G8MNY  > TECH     07.04.25 07:30z 56 Lines 2253 Bytes #14 (0) @ WW
BID : 29759_GB7CIP
Read: GUEST
Subj: Valve Heater delay Drake L-4/4B PA
Path: ED1ZAC<ED1ZAC<IZ3LSV<I0OJJ<I3XTY<GB7COW<GB7BED<GB7JED<GB7CIP
Sent: 250407/0723Z @:GB7CIP.#32.GBR.EURO #:29759 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                   (Update Apr 24)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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³ o            [|]³³       ³
ÀÄÂÂÄÄÄÄÄÄÄÄÄÄÄÂÂÄÙÀÂÂÄÄÄÂÂÙ

This USA made Drake PA as a pair of expensive (RCA=$750ea) 3-500Z valves in it.
On these unbiased power triodes, the grid goes quite +ve with resect to the
cathode with RF drive.

These have a seperate heater transformer in the PA unit.

To reduce the heater inrush current, that can eventually blow the bright direct
cathode filement, I added a 400R NTC 10W Thermistor (ex Valve TV heater chain)
instead of the bridging strap of the 2x 115V windings. The thermistor is put in
a heat proof silicon sleeving & heatshring on leads, as it is on mains circuit
& hot.
                                    CW/tune
LÄo-oÄÄÄÄo/oÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ\ oÄÄÄÄÄÄÄÄÄÄÄ¿_
 Fuse 1   on  ³                     oSSB        __)|| _
              ³ 200VA Heater        ³      TUNE³__ ||(
              ³____    __           ³_____________)||(  850V on
               115V)||(__ 5.5V                 ³__ ||(  TUNE mode
     400R _________)||(__ 30A                     )||(  or 1.1kV
     Therm_________ || __                     115V)||(  in SSB Mode
              ³115V)||(  18V                 _____)||(  @ 2A to
             FAN   )||(__0.5A               [o-o__ ||(  the Voltage
              ÃÄÄÄÄÙ                     Fuse 2   )||(  Doubler
              ³ Transformer                   115V)||(_
              ³   in PA                          _)||  2.2kW Peak
NÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ      HT Transformer

Results is, the heaters now take 4 seconds, rather than 0.5 second to warm up
now, & the filement volts have dropped a bit from 5.1V to 5V. But RF power
still 800W max on low power HT tune up setting & > 1kW on high. So the heaters
are bright (hot) enough!

 

See my Tech bul on "ALC line for Drake L-4B PA", "PTT Line for Drake L-4B PA"
"Drake PA 3-500 valve repair" & "Using 2 HF PAs".




Why Don't U send an interesting bul?

73 de John, G8MNY @ GB7CIP


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