[Amps] Gonset 903a cure for self-oscillation?

[Steve Katz]

I'd try the new socket, and hope you bought the correct one (with
internal screen bypass).

Yes, a bad screen bypass can cause oscillation -- the 4X150/4CX250 has a
tremendous amount of gain, right on up to 500 MHz.

I'd also add a 6 dB input attenuator, for sure.  The 903A has way too
much gain for an FT-857D driver, even if you "turn down" the 857D.
Every 903A or 913A amp I've ever used needed a 6-10 dB input attenuator
to function properly and in a stable fashion.  Even with a 10 dB input
attenuator (overkill), it can still be driven to "full output" power
with only 25-30W RF drive.

I've always installed the attenuator right at the input socket, since
the 903A needs outboard T-R bypass relay switching anyway.  This puts
the "pad" closest to the tuned input circuit and tames the amp down
well.

WB2WIK/6

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
On Behalf Of Larry
Sent: Thursday, April 10, 2008 7:44 PM
To: Amps at contesting.com
Subject: [Amps] Gonset 903a cure for self-oscillation?


     Hello again, guys. I'll get right down to the facts. I'm having a
bit
of trouble neutralizing this amp. With 10 watts input, I can get the
thing
"tuned" to show 250+ output, but when I unkey, the plate current will
increase to 350 to almost 400ma. As far as I know, this isn't healthy
for
the amp, so I quickly detune the amp to stop it from oscillating. I'm
trying
the neutralizing screw, but it really doesn't seem to do much. Is there
a
"standard" set of component values to use to stop this from happening at
144
MHz? Also, when the amp seems to be adjusted properly, the FT-857 I'm
using
will always show a high SWR - can my input match be affected by the
oscillation? I'll be honest - I haven't tried an SWR meter in line yet,
and
since I just lost another diode in the HV, I can't try it tonight to see
what it actually is. Could my whole problem be that I'm using a
transmitter
that's "too new" for this amp? This thing's really got me puzzled since
I
checked just about all of the components inside this thing and they all
appear to be fine. Well, except for the socket. It was shorted
internally,
so I took it apart & (hehehe) taped over the bad spots in the bypass
cap. I
am getting another socket, but didn't think this would cause the
oscillation. Is that bypass cap my whole problem since I'm sure the
value
isn't what it's supposed to be? I know that tape fix sounds stupid, but
I
wanted to try something in the meantime. An experiment. Thanks again.
...Learning in RI (Larry)


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