Argh! I started teaching this semester, so this is the first chance I've had to get back to trouble shooting this amp in six months. I just can't figure it out. The sound clip tells the story:
Everything powers up, but when you flip the standby to on, there is a low frequency oscillation that isn't effected by adjusting the master and gain controls, the switches, or the tone controls. Any ideas?
I know helping people like this can take a lot of time, so thank you in advance. I'd like to Paypal a little something to whoever helps me fix it as a thank you.
These are the pics. I redid each solder last night just in case, and some of them got too hot.
Sorry about that. There isn't any sound coming through. I tried switching the primaries when I was working on it six months ago and had the oscillation both ways.
Divide and conquer - if you have a power amp in jack that disconnects the preamp start there, if not lift the leg on the PI entrance cap on the preamp side. If the noise goes away it's in the preamp, if not its the power amp/PI.
Along those same 'divide and conquer' lines, I noticed that there is a audible click when touching the voltage probe to the preamp resistors when making the voltage measurements with a multimeter. Check the PI plate resistors. If there is no click from the speaker when you test them then perhaps you have mis-wired the PI, Power Tubes, or OT.
If it isolates to the output section you could try tracing your circuit connections from the speaker jack up stream to the PI. Either draw it out or compare to your schematic; you may discover something amiss. A lot of opportunities for dyslexic wiring errors there. (ask me how I know.... )
If you already know this, disregard, but you can make a cheap and easy signal injector.
1. Get chopstick, tape, hook up wire, and gator clip.
2. Get sacrificial instrument cable.
3. Get 600VDC/.1mF (virtually any mF value will do)
4. Cut off one end of the cable, and strip it back.
5. Solder about 18" of stranded hookup wire to the ground shield of the cable, and solder the gator clip to the other end of the hookup wire.
6. Solder one lead of the cap to the center lead of the cable.
7. Tape the cable/cap assembly to the chopstick with the free lead of the cap sticking past the end of the chopstick by about an inch.
To use it, plug the cable into a source, say the headphone jack of an stereo. Next, clip the gator to chassis ground. Touch the cap lead to where you want to inject signal.
For amp troubleshooting, start at the PI and move toward the front of the amp, because the point at which the amp stops amplifying the signal is a problem spot.
Argh! I started teaching this semester, so this is the first chance I've had to get back to trouble shooting this amp in six months. I just can't figure it out. The sound clip tells the story:
Everything powers up, but when you flip the standby to on, there is a low frequency oscillation that isn't effected by adjusting the master and gain controls, the switches, or the tone controls. Any ideas?
I know helping people like this can take a lot of time, so thank you in advance. I'd like to Paypal a little something to whoever helps me fix it as a thank you.
These are the pics. I redid each solder last night just in case, and some of them got too hot.
what value are the bleeder resistors on the 2 100uf B+1 filter caps?
EDIT - never mind they must be 220K it's hard to tell that third band is yellow
I'm not crazy about the PI to output grid wire(s) routing but I don't think that's causing your problem.
2nd EDIT - It looks like you're using isolated jacks for the speaker outs but I don't see where they ever get grounded to the rest of the amp - that would definitely cause your problem unless you disable the GNF (presence circuit).
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