60's Ampeg Reverberocket II no clean headroom

pine · Post by **pine** » Wed Jul 22, 2009 5:17 pm

It says GS12R on the faceplate and has tremolo, but there is no 7199 tube, I think the second 12AX7 is doing the function of the 7199. It doesn't follow the downloadable schematics very well. The values of the components as I found in the cathode resistor are also different.

skeezbo · Post by **skeezbo** » Wed Jul 22, 2009 7:43 pm

Pine:
One of the schematics I found shows half of a 6U10 as a split load inverter, the other shows the first triode in a 12AX7 as the inverter, with the second triode of the tube as a gain stage in front of it. Wiring it backwards like this hints at a plan that seems to have been abandoned: If yours is this second version, why not try a 12dw7 in that spot? The first triode in that bottle is half of a 12au7, a good choice for a split load inverter. Maybe this will help.
Skeezbo

pine · Post by **pine** » Wed Jul 22, 2009 8:33 pm

Is there any chance you could link me to those schematics, it would help me enormously. I'm at work now so the amp isn't in front of me, so I'm not sure which version I have. I'm interested in your idea for the 12dw7. The only ones I can find are redrawn with the 7199 tubes or don't include the accordion jack or the same size cathode resistor that I have

skeezbo · Post by **skeezbo** » Wed Jul 22, 2009 9:35 pm

Here are the two I found. They are from the hoffman web site.
http://www.el34world.com/charts/Schemat ... _GS12R.pdf
http://www.el34world.com/charts/Schemat ... GS12R_.pdf

Skeez

pine · Post by **pine** » Mon Jul 27, 2009 1:39 am

Have replaced most of the coupling caps and still have early distortion on the amp, Will do the filter caps as soon as they come in the mail, if that doesn't cure the problem, where should I look next?

pine · Post by **pine** » Fri Jul 31, 2009 1:04 am

Changing the big filter caps didn't help any, I was wondering if anyone has a suggestion to try before I tie a bandanna around my head, douse the amp with lighter fluid and torch the damn thing on stage while playing through it

Structo · Post by **Structo** » Fri Jul 31, 2009 3:57 pm

Hmmm, strange.

I wonder if somebody before you modded the amp by altering some cap and resistor values?

Might want to compare values in the amp to what's on the schematic.

Maybe somebody did something to cause the early distortion.

pine · Post by **pine** » Fri Jul 31, 2009 4:10 pm

I spoke with the owner, he said it stayed clean up to about 3/4 of the way on on the volume pot, he gave it to me to try to fix (blind leading the blind) because the OT burned up

Firestorm · Post by **Firestorm** » Fri Jul 31, 2009 4:28 pm

I think we need some more voltage measurements at this point. It sounds like this asmp most closely follows the second schematic linked by skeezbo above (correct me if I'm wrong) in which case it would be good to know the key voltages on:
the first gain stage (pins 6, 7 and 8 of the 12AX7 labeled V4 in the schematic);
the tone recovery stage (pins 10, 9 and 4 of the 6U10);
the driver (pins 6, 7 and 8 of the 12AX7 labeled V3);
and the split load PI (pins 1,2 and 3 of V3). The grids of the tubes can make a lot of noise when you measure them so be forewarned. We're looking for stray DC voltages which shouldn't be there EXCEPT on pin 2 of the PI, which should be 70-80 VDC.

If there are voltages out of whack anywhere here, it can help pinpoint a bad cap or resistor that could contribute to early distortion. The driver and PI in particular might be sensitive to anything significantly out of spec.

Firestorm · Post by **Firestorm** » Fri Jul 31, 2009 4:31 pm

Do you know why the OT burned up? A short? Blown tube? Have you checked the output tube sockets for carbon tracks from arcing? That can distort the hell out of a tube.

pine · Post by **pine** » Fri Jul 31, 2009 4:49 pm

The owner just said he switched it on and smelled burning then switched it off; I don't know if the OT just gave up the ghost or something else shorted. I lightbulb tested it and the bulb got dim, then very slightly brighter, but never as bright as the initial power on level. Are carbon tracks pretty obvious between pins on an output tube socket? I didn't see any glaring marks. I will measure all the voltages you mentioned when I get home from work and report back.

pine · Post by **pine** » Mon Aug 03, 2009 12:49 am

Okay, here's what I measured off of the pins on this amp:

V1, (12AX7 furthest from output tubes) 255; 0; 1.93; 48; 48; 199; 0; 3.5; 49

V2 (6U10) 50; 278; 9.5; 89; 151; 0; 0; 0; 70; 0; .22; 53

V3 (12AX7 closest to output tubes) 136; 0; 0; 53; 53; 208; .05; 1; 54

V4 (7591) 0; 0; 386; 385; 16.6; 0; 53; 385

V5 (7591) 0; 0; 385; 384; 16.5; 0; 53; 384

Firestorm · Post by **Firestorm** » Mon Aug 03, 2009 6:16 pm

Based on this, I am not sure we've found the right schematic, but it looks closer to the first schematic linked by skeezbo where pins 4, 9 and 10 of the 6U10 are used as a split load PI. Can you confirm that this tube is being used the way?

There is still something peculiar about the voltage readings. Across the board, the heater pins seem to have roughly 50VDC on them. This would make sense if someone biased the heaters up to reduce hum or control the heater to cathode voltage differential.

BUT the heater pins on the 7591 are 2 and 7 so that voltage should be present on both. You show zero volts on pin 2, so I'm confused.

To make sure we're dealing with a DC-biased heater supply, would you check to see where the heater center tap is connected. If someone has tweaked it, you should see an added dropping resistor of around 220K at 1 or 2 watts connecetd to the B+ rail (probably at the screen supply). Then a smaller load resistor to ground. The center tap of the heater winding would go to the junction of those resistors. There might also be a 75-100V cap there, too. If the center tap is still grounded, you may have meter issues.

V1 seems okay. On V3, you ought to be seeing a volt or so on pin 3, but you're getting 0 so make sure that hasn't shorted to ground.

Assuming the 6U10 is wired per the second schematic, pins 2 and 3 look okay, but you have a small voltage on pin 11 that probably shouldn't be there. There should be a .1 cap feeding that. It might be leaking. Pin 5 seems right, but there should be about 1-2 volts on Pin 6 so make sure the cathode resistor is okay and that the bypass cap hasn't shorted. Pin 9 should be around 30V so 70V is wrong. Pin 10 should be 250 and pin 4 should be 85-90, so that's what makes me think we still don't have the right schematic.

I don't suppose you could shoot a few pictures? I doubt this can go much farther without that.