[AMRadio] info wanted on 4-400 linear amplifier

Jim Candela jcandela at prodigy.net
Tue Aug 2 23:07:29 EDT 2005


  I always admire those about to undertake such a
project. I don't have a circuit for you, but I will
share a few ideas. If you do use grid driven, it will
likely need to be class AB1 (zero drive power to tube,
i.e, just voltage), or class Ab2 where the grid draws
a little grid current on modulation peaks. In either
case, 25 watts will be way too much. Given the above
choices, I'd use Ab1, and a passive 50 ohm grid input
resistor, and scale the drive just short of grid
current. Still you will need a grid bias supply, and
screen supply in addition to a plate supply. That is a
lot of stuff, but hey, why not if you want to do it?

Another approach, and I'm being a little simplistic
here, is to think of a 4-400 in grounded grid as
something similar to or about 4/5 a 3-500Z. A Heath
SB-220 does legal limit AM with under 25 watts
(carrier) of drive with two 3-500Z's in parallel G-G.
No screen supply needed, and bias if necessary is easy
with a power zener diode. So a 4-400 configured as a
GG linear should be good for 150 to 200 watts am
carrier output, and only need 10-15 watts carrier for
drive. My figures are a SWAG, but I'd bet they are in
the ballpark.

Thanks for reading,
Jim Candela
--- zuu6k at juno.com wrote:

> I am planning to build a linear amp around a grid
> driven 4-400 tube. Can
> anyone suggest where I can find a suitable circuit
> with component values.
> I would like band-switching for 80, 40 and 20
> meters. I will be driving
> it with a 25 watt carrier output rice box on AM. I
> don't like the one in
> the ARRL 1967 handbook because it requires you to
> open the lid and
> physically change taps on the coil and change tuning
> capacitors. I don't
> have any other older handbooks. Thank you.
> Ed K6UUZ
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