To: <amps@contesting.com>
> Date: Sun, 24 May 1998 10:19:44 -0500
> From: Jon Ogden <jono@webspun.com>
> Subject: Re: [AMPS] where to find info
> To: w8ji.tom@mcione.com, amps@contesting.com
Hi Jon,
> >> From what I have heard the 4-1000A has great IMD and audio performance.
> >
> >Nonsense. The typical 8877 or 3CX800 GG PA will provide -38 dB to
> >-42 dB third order IMD performance, while the best 4-1000A
> >in grounded grid (with no screen or grid bias supplies) will only
> >provide -25 to -35 dB.
> OK. It's different from what I've heard. But I'll stand corrected.
Most tetrodes are that way. Some fellow keeps trying to sell
Ameritron on a G2DAF circuit, claiming it has "good and fully
acceptable performance". He finally delivered IMD specs, and they are
-24 dB third order. An unbiased class C transistor will do that well.
My point? Acceptable means different things to different people. IMO,
less than -35 dB is unacceptable for high power PA's. For that
fellow, -24 dB is good enough. I hope his neighbor uses one of
his amps real soon, before my neighbor buys one!
> >4-1000A's, especially when the grids are not supplied proper dc
> >operating voltages, are notoriously trashy when compared to modern
> >focus beam triodes.
>
> I've never seen a 4-1000A cathode driven design that has biased grids.
I used regulated bias and screen supplies in all but my first few GG
4-1000 amps. I initially thought it would improve output power,
but it made next to no difference. I later learned that was because
of the heavy negative feedback inherent in a cathode driven PA.
One side effect I didn't expect was a fellow about a mile from me who
constantly complained about my signal suddenly asked what I did to
fix my amp. He brought over a spectrum analyzer and we measured the
same PA with and without grid voltages. There was about twenty dB
improvement in high order IMD products, with only that single
circuit change!
> >There is absolutely no comparison between a clunky old bottle with
> >long internal leads like the 4-1000 and a modern tube DESIGNED for GG
> >service.
>
> Tom, I won't argue with you at all here. You have far more experience
> than I. It's just a romantic old firebottle, IMHO. That's why I like
> it. Plus it's what the amp had when I bought it!
Nothing wrong with nostalgia Jon, but let's keep in mind the guys
question.
He wanted to get info on a 4-1000 PA because he wanted to build what
sounds like his first PA.
IMO, using an unstable tube that requires at least 5 kV to make
1200-1500 watts output with a 100 watt exciter is not a good
beginners project.
73, Tom W8JI
w8ji.tom@MCIONE.com
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