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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