60's Ampeg Reverberocket II no clean headroom

[Firestorm]

It's a dropping resistor, but not an added one. It's stock. See the first schematic skeezbo linked to. Let's look at these photos and see if we can identify the circuit.

[pine]

I see what you mean about pin 10 on the 6U10, we should have voltage there if skeezbo's first schematic is correct, the bias resistor in the amp is 250 ohms and the schematic is 140 ohms, but that may just be someone's tweak

[Firestorm]

Looks like schematic #1 is correct. You've got problems on the 6U10.

Pin 6 should have 1.5V so check the resistor and caps associated with it.

Pin 9 should only have 30V or so, so check the 5M6 resistor, the .005 cap and the 1M5 resistor.

Pin 10 is the big one; should have 250V; you've got nothing. Check the 68K resistor and associated wiring. Even if the tube was shot, I think you'd still have to see the voltage there.

Good luck.

[pine]

Pin 10 had a bad connection, now have 271 on there; but still no clean headroom; going to check on pin 6

[pine]

It's up and running again, sounds great. Problems were with the connections to pins 6 and 10 on that funky 6U10 tube, and it needed a recap and new OT. Still breaks up at about noon on the volume pot as I think Bob-I mentioned was the case with his, but it gets pretty damn loud before that happens. My sincere thanks to Firestorm, Skeezbo, Bob-I and everyone else who helped me figure this out, I couldn't have done it without you guys.

Pine

[Firestorm]

Cool. I'm still curious what you have on pin #9, now.

[pine]

Ack! I couldn't resist opening it back up. Still 70 volts DC on pin #9. 5M6 resistor checks ok, .005 cap checks ok, got called away before I got to the 1M5, will report back

[Firestorm]

Sorry to get you started again ...

But that pin 9 voltage seems like it could make the tube clip the peaks of the signal. If the 1M5 is okay, you might see what happens to the voltage if you lift the .005 cap.

[pine]

Okay, the 5M6 resistor checks, the 1M5 resistor checks, the .005 cap checks, and I still get 70 VDC when I lift the .005 cap. I did notice that on the hot side of the 5M6 resistor I'm getting 366 volts instead of the 330 the schematic calls for. It is 70 volts on the lower potential side of that same resistor. Would increasing the 5M6 be a valid way to lower the voltage on pin 9?

The amp has also somehow developed a "rushing " noise that is present even with no input signal, it disappears when I remove the 12ax7 next to the output tubes, I've swapped the tube out but the noise seems to be there no matter what tube is in there. Any Idea what could be causing the noise? I chop sticked around but couldn't find anything.

[pine]

I think I found the rushing noise, had to reflow the solder joints on the pins of the 12ax7 nearest the output tubes

[Firestorm]

Okay, the 5M6 resistor checks, the 1M5 resistor checks, the .005 cap checks, and I still get 70 VDC when I lift the .005 cap. I did notice that on the hot side of the 5M6 resistor I'm getting 366 volts instead of the 330 the schematic calls for. It is 70 volts on the lower potential side of that same resistor. Would increasing the 5M6 be a valid way to lower the voltage on pin 9?

Sorry -- I havent been paying attention. I assume you've got it buttoned back up by now, so I wouldn't suggest you go tearing it open just for fun. But if you do, the idea of a split load driver is to put the signal in the middle of the voltage swing available on both sides. If the signal goes too far positive relative to the plate supply, the peaks will clip; if it goes too far negative, relative to ground, the valleys will clip. You can do a split load driver with 0VDC on the grid, or you can bias the signal up like on the Ampeg. If you want to see if it makes a difference, try cutting the 1M5 to 1M2 or 1M and listen. Like I said, I might not be a problem, though.