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G8MNY > TECH 20.09.21 07:02z 63 Lines 2229 Bytes #999 (0) @ WW
BID : 47224_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4B PA
Path: HB9ON<IW2OHX<UA6ADV<I0OJJ<GB7CIP
Sent: 210920/0657Z @:GB7CIP.#32.GBR.EURO #:47224 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Feb 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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ÀÄÂÂÄÄÄÄÄÄÄÄÄÄÄÂÂÄÙÀÂÂÄÄÄÂÂÙ
THE PROBLEM
The output is 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 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
ACÄÄ100Ä´<ÃÁÄÄÄÄÄÙ100uF LEVEL
1N4001 (-25V) ÀÄÄÄÄÄ+120V Bias
RESULTS
I have good audio reports now, & 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 > 1.5kW,
which makes tuning up much easier. (400W Max @ Aerial in UK)
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".
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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