>Why mess around with all those tubes? Pick out the power you want and then
>find a single tube that will produce it.
? Good idea, Bob. However, for those who want to use an inexpensive
tube type, the 811 is an ok choice.
>Bob W6TR
>----- Original Message -----
>From: "2" <2@vc.net>
>To: <RFlabnotes@aol.com>; " AMPS" <amps@contesting.com>
>Sent: Monday, April 07, 2003 1:41 PM
>Subject: Re: [Amps] modern 811 designs?
>
>
>
>
>>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.
>end
>
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
>
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
end
|