Hi Jim,
Thanks, that is what I needed to get me going. I couldn't see the exact need
for the T2 circuitry since D17 and D18 were still the traditional high power
50W zeners. I would have thought since there was a secondary bias supply that
some sort of lower wattage bias circuit would have been in place but it appears
this is just a traditional bias.
I think there is no QSK because this amplifier is designed to be either keyed,
or off it is from a TX only system. I wanted to modify it for TX/RX by adding
a relay in the bypass line of R20 to put the amplifier in standby during RX.
After looking at the schematic for a while I just had to ask what the heck T2
was doing since it didn't really make sense it was needed.
Thanks for the explanation!
Paul
kg7hf
----- Original Message ----- From: MU 4CX250B <4cx250b@miamioh.edu> To:
Paul Decker <kg7hf@comcast.net> Cc: amps@contesting.com Sent: Mon, 05 May
2014 02:22:36 -0000 (UTC) Subject: Re: [Amps] complicated Bias circuit
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 wrote: > > > FYI, if it helps any, this goes to a 3cx800A7 amplifier
of some sort. > > > > ----- Original Message ----- > From: Paul Decker >
To: amps@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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