Good point, Bill, and if the added L lowers the anode circuit''s
natural VHF resonance to a frequency below grid-resonance, where the
grounded-grid begins acting like a shield, VHF stability should
improve. cheers
On Oct 2, 2005, at 10:07 AM, Bill Coleman N2BC wrote:
> Instead of lowering Cmin at Ctune, you can insert a small L between
> the tube
> and the blocking cap. This has the effect of raising the plate load
> resistance and increasing the minimum Ctune required at higher
> frequencies.
> The effect reduces as frequency goes down of course.
>
> I did this on a plasma heater I converted into a ham amp. It uses a
> 5CX1500
> which has a pretty big Cout. Adding a small coil (.3uH or .5uH, I
> don't
> recall) in series with the anode did the trick. You will have to
> calculate
> the effective plate load resistance for each band to determine the
> Ctune L
> Cload values.
>
> Also did this with a 2X4-400 rig to accommodate a vac variable I had
> on hand
> for Ctune.
>
> I used Ian White's (GW3SEK) "Netcalc" program to calculate the plate
> load
> with various values series L, then plug the effective plate load into
> your
> favorite PI net calculator. NETCALC here:
> http://www.ifwtech.co.uk/g3sek/netcalc/netcalc.htm Thanks Ian!
>
> 73, Bill N2BC
>
> ----- Original Message -----
> From: "Scott Townley" <nx7u@arrl.net>
> To: <amps@contesting.com>
> Sent: Sunday, October 02, 2005 12:02 PM
> Subject: [Amps] air variable min C?
>
>
>> Regarding air-variable capacitors:
>> Most folks refer to air-variable caps by their max C value, and
>> sometimes
>> include the plate gap and/or rated working voltage (particularly when
>> selling them!)
>> But much less frequently mentioned is the minimum C value, which of
>> course
>> is quite important in Ctune applications.
>> It's surprising (to me, the uninitiated) how few "vintage" caps
>> (Johnson,
>> Millen, etc.) are suitable for say a single 3-500z amp without
>> compromising
>> the loaded Q at 28MHz. Or perhaps I am missing something. (I am
>> purposely
>> excluding vacuum variables here). To me "suitable..." means around
>> 10-130pF at > 4kV.
>> So a few questions:
>> 1. What are the primary driving factors behind achieving small minC
>> in an
>> air-variable? Say I would homebrew one...what are the
>> mechanical/layout
>> considerations in the capacitor itself that tend towards small (c.
>> 10pF)
>> min C?
>> 2. Are there other sources/manufacturers beyond those I've previously
>> named? I have several old Johnson catalogs and it seems that only 4
>> of
>> the
>> many, many caps they make/made even come close.
>> 3. Or is padding for the low bands more common than I think? There
>> are a
>> few more Johnson caps that would work if you padded both 40m and 80m.
>> Thanks,
>>
>> Scott Townley NX7U
>> Gilbert, AZ DM43di
>> http://members.cox.net/nx7u
>>
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|