| Re: 200B 5 PA head hum/hiss issue [message #28131 is a reply to message #28121] |
Sun, 21 March 2021 18:09   |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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Thanks Steve, I did all of the reverb circuit testing with the reverb pan hooked up and the controls at max, as per the schematic notes. The testing I've been doing so far has just been with a generic digital multimeter. using the capacitance setting for caps, and using a diode test on each juncture of the transistors to see if any have failed. I just bought a cheap transistor and esr tester on amazon. I'll re-test all the components and see if anything is way out of wack. I know these are not super accurate or reliable testers but I'm hopeful it will give me a ball park. I'm about to go test all the mentioned components in the reverb circuit.
In the meantime I have done a little playing around with the head, just to hear how its performing, ignoring the noise. I can play over it with out too much issue. But I've run into some other concerns. the overall volume of the amp is much lower than I would expect for a 100w head. With the volume at noon and it plugged into my 2 8ohm 1x15 peavey cabs, its only about bedroom volume. My 15w orange head is louder than that with vol at 9 o'clock into the same two speakers. If I turn the volume up past 3 o'clock, it get very distorted. the volume gets quite on the attack, then fades back up as the note decays. It sounds like a transistor distortion pedal that's biased poorly.
I realize this is a PA head, and isn't really meant to be dimed like this, but I still expect more volume than this. Distortion at high volumes is desirable, but this isn't usable distortion. its the same on all 4 channels so I'm not convinced its in any of the preamp boards. Is this an issue of impedance matching? I'm aware of how tube amps react, but I'm not as familiar with solid state.
I've taken voltage readings at all 4 of the RCA 36892 output transistor leads and the voltages are pretty close to the values on the schematic. I don't expect them to be perfect on an amp this old. I'd like to take voltages on the 4 RCA 38736 transistors in heatsinks on the board, but I've got to find a way to do that without shorting anything out. After that I might need to remove them to try and get a reading on this new tester. I'll see if I can find the datasheets for value reference.
sorry this keeps becoming more and more of a rabbit hole every time I post, but I really appreciate the help.
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