G8MNY  > TECH     07.04.25 07:30z 65 Lines 2302 Bytes #14 (0) @ WW
BID : 29758_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4/4B PA
Path: ED1ZAC<ED1ZAC<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¿
³ÚÄÄÄ¿    _    _  ³³       ³
³ÀÄÄÄÙ   (_)  (_) ³³       ³
³ÚÄÄÄ¿    _       ³³       ³
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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