[Amps] Odd efficiency shift on 80m band

[Jeff Blaine]

Hi Larry,

And thanks for your comments.

The load cap is an 850 pf unit and I would agree with your 500 pf 
assessment, the control is about 2:00 on the 4.0M and around 10:00 on 
the 3.5 so that matches up fine.

Regarding the drive, I am not sure about what "signs of running hot" 
means exactly.  What I have noticed, with this particular setup, is that 
indicated grid currents of about 180ma show a stable grid current and 
power output level.   However, once you start pushing above this point, 
the grid current will push up, and then sort of drift back somewhat.  So 
I have set 180ma as the max from the drive parameter.  The official spec 
is 7W but it seems that without a set of operating curves that cover our 
application domain, none of the usual tools to estimate grid dissipation 
levels are of much help.

The actual mechanical or electrical reason for this has not been 
established clearly from what little data I have seen.  Lot of 
speculation out in the cloud, of course.  There clearly seems to be 
something happening at that point because a drop back in the Ig is not a 
normal behavior to my thinking.  A lot of guys with GI7B have seen the 
similar thing.  Now on the other hand, the late GI7B master Tony W4ZT 
told me that the max grid drive was 250ma per tube!  May have had that 
mixed up.  We never had the chance to clarify it unfortunately. 

The 180ma drive point comes on this band at about the 71w level +/-.  
There's nothing magic about that drive level or the B+ level.  The 
efficiency slope remains in tact regardless of the drive or B+ levels.

And don't worry about the B+.  The tubes in good shape will hi-pot to 
5K+.  Not suprising because they are pulse rated and in that mode, they 
put something like 9KV on the anodes. 

This 80m thing - sure has the feel of a resonance of some kind.  But 
looking for that has me wanting to grab a bottle of Jack.  :)

73/jeff/ac0c

Larry wrote:
> Jeff Blaine wrote:
>>
>> 4.00 mhz   73w drive   Po=985w      Ip=0.62A     B+=2420v    
>> Pin=1500w   60.8%
>> 3.75 mhz   71w drive   Po=985w      Ip=0.645A   B+=2400v   
>> Pin=1548w   59.0%
>> 3.50 mhz   73w drive   Po=1000w   Ip=0.710A    B+=2375v   Pin=1686w   
>> 55.0%
>>
>
> Looks to me like you are running too much loading for the amount of 
> drive. Do you have enough range on the load cap? I ran a few numbers 
> and it looks like you would need at least 500 pF additional load "C" 
> on 3.5 vs. 4.0.
>
> My 432 amp runs about 35 watts drive to a pair of GI-7B's, get 600+ 
> watts out with 2050 volts on the anodes. (My stripline half wave 
> resonator runs a measured Q of several hundred and has about 15% loss) 
> At 40 watts drive for the pair, the grids show definite signs of 
> running hot. Unless you have a very lossy input matching network, 73 
> watts seems like far and away too much power to run into those tiny 
> grids even on HF.
>
> With 2400 on the anodes (close to the point at which these tubes like 
> to flash over inside) the gain on HF should be at least 13 dB 
> including circuit losses which would mean 50 watts total drive 
> required for 1000 out.
>
> 73, Larry
>
> Larry - W7IUV
> DN07dg - central WA
> http://w7iuv.com
>