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Home built Twin AB 763 Reverb channel Problems.

maxwedge

Member
Messages
323
I built this amp myself and overall it has been a successful endeavor but it still has one nagging problem that has me pulling my hair out. There's a lot to this amp because its a blend of the ab763 power section and a 6G8 tone stack. It has also has switchable OP trannies with a cathode lift switch to take 2 tubes out. The reverb and vibrato section are all std ab763 twin. All the trannys are Mercury Magnetics.

All of that stuff works fine but I have a problem with the reverb channel. If I play it where it starts to get loud it sounds like a blown speaker. Also the reverb is always on even if the foot switch is disconnected (I can turn it off with the knob). The foot switch used to work and the jack wiring looks fine.

The normal channel is fine and I can crank it all the way up. It seems to me that this problem has crept up on me and maybe I caused it by having it on without the reverb tank connected (while bench testing, ect). The only thing that makes sense to me is maybe I've damaged the reverb tranny??

I'm at my wits end! Tried different tubes, voltages are all good, disconnected the vibrato circuit from the driver circuit and gone over the circuit many times.

I guess the next thing I could try is to bypass both the reverb and vibrato circuits to isolate the tone stack.

If anyone has any Ideas please help. I'm leaning to try a reverb trans.:confused:

Thanks and here's a few pics of it.





 

RussB

low rent hobbyist
Messages
11,165
I would guess you have a parasitic oscillation. The lead dress is "busy" in there. Could be issues with the two OT's as well.

Can you get it onto an oscilloscope?
 

maxwedge

Member
Messages
323
I don't have an oscilloscope. There is a bassman OP trans over there on the rt side ( looking from the rear) and I've thought of that also but I have zero problems with the normal channel. It doesn't matter which output setup I use, the reverb channel still distorts. I've tried to separate the wiring as possible.
 

RussB

low rent hobbyist
Messages
11,165
I like your idea of disconnecting the reverb circuit to track down the "Bug"
 

WaltC

Member
Messages
2,129
Go back and go over the reverb circuit wiring again. If the footswitch doesn't switch off the reverb then you got a major wiring mistake someplace because the Reverb F/S just shorts the reverb signal return from the tank (100%) to ground. If you still have reverb then (maybe) the F/S is mis-wired, but (more likely) you've got a problem w/ the reverb return part of the circuit. Where did you get the reverb tranny? Make sure you've got the primary and secondaries on that little bugger wired up correctly too, that can be a problem if they're not.

Won't hurt the reverb circuit to run the amp w/ the reverb tank disconnected, happens all the time with no harm done.
 

maxwedge

Member
Messages
323
Go back and go over the reverb circuit wiring again. If the footswitch doesn't switch off the reverb then you got a major wiring mistake someplace because the Reverb F/S just shorts the reverb signal return from the tank (100%) to ground. If you still have reverb then (maybe) the F/S is mis-wired, but (more likely) you've got a problem w/ the reverb return part of the circuit. Where did you get the reverb tranny? Make sure you've got the primary and secondaries on that little bugger wired up correctly too, that can be a problem if they're not.

Won't hurt the reverb circuit to run the amp w/ the reverb tank disconnected, happens all the time with no harm done.
I agree with everything you're saying but the switching function used to work and the reverb would stay off with the foot switch disconnected. The tank came from Tone Tube Amps, its a 4AB3AC1B. I think their out of biz now. I read a few posts here that said you can kill a reverb trans without the tank connected but I don't know if that is fact or not.

I'm going to run a bypass and run the reverb side as a normal side in the morning and see how it does. I found 2 other weird things that side is doing...it gets much worse if I flip the bright switch and the reverb pot works normal until about 1/2 way and then the volume drops way down.:NUTS
 

maxwedge

Member
Messages
323
I've concluded I have 2 problems at this time and they aren't related. The reverb staying on all the time, I'm leaning to the tank. And the distortion is from the reverb channel tone stack.

I lifted the coupling cap for the tone stack and connected it to the driver circuit input and also disconnected the reverb input to the driver. The distortion is still there, so it's in the tone stack. This coupling cap is .02 and I didn't change it to a .047 like on the normal side. I don't now if that matters for this testing.

Here's what it's doing. I don't have to turn the volume way up but if I crank the treble and/or the bass to max it it really gets nasty and much worse with the bright switch on. Maybe it's a gain problem, over driving the tube? I'm going to go check the resistors in the circuit next. It's wired exactly like the normal channel except the slope resistor is a 180k instead of a 220k. The treble pots are the 350k-70k tap from Weber.

When you guys look at these pictures you have to realize that it's a Brown/Blond 6G8 tone stack married to a black ab763

 

WaltC

Member
Messages
2,129
hmmm, a blonde tone stack.... but not really....

You've got a bright switch and cap hooked up to the volume pot, not blonde, but ok if the rest is right, but...

Where'd you get the 4 pole pot for the treble pot? And, the treble pot and volume pot don't look like they're wired up like the blonde example, but it hard to tell from the pic.

Where is the left pole on the vol pot grounded?, not to the case of the pot right? The .005 OD cap looks like it shorts the left most pole to ground, but that pole should be connected to the middle pole of the bass pot. Is the pot next to the treble pot a mid control? If it's the bass (like the layout above) it's not wired correctly, if it's a mid then the tone stack isn't like the blonde example and you need to make sure that the mid control is basically wired in in place of the 10K resistor to ground on the bass pot (which is really just a static mid control in fender tone stacks). And the treble pot *has* to be wired just as in the blonde example, with the bass control middle pole tied to the 70K aux tap on the pot, the left (or next to the left on weber-style 4 pole blonde-style pots) with the .005 to ground, the middle to the vol pot (as it looks like you have) and the right through a .0002 cap summed w/ the bass pot's right pole and 220K resistor to the next gain stage.

As for the reverb F/S and tank stuff. It is not possible, I don't believe, for a problem w/ the tank or reverb tranny to cause the problems you're describing. The tranny is dealing w/ high dc on the primary side and will either be open and no signal or shorted and you've got blown fuses and/or smoke and flames <G>, and impedance and voltage adjusted ac on the secondary side to drive the tank. It it fails all you get is no signal (open) or no signal (shorted to ground) except a loud hum <G> to the tank. If the tank transducers or springs fail all you get it no signal to the reverb recovery/footswitch jack.

Keep in mind that the 6G8 twin didn't have reverb and used the older style non-trem roach vibrato similar to the brown concert and the summing of the vib signal w/ the reverb signal can be problematic if you're not really careful.

Do you have a schematic of what you've built? Adding reverb to a blonde twin will take at least one more 12AT7 and you can use the unused half of one of the blonde's 12AX7s for reverb recovery I suppose, but what did you do?

Hope some of this helps... Good luck.
 

maxwedge

Member
Messages
323
The treble pot came from weber and the left leg of the vol is grounded to the brass plate. The pot has continuity to the plate. Yeah, not really blond 'cause I got that bright switch.

The schematic is in my head lol, I'd have to draw one up. It looks strange because the 6G8 is adapted to ab763 board at the rt side. If you look close you can see, but it's hard looking at pictures. I could do a close up of that area.

The original pot and weber's pot are different. Weber's takes the 70k tap at the right side then the next 3 to the left are normal. Look at my 1st group of pictures and you can see a small circuit board off to the right on the treble pot.

That's a mid pot in the center

I just swapped out the volume pot because I found it reading different from the normal channel. I doubt thats it though. I'm going to recheck all those caps next. The other channel is just like this one and it's fine:p

PS: Thanks for your help:cool:
 

maxwedge

Member
Messages
323
:RoCkIn Pin #2 for the grid of the 2nd preamp tube was loose! The socket was loose!!!
I got fed up and just turned the bass and treble all the way up, set the vol about 5/8 and the bright sw on. Started tapping wires with a chop stick and it just started farting like a stuck pig when I tapped the wire coming off of the input jacks. What a pain.

Now to wire the reverb back in, LOL.
 

maxwedge

Member
Messages
323
The reverb is also working normally now except at about 1/2 it starts to squeal. It's a high pitch, not like before. It has plenty of reverb by that point and maybe I can live with that. I'm not much of a reverb user anyway but just wanted the darn thing to work. Any ideas about that one?

Thanks for the help guys.
 




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