This is a timely question for me, too, since I'm considering making a
modification to my Drake L-7, deriving cutoff bias from a resistor in the
filament side, rather than the existing heat-producing voltage divider in
the P.S. which would be disabled (yes, there'll still be a bleeder). The 120
VDC from the divider is currently also used to develop ALC, but my exciter
is matched nicely to the amp, and I don't anticipate using ALC...therefore,
Jerome's question raises an issue applicable to my planned mod: is there an
unintended consequence associated with this approach to cut-off bias???
Inquiring minds want to know...
Chuck, N4NM
----- Original Message -----
From: "jerome schatten" <romers@shaw.ca>
To: "Amps" <amps@contesting.com>
Sent: Wednesday, June 11, 2008 6:42 PM
Subject: [Amps] 813's shot noise?
> I'm about finished rebuilding my pair of 813's (zero bias GG) amp. I
> acquired a nice almost new (1966) pair of 813's to finish things off.
> The new pair has almost double the output of my tired old pair (1943)
> which I've been using for almost 40 years.
>
> The set up is:
> Driver: Pro3 with its internal ant. tuner providing matching to the
> input of the amp.
> The Amp: two 813's gg (all grids tied together to chassis) Ebb=2200v
> no load; pi network output.
>
> In testing out the new pair of tubes, I noticed that with the amp
> manually put in operate position (ant relay in tx position, no drive,
> HV applied running idle plate current into a dummy load), I can hear
> popping noises with the higher gain receive preamp engaged (+20db or
> so of preamp). It sounds like arcing or popping and the noise blanker
> on the rx takes it out completely. The band-scope on the rx shows
> random narrow spikes all through the bandpass of the scope. It occurs
> with one, or both tubes in place. It occurs as well with cut-off bias
> applied. The bias is developed as a kind of cathode bias with a 2k ohm
> resistor in the HV return to the CT of the filament xformer. So, it
> essentially cuts off the tubes, but not totally.
>
> The popping noise does not happen with the tired pair, so I think I
> can eliminate arcing in either the power supply or the amp chassis,
> but would like opinions on this.
>
> So, my question is: Is this likely 'shot noise' that I can't do
> anything about; and if not, what else should I be looking for?
>
> The reason for my concern is the way I use the amp. If the receive
> signal is strong enough, I can, with the help of the receive preamps,
> listen to the band through the amplifier, and not have to switch the
> amp at all. Ultimately I'll put a QSK-5 in and the problem will most
> likely disappear, but til then, I've got the popping noise to deal
> with.
>
> Thanks for any insights,
> jerome - VA7VV
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>
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