Date: Thu, 15 Jul 2010 23:58:34 -0400
From: Roger <sub1@rogerhalstead.com>
Subject: Re: [Amps] Size of Parasitic Chokes
To: Carl <km1h@jeremy.mv.com>
Hi Carl,
Although I know you are not a fan of the RUSSIAN tubes, what about the
GU84B, the so called 4CX2500?
How stable and clean is it at 50 MHz?
I realize that size of components get a bit more more precise and that
both stray C and L can be problems, but I'm not familiar with an "output
C problem". I also realize this reveals my lack of experience in VHF and
UHF.
I've been thinking the 4CX3000A7 would do a nice job using an adaptation
of the 2-meter amp in the last couple of handbooks.
Are there problems using these tubes in a "conventional Pi, or Pi/L
circuit on 6-meters?
What about current in the coupling cap?
I'd like to stay some where within, or at least near the size of most
conventional amps.
## A buddy uses a 2 x GS35B on 6m... in GG. 4700W pep out on
50.125 mhz... with 215 w of drive from a FT-2000. If the FT-2000
is cranked up to 250w on 6m... it will blow the slug past 5 kw !
No parasitic suppressor's required, and no sockets either. No screen
supplies to mess with, neutralization, RVS screen current, cavities,
or anything else. It's as simple as it gets. On the single tube GS35B
6m amp, a vac tune cap was used, and an air variable was used for
the load cap. The load cap would heat up! The amp was scrapped,
and re-built as a 2 x holer. 2 x hole version used VAC caps for both
tune + load. NO heating at all. Simple PI-net used.. just a few turns
of CU tubing, that's it. The used slug on hand was a 10 kw 50-150 mhz
slug. A new 5 kw, 20-60mhz slug was bought directly from Coaxial Dynamics.
## On the single tube version, a 'T' input was used for the tuned input.
No amount of tweaking, and rebuilding, would get the input swr dead flat.
It wouldn't go lower than aprx 1.5:1 . On the 2 x holer, we used a PI net,
which consisted of 2 x small 5-150 pf air caps.. and one tiny coil between
em. [SSON had the caps]. Both caps are mounted below the chassis, but are
be tweaked from above the chassis. Now, the input swr is dead flat. Tank eff
is superb, and globs of power output. 3 x vac relay's used. One for input,
one for output, and the 3rd for simple RX cut off bias [ shorts out a
parelleled
pair of 3 w-100k resistors in the CT]. All 3 x vac relays are new gigavac
GH-1's.
All 3 x are sped up with an overvoltage. You can get em in either the 12 vdc
version
or the 26.5 vdc version. We used the 12 vdc version, and with 18 vdc on em,
+ drop resistor, they will not hotswitch. Bypass swr on 6m is dead flat. I
posted
loads of pix of this amp on the yahoo_amps reflector pix page.
B+ supply is external to save weight. With ur typ 100-250 W 6m xcvr, I
would
stick with GG triodes... like GS35B.... or 8877's. A GG triode, monoband amp
on 6m is abt as simple as it gets. Plan B is to get a hold of a junker HF
table top
amp, and convert it to 6M.. which most will do an easy 1000-1500w out. If you
want more power than that, a pair of metal triodes is the simplest,fastest
route.
if you need > 5 kw on 6m... you need a bigger yagi,stacked yagi's, or lower loss
coax. A simple 1/4 wave shorted stub on the output of these PI net amps will
provide plenty of harmonic suppression. That config would be easier than a
PI-L. A PI is simpler to get going.
later... Jim VE7RF
73
Roger (K8RI)
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