G8MNY  > TECH     31.08.25 08:00z 65 Lines 2306 Bytes #184 (0) @ WW
BID : 39898_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4/4B PA
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250831/0749Z @:GB7CIP.#32.GBR.EURO #:39898 [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
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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 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

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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