Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on June 01, 2020, 02:44:03 pm
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I think the original amp was a tube rectified 18w EL84 Marshally thing. But it is no more. My plan is to rebuild the cab to rehouse the slant chassis. I'd like to make a little Dumble voiced clean pedal platform amp. Are there any good layouts about for this?
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The Power xfmr has 590v CT/7v/5.3v secondaries. Can I build a 6L6 based amp with this?
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Iron that was originally used in an 18 watt EL84 amp is not well suited for a 6L6 amp.
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Check out the D'Lite w/ 6V6
If you just want to do the clean channel, that's even easier.
https://ampgarage.com/forum/viewtopic.php?f=4&t=32248&hilit=d%27lite
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Thanks guys.
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> Can I build a 6L6 based amp with this?
You can build a 6L6 amp with a juicy 9V battery.
Sound will be almost inaudibly tiny, and you wonder why you bought the big/costly 6L6 for such a wee sound.
Still, 590VCT comes near 400V DC, and many bold 6L6 amps have worked on 400V. But if the *current* is scaled for EL84, you can't run the 6L6 over half what they"could" do, and EL84 or 6V6 are more right-size.
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+1 for going with 6V6 output tubes.
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I drew this up some years ago but have not built it. Compare it to whatever Dumble version you like. I'm attaching the SCH editable version for you also so you can change it to 124, 102,183 values etc...…….
With respect, Tubenit
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And I have this version drawn up also with added one tube reverb. As always CHECK for ERRORS! Compare schematic with layout IF there is a discrepancy, go with the schematic.
With respect, Tubenit
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Lastly, when I think of a Dumble inspired clean channel but wanting to use it with FX pedals, I'd want an active FX loop.
I built a sorta version of that for a friend who is quite pleased with it. The amp sounds fantastic, IMO. Having the mid boost and PAB features (on mini-toggle DPDT) will actually give the amp some grind when they're engaged. Delay and reverb pedals sounds excellent inside the FX loop.
It is cathode biased 6v6's & has a PPIMV (which was useful to my friend with young kids at home but wasn't something I'd need given the FX loop controls as a master volume).
I have ExpressSCH versions of the schematic and layout IF anyone wants them?
The schematic is the current "tweaked" version after playing it a while. Compare the schematic and layout and go with the schematic version.
With respect, Tubenit
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Thanks Tubenit. That looks great!
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So after digesting this, I'm really tempted to build the D'mars. Of course, there goes my 18w iron. Oh well, I'll save it for a rainy day. Tubenit-I'm looking at the BOM for the D'mars. Any updates? Also, I know Doug isn't selling chassis any more. I assume this will fit in a Deluxe style chassis?
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I'm looking at the BOM for the D'mars. Any updates? Also, I know Doug isn't selling chassis any more. I assume this will fit in a Deluxe style chassis?
No real updates to the D'Mars. However, I think of the Dumblish inspired amps as an amp that one builds and then tweaks to the tone they like the best. So, if you follow thru and build it ………… post how you like it and what you might want changed in the tone and I (along with others) can help you tweak it towards that.
I built a D'Mars in a tweed style chassis that was 20"x 2.5" x 8". Using stand up PT and stand up OT, I think one can easily get by with a chassis around that size. (maybe even a 6" instead of 8" width?)
I've not built one with EL84's nor can I remember someone doing that? Are you using 6V6's?
With respect, Tubenit
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Yes, Tubenit. I plan on using 6V6's. One concern I have is with the tube rectifier. I will be using this amp with a little pub blues band. I have been using a 40w 6L6 Traynor YCV 40 amp and it's more than enough. I never run the volume past 3 and it's plenty loud. With a Strat I use boosts into the high headroom clean channel and it stays tight. I just don't know if a tube rectified 6V6 amp will keep up. Then again, I might like what it does to the sound. For comparison, would you say it's in the same ballpark as a Deluxe reverb? I know there are ways to tighten those up.
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I would say it's not as loud as a Deluxe Reverb which is 22 watts. Closer to 14 watts similar to a 5E3 Deluxe (as they are both cathode biased).
Typically, when I build an amp …………. I use a 275-0-275 or 300-0-300 PT. Then I use a 5Y3, 5V4, GZ34 or solid state plug in rectifier. It does allow me to change both volume and tone (sort of). I'm not saying you need that kind of PT, just using this as an illustration on how one can change voltages easily and increase volume.
With respect, Tubenit
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So I'm going with Ryan's TBM. Does Doug make a turret board for it?
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No. You'll need to make your own or use/adapt a tag board or pre made parallel turret board.
With respect. Tubenit
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Thanks again Tubenit for creating this. I've been studying your schematic and layout and per your advice, I've deferred to the schematic when there is a discrepancy. I took the liberty of making some markups to them where I saw a couple. I labeled the pots and I found 3 small component values that differed. Please have a look and see if you agree. I know you said this was tweakable and maybe those values are a tweak!
I ordered the bulk of the parts from Doug, so this build is on! Thanks again.
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So I received some parts I ordered from Doug. And I have a donor chassis and cabinet (an old Yamaha G100-112). I have a 30x30 turret board that I'm going to start loading up tonite. BUT I discovered that the OT I have is for a Blues Jr. Bummer! But I do have another OT (and PT actually) from a 50w Peavey VTX 100 series. It says a 100 series but it has only 2x 6L6 tubes. Can I use this OT for this build?
I don't have the knowledge, but I'm guessing that the primary impedance is the most important thing and it's wattage rating like a speaker, is OK to be higher but not lower?
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I don't know the answer to your question, so I hope someone who does will answer it for you. That Peavey amp appears to be all transistor up to the power tubes? I'm not sure if that means the OT could or could not be used?
For a PT, if you're using rectifier tubes ……….. I like a 300-0-300 with 140ma with 6L6's. However, you can use something higher like a 330-0-330 if still using tube rectifiers.
Regarding the OT, I'd advise something that would be in the 30-40w minimum. However, I've used a Hammond OT that was 20w for 6L6's and it worked fine. But it was almost as large as a Marshall 50w OT.
With respect, Tubenit
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try peavey, they have always been helpful in the past, just pretend you know what you're doing :laugh:
https://peavey.com/
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I measured the primary of the Peavey OT and my meter says 308Ω. I guess this doesn't mean much because we're looking for impedance...What I'm trying to find out is-can a dual 6l6 OT be used with 6v6's? I can't find the exact schematic, but the OT is the same one Peavey uses in the VTM60.
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typical 6L6 in PP ~ 5k, 6V6 ~8k.
all that is dependent on B+, current....
surf up "determining turns ratio in a transformer"
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Here's where I am. The donor chassis, Iron, boards, sockets. I'm looking for approval or suggestions on the layout.
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That looks OK to me and I would anticipate that it will work out fine. IF you could have the rectifier tube a little further away from the layout board and maybe lined up with the other 8pin sockets, you might consider that.
Regarding OT and impedence………….. Dumble purposely mismatched some of his Dumble amps using 8 ohm speakers with 6L6's. Kevin O'Connor of London Power states the following:
Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.
A: It is awfully expensive, and also a ridiculous suggestion. There are two issues here, though; one is the notion of “impedance matching”, and the other is simple design preference.
As stated throughout our TUT book series, speaker load impedances and reflected loads to the output tubes are all “nominal”. An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the “power bandwidth”. If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.
The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don’t behave like transistors.
The design issue for impedance matching comes into play when a designer takes the approach that “everything is critical”. In some circuits, this may be the case. Tubes don’t really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says “this is good”, “this is bad” and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
Food for thought! :icon_biggrin: With respect, Tubenit
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So as I'm proceeding thru this build, I've got a couple of questions.
1. Is the rectifier tube a 5V4 or a 5U4? and exactly how is it wired? I've never built with a tube rectifier before, so this is unfamiliar to me. My HV goes to pins 4 & 6. my 5v winding goes to pins 2&8 and the DC out is on pin 8? Where does the ground reference come from?
2. The two 2.2k screen resistors on the 6v6's should be 1w+?
3. If I don't want to use the PPIMV, can I just replace it with 2-220k resistors?
Thanks,
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I alternate between using a GZ34 and 5V4 rectifier. Note the 5V4 has limits to the 1st capacitor in the B+ rail. GZ34 is 1.3 x VAC and 5V4 is 1.2 x VAC.
Not for sure what your question is about ground reference? IF I am understanding the question correctly, on my amp, it's connected to the inside the PT. Look at a Deluxe Reverb schematic for wiring info on the rectifier tube.
I typically use 2w or 3w for 6V6 screen resistors. I'm thinking 1w would be sufficient though for 6V6? What do they use on Deluxe Reverbs?
Yes, just replace the PPIMV with the 220k grid resistors.
With respect, Tubenit
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the 5V4 has limits to the 1st capacitor in the B+ rail.
I don't understand this. Can you dumb it down for me?
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the 5V4 has limits to the 1st capacitor in the B+ rail.
I don't understand this. Can you dumb it down for me?
Different rectifier tubes have different ratings for the size of the filter caps you can use.
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the 5V4 has limits to the 1st capacitor in the B+ rail.
I don't understand this. Can you dumb it down for me?
read this...
http://tdsl.duncanamps.com/link.php?target=25A9B8
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OK, I have a 5u4 rectifier tube but I don't think I should use it because it's filament draws 3A. I'm ordering a cheap GZ34. I assume I can anchor my 2.2k grid resistors to pin 6 and that's also where I'll land the B+. I also just noticed that the 360Ω cathode bias resistor is the cathode bias resistor (!). Gotta make that a 10w. And speaking of the CBR, it goes to ground and in the interest of efficiency, is there any reason to NOT tie this to the ground buss you show on your layout? It ends at the NFB, but why not continue the buss all the way to the end of the board where the CBR is?
Thanks again Tubenit!
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And speaking of the CBR, it goes to ground and in the interest of efficiency, is there any reason to NOT tie this to the ground buss you show on your layout?
Out of Hoffman's excellent Library of Information. I follow this grounding scheme on all my amps. I never ground the power tube cathode resistor/cathode cap on the buss ground.
With respect, Tubenit
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90% wired. One thing that concerns me is the placement of the OT. The secondaries will have to be routed between V2 & V3. Will this cause any noise? I don't see any way around this unless I move it to where I've shown in green.
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I would do red not green. It's possible to do an OT near the preamp but there is possible risk of noise, so I'd not do green.
I have had the OT in similar spots where you indicate red and it worked fine for me.
With respect, Tubenit
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Thanks Tubenit for your continued support. I guess you kind of have a vested interest since I'm building Ryan's amp!
As you may remember, I'm using a Peavey output transformer which came out of a 50w 6l6 amp. After reading here and elsewhere, I've come to the semi informed conclusion that it will work based on the consistent opinions that for guitar amps, exact impedance matching isn't required. The impedance difference between the 6v6 (~5k) and the 6L6 (~5.6k) is almost negligible. One thing I don't understand is that if they are that close, why would one have to "halve the speaker impedance" if I use this xfmr with 6v6's?
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Just plugged it in with a GZ34 rectifier. (and my light bulb limiter) I'm getting 520VAC into the rectifier and 350VDC at PSU node A. Seems a little higher than 260 x 1.3 but probably not crazy. I don't have any other tubes plugged in. But I'm getting negligible voltage drop across the PSU resistors. Node B reads 349VDC, nodes C thru E read 347VDC. Is this expected?
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Is this expected?
yes
you can expect a decent VDC drop loaded
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The donor chassis is a Yamaha G100. I reused the mains power & power indicator light pretty much intact except I deleted the capacitors shown in the pics. The power switch is a 2 way (a 3PDT?) switch. As it is now, it only swaps the hot & neutral. I don't know what the caps did but before I go too far I guess I should try to understand what these 2 caps do. The cap attached to the switch looks like a death cap, but either position of the switch would still have it in the circuit, it would just move it from hot to neutral. It's got a 3 wire grounded power cord. What's going on here?
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here's a G100 schematic;
keep the parts, or not
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Thanks Shooter, but that doesn't show what I've got. I found this and the blue "Cap" I saw apparentyl is a 4A FUSE. What's the purpose of the .0047 cap to ground?
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Thanks Shooter, but that doesn't show what I've got. I found this and the blue "Cap" I saw apparentyl is a 4A FUSE. What's the purpose of the .0047 cap to ground?
Thats the death cap. Get rid of that thing.
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Thanks Shooter, but that doesn't show what I've got. I found this and the blue "Cap" I saw apparentyl is a 4A FUSE. What's the purpose of the .0047 cap to ground?
Thats the death cap. Get rid of that thing.
That cap is a line filter and it's properly rated to be put from line to ground. Plus the chassis is properly connected to earth. There is no danger. This is common on current production amps. The term death cap came about because amp manufacturer's were using common coupling caps back in the '50 and '60s. That was before Class X and Y caps became common. Class X and Y caps are designed to go across the line or from line to ground to be used as a line filter. And when they fail they will always fail open. It's all good now.
But if you see that cap in a '50s or 60s amp that only has a two prong power cord, get rid of it! And install a three prong power cord with the green connected to chassis.
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The on-off-on switch and 3-pin plug is a transitional scheme. If you have UN-grounded outlets (generally using a "cheater"), you flip switch for least buzz (get the cap to the more groundy side of the line). If you have proper 3-pin outlets, it just works.
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The amp is fully tubed now and using my lightbulb limiter, when I power up, I cant get anymore than 190vDC at the first filter cap. The limiter bulb is glowing orange as expected, but the voltage just hangs at 190. Could this be because of the light bulb limiter? With just the 3 preamp tubes the vdc at node 1 is 280. When I plug in the 6v6s I get 190.
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Could this be because of the light bulb limiter?
yes
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I've got sound! But I need to troubleshoot. Theres a hum. And my voltages seem high. Node A is 382, B is 369, C is 318, D is 308 and E is 300.
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I've got an amp with 6V6's in it and the voltages are: A/ 382, B/357, C/324, D/313 and E/300.
IF you pull tube V1, does it still hum
IF you pull V2, does it hum ………… and so on
Have you carefully chopsticked the amp and/or safely moved the heater wires to see if the hum lessens.
Have you tried different tubes?
Did you use Hoffman's grounding scheme? Are the pots connected to the buss wire?
With respect, Tubenit
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OK, I shielded the fx send & return lines and seperated the PSU grounds per Doug's diagram. I also had 2 ground points in the pre section and I got rid of one. Much quieter now. There's still a hum which I'll track down later. Now, the more important issue is the sound. The mid boost switch is a HUGE boost and it seems to affect how the Mid pot works too. With the switch out, the tone controls don't seem to do very much at all. And with the mid boost on and if I push the volume past 3 or so, the distortion sound is "ratty" sounding. Kind of like a real grainy fuzz box-farty sound. I double/triple checked my wiring and I'm pretty sure I interpreted the switch wiring as well as the fx loop wiring correctly. The fx send and return controls work as they should although I haven't used the loop yet. But both controls control the output volume.
Have I given any clues as to where I should look?
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you're using the schematic in reply #17 ?
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I'm using the one Tibenit posted in reply #9.
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put a 100k across the Mid pot; gator clip each lead to outside pins on pot, making the pot ~50K
did it help or hinder?
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Now, the more important issue is the sound. The mid boost switch is a HUGE boost and it seems to affect how the Mid pot works too. With the switch out, the tone controls don't seem to do very much at all. And with the mid boost on and if I push the volume past 3 or so, the distortion sound is "ratty"
Hmmm?? The PAB (preamp boost) somewhat disables all the tone controls and is a HUGE boost. The mid-boost is more subtle and doesn't disable the tone controls.
Look at the attached illustration. Is your PAB wired up the same way?
Without the PAB engaged and without the midboost engaged ................ how is the clean tone for you? Is it to your liking?
What kind of tube do you have in V1? Is it a 12AY7? or 5751? I find that I don't like 12AX7's in that V1 position.
With respect, Tubenit
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Tubenit, Ive got it wired like your diagram. Some of the cap values are slightly different comparing to the dwg I built to. One difference I noted is on the mid pot. The cap is tied to one side of the pot as well as the wiper. Your latest diagram shows the cap tied to the wiper only. I believe current takes the path of least resistance so it shouldn't matter, right?
And I'm using a 5751 in V1.
Edit- So upon further reexamination, theres something screwy with the wiring of the 330k fx resistor. The layout and the schematic dont show the same thing here. Am I reading this right? The layout doesnt show the connection to the .22 cap.
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Schematic is correct. The 330k doesn't connect with the wiper on the FX send pot. It connects with the non grounded side which is where the .22 cap is.
You still haven't stated how the clean tone is? Do you like how the clean sounds?
IF that's not right then the PAB and mid-boost aren't going to sound smooth. I would not be happy with a HUGE and buzzy boost tone.
You can wire the mid pot to just wiper OR connect wiper and side. Either is fine.
IF you have a 12AY7 handy, give it a try in V1.
With respect, Tubenit
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A little more listening and with no switches engaged, when I pick gently with a low output strat, the sound is ok at low volume, but when the volume gets past 2 or so, or when I pick hard, the result is bad distortion.
I dont have any lower output tubes.
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when the volume gets past 2 or so, or when I pick hard, the result is bad distortion.
Something is definitely not right with the amp. Ryan's amp has a super clean warm tone. Sort of a AB763 clean but (IMO) a warmer fuller tone. No distortion at all.
I can't find a soundclip specifically of Ryan's amp? However, his amp was essentially modeled after the Tweed BluezMeister that I had with ONLY the clean channel V1a & V1b going into the active FX and then into the LTPI (and NO V2 OD channel).
I did find a sound clip of my TBM with on board active FX loop where the first 30-40 seconds of the soundclip is just the clean channel. Your amp should sound similar in a clean tone to the first 30-40 seconds to this clip (after the 40 sec, I engaged the OD):
https://soundclick.com/r/s74j51
I once had the OT wires backwards and instead the typical loud squeal, I got a bad distorted sound and some oscillation. Any chance that could be the case here? One way to check would be to disconnect the NFB.
With respect, Tubenit
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I once had the OT wires backwards and instead the typical loud squeal, I got a bad distorted sound and some oscillation. Any chance that could be the case here? One way to check would be to disconnect the NFB.
With respect, Tubenit
I was thinking I was lucky when I first fired it up and didn't hear the crossed OT squeal. I'll look into this this afternoon after work. Thanks again Tubenit.
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I disconnected the NFB and theres not much difference. I also plugged my strat into the fx return to bypass a bunch of stuff and surprisingly (to me anyway) when I turned the fx return all the way up, it distorts. I would think there wouldn't be enough signal to drive the pi & output tubes.
Tubenit- I listened to your recording and I would love to get this amp there.
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I also plugged my strat into the fx return to bypass a bunch of stuff and surprisingly (to me anyway) when I turned the fx return all the way up, it distorts.
Two thoughts ………………….. I've never owned or played any amp at all where I dimed all the controls. I don't drive my car with my foot all the way down on the gas. My opinion is that amps are NOT made to have everything dimed especially when you have a goal of a clean tone. Probably any of my current 3 amps will distort into an unpleasant tone IF I dimed everything.
I have certainly plugged into a FX return and enjoyed a very nice clean tone doing that. I doubt I had the FX return or FX level above 5. I've played my Larrivee acoustic thru the FX return and loved the clean tone it had. I've done that with several of my amps and enjoyed a nice rich clean tone with my acoustic. Same good results with each amp I tried.
So, …………… IF you did not have the FX return or FX level cranked way up and it still distorted. Then I'd say something significant is wrong in the wiring, tubes or OT or speaker.
IF you have another amp that has an FX in it. Try playing your preamp thru the power amp of the other amp. In other words, take your send FX jack and plug it into the return of the other amp. IF that sound clean to you, then the problem is the FX return or power amp on.
Another thing you can do is remove the FX tube and jumper straight from V1b into the LTPI. How does that sound? IF it's sounding clean and good to you, then the problem is the FX wiring or tube.
You're trying to isolate where the problem is coming from.
I've built several Tweed BluezMeisters and then Ryan's amp. All of them had a very similar nice clean tone and sounded much like the early part of that sound clip.
Something is obviously amiss and I'm confident that you can get this figured out. :thumbsup:
Also check voltages, recheck resistor values and check for continuity. Carefully safely chopstick the solder joints and see if anything pops or crackles.
You might try a 12AY7 or 12AV7 in the FX. You might try a 12AT7 or 5751 in the LTPI.
With respect, Tubenit
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I've never owned or played any amp at all where I dimed all the controls.
+1
When I started building, the guitar guys always dimed them!! the good sounds, got ugly at 10! So I started "designing" them to be dimed, voltage dividers, lower gain stages... now you can dime away :icon_biggrin:
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I understand about "diming" amps, but I thought that plugging a very low output signal like a guitar into the fx return without any preamplification wouldn't be able to overdrive a power stage.
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Think of your amp as a preamp, FX & LTPI and power tubes. Isolate where the problem is with the suggestions in my previous post and give us an update, please.
And let us know if you've tried different known to be good tubes also.
With respect, Tubenit
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without any preamplification
The FX ret. feeds a gain stage, the PI has gain, not gonna do the math, but I suspect 2 gain stages with a hot pickup might bend 6V6's
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Tubenit, I tried your idea of isolating the pre and power sections using another amp. The preamp sounds fine and everything works as it should. When I tested the PA with a known good pre section, if I turn my fx return up past 2 or 3, there's the rattiness.
Remember, the OT I'm using is from a Peavey 6L6 amp. Their schematic shows it has 2 and 4 ohm secondaries. I think I'm correct in my 8 ohm speaker should be OK using the 4 ohm secondary with 6v6's. I do have what I think is an original Blues Jr. OT-part # B-049969. Maybe this would be a better match?
And just for reference, here are my voltages;
PSU nodes;
A-387
B-373
C-323
D-314
E-304
v1-1-176
v1-6-272
v2-1-205
v2-6-314
v3-1-234
v3-6-236
V4-3-371
V4-4-369
V4-8-27
V5-3-377
V5-4-369
V5-8-27
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Bypass the FX loop and let us know how it sounds, please? IF it sounds OK, the problem is the wiring in the FX loop.
I note quite a bit of a difference with V1-1 and V1-6 plate voltages. I'd recheck the values for the plates and cathodes on that.
And the FX V2-6 connects directly to B+ rail? Is that correct?
What value do you have for the FX return pot? Sometimes I don't use an FX return pot and instead have used either a 82k to ground or a 39k to ground. I guess I'm saying it wouldn't surprise me if the FX return pot past 3 would possibly sound ratty.
I am understanding your statement to mean you have an FX send, FX return and an FX level pot. Is this correct?
I think you need to try bypassing the entire FX system and see if the preamp into the LTPI sounds good.
With respect, Tubenit
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OK, I bypassed the fx ckt. No real difference. I also checked my values around V1, everything is as it should be. And I made a recording so you can hear the "ratty" sound. Enjoy.
https://soundcloud.com/mike-holtzinger-300757342/mikes-ryans-amp-test (https://soundcloud.com/mike-holtzinger-300757342/mikes-ryans-amp-test)
Recording was made with the volume on 2-3.
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You're saying that V2 (both triodes) are out of the circuit in the sound clip. And yes, that doesn't sound right at all with just V1 going straight into V3 (the LTPI). It should be a very clean tone.
I am presuming in the sound clip that the mid boost is not engaged, the PAB is not engaged and your volume is not higher then "5" (12 noon)
OK, based on what you're saying …………………….
1) The preamp tube V1 and V2a into another amp sounded just fine.
2) And you're saying with V2a & V2b out of the circuit completely. Both triodes of the FX, the problem still exists.
(per sound clip demo)
3) So that leaves V3 (LTPI) and the power tubes and the OT as the potential problem.
so I would ……………………..
- recheck all the component values from the LTPI on
- try a different 12A_7 that is known to be good in the LTPI
- try different power tubes that are known to be good
- try a different speaker that is known to be good
- try a guitar with single coils if you have one
- recheck the wiring for the 15th time (I say this because there are rare occasions that I miss something with multiple
rechecking of my wiring)
IF none of those reveal the problem, please post a photo of the inside of your amp chassis as currently wired. I've never had a bad OT before but perhaps that's an issue if nothing else proves to be the culprit?
With respect, Tubenit
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A bad OT could produce that sound. Best way to check an OT is substitution.
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Ok, I think I found the problem. I ASSUMED the Peavey OT's primaries were red and blue with brown being the center tap. Through process of elimination, I've narrowed down the problem to the OT. So with an ihm meter I measured the primary and I'm getting 179 ohms between red and blue, 119 between red and brown and 303 between blue and brown. I think red is the CT.
I guess the amp could have worked the way it did. What say you about my findings? I would think the ct would be closer to center than 119 & 179. Is this ok?
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Is this ok?
:dontknow:
although I believe blue and brown are "typical" plate wires and red is CT, so wire it, try it, buy a proper OT if that don't get you happy
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I tried it and I like it. Loud and clean. Thanks to all for your help and encouragement.
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Hooray! You did it! You got it solved! And now you'll have a fine sounding amp. Bravo! Perseverance paid off!
:headbang: :thumbsup:
With respect, Jeff
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I've noticed that I'm getting some distortion when using a delay in the loop. It only occurs when I push the front end. I assume I'm overdriving the input of the Boss DD3, even when the send level is barely cracked. Would putting a resistor on the wiper of the send pot fix this? Or between the .002 and the send pot?
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I've noticed that I'm getting some distortion when using a delay in the loop. It only occurs when I push the front end. I assume I'm overdriving the input of the Boss DD3, even when the send level is barely cracked. Would putting a resistor on the wiper of the send pot fix this? Or between the .002 and the send pot?
Try reducing the value of the send pot. (if it is the 250K on the schematic I'm looking at)
You can do this easily by strapping a resistor across the outside lugs. 27K will make the pot approx. 25K and that could be enough to take care of your problem.
What tube type did you use for the FX?....it look's like tubenit used a 12AU7
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Change the FX send to 25ka (if possible) and use a 12AY7 or 12AV7 or 12AU7
With respect, Tubenit
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I changed the pot to a 25k and I'm using a 12au7.It's a little better, but I've still gotta have the send pot below 2 to keep things clean. I know my pedals and cable are good. They work fine with my other amps. The fx send and return ARE the signal path to the PI and on. So the send level is used for matching to the input of the fx device and the return can be thought of as a "make up gain" level, right?
And on another note, A friend who is a knowledgeable amp tech suggested I use 32u caps for the first 2 filter stages (this is when I had the distortion problem) and I did. I put in a F&T dual 32 can. There seem to be no problems, but I was reading on Rob Robinette's site that increasing those cap values in an AB763 will tighten up the low end, but I shouldn't exceed 60uf which is the limit for the deluxe reverb's rectifier. 32+32 exceeds 60 and we're using the same GZ34 rectifier. Am I at any risk by doing this?
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I changed the pot to a 25k and I'm using a 12au7.It's a little better, but I've still gotta have the send pot below 2 to keep things clean. I know my pedals and cable are good. They work fine with my other amps. The fx send and return ARE the signal path to the PI and on. So the send level is used for matching to the input of the fx device and the return can be thought of as a "make up gain" level, right?
Are we still working off of the schematic that tubenit posted in Reply #9 (Ryan's amp)?If so, I would view the FX level control as just a master volume.
To solve your problem you might have to insert a voltage divider after the .02 coupling cap that feeds the effects send triode. You can tentatively tack in a pot to come up with values.
I would also have to try snipping out the 330K parallel path around the FX loop, to see if that helps.
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Where you have an 82k FX "return resistor" instead of a return pot, change that to 39k. You can also change the 5uf cathode cap to 2.2uf on the return FX triode.
You can experiment by paralleling another 82k with the 82k FX return resistor using insulated alligator clipped wires. Then adjust the FX send pot. Is it more to your liking now?
As an FYI, I never have my FX send pot more than "5" and typically around 3.5. I've tried 4 different delays and one reverb pedal and always end up with the same type setting.
I think it is sometimes a mental thing for us amp builders when we feel like some pot (vol, mstr vol, FX send etc...…) should be able to be set at " ______".
IF you can have the amp as loud as you want it and it sounds good and is clean as you want, then does it make any difference if the FX is set at "2"?
With respect, Tubenit
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I think it is sometimes a mental thing for us amp builders when we feel like some pot (vol, mstr vol, FX send etc...…) should be able to be set at " ______".
+1!!
I've had to "re-do" a few builds because the players just can't get past the 0-10 dilemma :dontknow:
might consider bypassing the tube feeding the FX send as a test, expect the pot might get to 7-10 then :dontknow:
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I understand the mental thing.
What I realized I don't understand is how the whole loop circuit works. My understanding is; the 330k is the signal path when nothing is plugged into the send jack. But how V2 is wired is unclear to me. I'm used to seeing the stage output signal coming from the plate but here, it comes off the cathode. Why is that? Anybody want to teach me some tube theory?
So once I find a good level setting on the send pot, I think I'm going to replace the pot with a fixed resistor. And the return level pot will then just be a master volume in my mind. An effects loop is something that I (and probably most guitar players) just want to be plug & play.
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but here, it comes off the cathode.
called a cathode follower, used for impedance "matching" typically < 1 for gain, no-inverting signal in/out
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What I realized I don't understand is how the whole loop circuit works. My understanding is; the 330k is the signal path when nothing is plugged into the send jack
No, that resistor provides a parallel path around your effects pedal chain so that some of the signal stays un-effected into the recovery stage.
If you have the switching jacks wired like in the schematic, the switched terminals provide a signal path when nothing is plugged into either one of them.
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So I came across a proper OT for this amp Classic Tone 40-18002. It has the same specs as the Hammond 1750H. Right now the amp has a xfmr out of a 2 6l6 amp with a 4 ohm secondary. I understand that running 6v6's into an 8 ohm speaker is a proper match. My question is does this "incorrect OT" give me any more headroom or output capability? I've heard that the Deluxe xfmrs compress due to saturation. I know this amp isn't exactly a Deluxe and has lower power.
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"incorrect OT"
the OT doesn't really care as long as you get the "tube math" correct and the current limits.
the OT is the tubes "load", if you get it wrong, the tube works harder, less efficient, the tranny doesn't really care though, unless you exceed its thermal or current ratings, then maybe
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The amp sounds good. But honestly I think I'm having more fun learning and tweaking than I am just playing thru it. With that said, I'm thinking about trying a SS rectifier to tighten up the sound. My reasoning is that based on my sound- I like to run the amp on edge of breakup and hit the front of the amp with boost pedals for my lead sound, I don't want the amp to mush out (sag?) too much. I understand that a SS rectifier will raise the B+ a little and I'll also need a standby switch...or will I? When I search standby switch...well, it's like watching the evening news!
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Two of my amps have a PT with 300-0-300 and 140ma.
Using a 5Y3GT, I can run 6K6's or 6V6's.
Using a 5V4, I can run 6V6's or 5881/6L6
Using a GZ34, I can run 6V6's or 5881/6L6's
Using a plug in solid state rectifier, I can run JJ 6V6's or 5881/6L6
I've tried and done all of them in both amps. Try out one of Hoffman's plug in solid state rectifiers and see if you like it better. I use GZ34 (or 5V4) almost all the time.
With respect, Tubenit
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I'll also need a standby switch...or will I?
never found a good use for SB switch (in an amp)
just consider your Ecaps, their VDC rating, before the filaments warm up, you may exceed the vdc while waiting for the tubes
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I've been playing with the circuit just to experiment and what I did was I took out the whole fx circuit. I'm going straight from the .02 coupling cap off of the plate of v2b into the input coupler on the P.I. I think I probably don't need both coupling caps? But I've noticed a drop in gain and volume. I think V2a being a cathode follower is unity gain at the most and V2b provides some gain in the circuit, correct?
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I
think V2a being a cathode follower is unity gain at the most and V2b provides some gain in the circuit, correct
Yes, that is my understanding.
Why remove the FX loop? You wanting less gain or is there some other issue?
As an FYI, you do have options of a parallel or series FX loop. I've come to prefer a parallel one.
If you tell me what you're trying to achieve, I may be of some help to you.
With respect, Tubenit