[Amps] 813's shot noise?

[jerome schatten]

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