Twin Reverb - help with troubleshoot

[überfuzz]

This circuit can be a bear to troubleshoot.

True, but VERY rewarding if you get it right.

[überfuzz]

I did a video of the issue. There's no guitar plugged into the amp. Reverb is off, volume is a min. Intensity is 9 and speed is min.
The noise [~9 Mb, avi]

(Sorry about the strange file format on the last file. Apparently my phone isn't able to record in standard format.)

[m-fine]

That sounds like 50 hz AC noise that is entering in the vibrato/tremolo section itself. Which twin reverb model do you have?

[überfuzz]

It's a ab763 board.

I tried to trace a leakage going into v5. I couldn't really find any, I think, 0.010 to 0.001 VAC in the bias supply. Is that going to mess up the vibro when it's ON..?

Might be off topic: The bias supply is -82 VDC. It says -52 VDC in the circuit layout, but as long as I get a good wattage through my power tubes it's OK, or?

[überfuzz]

I was able to get some time to measur the tube voltage on tube 5 and 6. Some of the values seems a bit odd, but I don't know whats causing it.

12AX7 tube 5

1. 435V [300V]
2. -49.9V [?]
3. ~0V [2.0V]
4. 388V [380V]
5. -49.9V [?]
6. ~0V [17.0V]

12AT7 tube 6

1. 230V [235V]
2. 47.5V [?]
3. 95.1V [100V]
   
4. 335V [245V]
5. 52.4V [?]
6. 95.1V [100V]

Edit: The stated values in the circuit are in brackets.

[überfuzz]

That sounds like 50 hz AC noise that is entering in the vibrato/tremolo section itself. Which twin reverb model do you have?

I have a multimeter with a function called: Hz/Duty. It's there to display frequency. Exactly what I want to do. Se if there's a AC leakage in my circuit. However, there's a note that's puzzling me.

Warning: Make sure that the circuit to be tested is "dead". Maximum input overload: 250 V (rms) < 30 seconds.

Is it possible to measure my circuit, the negative bias feedback?

[Bruce / Mission Amps]

Plug in the reverb/vibrato pedal and turn on the vibrato...check those voltages again.

[Lee_ranaldo]

Is that a twin reverb reissue? or an old silver/blackface.

what u can do is to measure the voltages that appear on the original twin reverb schematic.

i had issues with a super reverb and some users here guided me. The wiring was wrong and some path that was supposedly to be grounded it was connected to another tube.

That noise comes from the pre-amp i bet.

Check the input jacks.. coupling caps. solder joints (if u have the reissue PCB version). Remember to discharge the caps by letting the amp off for about a few hours or days (depends of the amp). You can always use a resistor and some clips from pin 3 or b+ to ground to discharge the caps. I don't recommend doing the Screw driver tecnique..

For me it sounds like a shorted audio path/faulty pre-amp tube or socket.

Check if the tubes are glowing (pre-amp tubes) and the voltages that appear on the original twin reverb schematic.

Good luck!

[überfuzz]

Plug in the reverb/vibrato pedal and turn on the vibrato...check those voltages again.

Believe it or not, I actually figured that one out my self. Eh well honestly... It was at the dinner table, I was going on about the amp and my wife listened half hearted. At some point she replied, so the voltages stays the same even if the tremolo is off?

Thats why asked about tracking pulses, 50 Hz, in the amp.

@Lee R - It's a AB763 thats been tinkered with. It's not modified in any way that I can detect, but many parts have been replaced. I've been checking shorts etc but didn't find any.

[überfuzz]

I vent through my private stash and found 3 caps; one 0.02 and two 0.01. They're tolerance labelled with a J, wish is better then the caps that where sitting there before. I figured what the BEEP and started replacing them.

Result: The hum is gone and I don't understand zip.

However the tahh-sound is there at the end of each wave. So my question now, hopefully my last regarding this amp... How do I eliminate tahh-sounds in the tremolo?

[Lee_ranaldo]

Some caps develop DC leakage when they get old.

I'm gonna check ur voltages and compare with the ones i had on an old super reverb i was fixing.

most of the fender amps that had an opto in the vibrato section
had some "ticking" or "popping" noise along with the rate of the oscilator

you can fix this with an 0,01uf cap in parallen with the 10M resistor
or with a cap with the same value tied to ground. I don't remember the exact cap value but some fenders already had this modification.

[bluesfreak]

He Mentioned that it is a AB763... if it is not a clone but on original it could be that the board got conductive. As fas as I remember they used waxed paper for these boards and over time they can loose they ability to insulate and get conductive. If this is the case he needs to rebuild the board completely...

just my 2c

[überfuzz]

The board is new. I actually feel a bit dumb, I should have gone for a better one. The new board looks like the old one, seems a bit paperish, 1-2 mm thick.

I've got a coupe of 0.01 capacitors, so I'll try the fender tremolo fix. googling it up right now.

[überfuzz]

Now the 0.01 fix is installed. The hiss-bop hiss-bop is gone, but...

When the tremolo is on now there's a minor hum. Right now it feels as if I could live with it. I need to play with the amp with a full setting before I can vote ai or nay. The thing is, it's only when the tremolo is on I get a hum. In all other aspect the amp is behaving really good. So, I don't know about the tremolo...

What do you say about more dampening off the tremolo. Maybe adjust some voltage by a resistor change. The intensity is really evident around 5 - 6, so I definitely have a margin to lower the tremolo, if it meant that the hum is lowered..? Hmm...

Comments on that..?

[bluesfreak]

The Tremolo is a bias tremolo so a slight hum can be introduced by either the mod you made, interferences into the tremolo circuit/tube or the tubes not being matched perfectly. However, I haven't heard a AB763 which is completely dead silent, they most have very slight hum noticable when tremolo is on....
Does the hum stay at the same level or does it fluctuate with the tremolo effect?