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?

[lowell]

Half way up on the volume might be normal for that amp. Is it grossly distorted or just starts to break up around half way? You might check output tube bias and voltage check every pin of every tube and report back. If you have a scope and generator can you post how much clean wattage you have?

[pine]

I have no scope or generator, but here are the pin voltages:



V1, (12AX7 furthest from thr 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 the 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

[melvin]

did someone put the filament center-tap on the output cathode resistor? (nice trick) I'd check the heater AC voltage.

Could be the speaker, a preamp tube, even a coupling cap...but I wouldn't think it would get a lot of clean headroom with a pentode into a phase inverter made from a single triode. upping the plate and cathode resistors there might help a tiny bit.

A lot of amps from the 60's start distorting halfway up. So you have to ask yourself this question: is the distortion musical?

If this amp is 50 years old, not much else has been done to it and you wanna be sure you got all your headroom, finish the cap job: replace all electrolytics (including cathode bypass caps), all resistors in the power filter section, all plate load resistors, re-solder the OT connections, re-solder the chassis grounds, clean and retension all tube sockets, check the current on each side of the output and check each preamp tube against a new/good one for clean, good tone. then it might be good for another 50 years. but it will probably start distorting on 6 instead of 5... but then you might like it.

[lowell]

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

V3 looks as if it has no cathode voltage on pin 3. According to the schem I found here http://www.schematicheaven.com/ampeg...erbrocket2.pdf there should be some cathode voltage, however this schem has a different PI tube and maybe your version has a different PI setup?

Aside from a bad cathode resistor heed Melvin's words.

[rudutch]

is the NFB lead attached?