----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Tuesday, January 11, 2011 3:02 AM
Subject: Re: [Amps] GI-7bT amplifier
> Glen Zook wrote:
>>I am in the process of building a single band linear for 160 meters
>>using a pair of GI-7bT tubes. The reason for single band is that I
>>have 3 other linear amplifiers that cover 80 through 10 but none of
>>them cover 160 meters.
>>
>>Looking at various circuits (gleaned from the Internet) I notice one
>>common factor and that is none of them, at least that I have found,
>>have any provision for monitoring grid current. Also, unless I am
>>overlooking something, I don't find any specifications for maximum grid
>>current for the GI-7b or GI-7bT.
>>
>>My 160 meter exciter puts out a maximum of about 80 watts which
>>definitely should be plenty to drive a pair of GI-7bT tubes. Anyway,
>>does anyone have any suggestions or information on if I should have any
>>worries about this. I am well aware of grid restrictions on tubes like
>>the 811A, 572B, 3-500Z, etc. But, this is my first venture using the
>>GI-7bT tubes.
>>
>
> This page is a central resource for information about Russian tubes:
> <http://www.nd2x.net/base-1.html>
>
> Half-way down, there are tables of designs for various triodes and
> tetrodes including the GI-7B. YU1AW's schematics suggest the following:
>
> Va = 1800=2000V
> IA0 = 30-50mA*
> IA = 350-400mA*
> Vb = 12-15V
> Pdr = 15-20W*
> Po = 400-450W*
> Igmax = 50-100mA*
>
> *per tube
>
> The suggested figures vary between different schematics, and the grid
> current will be strongly dependent on the drive level, output loading
> and anode voltage.
>
> Don't worry about grid dissipation. The GI-7B, GS-31B and GS-35B are
> 'planar' triodes, originally intended for UHF operation, and their
> construction is much more rugged than the other tubes that you mentioned
> above. The grids are massively built from reinforced mesh and can take a
> lot of abuse:
> <www.ifwtech.co.uk/g3sek/misc/gs35g.jpg>
>
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek
In addition most triodes in GG service will have their own grid current
sweet spot where the grid current peak and Pout maximum coincide. As long as
the Vp and drive isnt excessive the Ig will be within spec. This holds with
fragile tubes such as the 8874/8877/3CX800 to the robust 3CX3000A7/YC-156
variety.
Carl
KM1H
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