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Vintage threads from the first ten years

Re: Immediate Feedback at Startup

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8/21/1999 8:19 PM

Re: Immediate Feedback at Startup

Mook:

FWIW my PV Classic 50 runs about 392v to the plates, too. See how it sounds and try keeping the bias set below 30mA (the "sweet spot" with mine seems to be about 27mA). Maybe I have this backwards but I'd wait until the first set of EL84's were trashed before adding in the power zener. (There are many other examples of guitar amps pushing output tubes beyond their laboratory ratings and "getting away with it"...)

Good luck!

Steve Ahola

8/22/1999 2:52 AM

Legend has it......

I'm trying to get much lower voltage...

Legend has it that Ken Fischer ran his EL84s at 320V and 65mA.

But, when they say 65mA, does that count for 2 tubes (out of 4). So, each tube would receive about 32.5mA?

Does that sound right?

I don't see how a single tube could dissapate almost 21 Watts (320*.065).

Sounds like it should be more like 10.4 Watts (320*.0325).

Thanks,

Mook

8/22/1999 7:47 AM

Mook:

I tend to agree with you about that being 65mA per pair... But if you are going for the Trainwreck approach, why not go with cathode bias? (The example I listed was for fixed bias, not cathode bias.) We haven't heard from Bob S. (the Vox expert from Colorado) since this BBS went down but I seem to remember that he'd run his AC-30's around 300-320v, with the current in the mid to high 30's (for total dissipation of around 11 or 12 watts). I had been saving many of his posts to my download drive which got trashed. (Ouch!)

Steve Ahola

8/22/1999 6:42 PM
Bruce	He's been very busy mixing some TV shows the last week or two. I'm going up in the mountains fishing with Bob and his family next weekend for a couple days and I'll be picking his brain for a while! Bruce

8/22/1999 7:01 PM
Steve A.	Bruce: I sure hope that the archives come back so that we don't have to bug him with the same old questions all over again... (And I thought I was being smart saving all of his posts to disk files— "Why backup my download drive? I can always just download them again..." Argh!) Steve Ahola

8/23/1999 12:05 PM

Well, I did some measurements on mine this weekend?

V A Disp. Watts

---------------------------------

391 46mA 18 W

419 21mA 8.8 W

431 13.5mA 5.8 W

The crazy thing is this was WITH and WITHOUT a

Zener. The Zener is a 62V/50W. I wonder if the

Zener is defective. I used the OT shunt method.

I might use a DR tranny that I have laying around instead of the (overpowered) PT I have in there now.

Mook

8/23/1999 5:04 PM
SpeedRacer	I spoke with Dr. Z about this one time and he said the secret is to run them according to the Mullard Spec.. (like in that circuits for audio amps book they did).. so if you want 320V on the plates, you need about 330V B+ (since idle on an AC30 type is around 10V or so) If I were a betting man, I'd say Ken's amps are right in there too. ps: Z - you out there?

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