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G8MNY > TECH 30.10.10 23:25l 53 Lines 1757 Bytes #999 (0) @ WW
BID : 65107_GB7CIP
Read: GUEST
Subj: ALC line for Drake L-4B PA
Path: IZ3LSV<IK6ZDE<VE3UIL<GB7COW<GB7CIP
Sent: 101030/2219Z @:GB7CIP.#32.GBR.EU #:65107 [Caterham] $:65107_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Oct 10)
(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 an IC735 with a large cap on the ALC. This causes the ALC
to be too slow & give bad audio reports. The use of the PA's ALC is desirable
as there is an easy power adjustment on the PTT switch control on this PA.
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
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 (@ the transistor as an RF stopper),
the PNP is just an emitter follower to this reduced voltage. It is powered by
an negative rail derived from the relay supply.
ALC o)ÄÄ1kÄ¿ ÚÄÄÄÄÄÂÄÄ1kÄ´>ÃÄÂÄÄÄÂÄÄ´ÃÄÄÄÄ<Cathode
-10V e\³ 100k === ³ ³ 47pF Drive
1kê PNPÃÄÄÄ´ ³5nF 15k ===
Source /³ 100k ³ ³ ³47pF
³ +ÃÄÄÄÄÄÁÄÄÄÄÂÄÄÄÄ)ÄÄÄÁÄÄÄÄÄÄÄÄÄ
18V ³ === 100k<ÄÙ
ACÄÄ100Ä´<ÃÁÄÄÄÄÄÙ100uF LEVEL
1N4001 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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