Oscillation question- pre or power?

[Sherman]

I was listening to my new SE KT88 monoblocks driven by an ECC82 preamp. The music was acoustic but with a lot of bass (string bass). The volume was relatively high but I've listened to it higher. Suddenly the sound became distorted and seemed to oscillate or motorboat at maybe several cycles per second.

I jumped for the volume control and turned it down. Immediately everything returned to normal. I turned it back up (but not to the same level as before) and had no problem. I am not anxious to repeat this experience so I didn't make it happen again in order to put a scope on things.

This system has never done this before and I have had the volume set much higher. However I do think the amount of bass in this passage was exceptionally high.

I am assuming the power amps were the culprits but I suppose it could have been the preamp which is a Lighthouse Electric STA-1LG with DC on the heaters.

Any clues as to where to start? I assume I'll have to induce the condition again in order to find the cause. How worried to I have to be about damaging the system?

Thanks for any help!

[analog_sa]

How worried to I have to be about damaging the system?

Not very. A SE KT88 might only be capable of damaging headphones.

I would suspect the problem to be a shared PS between several stages in the power amp. You may try to further decouple the PS to the input tubes.

[Sch3mat1c]

Originally posted by analog_sa


Not very. A SE KT88 might only be capable of damaging headphones.

I would suspect the problem to be a shared PS between several stages in the power amp. You may try to further decouple the PS to the input tubes.

Er, decoupling with more than one stage will potentially cause phase shift oscillator style motorboating.

Does it have NFB? Use smaller coupling caps. If you have a scope and function generator, sweep it from 1Hz to 100kHz or so. Motorboating appears as a large rise in gain at LF.

Tim

[analog_sa]

He,he. 'further decoupling' does not necessarily mean extra stages. IME it usually means a separate transformer/rectifier etc. Is it an overkill? By all means, but good sound usually is.

[Sherman]

Originally posted by Sch3mat1c



Er, decoupling with more than one stage will potentially cause phase shift oscillator style motorboating.

Does it have NFB? Use smaller coupling caps. If you have a scope and function generator, sweep it from 1Hz to 100kHz or so. Motorboating appears as a large rise in gain at LF.

Tim

Well it seems the consensus is that the issue lies with the power amps rather than the preamp so I'll start there. I do have a scope but no function generator though I do have some function generator software for my PC that would probably work for this purpose.

I've attached the schematic.

[analog_sa]

So, no decoupling whatsoever. I would include something like 5k/47uF for the input tube. You may need to slightly tweak the cathode resistor to compensate for the reduced PS.

[Sherman]

Originally posted by analog_sa
So, no decoupling whatsoever. I would include something like 5k/47uF for the input tube. You may need to slightly tweak the cathode resistor to compensate for the reduced PS.

Thanks,
That gives me a starting point (and an easy one at that). Time to look in the parts bin.

[Sherman]

I finally got a chance to sit down with the monoblocks yesterday and do some testing. I got some odd results.

First I put on the music that gave it problems before (for the record- Damien Rice

I found that the volume setting had to be quite high to get it to repeat the oscillation and it didn't do it every time! But I could get it to oscillate about 60% of the time.

Next I used my PC as a source using some function generator software and fed it sine waves from 20Hz to 200Hz. It won't oscillate with only a sine wave input, it just gets louder until it distorts.

I suppose I should just give analog_sa's suggested fix a try but I would really like to find a way to nail it down.

If I get a chance this weekend I'll put a scope on the B+ and see what it is doing while the amp oscillates.

[analog_sa]

If I get a chance this weekend I'll put a scope on the B+ and see what it is doing while the amp oscillates

I doubt you'll see too much. Exactly the same volume dependent oscillation may also be due to bad grounding. IME you just have to go through all possible causes and eliminate each until the problem disappears.

If a more scientific approach happens to work it's due to pure chance

[Sherman]

Originally posted by analog_sa


... If a more scientific approach happens to work it's due to pure chance

Over the years I have found that I learn best in one of two ways; the first is to have to teach something to someone else and the second is to screw something up and have to fix it myself. This fits pretty well into the second category. (Though with all the help I've gotten on this forum I'm hardly doing it by myself.)

One thing I noticed just this morning is that the exact same track makes the stock stereo in my Miata go goofy as well. (It is the third track "Blower's Daughter".) Maybe I should just not listen to that CD any more!