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/
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|