With out some load the voltage gain of the amplifier will be very high.
Also, if no signal has been applied the tuning positions of both the input and
output
capactiors may be abount anywhere on any frequency. It is likely unless you have
very good shielding and no other feedback paths other than via grid plate
capacitance
to get oscillation. Mind you there are other paths for feedback , internal
wireing even if bypassed,
common current paths for some circuits on the chassis etc. that the
neutralization capacitor can't
correct for. Some input swamping may go a long ways with out soaking up too
much of your
drive.
You see, you may adjust the neutralization capacitor for minimum feed thru
but if there are other
paths, and there are always some, that minimum may not be ZERO and enough for
oscilliaton with input and
outputs unloaded.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Carl [km1h@jeremy.mv.com]
Sent: Sunday, August 15, 2010 8:34 AM
To: Vic K2VCO; Amps reflector
Subject: Re: [Amps] Initial test of 813 amplifier
----- Original Message -----
From: "Vic K2VCO" <vic@rakefet.com>
To: "Amps reflector" <amps@contesting.com>
Sent: Sunday, August 15, 2010 3:04 AM
Subject: [Amps] Initial test of 813 amplifier
>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
I think you answered your own question Vic. Once that is done touch up the
neutralization and then see if it holds at power. Even some older commercial
amps are marginally stable which is why they use a lot of cut-off voltage in
standby.
Carl
KM1H
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