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Old 25th Oct 2018, 7:41 am   #32
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
Default Re: 120W PP EL34 amplifier re-born.

Interesting, about it being in the Mullard book, I lifted the circuit from the Valve Wizard. I'll do some testing of the IP stage, though so far I've no complaint's apart from it caused a loud squeak and thump on SW off which I cured. Your concerned about the grid having no reference to ground I presume, will check. It does have less gain less than 9x, and less IP Z, about 1M5 from memory, but wow, FR to die for.

"Interesting though that you applied a bit of local feedback around the first stage rather than rely on the global feedback" This is another instance of the amp evolving rather than being designed from scratch. When I applied global NFB I was limited as to how much I could apply as the amp took off with a speaker load, so I had throttle back a bit and use a 5k NFB R instead of a 4k. That might have been better as far as FR is concerned, but I doubt it would have been better than as is.

Using NFB early in the front end really makes a big difference, another way is to apply FB from the following PS stage, but I was unable to do that here without altering the PS, as it was the IP just bolted on.

The thing with front end's is that to get very low THD, you have to use a big anode resistor, so low THD, too much gain. Local NFB solves this problem.

Yep, if you a have a reasonable selection of signal pentodes and triodes your spoilt for choice. I bought some EF37A's a while back that I've been itching to use, but havn't had a chance.

Which brings me to building valve amp's. The Eco Hippy in me hates wasting 40W ish just to warm the cathodes, but then they look nice, daft. Oooh, should I use octal or B9A type valves, again not a good sound engineering choice, but that's the beauty, it's part creative art work, part hard no nonsense engineering. good craic.

Thanks for your IP and interest, Andy.
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