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Old 26th Jun 2017, 4:20 pm   #206
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: 807 (maybe) amplifier build. Now EL34

Tried to apply NFB today, but the amp didn't like it, see - https://youtu.be/QxWrAEf4G8U . This is odd as when it was a "prototype" I applied NFB with no bother. I got the same fault as before when it was mis behaving, IE the OPT was "squeeking". I can here the squeek coming from inside the shroud. Time to have a look inside methinks.

Tried various small caps (1000p ) from anode taps - primary of OPT to 43% taps and HT to anodes. No attenuation of fault. I'll get it on the big Tek scope tomorrow which has a higher range and is happier with a higher voltage IP.

One thing I noted whilst monitoring the neg bias of the OP stage was it went from - 35v to around - 100v. No idea if this is indicative.

A.
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