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.
Sounds like it could be a grounding issue between systems. Have you changed anything in your wiring / connectors / grounding?
Regards, Gerrit
Regards, Gerrit
An instability has crept in somehow. Check and see whether the problem happens
with shorted inputs, with a direct generator input (no preamp), and with open inputs.
In both channels? At what amplitude and frequency? Do not hook up speakers until fixed.
with shorted inputs, with a direct generator input (no preamp), and with open inputs.
In both channels? At what amplitude and frequency? Do not hook up speakers until fixed.
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It's a mono-amp, so one channel. Occurs at all amplitudes and multiple freqs.
I'll check the other ideas you recommend.
thx
I'll check the other ideas you recommend.
thx
Perhaps a dryed out electrolytic ?
By the way, turning up the bias doesn't give more output power.
Mona
By the way, turning up the bias doesn't give more output power.
Mona
Swap the tubes over from the other amp, just to make sure it's not as simple as a tube going bad.
jeff
No; front end tubes and rectifier are making correct voltages; but, what the heck; no reason not to try.
> summy load, .... 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.
That's normal.
The screech isn't; but you do not get that on the test bench?
That's normal.
The screech isn't; but you do not get that on the test bench?
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
Your method of testing a tube amp with NO speakers and injecting signal into it gives me the creeps.
OT singing? so what? no big deal.
You probably bumped the chassis against something and the OT became loose.
Hope you haven't damaged it by now.
OT singing? so what? no big deal.
You probably bumped the chassis against something and the OT became loose.
Hope you haven't damaged it by now.
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
During this one year time frame was there ever a kt88 failure? Have you checked for continuity on or between the transformer windings?
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