Silver Twin Reverb Tremolo Volume Drop

[starfish]

Hi, long time lurker first time poster here. I inherited a 1972 Twin Reverb that was not functioning. Replacing the filter caps brought it back to life and it sounds great, however the tremolo has some issues. The tremolo isn't audible until the intensity is up to 8 or so and as I increase it to 10, the volume of the amp drops about 60-70%. There is also some distortion. The bug is flashing in time with the tremolo. Replacing v5 with another tube helped decrease the volume drop, but not much. I'm trying to learn more about amp repair and don't want to start indiscriminately replacing parts without an idea of why.   

I have searched and found some info, but not answers for this specific problem. 

Thanks, Michael

[EL34]

testing to see where this post shows up :)

[sluckey]

This is another thread that I had posted a reply and it disappeared. That makes two weird problems that I've seen in the last few days. I would have never known if I had not been posting in those threads. Wonder if the same has happened to other threads?

[PRR]

While the forum is being fixed(?), let's attack your trem.

Before parts! Take voltage readings! They often point to a part.

Also Observations. Does the trem rate change smoothly or does it crap-out at an extreme?

I'm always uncertain about names like "1972 Twin Reverb". There are so many variants. Compilations like Amp Model Differences make me dizzy.

I'll work from this plan.
https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Twin_Reverb_Schematic.jpg

Voltage are noted but for trem jack grounded, NO-trem mode. Lets get P G and K for both sections of the trem tube, also the voltage at node B.

[starfish]

I can check some voltages tomorrow. Don't know if this is the right schematic, but it is a master volume model with no pull boost. Thanks

[starfish]

So I took some voltage readings. Tremolo off via footswitch. (I thought grounding the jack turned it on?)

1   374
2   -13
3    0

6   363
7   -12
8     0

Node B on Rob's schematic   452

The trem rate changes smoothly.

Another thing I noticed is that the volume drops as I turn up the intensity, even when the tremolo is switched off.

[pdf64]

Does the 'vol drop with increasing setting of trem intensity control' only occur with the amp chassis out of its cab?
If so, the issue may normally disappear when the amp is re-assembled, and may be that the bit of heatshrink that joins the neon to the LDR having a bigger than ideal gap at one or both ends.
A bit of black insulation tape can help to cover up any gaps left by the heatshrink, and so allow correct operation to be confirmed whilst the amp is still on the bench.

[sluckey]

So I took some voltage readings. Tremolo off via footswitch. (I thought grounding the jack turned it on?)

1   374
2   -13
3    0

6   363
7   -12
8     0

Node B on Rob's schematic   452

The trem rate changes smoothly.

Another thing I noticed is that the volume drops as I turn up the intensity, even when the tremolo is switched off.

Grounding the jack does turn the trem on. Those voltage readings indicate the trem was off when you measured them. I would expect pins 1 and 6 to be much higher though. How much voltage do you have on pin 4 of the power tubes?

Have you replaced the bug yet?

[2deaf]

Maybe the volt meter has a 1M load.

[PRR]

Yes, I got ON/Off reversed. Sorry.

Zero at both cathodes strongly suggests the cathode capacitors have gone short. Power off, ohmmeter cathode to chassis should go to zero then come up to the value of the cathode resistor. If it just goes zero, put in a fresh cap.

[2deaf]

-52V on the grids tends to give 0V on the cathodes, also.

[PRR]

-52V at grids is to turn the trem oscillator OFF. (Also to kickstart it ON.) Once running (trem switch/jack shorted to ground), the oscillator tube grid starts at zero but then over-drives itself to one/two Volts negative. Meanwhile the cathode voltage will rise to say 3V positive then settle to say 2V positive.

The lamp driver grid also goes to zero, then may fall-off somewhat negative. The cathode should go positive. This stage runs far outside our usual 12AX7 experience and it is a little hard to guess what the cathode voltage will be. It also bops around madly and different meters will read the bops different.

Still ZERO on both cathodes suggests either no current or shorted cathode networks. Since it DOES trem, and couldn't with zero current, I'm saying to find both shorts on cathodes. E-caps are an obvious suspect.

> 3    0
> 8     0

[2deaf]

Still ZERO on both cathodes suggests either no current or shorted cathode networks. Since it DOES trem, and couldn't with zero current, I'm saying to find both shorts on cathodes.

The voltages he posted were with -52V on the grids.  He didn't post any voltages with trem going. 

[sluckey]

The voltages he posted were with -52V on the grids.  He didn't post any voltages with trem going.

Well, the numbers he gave were -13v on the grids, not -52v. But his plate voltages were low too. That's why I asked him to measure pin 4 of the output tubes. All those voltage readings are suspect. Maybe a faulty meter?

Hopefully he'll reply and straighten this out. Anyhow, since the bug is flashing there's no need to analyze the trem circuit. He needs to focus on the LDR side of the bug, or INT pot, or how the pot is connected into the circuit. I suggested he replace the bug several days ago and he ordered one. He should have it if he ordered from Doug.
 

[2deaf]

If you put a 1M load on his test points, you get the voltages he posted.  He's got 452V on node B.

It doesn't matter, anyways.  You already know what's wrong.

[starfish]

The problem exists with the chassis in or out of the cabinet.

463v  on pin 4 of the power tubes.

Bug just arrived, but I haven't put it in yet.

Measuring cathode to ground reads value of cathode resistor.

-55v coming from bias supply

Moving voltages with tremolo on. Cheap DMM

1    195/280
2    -1/1
3    1.8/2.6

6     328/355
7     -3.6/0
8     18.7/20

Thanks

[starfish]

Replaced the bug and problem solved. Reading same voltages with tremolo off. What should the voltages be? How do I know if the meter is faulty?

Thanks!

[sluckey]

When the tremolo is off, the tube voltages will be about 460v on plates, -50 on grids, and 0 on cathodes. Your meter is loading the real voltage readings. Your meter is not faulty, it's just cheap. What brand and model number is your meter?

[starfish]

velleman DVM850BL  I guess its time for a new meter...

[PRR]

> velleman DVM850BL

https://www.vellemanusa.com/products/view/?country=us&lang=enu&id=350297

That's a fine $20 DMM.

One thing is: DCV input is 1Meg. In tube-work we like 10Meg. The difference of loading is usually small, but this trem's impedance makes your meter input non-negligible.

Velleman DVM890L may be more generally useful around vacuum tubes. Amazon $26.

[starfish]

Gotcha. Thanks