[Amps] Question re GU-74B grid current

[Paul Christensen]

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 at vip.bg>
To: <Amps at 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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