[Amps] complicated Bias circuit

[MU 4CX250B]

Paul,
In standby, the 24V from T2 just biases the tube(s) to cutoff. R20 by
itself would almost do this, but without the 24V there would still be
some small cathode current in standby which could generate noise. When
the amp is operating, the 24V biases the Zener diode that establishes
the tube operating bias. R22 sets the bias current through the Zener
diode. Without the 24V, the tube cathode current would bias the Zener
diode, but the regulation would suffer (causing non-linearity) at low
currents. The 24V eliminates that possibility.

It strikes me as a bit odd that, since the designers went to the
trouble of building a 24V power supply, that they didn't just go ahead
and implement electronic bias switching, which facilitates QSK
operation.
73,
Jim W8ZR

Sent from my iPhone

> On May 4, 2014, at 7:49 PM, Paul Decker <kg7hf at comcast.net> wrote:
>
>
> FYI, if it helps any, this goes to a 3cx800A7 amplifier of some sort.
>
>
>
> ----- Original Message -----
> From: Paul Decker
> To: amps at contesting.com
> Sent: Mon, 05 May 2014 01:18:47 -0000 (UTC)
> Subject: complicated Bias circuit
>
>
> Hi builders.
>
> Hopefully you all can view this schematic.
>
> I'm trying to understand what T2 and associated circuitry do.  D17 and D18 seem like a simple bias circuit as SW3 selects between "class C" and "class AB1"  It seems opening R20 puts tube in standby (RX).
>
> What is the point of having 24VDC from T2?  Is it to have a smaller wattage D17 and D18?
>
>
> https://scontent-a.xx.fbcdn.net/hphotos-frc1/t1.0-9/10289874_10202768703951918_1823401065078063020_n.jpg
>
>
> Thanks in advance,
> Paul
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