I have a pair of DIY, mono-block, push-pull, KT-88 amps. They’ve been running properly for over a year; but, I do tend to tweak and experiment a little more than is probably necessary. I’ve been looking for around 50 watts. I read that at 450 b+, 65 mA would be max bias; so I’ve been working my way up to about 57 mA and getting pretty close at 19.5 vrms.
Last time I put one of amps back into the system, I noticed a low hum through the speaker. A couple of days later, the hum turned into a screech. On the bench I’m able to set all dc voltages with no problem. At idle, hooked up to a preamp, an 8 ohm summy load, a signal generator, and a scope, the amp sits quietly until I start to apply preamp power. Then the signal becomes audible and increases in volume with increases in preamp power. There is no speaker in the system; so, I'm assuming the noise is coming from the OPT.
I could supply the schematic, but before getting into that, I wonder if anyone could just steer me in the right direction and let me know where you would start to troubleshoot this situation.
Thanks in advance.
Last time I put one of amps back into the system, I noticed a low hum through the speaker. A couple of days later, the hum turned into a screech. On the bench I’m able to set all dc voltages with no problem. At idle, hooked up to a preamp, an 8 ohm summy load, a signal generator, and a scope, the amp sits quietly until I start to apply preamp power. Then the signal becomes audible and increases in volume with increases in preamp power. There is no speaker in the system; so, I'm assuming the noise is coming from the OPT.
I could supply the schematic, but before getting into that, I wonder if anyone could just steer me in the right direction and let me know where you would start to troubleshoot this situation.
Thanks in advance.
It's a mono-amp, so one channel. Occurs at all amplitudes and multiple freqs.
Swap the tubes over from the other amp, just to make sure it's not as simple as a tube going bad.
jeff
Amp 2, works fine. The screech in amp 1 started with my system, 6SN7 preamp. I've got a small 3W line level pre and a Realistic, 10W, 100-C amp, both of which I use for quick tests. Both showed complete sine wave deterioration beginning at >1 vrms output. I've got working a Mac C27 sitting around here that I'll try, but I don't think it will change anything.
Your ideas of testing shorted inputs and eliminating the pre are worth a try. But, after this discussion, I'm thinking that I'll probably have to work back through all of the output wiring, checking for broken/cold joints, shorts, etc.
thx
Your ideas of testing shorted inputs and eliminating the pre are worth a try. But, after this discussion, I'm thinking that I'll probably have to work back through all of the output wiring, checking for broken/cold joints, shorts, etc.
thx
I've eliminated all the suggested alternatives: swapped front end tubes and tested with open and shorted input - amp sits quiet. Wavegen input w/o pre: same as with preamp: opt noise and wave deterioration; however the gen does supply about 600 mA.
I'll keep messing with it and when I figure it out I post. Thanks for the thoughts and help.
Warren
I'll keep messing with it and when I figure it out I post. Thanks for the thoughts and help.
Warren
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