I don´t see anything special with the Pi-net circuit.
It is common practise to use fixed anode capacitors (including internal C)
when using a roller inductor, in that case to have a separate 10m coil in
front of the roller is another advantage.
Having all Pi components variable might lead to circuit Q far away from
optimum or bad adjustment.
To avoid the problems on the high bands, especially on 10m, I should like to
suggest to use a small compensation coil between the tubes and the anode
capacitor, keep all leads as short as possible and keep stray capacitances
small.
Concerning the RF choke I think to switch just two of them will be good
enough; at least in the West where vacuum switches are expensive.
73
Peter
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Thomas Hoeppe
Sent: Sonntag, 11. Januar 2009 19:05
To: Alex Malyava; DAVE WHITE
Cc: amps@contesting.com
Subject: Re: [Amps] GU81-M on highbands
Thanks for all infos! The EX8A design is going an interesting way. If I
understand it right the main trick is using L matching for the higher bands
(C plate is already "in the tubes"), and the tuned coil instead of the
missing C plate. But I don't see the need of the effort with all the taps in
the HV-side. Is it only to prevent resonances in the anode choke, or are
there any effects on the high band behaviour I do not understand?
Thank you for all comments!
Tom, DJ5RE
----- Original Message -----
From: "Alex Malyava" <alex.k2bb@gmail.com>
To: "DAVE WHITE" <mausoptik@btinternet.com>
Cc: <amps@contesting.com>; "Thomas Hoeppe" <thomas.hoeppe@asamnet.de>
Sent: Sunday, January 11, 2009 6:51 PM
Subject: Re: [Amps] GU81-M on highbands
> there is an EX8A schematic - pair of GU81M with double pi-networks -
> one of them instead of plate choke.
> that amp works up to 10m no problem and gives 3kW out.
>
> http://www.cqham.ru/HiFi_Contester_EX8A.htm
>
>
> On Sun, Jan 11, 2009 at 7:03 AM, DAVE WHITE <mausoptik@btinternet.com>
> wrote:
>> Hi Tom - Somewhere I have a circuit from a Hungarian amateur that uses a
>> single GU81M on 80-15m. I'll try to find it (a major operation in my
>> library, and not one guaranteed of success!!) and scan it for you if you
>> need it.
>>
>> cheers
>>
>> Dave G0OIL
>>
>> --- On Sun, 11/1/09, Thomas Hoeppe <thomas.hoeppe@asamnet.de> wrote:
>> From: Thomas Hoeppe <thomas.hoeppe@asamnet.de>
>> Subject: [Amps] GU81-M on highbands
>> To: amps@contesting.com
>> Date: Sunday, 11 January, 2009, 6:15 AM
>>
>> Has anybody used a single or a pair of GU81-M on highbands? Of course I
>> see the
>> problems with the high output capacitance, but there can be solutions
>> like
>> L-tuning or a small L before between tube an plate-capacitor.
>>
>> Has anybody done that in practice?
>>
>> Tom, DJ5RE
>>
>> www.hoeppe.name
>>
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