[Amps] Initial test of 813 amplifier

Vic K2VCO vic at rakefet.com
Sun Aug 15 00:04:22 PDT 2010


I did some testing of my 2 x 813, class-c grid-driven amplifier under HV for the first 
time today. After adjusting the neutralization (with HV off), I applied HV and set the 
bias for 50 ma ZSAC (there's a grid resistor to supply the additional class-c bias when 
drive is applied).

I connected a dummy load and set the plate tank for approximate resonance on all bands and 
diddled the grid tuning and loading to see if the amplifier would take off. I didn't see 
any flicker in the plate current, or any grid current. So I believe it is stable, at least 
under these conditions.

However, when I removed the dummy load, I was able to create an arc in the plate loading 
capacitor under some tuning conditions.

Note that all of the above was with no drive, fixed bias and screen voltage.

So it is possible to make it oscillate when there is no load connected. I didn't determine 
whether it was a VHF or HF oscillation.

The shield enclosure and bottom plate are not in place yet.

Here's my question: Should I bother with this oscillation at this point? Or is it normal 
to have instability when no load is connected to the amplifier?

I suppose the fact that there's no enclosure or bottom plate is also a cause...
-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/


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