Quad Reverb bias conversion?

[Mars-Hall]

Quad Reverbs, from what I understand, are really a Twin Reverb circuit.  I'm replacing caps and converting the bias balance to a bias adjust on a 72 Quad Reverb.  Looking at the bias conversion chart in the library, this amp has another white wire that goes from the rear lug of the bias pot to the middle of the board.  Should this be simply disconnected?

[sluckey]

Looking at the bias conversion chart in the library, this amp has another white wire that goes from the rear lug of the bias pot to the middle of the board.  Should this be simply disconnected?

No. Connect the white wire to the anode of the bias diode.

That white wire should connect to a 2.2M resistor on the main board and is used to turn off the tremolo oscillator.

[Willabe]

Quad Reverbs, from what I understand, are really a Twin Reverb circuit. 

Yes but a 100w with 4X12" speakers.   


              Brad     

[stratele52]

Quad Reverbs, from what I understand, are really a Twin Reverb circuit. 

Yes but a 100w with 4X12" speakers.   


              Brad     

There are 100 W Twin Reverb too . I have one at home I fix.

[stratele52]

  I'm replacing caps and converting the bias balance to a bias adjust on a 72 Quad Reverb.  Looking at the bias conversion chart in the library, this amp has another white wire that goes from the rear lug of the bias pot to the middle of the board.  Should this be simply disconnected?

Just rewind your pot like any Blackface Twin Reverb bias adjustment . Look at any shematic or Layout

If needed relocate one of the 220K feeding bias to a pair of  output tubes's grid

[stratele52]

Look here ;


http://www.el34world.com/charts/bias_conversions.htm

[sluckey]

Look here ;

http://www.el34world.com/charts/bias_conversions.htm

That's where he started. Unfortunately, that info does not answer his question.

[Mars-Hall]

I have that issue solved thank you sluckey.  I have also noticed that the electrolytic cap just left of the trem bug is a smaller value (5uf/50v), on later circuits than the AB763 (25uf/25v).  Is it ok to use the BF value here?

Also, what of the cap attached to the vibrato foot switch jack to pin 3 of the tube?  Can it simply be removed or should I replace it?

[sluckey]

Is it ok to use the BF value here?

yes

what of the cap attached to the vibrato foot switch jack to pin 3 of the tube?

The only cap on pin 3 of the tube should be a 25µF@25V cathode bypass cap. Maybe it's connected to the ground lug on the vib footswitch jack? You do need that cap.

However, if the tremolo is working just fine I'd leave both of the original caps in.

[Mars-Hall]

Is it ok to use the BF value here?

yes

what of the cap attached to the vibrato foot switch jack to pin 3 of the tube?

The only cap on pin 3 of the tube should be a 25µF@25V cathode bypass cap. Maybe it's connected to the ground lug on the vib footswitch jack? You do need that cap.

However, if the tremolo is working just fine I'd leave both of the original caps in.

Thanks once again sluckey!  I had been comparing the later circuits to the AB763. The BF circuits don't have a cap on the footswitch jack.  I'll try it out before replacing the cap.  However, I have already removed the cap next to the trem bug, thats when I noticed it being a different value. Should I put it back and try it or replace it, since it's already out?

[sluckey]

Should I put it back and try it or replace it, since it's already out?

...if the tremolo is working just fine I'd leave both of the original caps in.

Tell me about the cap on the footswitch. What value is it? You say it connects to pin 3 of the tube. Does the end that connects to the FS jack actually connect to the ground lug on the FS jack?

[Mars-Hall]

Should I put it back and try it or replace it, since it's already out?

...if the tremolo is working just fine I'd leave both of the original caps in.

Tell me about the cap on the footswitch. What value is it? You say it connects to pin 3 of the tube. Does the end that connects to the FS jack actually connect to the ground lug on the FS jack?

It's a 25uf/35v cap from the ground lug of the vibrato jack to pin 3 of V5.

[sluckey]

Thanks for the pic. It's as I suspected. That is the cathode bypass cap for the trem oscillator. That cap is located on the board for most of the blackface and silverface amps. The connection is the same in both cases, pin 3 to ground. Don't know why they mounted it like that on your amp. Looks like a factory job.

Anyhow, you need that cap.

[Mars-Hall]

Thanks for the pic. It's as I suspected. That is the cathode bypass cap for the trem oscillator. That cap is located on the board for most of the blackface and silverface amps. The connection is the same in both cases, pin 3 to ground. Don't know why they mounted it like that on your amp. Looks like a factory job.

Anyhow, you need that cap.

With enough investigation of the circuit, I think I would have eventually seen it.  Thanks for always taking my questions.  My only guess for putting there is, it's easier than crowding the board with it.

Would you suggest leaving the other coupling caps alone, as well?

[sluckey]

Would you suggest leaving the other coupling caps alone, as well?

Yes. Don't fix it if it ain't broke.

[Mars-Hall]

Yes. Don't fix it if it ain't broke.

Ok, the big caps did show signs of age, bubbles and fluid leakage.  Even at that, when I fired the amp up it was quiet.  I've always changed all electrolytics as a preventative measure.  Comes from watching those Gerald Weber videos!   What kind of issues would I expect to hear from a bad coupling cap?

[PRR]

> cathode bypass cap for the trem oscillator. .... Don't know why they mounted it like that on your amp.

The recovery amp's other input pin, the cathode, "should" go to the signal source. In this case the cap (audio path) is more critical than the cathode resistor (DC path). So doing it like that is "logical". Especially if there is significant ground-difference between ground on the board and ground at the RCA jack.

I must say I've rarely seen it done (only on transistor mix-amps).

It suggests there's some other, heavy, multiple ground issue on the chassis which was (since it does look factory) quick-fixed. Perhaps by theory, perhaps by guess-and-gosh.

Until it needs a rotisserie restoration, just replace that cap the exact same way.

[PRR]

> What kind of issues would I expect to hear from a bad coupling cap?

If sufficiently leaky, it will melt tubes.

Preamp tubes are unlikely to melt, but they can block-up to pass small or no sound.

At least check that all plates are near their nominal voltages. A leaky couping cap will make the tube suck more and the plate voltage will be way low. Leaky cathode bypass caps will do the same.