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G8MNY > TECHNI 05.07.16 23:27l 62 Lines 2157 Bytes #999 (0) @ WW
BID : 10075_GB7CIP
Read: GUEST DK3UZ DG4IAK OE7FMI
Subj: ALC line for Drake L-4B PA
Path: DB0FHN<DB0PM<OE5XBL<OE5XBR<OE1XAB<HG8LXL<GB7YEW<PI8CDR<DB0RES<DB0OVN<
DB0GOS<ON0AR<F1OYP<IW8PGT<CX2SA<GB7CIP
Sent: 160705/0916Z @:GB7CIP.#32.GBR.EURO #:10075 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Feb 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄÄ¿
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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)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 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
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 adjust power level from 100W > 1kW, which
makes tuning 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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