Fender Twin Reverb - 1970 AA769

[bbmade]

Hello all, working on a Fender Twin Reverb made in 1970.  It was modded in 1972 with a presence control, push/pull master volume and a couple other push/pull's in the tone stack BUT...not worried about those at the moment.

On V5A I'm getting a plate voltage of 405VDC which is basically the B+ supply.  I've checked the 220K resistor value and it checks out fine for value and it has 422VDC on one side and 411VDC on the other side.  I've checked the .02uf/.01/.01 caps in series and found .3% leaking on both .01uf caps.  I replaced them because of some cracking in the ceramic discs but still no change to that high plate voltage.  According to the schematic it should be +245VDC at that plate.  I'm a bit stumped.

V2 A and B have low plate voltages V2A has 150VDC and V2B has 145VDC, schematic calls for 210VDC for both plates.

Other work done so far:

1. Replaced original filter caps and dropping resistors (metal film 3 watt).

2. Replaced cathode bypass caps for V1, V2, V3 and V4.  All were original and leaking 3 - 7% voltage loss.

All other voltages are spot on so far and appear stable.  So, stumped on V5A and not sure I should be overly concerned about V2 plate voltages at this point.  Any insight is welcome.

I'm going to check the tubes right now but don't think that would throw it off that much.

THANK YOU in advance.

UPDATE - The main AC voltage coming in is 117 volts from my variac.  Just FYI.

[sluckey]

Jumper the vibrato footswitch jack to chassis ground.

[bbmade]

Thank you, tried that based on your reply and still have the same plate voltages.  Appreciate your help, thank you.

[sluckey]

What voltages do you measure on pins 2 and 3?

[bbmade]

Pin #2 is -47.2 VDC which seems way off?

Pin #3 is .001 VDC

Pin #6 is 360.8 (good) schematic shows 380

Pin #7 is -47VDC

Pin #8 is .04VDC

[bbmade]

On the schematic and layout they show a cathode bypass capacitor across the 2.7K resistor.  This one did not have that and the solder joints seem undisturbed.  I don't see how that would affect voltage on pin #3.  Schematic shows it should hav +2VDC and V5B +18VDC.  Checking those again....

Yes...V5 has some issues with Pin #2 and #7 measuring -47 VDC each.

V5 Pin 3 is .001VDC and Pin 8 is .042VDC

[sluckey]

Pin #2 is -47.2 VDC which seems way off?

That's the correct voltage when the vibrato is switched off, ie, no ground on the tip of the trem FS jack. That's also the reason the plate voltage is so high. The tube is cut off hard.


So, try it again... Put a ground jumper on the VIB FS jack. Voltage on pin 2 should go to zero and allow the tube to conduct, which will bring the plate voltage down. If you cant put a ground jumper on the FS jack, then plug in the footswitch. Click the VIB switch once and the voltage on pin 2 should be zero. Click the switch again and the voltage on pin two should be  approx. -47.

[bbmade]

I really appreciate your help.  It just tripped me out seeing 405 volts and the schematic says 245.

I will try jumping the jack to ground one more time and check.

Thanks again.

UPDATE - This time it worked exactly as you described.  Not sure what I did wrong the first time but yes, working as it should.

[sluckey]

It just tripped me out seeing 405 volts and the schematic says 245.

Anytime a tube is hard cut off (not conducting) the plate voltage will be the same as the supply voltage. When no current is flowing through the tube there can be no voltage drop across the plate resistor. Nothing to trip over.

[bbmade]

That makes complete sense and maybe (Maybe) if I had paid closer attention to what the job of V5 was early on I wouldn't have been as confused.  Maybe.

[sluckey]

So, does the amp work? Haven't seen any issues yet.

[bbmade]

The amp is finally working.  I put the intensity control back where it originally was and moved the pot that was there to the back hole.  It is indeed a master volume of sorts and now it's on the back of the amp.

The normal volume is a push/pull pot and it basically selects between two sides of the 470K resistor coming off the reverb control (lug 2).  This gives it quite a boost and with the master volume you can get some good overdrive going even at lower volumes.

Now I get why they moved the intensity control to the back and will have to think about whether I want to leave it there.  I think most people would want that master volume on the front, I personally would crank it up and forget about it so being on the back is no big deal.  Trying to figure out what a hypothetical next owner would want is tough.  I'd prefer to have the intensity control on the front where it belongs.

Thank you Sluckey for the advice and help.  I tend to get blinders on when I work and it makes me miss seemingly simple things.  I had hooked the master mod into the normal channel by mistake and once I caught that everything works fine.

I'm also pondering getting a matched quad of Apex matched 6L6GC's for this amp but I'll be lucky to break even and a buyer might prefer seeing some old RCA's in there.  I have a sense they're worn but only one is  a tiny bit microphonic.  Not bad enough to cause issues to my ears.  If this amp were for me I'd put new output tubes in it.

Any opinions on any of this is welcome.  Thanks again!

bb