[Amps] complicated Bias circuit

[Paul Decker]

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 at miamioh.edu>  To: Paul Decker <kg7hf at comcast.net>  Cc: amps at 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 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  > _______________________________________________  > Amps mailing list  > Amps at contesting.com  > http://lists.contesting.com/mailman/listinfo/amps