Message 24068 in TECH.WW
From: G8MNY
Date: Fri, 12 Jun 26 07:02:00 Z
Newsgroups: TECH.WW
Subject: Valve Heater delay Drake L-4/4B PA
Message-ID: <62212_GB7CIP>
Path: N2MH4|GB7BED|EI2GYB|GB7CIP
R:260612/0712Z 32321@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260612/0712Z 12771@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260612/0711Z 55212@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
R:260612/0702Z @:GB7CIP.#32.GBR.EURO #:62212 [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)
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 and heatshring on leads, as it is on mains
circuit and hot.
CW/tune
400R _________)||(__ 30A )||( or 1.1kV
Therm_________ || __ 115V)||( in SSB Mode
FAN )||(__0.5A [o-o__ ||( the Voltage
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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