On the schematic a choke, Dr 401, is shown between the six-pulse bridge and the
filter capacitor.
It is bridged by an overvoltage protector.
As you do not generate the screen voltage from the plate supply, modulation
from the screen supply can be ruled out.
Left is the 3-phase supply itself.
Made a "back of the envelope" calculation of the ripple reduction that a 5 H
choke and a 8µF capacitor is capable of;
which in the order of 40 dB or 1:100.
Add this to the inherent ripple modulation factor of about - 30 dB in an AB1
amplifier and you should end up in - 70 dBc or so, The Collins gear I worked on
during the 90s usually showed a total 300 Hz ripple modulation of -70 to -75 dB
down at full power (10 kW).
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : g0ruz@g0ruz.com
Datum : 2021-05-23 - 20:05 (CEST)
Till : sm0aom@telia.com
Ämne : RE: [Amps] 300Hz sidebands
Hello Karl-Arne. I do not see a choke in the schematic. I must get the supply
out of the rack and need some help, it is too heavy for me. Perhaps the DC
smoothing capacitor is defective. When I get the supply out I can test it.
The screen supply is run off a single phase independent trafo with a zener and
mosfet shunt stabiliser, I do not expect that it will follow the anode supply
ripple.
73
Conrad PA5Y
-----Original Message-----
From: sm0aom@telia.com <sm0aom@telia.com>
Sent: 23 May 2021 19:53
To: Conrad PA5Y <g0ruz@g0ruz.com>; amps@contesting.com
Subject: Re: [Amps] 300Hz sidebands
I would suggest checking both the choke and the capacitor, If there has been a
short in the overvoltage protector across the choke, it would be out of the
circuit.
Without the choke, the ripple increases considerably but normally class AB
amplifiers are quite tolerant for plate power supply ripple.as long as the
screen voltage is kept constant.
The way the screen voltage is derived, however, makes the screen voltage follow
the ripple content of the plate voltage directly to the screen. This could
cause considerable ripple modulation in tetrode amplifiers.
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : g0ruz@g0ruz.com
Datum : 2021-05-23 - 18:35 (CEST)
Till : amps@contesting.com
Ämne : [Amps] 300Hz sidebands
Hi all.
I have several tetrode amplifiers that run from a 3PH 50Hz supply. With the
advent of the FT8 I have become aware that I have 300Hz sidebands at only
-50dBc, this is obviously unacceptable. The supplies are from Rhode and
Schwartz and are very well made and use an 8uF oil filled capacitor for
smoothing. Some of the EU amateurs will know the supply, it is known as an IN
026 A1.
I suspect that this problem has always been there.
What is interesting is that Class AB1 or Class C, 100W out or 1kW out from the
PA the sidebands are -50dBc. I did consider the G1 and G2 supplies but 300Hz
suggests the 3PH supply and this happens with 3 different amplifiers, the only
common factor is the HT supply. If it was ripple, I would have expected this to
change depending on Anode current, but it doesn't. Unless of course the
capacitor is completely shot. I will check that next, it's a little difficult
to get to.
The PSU details can be found here:
https://www.dropbox.com/s/l71qj1g3qv7zh73/Servive%20Doc%20UHN284.pdf?dl=0
Any ideas?
Regards
Conrad
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