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G8MNY > TECH 06.11.26 17:12l 56 Lines 2272 Bytes #7 (0) @ WW
BID : 10329_GB7CIP
Subj: Valve Heater delay Drake L-4/4B PA
Path: JH4XSY<N3HYM<K5DAT<VE2PKT<PY2BIL<GB7CIP
Sent: 261106/0809Z @:GB7CIP.#32.GBR.EURO #:10329 [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)
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿﾚﾄﾄﾄﾄﾄﾄﾄｿ
ｳﾚﾄﾄﾄｿ _ _ ｳｳ ｳ
ｳﾀﾄﾄﾄﾙ (_) (_) ｳｳ ｳ
ｳﾚﾄﾄﾄｿ _ ｳｳ ｳ
ｳﾀﾄﾄﾄﾙ (_) � ｳ ｳ
ｳ o [|]ｳｳ ｳ
ﾀﾄﾂﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾂﾄﾙﾀﾂﾂﾄﾄﾄﾂﾂﾙ
This USA made Drake PA as a pair of expensive (RCA=$750ea) 3-500z valves in it.
On these unbiased power triodes (class B2), 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 and heatshring on leads, as it is on mains
circuit and hot.
CW/tune
Lﾄo-oﾄﾄﾄﾄo/oﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ\ oﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ_
Fuse 1 on ｳ oSSB __)|| _
ｳ 200VA Heater ｳ TUNEｳ__ ||(
ｳ____ __ ｳ_____________)||( 850V on
115V)||(__ 5.5V ｳ__ ||( TUNE mode
400R _________)||(__ 30A )||( or 1.1kV
Therm_________ || __ 115V)||( in SSB Mode
ｳ115V)||( 18V _____)||( @ 2A to
FAN )||(__0.5A [o-o__ ||( the Voltage
ﾃﾄﾄﾄﾄﾙ Fuse 2 )||( Doubler
ｳ Transformer 115V)||(_
ｳ in PA _)|| 2.2kW Peak
Nﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ HT Transformer
Results is, the heaters now take 4 seconds, rather than 0.5 second to warm up
now, and 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 and > 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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