Browned out plate voltage

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Browned out plate voltage

Postby Mars Hall » Tue May 19, 2009 10:05 pm

I ordered a matched pair of the Chinese KT66's for my JTM45ri. When ordered I asked that the seller to send me a pair that would have a good amount of headroom. Using the GT scale, I asked for an 8 or 9.

The tubes I received seem to have less headroom than a pair of GT KT66HP rated 5. When I biased the tubes, it required 45ma and the plate voltage dropped to 378. The prior set biased at 40ma with pv at 425.

Is this what I should expect from tubes with more headroom?

Would changing choke value from a 3H TO 20H cause this change in plate voltage? I did this modification at the same time.
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Re: Browned out plate voltage

Postby Jeff West » Mon May 25, 2009 11:44 am

Mars- It's true you can expect measurable plate voltage drop with higher idling current (including across same pair of tubes at different settings) but I don't think you can account for 47V under the conditions you describe. I'd swap the old choke back in to see what you get then (or have you already?). Brian or others can likely say more, but that's a significant difference across chokes.

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Re: Browned out plate voltage

Postby BryanL » Tue May 26, 2009 8:15 am

Having had a look at Choke specs here in the UK; a 5H choke capable of carrying 100mA with a resistance of 170 Ohms weighs just over 1lb, while a 30H choke capable of carrying 100mA has a resistance of 100 Ohms and weighs 7lb: if the two chokes seem to be similar in weight, then the voltage loss will be due to the higher resistance of the wiring.

It's many years since I got involved in amplifier power supplies, but I see the discussions haven't changed much!! I always believed in large paper capacitors with low inductance(-can't seem to find specs for that nowadays-)and high H/low R chokes, in the Pi-filter configuration. The first capacitor shouldn't be so big that it's chargeing pulses strips the coating off the tube rectifiers cathodes as they heat up; semiconductor diodes usually need a slow run-up device to prevent the ampslifiers tubes suffering a similar effect. Oh, by the way, a 500mA choke at 10H is likely to weigh 18lb!
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Re: Browned out plate voltage

Postby BryanL » Tue May 26, 2009 8:18 am

Finger trouble--wiring should read winding.
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