Hello
It is a surprise that a grounded G2 and G1(via caps) can oscillate.
My experience is with a 432MHz PA with YL1111 (eq.RS1064C) according to
Dubus and DB6NT.
G2 is physically grounded and G1 is AC grounded with a teflon ring disk
capacitor that is a integral part of the socket. Output is 750w with 15w
input. Isolation was measured with a generator attached to the output and
measuring with a Boonton 92EA level meter at the input.
No voltages applied, except heater and g1 bias (if these are off should not
make much if any difference).Transmitting at Input and measuring at output
should give comparable result if PA is tuned at operating frequency on
input and output.
Measuring loss from generator to meter, this must be more than the gain in
the PA or else you will have a power oscillator. Margin should be at least
6dB. I measured this to be >20dB as I remember it. May have been >30dB.
Anyway stability margin was very good with this design.
A AC(or DC) grounded G2 and G1 amplifier shall have a good stability margin.
If not decoupling capacitors should be suspected.
First of all try to measure isolation between output and input and compare
this to gain.
If this is close to zero, I would inspect the 470pF capacitors. 8x470pF
seems much for 432MHz.
Try to remove capacitors in a symmetrical pattern. Any chenge in measured
isolation ?
Try to measure with another tube.
Helge
LA6MV
-----Original Message-----
From: Andreas Lohmann [mailto:fe7x231@public.uni-hamburg.de]
Sent: 30. oktober 2002 10:16
To: amps@contesting.com
Subject: [Amps] Thomson YL1050 Cavity Neutralization 432 Mhz
Hello all,
have got two YL1050 432 Mhz cavity amps with quarter wave output an folded
back 3/4 wl re-entrant cavity here. Screen grid is grounded and grid 1 is
bypassed by 8 x 470pF leadless disk capacitors.
This amplifier is not stable. At cathode currents above around 600mA the
amplifier tends to turn into a run-away condition. This means that cathode
current,screen current and Power Meter(1,5KW) come up with a full scale
reading, which is obviously not good for the internal tube structure.
Removing drive power restores the normal quiescient current after around
two seconds.
Originally the cavity tuned from 225-400 Mhz (R&S design). After minor
changes it now tunes up to 500 Mhz but here is my question: How can I
neutralize this beast??? Obviously there must be some unwanted feedback
through the tube causing the trouble. I need a very simple way to get rid of
this because access and space within the cavity structure are limited ! Any
ideas are welcome.
Andy DL4XX
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