[Amps] 300Hz sidebands

[sm0aom at telia.com]

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 at g0ruz.com
Datum : 2021-05-23 - 20:05 (CEST)
Till : sm0aom at 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 at telia.com <sm0aom at telia.com> 
Sent: 23 May 2021 19:53
To: Conrad PA5Y <g0ruz at g0ruz.com>; amps at 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 at g0ruz.com
Datum : 2021-05-23 - 18:35 (CEST)
Till : amps at 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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