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G8MNY > TECH 07.04.25 09:21l 65 Lines 2302 Bytes #999 (0) @ WW
BID : 29758_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4/4B PA
Path: IZ3LSV<I0OJJ<GB7CIP
Sent: 250407/0720Z @:GB7CIP.#32.GBR.EURO #:29758 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jun 24)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄÄ¿
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³ÀÄÄÄÙ (_) ø ø³³ ³
³ o [|]³³ ³
ÀÄÂÂÄÄÄÄÄÄÄÄÄÄÄÂÂÄÙÀÂÂÄÄÄÂÂÙ
THE PROBLEM
The output of these 1965 PAs are designed for a valve Tx, where the load
impedance & voltage are much higher than for my IC735 which has larger cap on
the ALC line. This causes the ALC to be too slow, & hence give bad audio
reports. The use of the PA's ALC is desirable as there is an easy power
adjustment on the PA's On/Off Volume control PTT switch.
L I N E A R ' s A L C C I R C U I T
ALC o)ÄÄ1KÄÄÂÄÄ1KÄ´>ÃÄÂÄÄÄÂÄÄ´ÃÄÄÄÄ<Cathode
-20V === ³ ³ 47pF RF Drive
into ³5nF 15K ===
1Mê// ³ ³ ³47pF
1nF ÄÄÄÄÄÄÁÄÄÄÄÂÄÄÄÄ)ÄÄÄÁÄÄÄÄÄÄÄÄÄ0V
PTT SW ³ ³
& LEVEL 100K<ÄÙ
POT ÀÄÄÄÄÄ+120V Bias
This problem may occur with many Valve PAs driven from modern rigs!
MY CURE
I added a high impedance 2:1 attenuator to reduce the voltage into a PNP
transistor emitter follower, (the resistors are located at transistor base
also act as RF stoppers). It is powered by an negative rail derived from relay
supply.
N e w E x i s t i n g c i r c u i t
ALC o)ÄÄ1KÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÂÄÄ1KÄ´>ÃÄÂÄÄÄÂÄÄ´ÃÄÄÄÄ<Cathode
-10V e\³ 100K === ³ ³ 47pF Drive
1kê PNPÃÄÄÄ´ ³5nF 15K ===
Source /³ 100K ³ ³ ³47pF
³ +ÃÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÂÄÄÄÄ)ÄÄÄÁÄÄÄÄÄÄÄÄÄ0V
18V ³ === 100K<ÄÙ
ACÄ100RÄ´<ÃÁÄÄÄÄÄÙ100uF LEVEL
1N4001 (-25V) ÀÄÄÄÄÄ+120V Bias
RESULTS
I have good audio reports now, and at the touch of the PA's PTT/ALC control I
can control if the PA is in or out, & the adjust power level from 100W >~ 1.4kW
which makes tuning up much easier. (Max was 400W @ Aerial in UK but now 1kW)
See my Tech bul on "PTT line for Drake L-4B PA", "Using 2 HF PAs", "Power
Factor of Drake L-4B PA", "Valve Heater delay Drake L-4B PA" & "Drake PA 3-500
valve repair".
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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