[Amps] GS-35B Efficiency Varies with frequency?

[Paul Baldock]

Problem is solved:

Originally I had about 8" of RG-58 between the PI input and the 
Cathode. I thought this would be short enough.

After reading info from W8JI's web site I now have 3 pieces of RG-58 
in parallel from the PI input to the Cathode. I also moved the 10M 
cap and put it right on the Cathode, and compensated by reducing the 
cap for all other bands.

Efficiency is now better than 60% on all bands and about1300W PEP 
output on all bands with 70W (of dits) drive and around 3.1KV on the plate

For those that asked, the reason the plate Q is high on the WARC 
bands is that I am using the next higher standard band for double 
duty so I have less inductance than would be ideal. There is not 
enough position on my switch to do it correctly

Thanks for all the suggestions.

- Paul KW7Y




At 08:02 AM 12/13/2012, Carl wrote:
>That is characteristic of a tired tube and the poor PS regulation doesnt help.
>
>Carl
>KM1H
>
>
>----- Original Message ----- From: "Paul Baldock" <pbaldock at frontier.com>
>To: <amps at contesting.com>
>Sent: Monday, December 10, 2012 7:50 PM
>Subject: [Amps] GS-35B Efficiency Varies with frequency?
>
>
>>I am building a GS-35B amplifier. The efficiency drops off a lot 
>>between 80M and 10M. Although the gain is quite constant. Does any 
>>body have suggestions as to why this is or whether this is typical.
>>
>>The input PI network has a Q of around 2. The PI output has a Q 
>>between 10 and 20
>>
>>Here's some results with 70W Drive:
>>
>>3.80MHz. Q=10, Output Power 1.1KW, Vp 3000V, Ip .54A, Ig .26A, Eff 68%
>>7.10MHz. Q=13, Output Power 1.1KW, Vp 3000V, Ip .60A, Ig .28A, Eff 61%
>>10.1MHz, Q=20, Output Power 1.0KW, Vp 2900V, Ip .68A, Ig .30A, Eff 51%
>>14.2MHz, Q=13, Output Power 1.1KW, Vp 2900V, Ip .70A, Ig .29A, Eff 54%
>>18.1MHz, Q=16, Output Power 1.0KW, Vp 2850V, Ip .78A, Ig .31A, Eff 45%
>>21.2MHz, Q=13, Output Power 1.0KW, Vp 2800V, Ip .82A, Ig .37A, Eff 43%
>>24.9MHz, Q=19, Output Power 1.0KW, Vp 2850V, Ip .79A, Ig .35A, Eff 44%
>>28.5MHz, Q=16, Output Power 1.1KW, Vp 2750V, Ip .85A, Ig .38A, Eff 47%
>>
>>You will note as efficiency goes down the Grid current goes up (in 
>>general) for the same drive. Is this normal?
>>
>>Things I have tried:
>>
>>1. Checking that the bias voltage is not affected by RF. No signal 
>>Vp=3600V, Ip=140mA, Vb=34.5V.
>>2. Rebuilding 10M PI Output in a different configuration. (It is 
>>actually an L-PI)
>>3. Checking that the metering is not affected by RF
>>
>>Any Suggestions?
>>
>>What is a safe Max Ig for the GS-35B?
>>
>>Thanks
>>
>>- Paul KW7Y
>>
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