60's Ampeg Reverberocket II (GS-12-R) Reverb Not Working - Advise Needed

[Wittgenstein]

I am in the process of fixing up an old 60's Ampeg Reverberocket II combo amp that had had a broken fuse holder and had not been played in a few years. I gave it a deep cleaning including the tolex, input jacks, pin sockets, tube pins, etc..., installed a new fuse holder, and fired it back up. The tremolo works just fine but the echo or reverb is not heard even when turned all the way up.

Here is what I have checked so far:
RCA cables tested good
Reverb tank tested good on both sides (168ohms + 184Ohms)
When I bump the tank while the amp is on I can hear the springs crashing around so I assume the recovery side of the circuit is good and the drive side is suspect
I also briefly unplugged the output side of the RCA cable while the amp was on and touched the center lug and heard a loud hum / buzz as I should.
Tested all pins on the 6U10 back to the circuit board for continuity - Unable to test this 12 pin tube on my tube tester
I opened up the footswitch and tested them back to inside the amp and they appear to be working
I also ohmed out the echo potentiometer and it appears to be good as well.



Then I started looking for drift in any of the resistors tied to the 6U10 tube. I noticed that I am not able to get a reading from the 10K wire-wound resistor that comes off pin 10 of the 6U10 (SEE attached photo at bottom). I did try alligator clipping in a new 10K 5W resistor just to test it, but it did not seem to make any difference. I would assume though that if it took out the 10K resistor it probably took out the .47 and/or the .02 capacitor that is tied to it, but I do not have a capacitor tester.

I am currently ordering a new 6U10 triple triode to swap in and see if that fixes it.


So my questions are:
What is the wattage of the 10,000 Ohm Milwaukee wire-wound resistor that comes off pin 10 of the 6U10? Is it a 10Watt or higher?
Would you suggest that I swap out that resistor and two adjacent caps and try it then?
Is there something else that caused these parts to go bad that I should be checking first before blowing new parts?
Any suggestions, tips or something else that I should be checking?

[Leo_Gnardo]

What is the wattage of the 10,000 Ohm Milwaukee wire-wound resistor that comes off pin 10 of the 6U10? Is it a 10Watt or higher?
Would you suggest that I swap out that resistor and two adjacent caps and try it then?
Is there something else that caused these parts to go bad that I should be checking first before blowing new parts?
Any suggestions, tips or something else that I should be checking?

That 10K is a 7W in many old Ampegs I've seen. A 10W replacement is what you'll likely find easiest. And IIRC 0.47uF cap blocks the DC from getting into the tank. Either one can fail - I've seen both. You could tack in whatever you have on hand, 0.1uF will do fine for an operational test. Also make sure the signal to the reverb drive circuit isn't muted, maybe by a stuck button on footswitch. Unlike the Fenders, Ampeg did their reverb mute on the drive side, so clicking reverb off didn't result in an un-natural sudden stop in the reverb "tail".

[Wittgenstein]

That 10K is a 7W in many old Ampegs I've seen. A 10W replacement is what you'll likely find easiest. And IIRC 0.47uF cap blocks the DC from getting into the tank. Either one can fail - I've seen both. You could tack in whatever you have on hand, 0.1uF will do fine for an operational test. Also make sure the signal to the reverb drive circuit isn't muted, maybe by a stuck button on footswitch. Unlike the Fenders, Ampeg did their reverb mute on the drive side, so clicking reverb off didn't result in an un-natural sudden stop in the reverb "tail".

Thanks for the reply Leo! I just edited my original post to include that I did test the footswitch and it seemed to be working. I also tested the echo pot and it ohmed out correctly.

When it comes to doing the operational test, can I just leave the old parts still hooked up and alligator clip in the new ones, or would I want to remove them first?

[Jazz P Bass]

Ampeg GS12R Schematic

Have you taken any voltage measurements.
Plate & cathode voltages of the reverb send will tell if the tube is conducting.
Signal voltages (Vac) would also go a long way to telling you what is wrong.

[Leo_Gnardo]

Thanks for the reply Leo! I just edited my original post to include that I did test the footswitch and it seemed to be working. I also tested the echo pot and it ohmed out correctly.

When it comes to doing the operational test, can I just leave the old parts still hooked up and alligator clip in the new ones, or would I want to remove them first?

If that cap was shorted, it would have roasted the transducer in the reverb. So it's either open, or working. To be on the safe side I'd replace it anyway. You could clip in the 10K ( anything close will do for an op. test say 12K to 50K, whatever you have on hand ) but why not commit to replacing it since you measure it as "open".

Ampeg saved themselves the trouble & expense of putting in a transformer drive for reverb, and we have this cap & resistor to hassle with instead.

[Wittgenstein]

Have you taken any voltage measurements.
Plate & cathode voltages of the reverb send will tell if the tube is conducting.
Signal voltages (Vac) would also go a long way to telling you what is wrong.

I have not taken any voltage measurements yet, but I was planning on it. The schematic that I attached (from the back panel) in the OP conveniently has the voltages written on it (note this amp uses a 7199 and not two 12AX7's as in your schematic). That is a great suggestion though as I can get a better idea if the tube is bad before ordering a new one. I will be sure to measure it all tonight and post the results here.

[Leo_Gnardo]

You won't see proper voltage on one of the 6U10 plates/cathodes unless that 10K is in place. Likely the tube is OK but monitor the plate voltage on the reverb drive triode with the 10K in place, in case that triode is shorted (unlikely but possible). You know for sure the 10K is defective so start with that. Note - if the reverb tank isn't connected you will develop a DC charge on the reverb side of that cap, enough to give you a shock. (I found out the hard way - OW!) And it will show up on the tip of the RCA plug so 1) avoid the shock and 2) discharge before you plug into the reverb so you don't wipe out the transducer coil with a little lightning bolt.

[Wittgenstein]

6U10 Questions



I am trying to get a better understanding of how the 6U10 works in this circuit. From what I can tell and please correct me if I am wrong, "section 1" handles the drive side of the circuit, "section 2" handles the recovery side of the circuit, and "section 3" is a gain stage to boost the signal after the tone stack?

Pin 1 & 12 are heaters - Pin 8 has no connection
Section 1: Pin 4 - Cathode, Pin 9 - Grid, Pin 10 - Plate
Section 2: Pin 6 - Cathode, Pin 7 - Grid, Pin 5 - Plate
Section 3: Pin 3 - Cathode, Pin 11 - Grid, Pin 2 - Plate

[Leo_Gnardo]

Looks like you're reading it right. Note the reverb mute switch keeps the output cap shorted to ground when in mute mode, to keep "mirror" charge from building up. No fair "shocking" the skinny wire in the reverb transducer. Similar wire to what's in a guitar pickup, @ 42 ga more or less. Could act as a fuse - then it's shot.

[Enzo]

Why are we just changing parts without troubleshooting the circuit?

What does "can;t get a reading" mean on the resistor? it is open? it won;t settle on the meter?

Your recovery seems to work, good. SO you need drive. You already verified the pan is not open, good. Did you check the two cables themselves?

Oh foo, the two schematics do not agree.

A loss of function is not likely due to value shift. That resistor ought to be 10k, but if it were 15k, it would still work.

Pin 10 is the plate of the drive stage. You get B+ on that pin? If so, the resistor is intact. Pin 4 is the cathide. Got a couple volts there? Put a signal into the amp input, does it show up at pin 10? If so, does it show up at the tip of the plug for the reverb pan?

[Wittgenstein]

Why are we just changing parts without troubleshooting the circuit?

Only the broken fuseholder was changed, but the wirewound appeared to be broken open.

What does "can;t get a reading" mean on the resistor? it is open? it won;t settle on the meter?

I believe it is open. The meter would read something for just a brief moment but then goes back to 1, as if it was broken open.

Your recovery seems to work, good. SO you need drive. You already verified the pan is not open, good. Did you check the two cables themselves?

I did check the RCA cables and they have continuity back to the inside of the amp.

Pin 10 is the plate of the drive stage. You get B+ on that pin? If so, the resistor is intact. Pin 4 is the cathide. Got a couple volts there? Put a signal into the amp input, does it show up at pin 10? If so, does it show up at the tip of the plug for the reverb pan?

I did not have a chance to measure the amp while running last night, but I should tonight. I see on the schematic it should be around 250V on pin 10 and 9V on the cathode (pin 4).

Sorry for my ignorance, but I am still learning. When you say put a signal into the amp input and look for it to show up at pin 10, would that require a o-scope? Can I just plug a guitar in and strum and measure using a multimeter?


Thanks for all the suggestions and input. I really do appreciate all the help that I have received on this board.

[Jazz P Bass]

When you say put a signal into the amp input and look for it to show up at pin 10, would that require a o-scope? Can I just plug a guitar in and strum and measure using a multimeter?

That will work fine.
With the meter set to read volts ac.

[Wittgenstein]

That will work fine.
With the meter set to read volts ac.

Sweet... thanks Enzo and Jazz P Bass. That is what I needed to know and I will give it a shot tonight.

[Enzo]

It is easier with a more steady signal. I use a line from my cheap shop stereo receiver tuned to music. That doesn;t tie up my hands strumming. You could also get a signal generator, but I bet you already have a reveiver or a CD player or even a tape deck. ANother test signal I used to use was a cheap little Yamaha personal keyboard - the kind with half-size keys. Pick a sustaining patch cuch as organ or strings, and hold a key down with something heavy. Viola, instant signal. Or I hit the DEMO button, and it plays "Don't go changin'" over and over.


Yes, and AC voltmeter will track it. A scope is ideal. And google up "signal tacer" for a zillion little articles on making one of those. A tracer is really just a probe plugged into some other amplifier. This allows you to probe any part of a circuit and hear the results through the other amp. Like a doctor's stethoscope.

[olddawg]

There are a bunch of free iPhone signal generator aps out there. FreqGen is the one I use.