"Highland Ham" wrote in message
...
John Siegel wrote:


Jimmie D wrote:
a guy has a 6m amp for sale that uses an 811a. in GG configuration. Is
this possible with reasonable gain? The amp appears to be built well and
I will probably buy it just for the parts before the week is out but
should I expect this thing to really work on 6M.

Jimmie

I used an amp with 572Bs for many years. With an untuned input, output
ran up to 600W PEP for 100 in.
This suggests that the 811A would work but at some lower power. What
does the owner say about input and output power?
John

============================
Power amplifiers in the past had untuned input circuits (see above)
because the driving transmitter (with valves in the output stage) had an
adjustable 'Pi-tank' for matching.
However nowadays most driving transmitters (usually transceivers) have a
solid state output stage requiring a resistive load of near 50 Ohms .
So when using these for driving a valve power amp ENSURE the amp has a
tuned input circuit . If not ,a matching circuit will have to be added.
Alternatively the valve power amp can be modified with a simple passive
input circuit comprising an 1:4 impedance transformer and a 200 ohms
non-inductive resistor capable of absorbing the driving power .

Frank GM0CSZ / KN6WH
Also worth noting:

The solid state rigs really like to see the Fly Wheel provided
by even a low Q circuit on the input; a class B amp. will
otherwise see an unsymmetrical load over portions of
each cycle for the driving signal .