Pro Reverb Low Volume

[Shawnobi]

Hello again, after some troubleshooting guidance with my pro reverb. The volume is very quiet with all knobs turned to max. Its a 70w Ultralinear Silverface.

Schematic here:
Fender-Pro-Reverb-Ultralinear-Schematic.pdf (thetubestore.com)

So far I have checked cables.
Checked both channels (Normal & Reverb) exhibit the same effect (They do).
Swapped out V6 Phase Inverter tube
Swapped out Power tubes.

Voltages look ok to me on V7 & V8
-55/0/511/512/-55/513/0/0
-55/0/512/512/-55/510/0/0

Looking for the best way to go from here to ascertain where that gain is going!

Thanks

[Jon Snell]

I would follow the signal path with my oscilloscope, that would be the easiest way to solve the problem but check the master volume control for a broken track first.
Don't know what your voltages mean as they are DC levels and not signal levels.

[Helmholtz]

Speakers good and well connected?

I'd start with cleaning all output jack contacts.

And please specify tube pins with voltage measurements.

[pdf64]

Look out for the output jack arrangement on these amps, it's unlike previous Fenders, with load impedance switching on the ext jack. Not as complicated as the TR or SR though, which have the jacks wired in series!
See note #5 and typical signal Vac in oval boxes on the schematic.

[Leo_Gnardo]

Hello again, after some troubleshooting guidance with my pro reverb. The volume is very quiet with all knobs turned to max.

Have a close look at the 100R feedback ground reference resistor. For reasons I cannot fathom, this little so & so often becomes unstuck in Fender amps. Also have a look at the wires that tie ground points on the circus board to chassis. Any of those loose and your volume will be as you note next to nothing.

[Helmholtz]

I think the 2 Ohm Super Reverb arrangement in the schematic wires the ext. speaker in series.

[Shawnobi]

OK,
Bit of an update.V6 had some funny voltages that didn't tie up with the schematic. I started pulling legs on the resistors around it and found that one of the resistors on the voltage divider from pins 1 & 6 was open circuit. Now those resistors seem to have been changed. Scheme says 47K but one was 100K and the broken one appears to be 82K from the colours (First band isn't clear).
I popped a fresh pair of 100K in there anyway and now I have motorboating with the volume down which turns into a squeal as i turn up the master volume.

So I'm a little confused here. Not sure whether to put the resistors back to 47k (Didn't have any to hand) and wondering if the 82/100k was holding back the motorboating?

Something else I noted weird the 330K resistors that feed pins 2 & 7 have also been changed to 1K at some point.

V6 voltages are still off. That may just be because of these 'mods' however.

V6 voltages going from 1 to 9:
180 - 66 - 94 - 0 - 0 - 183 - 64 - 94 - 0

[Helmholtz]

OK,
Bit of an update.V6 had some funny voltages that didn't tie up with the schematic. I started pulling legs on the resistors around it and found that one of the resistors on the voltage divider from pins 1 & 6 was open circuit. Now those resistors seem to have been changed. Scheme says 47K but one was 100K and the broken one appears to be 82K from the colours (First band isn't clear).
I popped a fresh pair of 100K in there anyway and now I have motorboating with the volume down which turns into a squeal as i turn up the master volume.

So I'm a little confused here. Not sure whether to put the resistors back to 47k (Didn't have any to hand) and wondering if the 82/100k was holding back the motorboating?

Something else I noted weird the 330K resistors that feed pins 2 & 7 have also been changed to 1K at some point.

V6 voltages are still off. That may just be because of these 'mods' however.

The two 47k resistors "between" the PI plates are no voltage divider. Rather these are 2 individual plate resistors connecting to a supply node, being a signal ground.
I would not change values, as that changes operating conditions, tube voltages, gain and stability.

The motorboating might indicate bad filter caps.

And please restore the 330k grid resistors. 1k there makes no sense and will considerably lower volume.

BTW, with tube amps most resistors can be measured in circuit. Just make sure the filter caps are discharged.

[Leo_Gnardo]

Something else I noted weird the 330K resistors that feed pins 2 & 7 have also been changed to 1K at some point.

As Helmholtz notes, that would kill your signal for sure. The "classic" value for these was 1 Meg, that shifted to 330K in more recent models. I wonder if the mod genius didn't know the difference between 1000 and 1000000. Good thing he doesn't run a bank. Heck, these days, maybe he does.

Also, a broken / open circuit 82K would definitely stop the signal in its tracks also. Whether you stick with the "classic" 100K/82K plate R pair, or select 47K both that this version originally had, is up to you. In any case, the voltages you see on V6 plates tell us that 82K isn't broken. If it was, you'd see about zero volts on the plate it's wired to. Whichever values, that wouldn't lead to motorboat/squeal as you describe. That kind of noise is usually attributable to either output transformer wired inverted polarity, or insufficient or missing hi voltage filter caps. The first condition is easy enough to test - swap plate leads on the output tubes. If the motorboat/squeal ceases, you win. If not, look further: filter caps may be defective or disconnected.

[Shawnobi]

Need to check again that those 1k aren't actually 1 M and I read wrong. I've had this amp years and it ran fine. The 82k was definitely bust. The V6 values I posted there were after I had replaced those resistors with a pair of 100k

[vintagekiki]

The volume is very quiet with all knobs turned to max.

If volume is low, and (or) thin bass, and everything else is ok, check the output transformer.

[Leo_Gnardo]

The V6 values I posted there were after I had replaced those resistors with a pair of 100k

Pair of 100K, close enough for rock n roll. Might make for a slightly "lumpy" look if you pass a sine wave thru the output department but that's of little importance when the big problem is a signal that's gone mostly missing. Put a meter on those "1K" R's & let's find out what they really are.

[Shawnobi]

The 1K's were actually 1M. My bad.

So I've gone on to investigate the power filter caps based on the motorboating that's started since I replaced the broken 47k (82k!) plate resistor.
There is a 70uF and and 80uF which are now both 80uF as I replaced them with new F&T's when I got the amp some years ago. The voltages on the negative sides look to spec (-56 & -60). On the positive side (which are linked) I'm getting 0v. Nothing on the scheme to say what that should be. But that feels very wrong to me! My head is going around in circles trying to chase the Pos side and work out what could be pulling it down. Although I guess if you have -60V one side then 0V is +60V relative to that?

[pdf64]

...There is a 70uF and and 80uF which are now both 80uF as I replaced them with new F&T's when I got the amp some years ago. The voltages on the negative sides look to spec (-56 & -60). On the positive side (which are linked) I'm getting 0v. Nothing on the scheme to say what that should be. But that feels very wrong to me! My head is going around in circles trying to chase the Pos side and work out what could be pulling it down. Although I guess if you have -60V one side then 0V is +60V relative to that?

Those are the bias supply caps. Their +ve terminals must be connected to 0V / circuit common./ chassis.
I acknowledge that the schematic does get a bit convoluted in showing that

[Leo_Gnardo]

The 1K's were actually 1M. My bad.

The voltages on the negative sides look to spec (-56 & -60). On the positive side (which are linked) I'm getting 0v. Nothing on the scheme to say what that should be. But that feels very wrong to me! My head is going around in circles trying to chase the Pos side and work out what could be pulling it down. Although I guess if you have -60V one side then 0V is +60V relative to that?

Those caps you describe are filters in the bias supply, which is negative with respect to ground. So... for the - end of the caps to be -56 and -60 as you note, that is normal. The + end of those caps is wired to chassis ground, in other words 0.0 V, zero volts. Everything's perfectly normal more or less. Why more or less? With a bias voltage applied to the output tube grids around -55v as you described in your initial inquiry, it's possible even probable your output tubes are barely conducting at all, and that might explain your symptom of almost no signal, and what signal there is sounding very distorted. Note on many Fender schematics, -47V is given as the bias voltage figure. And remember plus or minus 20% is advised on the schemo, so -47V is not a hard and fast rule.

How to adjust bias voltage? Take a look at your "balance" pot. This control allows you to adjust the bias voltage applied to one pair of output tubes relative to the other. More on this later. Note there is a 33K resistor from the wiper of this pot, and grounded to the metal on the back of the pot. Reducing the value of this resistor will reduce the bias voltage, which will have the effect of increasing bias current in the output tubes. I hope you have a couple of clip leads in your workshop, and maybe some resistors. Find a 100K resistor, and clip it parallel to the 33K. You should see the bias voltage drop noticeably, probably now in the lower -50's. Then have a listen to the signal coming through the amp. Is it stronger than before? Good, we're heading in the right direction.

It would be good to be able to measure bias current now that you're adjusting it. Here's a method I learned on MEF and it works quite well. All it takes is a voltmeter/ohmmeter and a calculator. With the amp OFF, and power supplies drained, measure the resistance from output transformer center tap to the end of each plate winding, iow pin 3 of the output tubes. I expect you'll see 50-ish ohms. No worries if it's more or less. Write those numbers on a notepad. Now switch your meter to DC volts measuring function, and power up your amp. You're going to probe the same points, and both probes are going to be somewhere around 450-500V so be careful, pay attention, don't get distracted. Once the amp is warmed up, measure the voltage from OT center tap to output tubes pins 3, same place you measured resistance before. You might see somewhere around 1 volt there. No worries if it's a little more or less. Power down, write down the voltages you read next to the resistance readings you got before. Now a little arithmetic. By Ohm's Law, V=IR. We're solving for I, that's current, so the appropriate form of the equation is I = V/R. Punch into your calculator the voltage you read, divide by resistance. The result will be bias current, in amps. Multiply by 1000 for milliamps. Let's say you read 1.0V, and the resistance was 50 ohms, the current will be 20 milliamps.

I like to see bias current around 30 mA for this amp, a little more or less, no harm. Anywhere from say 25 to 40 mA, close enough. You can try different resistors, say 82K, 68K, 47K in parallel with that 33K bias setting resistor. Whichever gives you a result of 30 mA more or less, that's the one you want to solder in parallel with that 33K bias setting resistor. Once you settle on a value, measure the bias voltage again, pins 5 on the output tubes, you might see it anywhere from mid -40V to low -50V range. Anywhere in there is normal.

Be safe, don't poke around with the power on, except maybe for taking those voltage measurements on the OT primary. Hope this helps.

If you've done all this, and bias is in an acceptable zone, and you still don't get much signal through the amp, then we must look further, into the preamp or possibly output transformer. But at least we know the power amp tube section is prepared to amplify properly, and we wouldn't be able to make any further progress unless that was the case. There remains some small possibility the output transformer is defective though, and that also would keep you in the low & distorted signal situation even if everything else is working properly. There's a way to test that doesn't require replacing the OT - I'll leave it to someone familiar with the neon bulb test to describe it. Since I work on tube amps all the time, I have spare OT's in the shop. But I don't expect you have one, so stay tuned for the neon bulb test lesson ifshould it become necessary.