Message 23971 in TECH.WW
From: G8MNY
Date: Thu, 11 Jun 26 08:51:00 Z
Newsgroups: TECH.WW
Subject: ALC line for Drake L-4/4B PA
Message-ID: <62118_GB7CIP>
Path: N2MH4|N2MH|VE3KPG|GB7CIP
R:260611/0853Z 32238@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260611/0853z @:N2MH.#NNJ.NJ.USA.NOAM [via AREDN Mesh] jnos 2.0p $:62118_GB7CIP
R:260611/0853Z 24125@VE3KPG.#ECON.ON.CAN.NOAM BPQK6.0.25
R:260611/0851Z @:GB7CIP.#32.GBR.EURO #:62118 [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)
THE PROBLEM
The output of these 1965 PAs are designed for a valve Tx, where the load
impedance and voltage are much higher than for my IC735 which has larger cap on
the ALC line. This causes the ALC to be too slow, and 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
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
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, and the adjust power level from 100W >~
1.4kW which makes tuning up much easier. (UK Max was 400W @ Aerial, 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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