercial amp already. You just did that because PSUD does not allow arbitrary placement of dropping resistors.
Skip R1 and C2. Tell PSUD that L1 is 4H 1100r. It won't know you really mean a 4H + 100r plus a 1k.
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...You just did that because PSUD does not allow arbitrary placement of dropping resistors.
Yup, that's exactly why. :icon_biggrin:
Skip R1 and C2. Tell PSUD that L1 is 4H 1100r. It won't know you really mean a 4H + 100r plus a 1k.
Perfect. Thank you PRR.
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If I do decide to use a 1K dropping resistor before the choke, is this math correct?
20V dropped across 1K resistor with around 22mA of current (6V6 screen current + three 12AX7's current draw):
20 Volts x .022 Amps = 0.44 Watts
So, a typical 3W resistor would be more than enough for this location? Or, am I not seeing all of the current?
Thanks.
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20V dropped across 1K resistor with around 22mA of current (6V6 screen current + three 12AX7's current draw):
20 Volts x .022 Amps = 0.44 Watts
So, a typical 3W resistor would be more than enough for this location? ...
Yes and Yes
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Thanks tubeswell!
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I've come up with this. Nothing really original. As the title block on the schematic says, it's an AA864 preamp with some TW Express component values, and a cathode biased power amp based on the 5E3. I do know that I really like this preamp from my 6L6 version.
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Nice schematic. Lot of potential.
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Thanks sluckey! I still want to go the Hoffman Stout chassis route. So, the layout you designed is most of the way there, already.
What's great about this hobby is that everyone can go in their own direction - as long as the design is safe and stable. I doubt I'll ever get into the pursuit of replicating an exact clone of some famous/iconic amp. For now, my interest seems to be "simple, inexpensive, and (relatively) easy to troubleshoot."
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For zero effort, DC heater elevation could be implemented, by referring the 2x100ohm heater balancing resistor node to the 6V6 cathodes, rather than 0V common.
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For zero effort, DC heater elevation could be implemented, by referring the 2x100ohm heater balancing resistor node to the 6V6 cathodes, rather than 0V common.
Thanks PDF64. That's something I've never tried, but have thought about. The Stout chassis is compact, and there's not a lot of open area to keep high-current wires far apart from each other. Your suggestion is a great one.
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I've ordered the chassis and parts for this build. Now I'm on the fence as to whether I should go cathode, or fixed bias. I have the parts for either, and I'm finishing up layout drawings for both. I've been reading several threads over at Ampgarage about cathode biased Expresses (some dating way back). Some say it rocks, others point out the drawbacks like blocking distortion and flubby, loose low end. The purists think it's sacrilege. Cathode resistor and bypass cap choices are all over the place. I've gotten the impression that it can work well, but has to be dialed in with some trial and error.
Anyway, I'm leaning towards fixed bias with Express component values. It won't be a clone, but most of the signal path will closely resemble one or more of the (many) Express schematics found in the library here.
But, like I said, I'm still on the fence. :icon_biggrin:
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Due to the high input impedance on the LTP, you can get away with a lot smaller coupling capacitance feeding the LTP input/inverting triode (without losing any useable 'guitar bandwidth'). So you might want to replace the 0.022uF with 1000pF
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Now I'm on the fence as to whether I should go cathode, or fixed bias.
If you have room, you could do both - switchable. Sluckey has a sketch in his scrapbook. Outside of that, I would build it as Ken intended, although I know its not a full clone, and see what you think. Pretty easy to convert an amp from fixed to cathode bias. Although I really like cathode bias amps, I once converted a Princeton Reverb to cathode bias, and did not like it at all. But I did not give it much of a chance - 20 minutes or so. :icon_biggrin: It was like the original designer knew what he was doing, better than me :dontknow:
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Thanks tubeswell and bmccowan.
One reason I'm set on using turrets is that they make it easy for me to switch things like coupling caps. Just reflow a dab of fresh solder and parts come right out.
...I would build it as Ken intended, although I know its not a full clone, and see what you think. ...It was like the original designer knew what he was doing, better than me :dontknow:
Yeah, going against that logic is what's nagging me. Fixed bias it is.
I also went through a similar thing with my AC15 where I grafted in the Princeton Reverb cathodyne PI, never bonded with it, changed it back to the Vox LTP, and now it sounds "right."
Thanks.
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I've been thinking about the Express's high-pass filter before the third triode with its .002uF coupling cap and 150K resistor. From what I've been reading this is an important part of the preamp's character.
So, I'm wondering if the third circuit (on the right) in this drawing will maybe preserve more of that high-pass filter's "quality" than the middle Musing circuit. Or, is there a better way to wire the gain pot?
Thanks for any input!
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The -3dB cutoff point with the Express's .002uF cap/150K resistor is about 581 Hz.
A 750pF cap and 500K I think works out to about a 425 Hz cutoff point.
That's a bit more low end, but close, if I'm doing this right.
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Well, I've come full circle again. :icon_biggrin:
It's back to basically the AA864/Musing design, but with a few of the Valve Wizard's recommended anti-blocking distortion tweaks in the PI, and his use of a warmer-biased third gain stage.
Suggestions/criticisms welcome!
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:wink:
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Well, since I added back the global negative feedback, I would have to answer yes to Billy.
But, I do seem to brainstorm on this forum. Which is good, because a fair number of my "ideas" are solidly based on a lack of knowledge.
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Here are my drawings so far.
Don't think of them as final.
Will I run into any trouble grounding the heater artificial center tap on the board along with the power and bias grounds as shown in the layout?
Thanks
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Which is good, because a fair number of my "ideas" are solidly based on a lack of knowledge.
Very funny :laugh:
What I like is that you spend a bunch of time thinking about your builds and asking questions. It's the Socratic method of amp building. I should do more of that. I tend to just jump right in - build it - and then start making changes. :icon_biggrin:
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Will I run into any trouble grounding the heater artificial center tap on the board along with the power and bias grounds as shown in the layout?
no. You're getting pretty good with that layout program. :thumbsup:
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sluckey, it's getting maybe a bit obsessive. I created my own terminal lug drawing. :help:
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Hahaha. I totally get it!
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I've finally gathered all the parts for this project. One bit of good news is that JJ tubes - from Slovakia - are reappearing as "in stock" at some of the online merchants I've visited. The prices are somewhat higher, but not ridiculous (I got a matched pair of JJ 6V6's for $48).
So, I've reverted back to the cathode biased version. And, I'm going to try the elevated heater method that pdf64 suggested.
I'm sure there are mistakes in my drawings.
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Seriously, this time. The noodling is done. This is how I'm building it:
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Looks like a plan. :thumbsup:
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Thanks. Yeah, this should give me a pretty interesting range. With the TMB and NFB switched out it won't really be a 5E3, but it should lean in that direction. And, having the gain control on the third triode is a nice extra.
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Some progress.
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:thumbsup:
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Some progress.
What chassis did you use?
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Thanks!
66Strat, I used Hoffman's Stout chassis.
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New summer work schedule = less time during the week to do amp stuff.
I did finish and install the board, and I mounted the odd bits like the terminal strips and cap can clamp. So, hardware is done.
You can see all of the diode rectifier and fixed bias holes in the board that I filled in with epoxy.
Tomorrow I'll install the cap can and solder up all of the PT and the OPT wires.
I should finish before the first day of summer. :icon_biggrin:
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The soldering is finished. I'll trace over everything one more time, and dim-bulb test it in the morning.
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Nice work. :thumbsup:
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I still get shaky and nervous whenever I fire up a new project for the first time. :icon_biggrin:
OK, so Antique Electronic Supply does a good job of matching tubes:
JJ 6V6S #1 - 323.4VDC plate to cathode
JJ 6V6S #2 - 323.2VDC plate to cathode
Voltage across 270 Ohm cathode resistor - 20.05VDC
Here's a screen shot from Rob's calculator. To be safe, I aimed low and landed low. :icon_biggrin:
80.7% PD with an average of 11.3W per tube.
35.1mA average per tube.
I need to breathe for awhile and I'll plug in my guitar.
Edited to correct: The cathode resistor actually measures 266.6 Ohms. So, PD is really 82.1% at 11.5W per tube average, and plate current is 35.6mA average per tube. At any rate, I can switch to a 250 Ohm later on, or two parallel 470 Ohm 5W's for about 235 Ohms to up the PD, if I want.
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Everything works as intended. Next, I'll hook it up to the 12" speaker in my 6G3 cabinet. The little 8" test speaker sitting on the desk doesn't tell too much.
I thought I might get pops when I flipped the tone stack and NFB switches, but no noise.
There's an expected jump in gain when I switch off the TMB.
I ended up using a JJ medium gain 12AX7 at the input, a regular 12AX7 in the second position, and a 12AT7 for the PI.
There is a little bit of hiss, but not bad. There's literally no hum. I'm guessing that grounding the heaters at the power tube cathodes helped with that. Thanks to PDF64 for that suggestion. Anyway, I'm not going to tweak anything for now. I just need to play through it.
Thanks for all the guidance and encouragement everyone!
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I need to breathe for awhile and I'll plug in my guitar.
:laugh:
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There's an expected jump in gain when I switch off the TMB. Anyway, I'm not going to tweak anything for now.
Surely not ‘unexpected’? TMB tone stacks are significantly lossy, so if you use a TS lift, you’ll get a significant jump in gain.
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Surely not ‘unexpected’? TMB tone stacks are significantly lossy, so if you use a TS lift, you’ll get a significant jump in gain.
Yes, I did mean "expected." I knew the gain would jump up. The next bit about tweaking was unrelated to that, but I see how it looks like it was.
I replaced the gain pot return wire (to the 220k/220k divider) with a shielded wire and the hiss dropped quite a bit.
It also sounds a lot better through the Celestion Creamback in my AC15 cab.
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I like the looks of a big ole amp in a tiny chassis. :thumbsup:
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Thanks tubeswell. Yeah, it is basically like a boost switch. You can tell the difference without the bright cap, but there's still plenty of treble by using the guitar's tone control. IOW, it doesn't turn into a dark amp when you flip the TMB off. You just sort of lose the bright cap part, and it sounds more full.
Thanks sluckey for your help and support with this project. Some of the soldering was a challenge in the confined areas. But, I agree, it's cool having six tubes on a 12" chassis.
So, here's a question for anyone. I'm at about 82% dissipation at 11.5W per tube. How would bringing the dissipation up closer to 100% change the tone? I haven't had time to play the amp enough yet, and it sounds good now. But, it seems like most people prefer to run cathode biased amps near the limit.
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closer to 100% change the tone?
SE tone comes mostly from the pre since they're basically Class A - Amplify what comes in without change.
80 - 100 changes the "operating point" - Vk. it should give a little more headroom.
I would play it til Your fingers bleed, then make a change and evaluate. It's more about your happy than the tubes. If your happy has tubes melting at 6mths, then tubes melt at 6mths.
don't forget to use ANY and ALL pedals while evaluating.
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Thanks shooter.
You're right. I need to just play it. There are four different ways to run this amp: tone stack with or without NFB, and tone stack bypassed with or without NFB. So, it will take some playing to really check out all the differences. Plus, I can swap around preamp tubes - try a 12AY7 for the input, 12AX7 for the PI, etc.
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I can see the value of using a switch to lift the tonestack, but I wanted to throw out this idea that I sometimes use. I replace the 25K mid pot with a 250K-A pot (10% taper). The pot acts as a standard mid pot for the first half of rotation and acts as a variable tonestack lift (raw) for the rest of the rotation.
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sluckey, that's without doubt a more efficient and useful solution. I think I'll keep the switch for the bright cap, and swap in a 250K-A pot. Having those two things independent of each other is even more versatile.
Thanks!
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I'm lovin' this amp. Here's a pic of my "home rig" that lets me push things into overdrive through the 12" WGS G12C/S in my 6G3 cab. It can do clean/bright AB763, and some kind of meaner raw gain thing. And whatever in-between. At slightly bigger than a lunchbox - thanks Mr. Hoffman for your chassis.
(I'll post my final drawings with all the voltages when I get a chance to measure them)
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Here are my drawings with the DC voltages entered on the schematic. With all the help I get here I like to share them since only a part of it is really my work.
Thanks again to everyone. This is a great place to build amp projects!
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Great job, man, thanks for sharing this!
m.
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So, I'm dropping 28.8 volts across a 1K 3W resistor in the B+ right before my choke (which feeds the 6V6 screens).
That equates to .0288A and therefore .839 watts.
Is a 3W resistor enough leeway, or should I go with a 1K 5W resistor?
Thanks!
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I usually double the expected watts and use that as a rough guide to rating. So in your case, I think a 3W resistor should be more than enough.
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a 1K 3W resistor in the B+ right before my choke
we used to call that a brother-in-law circuit :icon_biggrin:
the inductor should be good enough for the screen since inductors really hates changes in current, thus protecting G2 from large swings. My sense is the resistor is just along for the ride, but hey, you helped out your sister. :laugh:
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Thanks acheld, I suspected that it's fine.
Thanks shooter. I was sort of looking at the 5E3 power amp when I drew this project up. I guess I wanted to lower the screen voltage and benefit from using a choke. The amp works fine, so I'm just leaving both in place. :icon_biggrin:
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I've been reading Merlin Blencowe's (a.k.a. Valve Wizard) new 2nd edition of Designing Power Supplies for Valve Amplifiers and I came across this section on suppressing the resonant peak of chokes.
He gives an equation for determining the damping resistor value.
The choke I'm using is 4.8H and 100Ω. The capacitor is 32uF. I'm no good at math, but I get 775Ω. I happen to have a 680/5W that would give a total resistance of about 780Ω. And, dropping down from 1K to 680 will also increase the B+ to the screens and preamp a little bit, too.
shooter, maybe that brother-in-law is not completely worthless. :icon_biggrin:
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This morning I went back into this amp and made a few changes. It was already a good sounding amp, but now I think it's really good.
First, I went from the 270Ω cathode resistor to a 250Ω 1%, and a 620Ω 1% in front of the choke (using Valve Wizard's resonant peak suppression formula).
This biased the amp hotter at about 93%.
Then, I disconnected the tone stack bypass wiring, but left the Bright cap switchable.
Finally, I went with some TW Express resistor values on the three triodes:
V1 - 100K load/1.5K cathode
V2 - 150K load/2.7K cathode
V3 - 100K load/10K cathode
With the bright cap on, the volume up around 6, and the gain at around 3 it's bright (but, not piercing) and slightly crunchy.
With the bright cap off, the volume around 3, and the gain at 6 it's clean and warm - great for jazz chords.
I'm really happy with what this amp can do now. I think I've found the optimal values for what I want. As I've said before, I'm just borrowing the work others have done before.
Here are the latest drawings. I didn't take all the voltage readings, and the ones on the schematic were measured across the tubes, not to ground.
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I played the amp for a good three hours tonight through the 12" WGS speaker in my 6G3 cabinet.
I put a BYOC Tube Screamer clone in front of it.
With the gain dimed and the volume around 9 o'clock it's aggressive and edgy, and does the live Midnight Rambler thing pretty good.
With the volume dimed and the gain around 9 o"clock it does that creamy Mick Ronson Moonage Daydream thing great.
In between, it's got a whole range of clean to crunchy sounds.
It's the best amp I've built, and the most versatile.
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I needed something to work on, so I changed the preamp components in this amp back to Fender values.
I also took a bunch of voltage readings and added them to the schematic.
Then, I spent some time in PSUD2 creating a sim that is very close to the actual amp voltages. I did this mainly to get better at modeling power supplies.
The amp sounds better.
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You're probably all tired of hearing about this project. :icon_biggrin:
I made a few more small changes (for the better, I think).
I'm now using 2.2k > 4.7K dropping resistors that brought the input tube B+ right near 300VDC.
The first two triodes are biased at -1.5V.
But, for the third gain stage I went from first using a cold clipper, to a center bias, to now using a warm bias with an 820 Ohm resistor and -1V on the grid. Of the three, I like this the best. My second gain/volume pot is connected to this grid, so I can crank the first volume pot (right after the tone stack) and then control how far into grid-current clipping I go with this stage. In this amp that sounds better than cold clipping into cutoff - at least to my ears. But, it's subjective and subtle.
The 6V6's are cathode biased with a B+ at around 350V, so it's sort of like a 5E3 power amp with an AB763 input and TMB tone stack, and an AA864 extra gain triode that's warm biased with a gain pot in front.
It's a simple amp that has a wide range of sounds with only a bright switch and an NFB bypass switch. With JJ tubes, Hammond iron and a Hoffman Stout chassis it was something I could afford to build.
Thanks for reading.