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G8MNY > TECH 16.11.24 11:32l 56 Lines 2253 Bytes #999 (0) @ WW
BID : 19995_GB7CIP
Read: GUEST
Subj: Valve Heater delay Drake L-4/4B PA
Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP
Sent: 241116/1022Z @:GB7CIP.#32.GBR.EURO #:19995 [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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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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