[Amps] Low output from a pair of 4-400s [SEC=UNCLASSIFIED]

[Carl]

Can you describe the input and output circuitry in some detail?  One way to 
test your input theory is to eliminate it completely and see what happens 
thru a .01 besides aggravating a SS rig. Thats why I always have an old 50's 
tube rig close at hand which will load into anything.

Carl
KM1H


----- Original Message ----- 
From: "Wood, Mike" <mike.wood at dsto.defence.gov.au>
To: <amps at contesting.com>
Sent: Tuesday, August 03, 2010 10:22 PM
Subject: [Amps] Low output from a pair of 4-400s [SEC=UNCLASSIFIED]


> UNCLASSIFIED
>
> Gents,
> I have built a GG linear using a pair of 4-400s. The circuit is similar
> to the ARRL 'junk-box' linear of 1983.
> My problem is that I only get about 500 watts out; CW or pep, no matter
> how hard I drive it (measured on a Palstar PM-2000).
> This power output is with two tubes, or one in either socket. I have
> tried combinations of 4-400s and Philips 3.5-750s with no improvement.
> I get this with about 35 watts in, and using an LDG tuner on the input
> form the exciter (0.5 metre length of co-ax to input from tuner)
> Also, this output is only available on 160, 80, 40, 30, but I cannot get
> any output on 20 or 15! (only the input drive appears at the output, no
> tune and load dips!).
> this, no matter what tapping I try on the tank coil.
> The HT is rock solid at 3.8kV (big 3kVA transformer, full wave
> rectifier, and 25 uF smoothing), and only varies about 100 volts when
> driven to 500 watts out, idling current is about 120 mA. Heaters are
> good at 5.0 volts at the pins).
> I did a frequency sweep on the input circuit and it is flat 1-30 MHz.
> The amp seems very stable at the available output power.
> I get the impression that the amp has too much effective capacitance at
> the input and is acting like a LPF and rolling off beyond 10 MHz. But
> why?
>
> Can you Guys suggest anything I could do for further elucidation of this
> problem.
>
> Cheers from Oz
>
> Mike Wood  VK5SF
>
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