Another Bassman Build With Issues (AA864)

[mazzas]

Assuming you are inputting with a signal generator, 100mv input at max volume, should drive pin 5 to about 100 volts ac. Measure pin 5 to ground & sweep the volume control up & down. Look at the same point with your scope. The signal should be symetric ( top & bottom should look the same) If all is well, look at pin 3, the plate. Same thing. Meter on ac, use the scope. If the plate is behaving properly & you have a weak output, then your output transformer is bad.

That all sounds very nice and makes me wish that I had access to a signal generator and a scope.

[loudthud]

How high can I go with this value? I'm not certain that I've got a suitable replacement available. I've got a .047 in an acceptable voltage... but that's pretty high.

How much trouble can I get myself in by swapping the OT leads... just to make sure I've got them right?

A .047 will work. The bass frequencies will just be very strong. It will allow the amp to work better so the real problem can be tracked down.

[mazzas]

After changing the capacitor at pin #2 of the PI (from 500pF to .047), I now get 0.5M reading on the resistor with the power off (changed from 0.08M).

Voltage readings across the resistor at pin #2: 0.58 (barely changed from 0.05)
Voltage readings across the resistor at pin #7: 0.025 (changed slightly from 0.05)

Substitution of a known good 12ax7 for the 12at7 causes the voltage at pins 1 and 6 to oscillate and my meter can't get a reading on them.

[mazzas]

I want to sincerely thank all of you for your thoughtfulness and help. I've located the problem: a short in the signal path. It was a stray strand of wire from the OT signal lead on the speaker jack that was touching the chassis. The strand came from "underneath" the connection so that I could not see it. I found it when, in desperation, I decided to wire a single output jack.

I learn quite a lot from these builds and even the false leads and blind alleys show me something that I didn't previously know about the amps I build. I am greatly reassured that we have such an engaged and talented community.

Please feel free to PM me and I'll send you a beer.

[mazzas]

I am assuming that it should not induce a fairly loud tapping noise when I tap on the PI tube. If my assumption is correct, can I further assume that the tube has gone microphonic? Or is this somewhat standard for these tubes? Or is this indicative of something wrong?

[loudthud]

Usually it's the first preamp tube that will develope a microphonic 'ping'. Many times the tube will be ok further down the signal path because the signal is bigger. If the PI is making some other kind of noise I would suspect a bad tube (one with a broken weld or melted grid) or poor contact with the socket. On a new build, a bad solder joint is a possibility. The so called 'chop stick' test helps locate these type faults. Poke around at the connections and wires with something insulated while the amp is on. Don't use a lead pencil !

[mazzas]

I've ordered a couple new tubes and a new socket. I've noticed a slight tendancy to cut out if I move the tube in the socket too much. I think it should be making better contact than that. Of course my soldering is flawless. Thanks for the advice.