[Amps] modern 811 designs?

[2]

>Dan:
>       Someone suggested that you use 8x811A in parallel. Before starting to 
>punch out the holes for all of those sockets (not to mention the 200W 
>filament transformer) I would suggest you consider the following basic 
>electronic facts that would confront you.
>
>       Assuming anywhere nearly a normal operating regime, the optimum plate 
>load for that bank of tubes operating anywhere near their maximum,  will be 
>in the range of 500ohms. 

€  Close enough.  With 1800v on the anodes, I calculate 590-ohms (‡) of 
RL..  

>At 10 meters, for a properly designed PINET plate 
>circuit, you will end up with a target L value of arounf 300nH, around 500nH 
>for 15M. 
>
>       Next consider the fact that with that physically large bank of tubes, 
>the total anode lead from the distributed anodes to the output capcitor, and 
>through that to return to the GG, is not likely to be less than a foot long. 
>With reasonable size conductors, you will be facing the nominal 25nH per 
>foot of inductance, which when multiplied by your ca. 1 foot of wire, uses up 
>your ENTIRE budget of inductance. 

€  new math ?  

>In other words, most of the L in your tank 
>circuit will be the anode wiring itself. This is necessarily a low-Q way to 
>build an inductor and will result in considerable losses. 

€  I can layout a 3 by 3 square for 8, 811As, including the anode choke 
that would need c. 8-inches of anode lead between the collection point 
and the Tune-C.  8" of Cu strap has c. 80nH of L.  For a Q of 15 at 
28MHz, one needs 145pF of Tune C.  Since the 811A has c. 5.6pF of 
anode-C, the total anode-C for an 8-holer is c. 45pF.  Thus, a variable 
Tune-C of at least 100pFmax  would be needed.  The needed load C would be 
484pF, and the needed L is c. 290nH, however, the 80nH of anode lead 
transforms the 590-ohms to a slightly lower value, so 290nH should be a 
bit too high.

>Hence, your 10 
>(and possibly 15) meter efficiency will stink.
>
€  only for those who ignore skin effect when choosing tank conductors.

>       I have found this basic issue slightly troublesome in the 4 tube 
>designs, which always tend to show decreased efficiency on 10. 

€  The only amplifier that I have seen which used tank conductors based 
on skin effect is the TL-922.  However, even if the tank efficiency was 
so low on 10m that it caused an additional 20% loss due to heat, the guy 
on the Rx end would never  notice the difference.

>Good designs 
>get the 10 meter tank coil as close as possible to the anodes to reduce this 
>effect.

€  True, Eric --  at 28MHz, the Z-transformation process pretty much 
starts at the anode(s).  
>
>       The solution is obvious - you need to raise Rplate, by raising the 
>plate voltage and lowering the peak current. This will lead right to the 572 
>or similar. 

€  The above calculations show that this project is do-able with 
1800V/1.6A.  

>This also reduces the drive requirement considerably by not 
>having to drive the plate current as high (for a given tube).
>
€  a FT-1000D will easily drive 8, 811As.



-  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, 
www.vcnet.com/measures.  
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