Hi John,
It's tough to get an auto-tuner to work into a PA, because the inherent
negative feedback and varying grid current affect the input impedance as
drive level changes.
John Bradley wrote:
>
> 1. Was I correct in concluding that the 100 ohm resistor should be in the
> input circuit?
It actually should have series swamping resistance as well. The 8875's
are easily overdriven by any 100 watt PA.
> 2. Do you think my problem with 10 and 15 meters is caused by excessive
> inductance of my 3 in. wire from the input signal to the 100 ohm resistor?
> If not, any ideas?
No, the PA needs a input matching system.
> 3. What kind of power should I be getting out of this amplifier? The
> power
> in is between 80 - 100 W. (depending on band).
Well over a kilowatt, unless the tubes are soft.
> 4. Is there any simple modification to the input network that would
> improve
> the match with a driver like the TS-450?
No.
> Any other advice regarding this amplifier would be welcome as well. Thanks
> for any help on this; and thanks to everyone contributing to the reflector,
> it makes for enjoyable reading.
Add a grid overload circuit to disconnect the tubes if you mistune the
amp. It only takes a matter of seconds to ruin the grids if the PA is
underloaded.
A grid protection system is the single most important thing you can do
to improve tube life. A series energy absorbing resistance in the anode
supply lead is another.
73, Tom
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