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BRSHiFi 14th March 2012 03:49 PM

Stubborn Oscillation
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I am almost done with my KT88 push push parallel amplifier. There is a stubborn stability issue I am having with it that only occurs when a speaker load is connected. An 8 ohm dummy load presents no oscillation. One speaker I tried it oscillated at 312k, and at present it is about 125k. Sometimes the frequency and amplitude can vary but it is relatively consistent. Another symptom is a squeal that lasts several seconds as the amp is warming up, while the oscillation is climbing up to maximum amplitude.

I added an RC network after the upfront stage with a 10k POT in an effort to stop the resonance. No cigar after trying a few caps and sweeping the pot.

One person suggested placing an RC network across the OPT primary. I tried that with a 270 ohm and .01uF capacitor, as I do not have picofarad capacitance high voltage capacitor stock yet. No cigar with that.

Another person said to try placing a small cap between the phase splitter plates. I have not yet tried this as I am waiting for my high voltage caps in the mail.

I also tried a few resistors and caps across the output. That did nothing either.

Am I on the right track, or is there something else I should be looking at? The schematics are attached.

Osvaldo de Banfield 14th March 2012 04:01 PM

As a first try, remove 39pf at the feedback resistor loop. If the oscillation stops, then, the amplifier is not unity gain stable, so you must stabilize it after closing the loop. If it doesn´t stop, the chech abnormal loops like trough power lines, capacitive coupling between in and outs´etc.

Best regards. keep us posted.

BRSHiFi 14th March 2012 04:30 PM

The cap has been removed. There is still an oscillation.

Osvaldo de Banfield 14th March 2012 04:55 PM

Re-check the amplifier whit only a single push pull, without the parallel, then only a tube in each half primary. May be some interation between tubes.

BRSHiFi 14th March 2012 05:55 PM

Nope, half the tubes out still produces the oscillation.

With the scope I checked:

- The output of the SRPP upfront stage.

- The speaker output.

The oscillation is affecting all stages of the amp, not just one.

TheGimp 14th March 2012 06:30 PM

First I'd break the feedback loop until you eliminate the oscillation.

Try changing your zobel from a single pri-pri to two each pri-screen.

BRSHiFi 14th March 2012 06:40 PM

The oscillation goes away when the FB loop is removed.

Osvaldo de Banfield 14th March 2012 07:11 PM

reverse the output transformer leads. The can be wrong wired.

Wavebourn 14th March 2012 07:31 PM

Play with R12 and C31.

One more option is to add a loop across output tubes, like 240K 2W resistors from anodes of output tubes to anodes of driver tubes.

DF96 14th March 2012 07:47 PM

Temporarily remove the 39pF across the 10k feedback resistor, then try increasing the 10k until the amp becomes stable. This will give you an idea of how far off stability you are.

One option to try is splitting the 10k into two resistors, with a small cap across one of them.

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