[Amps] 300Hz sidebands

[Conrad PA5Y]

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 at contesting.com> On Behalf Of jim.thom jim.thom at telus.net
Sent: 26 May 2021 16:25
To: amps at contesting.com
Subject: [Amps] 300Hz sidebands

Date: Sun, 23 May 2021 20:51:52 +0200 (CEST)
From: "sm0aom at telia.com" <sm0aom at telia.com>
To: g0ruz at g0ruz.com, amps at 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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