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Re: [Amps] Question re GU-74B grid current

To: <Amps@contesting.com>
Subject: Re: [Amps] Question re GU-74B grid current
From: "Paul Christensen" <w9ac@arrl.net>
Date: Mon, 22 Oct 2012 15:18:14 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Val,

For the sake of completeness, I believe the typical "three state" bias voltages for the ACOM 2000A (GU74B) are as follows:

-124V on standby;
-75V during plate idling with PTT key line enabled; and
-52V when input RF is sensed.

When using VOX, the amp is quickly transitioning from Class C to Class B. -124V at level 1 should represent Class C. I'll stand corrected if it's not.

Paul, W9AC


----- Original Message ----- From: "Val" <val@vip.bg>
To: <Amps@contesting.com>
Sent: Monday, October 22, 2012 3:10 PM
Subject: Re: [Amps] Question re GU-74B grid current


Jim,

ACOM1000 and ACOM2000A are without doubt the most popular amplifiers based on GU74B. Many thousands of them are working under the following conditions at1000W output:
Plate  2350-2400V
Screen 340V
Anode current 510-580mA
Bias between -45 and -65V for 220 mA idling current (the EBS keeps the bias between -65 and -75V for idling current of 70 mA when no RF drive) Normally the grid current is well below 10mA where the grid protection trips.

73, Val LZ1VB


Could somebody advise me as to the resting plate current and operating ctrl grid bias voltage they are using on a GU-74B (plate voltage is 2500V, screen grid 300V)? Also, does the tube draw grid current? I realize that in AB1, no
grid current should be drawn, but the tube characteristic curves predict
about 15mA grid current at full output (1000W output, 600mA anode current
(per tube) and 250W resting dissipation. I am interesting in what builders
are actually getting, and not information from data or spec sheets.

Tnx and 73,

Jim W8ZR

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