6528 SE

[fooeywuffle]

I just finished an amp using 6528 tubes, 2 per channel push pull. It didn't work. Single tube works fine, but just paralleling the 2 triodes caused it to oscillate at a very high frequency. Finally I crossed connected the triodes in each tube and that works fine. Anybody know why this happened?

 

[SY]

Yes. The 6528 is a very high transconductance tube- that means you're always skirting with oscillation. One way to stabilize a push-pull stage is by neutralization (this is very common in RF linear amplifiers). In essence, it involves cross-connecting small capacitances between push pull phases. With your second connection, you may well have inadvertently neutralized the output stage using stray and interelectrode capacitances!

 

[fooeywuffle]

I don't know why I didn't think of it. I have built and sold hundreds of Linear Amps, but they only had the parallel resistor coil thing on the plate cap. Never had one of them do strange things. They were all cathode driven grounded grid type, so that may be the reason no problems. I have neutralized grid driven amps, but those were already made, so was just an adjustment. Seems we forget things as we age.
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If brute force doesn't work, your not using enough of it!

 

[vdeakin]

I prototyped the class A self splitter in my bench with a regulated power supply running B+ at 240 and the 317hvt current source at 250ma. It sets the bias point at close to -20v.I used a 25 ohm pot to balance the current in both tubes. I used a 100w power toroid transformer as opt that gives me 9800 ohm p to p. I checked the frequency response into a 8 ohm load at 1 watt. The lows are low, -3db at around 5 hertz with visible distortion, at 25 hz it is full level without visible distortion. The 3db rolloff at high frequency is at 110khz.

Also put a square wave at 1khz and the output is a perfect square wave.

I will be building the amp soon, think i will create a new thread on the build.

Alfredo

Hello Alfredo. I wonder what happened to this project of yours. I've been researching as to what to do with a few 6528s I've got and stumbled over your post. Surprisingly there's very little on this subject. Well, if you've finished it and got some nice results could you share the schematic or your thoughts? Or any thoughts that you have on this matter?
Regards,
Victor