Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wittyjeff on March 08, 2021, 06:51:03 pm
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I built a version of Steve's All American Duo. (Thanks SLUKEY for the great circuit by the way!) In my build (see attached Schematic), I used an old Triad Power Transformer I had, but as it turned out... it produced lower voltages than I expected. Consequently I used a pair of 6L6WGC power tubes instead of the 6L6GC I originally had in there... thinking that maybe since the WGC had a lower plate dissipation etc. it would be more appropriate for this underpowered amp. Regardless, it actually all works fine, produces reasonable volume (although I was expecting a bit more) and has really nice tone. Here are the remaining two issues that I may or may not be able to fix and would like some input on:
1. The Tremolo, while it works great, it's a bit "thumpy/clicky" when engaged and guitar is silent. I've already switched the tube from a 12AX7 to a 12AT7 (I read somewhere that it could reduce the thumpies) and I've added a bypass cap of 0.022uF across the 10M resistor on the OptoCoupler. I tried a large variety of capacitors using a Capacitance Substitution Box but none of them seemed to do anything so I just put in a 0.022uF since that's what others have done. Unfortunately, these mods had no effect on the background thumping-clicking. Are my voltages on V3 whacked? Any other ideas on what could be done to reduce the thumping a bit?
2. The two channels sound great, but are vastly different in volume. To blend the channels and/or have equal volume I usually set the Fender Channel on 4 and the Ampeg Channel on 9 (and this is AFTER my mods mentioned below). IMO the Fender channel volume seems normal but the Ampeg Geminii channel volume seems low. In order to get them to be more equal to each other, I changed a bunch of the resistors. First, I thought maybe the voltage was too low on V1 (Ampeg channel) so I changed the supply resistors to increase plate voltages at bit. Next I tried lowering the resistors on the input side which did increase the volume, but nowhere near the Fender channel (again, still about 50% difference). Any thoughts on why this would be the case?
Thanks in advance for your help and comments.
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Varying B+ changes overload point but makes very little different on gain.
The "Fender" side has gain like 350. The "Ampeg" side like 62. So like 5:1 difference.
You have a 50:50 mixer. Why? I think you want a 5:1 mixer.
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Disconnect the INT pot from C17 and reconnect to the junction of C16 and R28. This may affect the thumping and also increase the Ampeg volume.
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Varying B+ changes overload point but makes very little different on gain.
The "Fender" side has gain like 350. The "Ampeg" side like 62. So like 5:1 difference.
You have a 50:50 mixer. Why? I think you want a 5:1 mixer.
Totally makes sense. Apparently I'm not up on my mixers... I kinda thought that they (resistor sizes) were rather static and just there to prevent the channels from affecting each other. Probably because that's mostly what I've seen and that's probably because most channels I've seen are more similar/balanced in their gain. This will definitely help. ...and it's a valuable lesson to have learned. Thanks!
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Disconnect the INT pot from C17 and reconnect to the junction of C16 and R28. This may affect the thumping and also increase the Ampeg volume.
Thanks! I will definitely try that. I was kinda wondering about this since most of the tremolo insertion points I've seen out in the wild have been "behind" that mixing resistor... but I thought that was just arbitrary since they wanted Trem only on that one channel. I can see where it might be electrically "happier" there.
I'm guessing that will cause the Trem to only be available to the Fender channel, but that's not an issue.
I'll do this and see what happens to the symptoms.
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Well, guess what? I implemented those suggestions and I dare say all is perfect! The two channels are within 1 or 2% of each other volume-wise and the thumping/clicking is down to within acceptable levels. I now really LOVE this amp!
Thanks again for your help.