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G8MNY > TECH 05.07.12 00:17l 55 Lines 1902 Bytes #999 (0) @ WW
BID : 49623_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4B PA
Path: IZ3LSV<IV3SCP<SR1BSZ<ON4HU<F6IQF<IK6ZDE<I0OJJ<GB7CIP
Sent: 120704/2109Z @:GB7CIP.#32.GBR.EU #:49623 [Caterham Surrey GBR] $:49623_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Mar 12)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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)ÄÄÄÁÄÄÄÄÄÄÄÄÄ
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 as
an RF stopper). 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)ÄÄÄÁÄÄÄÄÄÄÄÄÄ
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 control I can
control if the PA is in or out & the power level, which makes tunning up much
easier.
See my Tech bul on "PTT line for Drake L-4B PA" & "Power Factor of Drake
L-4B PA"
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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