[ARC5] Switching the dreader B+ line.

[J Mcvey]

Les,
Here is a link to 1200V mosfets:  1200v mosfets
  
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1200 V MOSFET | Mouser
 1200 V MOSFET are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for 1200 V...  |   |

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These were sometimes used in the last generation of CRT TV horizontal deflection circuits. The pulse on the primary of the FBT was often 900V and the "charge current" was around an amp or so, depending on the requirements.
There are bipolar transistors that were used for the same task as well.Even though they are 'pulse rated" , I think they should be able to pass continuous 150 ma.So to figure the what is lost in heat,  you need to calculate (Id)^2 * Rds. I was looking at one that had 80 milli-ohms on resistance, so the dissipation at 150 ma would be nominal. 
There will be dissipation issues with bipolar transistors too. that would be Vce sat * Ic
Most tube "modern" transmitters leave the PA Plate B+ live and manipulate the bias in one way or another when in tx mode.Opening the cathode contacts puts the PAs in cutoff.

You could also:Permanently engage the antenna relay and key the select relay independently while ALL buss voltages osc/screen/plate remain live all the time.
  


 

    On Saturday, April 1, 2017 9:45 PM, Leslie Smith <vk2bcu at operamail.com> wrote:
 

 

Hello All,

Jim asked me why not use Dave Stinson's scheme to put a "command"
transmitter on the air.  In my reply, I omitted the most important part
of the discussion - a circuit diagram of the SCR-274 transmitter.  I'm
certain most reading this list will have that - but not all.

SO I send that now.
Looking at J64 (the connector on the rear panel of the transmitter) we
can see that the B+ line 

(1) to the PA comes in via pin #7, (to the 1625 circuit).
(2) to the screen supply comes in on pin J64.4 
(3) the supply to the oscillator from pin J54 pin #3 and K-53.

The HV supply (to the PA,  J64.7) is switched in the BC-456 modulation
circuit.  (See J64.7 then PL154.10 and then K-52 - fed from the B+ line
on the dyno.  (I don't supply this part of the circuit.)  So the actual
switching for the PA B+ line takes place in the BC-456.  Closing a key
in the +24V heater line pulls in the relay, and the B+ supply.

It would be possible, of course, to leave the PA B+ permanently "on"
because the oscillator is switched in the transmitter, K-53.    I'm sure
some will do that - but a safer method is to switch the B+ line "on"
using the key only when the transmitter is "on-air".  I'm want to do
that, using a FET rather than a relay.  I'm simply duplicating the
function of K-52, but with a solid state device.

Some will say that switching the B+ line with a relay is safer than
switching with a FET.  That is correct, without doubt.


Les
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