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

[Paul Baldock]

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