IW8PGT

G8MNY  > TECH     19.05.15 07:52l 62 Lines 2131 Bytes #999 (0) @ WW
BID : 30634_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4B PA
Path: IW8PGT<IZ3LSV<IW2OHX<IR2UBX<IK2XDE<DB0RES<ON0AR<GB7CIP
Sent: 150519/0616Z @:GB7CIP.#32.GBR.EURO #:30634 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                 (Updated Mar 12)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄÄ¿
³ÚÄÄÄ¿    _    _  ³³       ³
³ÀÄÄÄÙ   (_)  (_) ³³       ³
³ÚÄÄÄ¿    _       ³³       ³
³ÀÄÄÄÙ   (_)   ø ø³³       ³
³ o            [|]³³       ³
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)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
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)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/ALC 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", "Using 2 HF PAs", & "Power
Factor of Drake L-4B PA".


Why Don't U send an interesting bul?

73 de John, G8MNY @ GB7CIP


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