Hello Conrad,
It is not that difficult to find out, what is happening.
If you have 4% 300Hz ripple, a clas C amplifier will be platemodulated with
4% 300Hz modulation.
If the amplifier is class A, the platemodulation will be very close to 0%.
If class AB1, AB2, B, the modulation will be somewhere between 0% and 4%
A 3phase fullwave rectifier will without capacitor have 4,2% 300Hz ripple,
the same 4,2% unloaded and under full load!
If the rectifier is loaded with 4kohm, 2000V o,5A or 4000V 1A or-----, then
the capacitor will work as follows:
This supply with 4kohm load and 4uF or even 8uF filter cap will still have
4,2% ripple!
16uF filter cap will reduce the ripple from 4,2% to around 3%.
32uF and 4kohm load will result in around 1% ripple.
I do not understand why we see so many 3phase supplies with from 2uF to 8uF
filtercaps.
I had the problem years ago, now I use 36uF filter cap., now the signal is
pure.
I hope this will help to see, what is happening.
Vy 73
Finn
oz7yy
-----Oprindelig meddelelse-----
Fra: Amps [mailto:amps-bounces@contesting.com] På vegne af Conrad PA5Y
Sendt: 26. maj 2021 18:39
Til: jim.thom jim.thom@telus.net; amps@contesting.com
Emne: Re: [Amps] 300Hz sidebands
Hello Jim, it is a grid driven tetrode in AB1, the G2 supply is shunt
stabilised and from a separate single-phase transformer. I can assure you
that the drive level and therefore the anode current does not vary the
relative amplitude of the 300Hz sidebands. This surprised me as well, but it
is -50dBc.
I would also like to understand the additional ripple reduction.
Regards
Conrad PA5Y
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of jim.thom
jim.thom@telus.net
Sent: 26 May 2021 16:25
To: amps@contesting.com
Subject: [Amps] 300Hz sidebands
Date: Sun, 23 May 2021 20:51:52 +0200 (CEST)
From: "sm0aom@telia.com" <sm0aom@telia.com>
To: g0ruz@g0ruz.com, amps@contesting.com
Subject: Re: [Amps] 300Hz sidebands
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 8?F 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
## whoa. With little or no load on the B+ supply, the 300 hz ripple
should be through the floor. Typ 3 phase B+ supplies ( on 60 hz) and NO
filter, choke, nor cap at all, are typ 5.2% ripple ( 360 hz ripple in this
case of using 60 hz incoming mains). The bigger the load, the ripple gets
worse, normal.
## what do you mean by... the inherent ripple modulation factor of
-30db in an AB1 amplifier ?? Are you talking about a grid driven,
class AB1 amp, or also a garden variety GG, cathode driven Triode ??
This is a new one for me. Where is this extra 30 db of ripple
reduction coming from ????
## On my oem Drake L4PS supplies, with oem 8 x 200 uf caps, the
ripple is 207 V peak to peak, = 2.8%... = -31db ripple redux. Thats
what I get on the RF scope. Plane as day with a 1290 watt dead cxr on the
scope. I since tripled the uf in the drake supplies....to 75 uf....and
ripple is down to 1%, 69 V peak to peak = -40 db.
Jim VE7RF
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