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Old 8th Jan 2019, 8:34 am   #12
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
Default Re: EL34 power amp reworked as a stereo amplifier

It is almost certainly the EF86 grid Andy, as you've sussed out. Disconnect the grid resistor and temporaraly ground the grid - what hum do you get? Be careful when mucking about with grid stopper, it will effect bandwidth. If you reduce gain you might not need the stopper at all. The problem is that as you reduce Ra, THD goes up, so it might be provident to reduce HT as well as reducing Ra a tad. Is the EF86 decoupled from HT?

If it were me I'd disconnect the NFB and find out how much open loop gain you need, then work from there. As mentioned earlier do you really need the 86? Would you be better using a triode? It is better to design the IP exactly as it should be rather than trying to make the old IP stage fit.

Just some thought's, keep up the good work, Andy.
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