I ran 2x 4cx250B grid driven with passive grid in AB2.
Grids loaded by 50ohms dummy load and bias feed on
"inside" of grid coupling C via RFC (could have coupled
bias to ground side of dummy R but then I would have
needed a real RF bypass cap there to handle the drive power.
I got away with a regular 200V disc :-)
G2 fed with normal voltage and bypassed to ground. I got into trouble
with my first screen supply as those tubes draw negative G2 current
over part of operating range. That solved ,-this little amp performed
flawlessly more than three years. The driver was a FT901 (tuned output)
so the IMD was not too bad. Pwr out was in the range 650 watts. The amp
was a monoband 75 meters job. Worked a lot of good DX phone on it !!
Telling this just to show that it does not take much to get a respectable
signal, not much work or professional skill either really.
My next project was a very similar thing, just BIGGER :-) using QBL5-3500's
Those are 3KW plate diss jobs... and tuned grids this time,- but that is a
different
story...nearly burned down my barn on the farm here.... too tight coupling
to
something apparently :-) Did not work much more DX on that one, I just did
not
have to spend the same amount of time in the pileups....
Now I am 61 and have become an appliance operator...
Peter
----- Original Message -----
From: "Fuqua, Bill L" <wlfuqu00@uky.edu>
To: "Rob Atkinson" <ranchorobbo@gmail.com>; "Carl" <km1h@jeremy.mv.com>
Cc: <amps@contesting.com>
Sent: Friday, April 01, 2011 7:28 PM
Subject: Re: [Amps] 813 Amplifier Article
>I once knew a guy in Nashville, John Brown, who loved building amplifiers
>in the 1960's.
> He made a RF deck that he drove with his TR3 or TR4 which he could change
> sockets and
> try different tubes that most people would not try or had be advised not
> to try.
> One setup was four 4-65As in grounded grid. Talk about hot plates when
> he
> pushed up the plate input power. They were a bright yellow
> Another set up was 4x 4CX250bs or were they 4X150A/7034? No difference
> really.
> He tried direct grounded grid, they were toasted. Then connected the g1
> to k and low
> power gain but worked well and lastly he tried g1 to ground via a resistor
> to limit grid dissipation.
> None worked out to his satisfaction.
> 73
> Bill wa4lav
>
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