60's Ampeg Reverberocket II no clean headroom

[pine]

I haven't worked on these before, this one had a burned up transformer which I replaced, also replaced the 7591 tubes. Looks as though at least one of the filter caps has been replaced, the one that connects to the center tap of the OT. I now have sound, but no clean headrooom, volume only goes up about half way before distortion. The two preamp tubes test good, although I'm not sure about the 6U10 tube. Can you give me an idea about where to look next to try to get the proper amount of clean headroom?

[Structo]

If the amp is biased hot it will break up sooner.

[pine]

If the amp is biased hot it will break up sooner.

It's the original bias resistor, are they prone to drifting? This amp worked fine right up until the owner turned it on and saw smoke from the OT

[Structo]

I assume it was the OT you replaced?
What did you replace it with?

What are your plate and screen voltages?

What is the voltage on the bias cap and resistor?

Does it have a 140 ohm bias resistor and 25uf cap?

[pine]

I replaced it with a Mojotone Deluxe Reverb OT;

I'm getting 390V on the plates and I think the schematic calls for 360V.

The bias current I'm getting is 17ma, I think I need about 26 at that much plate voltage.

There is a 250 ohm 10W bias resistor and a 25uf cap.

Is there it better to lower the plate voltage (and how would I do this) or would it be better to try a 500 ohm 10watt bias resistor?

[Firestorm]

The bias current I'm getting is 17mv,

I assume you mean 17mA. How are you measuring it? What's the voltage at the top of the cathode resistor?

If you're only getting 17mA, you sure don't want a bigger cathode resistor.

[pine]

doh! Yes I meant 17ma, doing too many things at once, post edited, will report back shortly

[jjman]

Those look to be pretty high current tubes:

http://tdsl.duncanamps.com/show.php?des=7591A

I guess that's why the schematic calls for only 140ohms on the cathode resistor, as Structo mentioned:

http://www.schematicheaven.com/ampegamp ... ocket2.pdf

If you truly have a total of only 17ma of current (whether for both tubes or for each tube) they are running way too cold and will distort early from that.

Change the cathode cap if not already done to ensure it's not shorting at all, and double check the voltage and resistor value on the cathodes.

A Deluxe Reverb OPT expects to see 6v6 tubes which use less current. But many people say they use 6L6s in them so perhaps those OPTs have the stamina to handle the higher current. I don't know how the 7591 compares in the impedance department to what that OPT expects.

[Firestorm]

A Deluxe Reverb OT has a 6600 ohm primary so that looks pretty good for 7591s at that voltage. I'm sure the OT is hardy enough to handle the 7591s -- Fender ran the 6V6s in the DR at 415 volts so they were pushing well over 100mA when cranked; same zone as a cathode biased 7591A. The 17mA idle current is still troubling. That's got to induce serious crossover distortion when turned up. I also wonder about the health of the 7199. Always hated that tube.

[pine]

whoever worked on this one put in the 250 ohm cathode resistor for some reason. there are 16 volts on the top of the cathode resistor now, I changed the cathode cap as jjman recommended, that helped a little, now I am getting a whacked out reading of .680 on my DC ma scale for a bias current. I tried to calculate the plate dissipation and came up with 25 watts (do I divide that by the number of tubes?) Sounds ok until the volume is halfway up, then nasty distortion, still about 390 volts on the plates, do I need to get that down to 360?

[jjman]

I don't think the plate voltage is a problem. Also, it will come down if/when you re-bias the current upwards.

I'm not sure how you are measuring the idle current. Maybe you are using a bias tool like the bias-rite that taps the pin 8 current? That would be the screen on a 7591 so those tools won't work on that tube.

If the resistor is still ~250 ohms, the 16 volts means the tubes are idling at a total of 64ma. 32ma each is about 11 plate watts each when subtracting for some screen current.

The link to that tube shows a max plate of 19watts. I would expect the amp to want an idle around 14-16 watts idle each but it could have been designed for more. Many cathode biased amps run the tubes at or above the alleged gospel max. Using a 140 ohm on the cathode would probably result in an idle around that gospel figure.

I would try a lower value cathode resistor but something above 140. Today's higher wall voltages will make the ideal value higher. (It's an old amp yes?) I like to keep an array of 5watters handy but you need a 10watter in this case.

If the other filter caps are more than 15 years old they should also be changed. I worked on an Ampeg from 1959 and most of the Astron coupling caps were leaking DC. I've read many times that the old Astrons go bad like this. Do you have them?

Of course there could be other causes of the problem but getting the output bias closer to stock is a good 1st step. Maybe some 7591 connoisseurs can help. Or google up some recommendations.

[Firestorm]

I don't know how you're trying to measure your idle current, but in a cathode-biased amp, all you have to do is calculate it from the voltage drop across Rk. The formula is E/R=I, so 16 volts at the top of the cathode resistor divided by 250 ohms is .064 amps, or 64 milliamps. Since there arew two tubes, that's 32mA a tube. Your actual effective plate voltage (plate voltage minus cathode voltage) is 374. Multiply that by the current to see where you are as to plate dissipation: 374 x .032 = 11.97 watts. The 7591A is a 19 watt tube, so you're running them at 63% of max dissipation. Nothing wrong there.
I think you need to look at the PI: caps, resistors and the 7199 itself. A bad PI will screw everything up.

Edit: Damn! jjman types faster than me! But I do think the problem is unrelated to bias. Leaking coupling caps is definitely a possibility. Ampeg did sometimes use Astrons and it's almost certain they've turned to crap by now.

[Bob-I]

I'm not so sure you have a problem here. I had one of those amps when they were new, I "upgraded" from a Princeton. At the first jam I wished I had my Princeton back because I could dime that thing all day and no breakup, but the Ampeg broke up at about halfway, very dissapointing.

[pine]

There is no 7199 in this amp, it has 2 12ax7s, the 6U10, and the 2 7591s, I will replace those Astrons and see whats what with them. By the way, thank you all very much for your help with this.

[Firestorm]

Ampeg can drive you nuts -- they revised amp circuits more often than anyone else (except maybe Gibson). Do you know the model number so we can try to find the right schematic? Only two 12AX7s? No tremolo?