A low value carbon resistor right at the grid pin may be enough to finish
the taming once youve completed the rest of the work. Ive used 10-33 Ohms in
several tetrode rigs and amps that were otherwise a bit flaky as shipped. In
my own stuff they are installed as a part of the build. as well as a 100 Ohm
right at the screen terminal and bypassed on both sides.
Its just another one of those things I learned at National that has saved me
grief in the debugging stage.
Carl
----- Original Message -----
From: "Vic K2VCO" <vic@rakefet.com>
To: <amps@contesting.com>
Sent: Sunday, August 15, 2010 11:23 AM
Subject: Re: [Amps] Initial test of 813 amplifier
> 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/
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