[AMPS] Feedback path 3-500Z

Maurizio Panicara i4jmy@iol.it
Fri, 29 Sep 2000 23:20:15 +0200

Hi Tom,

stated that PI is "transparent" and the dangerous reactive load has to be
produced across amplifier output connector, please give some examples, among
the hundreds, with real HF antenna impedance at 200 Khz and reasonable line
lenghts that produces that sort of reactive load.
The extimated 200 KHz resonance of grid circuit is probably higher than
real, since I've memory that the coil is more than 1 mH. If I'll have the
proper mood and the time, I'll measure the resonant frequency in the next
My (ex) SB220 has a single ceramic tube, L is there and capacitors are
doubled  (taken from the empty socket).
Not only I say that's difficult VLF oscillations occur with SB220/TL922
because hardly an external load can resonate what's inside with high Q, but
I'm quite convinced that even artificially connecting the "risky" load, and
tuning for exact reactance, the positive feedback is still not enough to
start oscillations at VLF.
Practically, a piece of wire from grid to GND change nothing neither such
low Q parallel of L and C increase risk of parasitics.


----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <amps@contesting.com>; <i4jmy@iol.it>
Sent: Friday, September 29, 2000 8:22 PM
Subject: RE: [AMPS] Feedback path 3-500Z

> It's a big world out there Mauri, with many kinds of loads having
> many kinds of characteristics, I never said it was a frequent even in
> the real world, I just said it could be made to happen and can
> happen. The wrong impedance can occur in hundreds of ways you
> haven't expressed.
> > My point stay that real chances to have VLF oscillations because of a
> > to ground low Q parallel resonant circuit at 200 KHz (or lower) are
> > minimal, basically equivalent when grid is directly grounded. Final step
> > is if anyone want's to try to start VLF oscillations with an
> > The circuit must be connected to PA output connector and must have an
> > equivalent series impedance of about 5 +j 870, a 5 Ohm in series with a
> > 700uH variable inductor could be proper.
> First, everyone else decided it was at 200 kHz. Second, everyone
> said it can't happen.

> Does your SB-220 have those needless chokes in it?
> 73, Tom W8JI
> w8ji@contesting.com

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