[Amps] Initial test of 813 amplifier

[Vic K2VCO]

My t/r circuit applies bias beyond cutoff when the ptt is open. When it's closed, the bias
drops to a point which allows 50 ma of zero-signal plate current (about -50V). When drive 
is applied, rectified grid current through a resistor supplies the rest of the class-C 
bias (a total of about -175V). So the tubes are normally cut off, except for a period of a 
few ms before and after keyed elements in QSK mode. Of course I don't want it to oscillate 
then, either!

The idea is that the amplifier will be operating in a linear mode as drive rises, reaching 
full class-C along with the rise time of the keyed pulse. I HOPE that this will keep the 
amplifier from sharpening up the keying and producing clicks. If it doesn't work, I'll add 
some delay to the screen circuit.

Just in case anyone thinks I'm nuts, the intention is to use it on CW only. I probably 
should put a sign on it: "anybody using this amplifier on SSB will be shot!"

And yes, it would have been easier to use an extra 813 or two and make it a linear. But I 
want to test my theory.

On 8/15/2010 7:08 AM, Fuqua, Bill L wrote:
> One other thing. It is always a good idea to very high negative bias on the tubes when
> not transmitting. One is to prevent the possibility of an oscillation the other is
> less obvious.  If you are using an electronic TR switch the output of the transmiiter
> is always connected to the antenna. Even if you are using a relay it is possible have
> enough coupling through to still have a problem with the Shot-Noise from the tube.
> Power tubes are great white noise generators due to the random arrival of electrons on
> the anode. When using an electronic switch this will be enough to blanket the band
> with S9 noise.  I once had set up my Viking Challenger and receiver for an High School
> demonstration. I used seperate transmit and receive antennas so I would not fry my
> receiver. For the demo I was using voice, and the screen grid modulator( clamp tube)
> left quite a bit of plate idle current. I had to switch off the transmitter while
> listening to the other side because noise was just too high to receive the far
> stations.
>
> 73 Bill wa4lav


-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/