[AMRadio] Home-brewing construction considerations


jeremy-ca km1h at jeremy.mv.com
Thu Jan 17 13:49:48 EST 2008


Have you considered rebuilding for parallel operation and use a relay 
switched toroid input and a pi-net out? There are 6 band circuit boards 
available.

It makes band changing a lot easier when the deck is shielded. Im converting 
a similar era deck that way. A further benefit is a coax fed antenna to 
further minimize stray RF radiation.

There should be no problem getting to 15M with a standard air variable based 
upon the tube output C, the cap minimum and strays. I'll be using a vacuum 
variable and coverage will be 160-10M.

With the tubes enclosed you will also need some fan assisted air exhausting.

Have fun, 250TH decks seem to be everywhere lately.

Carl
KM1H


----- Original Message ----- 
From: "geoff" <ars.w5omr at gmail.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Thursday, January 17, 2008 12:49 PM
Subject: [AMRadio] Home-brewing construction considerations


> In a balanced push-pull Class C RF Final with plug-in coils, I've seen
> things done a couple of different ways.
>
> One is, to split the B+ so that the potential is fed with the B&W HDVL
> jack-bar to the inside of the two coil's.  The outside of the coils is
> then connected to the tubes.  That's the way the home-brewed 250TH final
> I'm running was done, back in the mid '50's.  There was also a couple of
> finger-stock pieces across the B&W butterfly tuning condenser to
> facilitate adding a vaccuum capacitor across it for allowing the circuit
> to resonate on a lower frequency (like 75, or 160m).
>
> I understand that the circuit would no longer be 'balanced' were that
> condition to occur.  It would go back to being balanced if a pair of
> capacitors at half the required value were used, and where the two meet
> in the middle were tied to the stator of the Butterfly.  THAT would keep
> it balanced.  That's the reason for the split-stator capacitor across
> the final, now.  But, that's not my question...
>
> I recently picked up another home-brew 250TH final that has had more RFI
> considerations involved in it's construction (than the one in current
> operation) as it was built sometime in the '60's.  Screen and mesh
> aluminum, etc... It's nice and hopefully will keep me out of the
> neighbors TV's, microwaves, blenders, etc... (grin) But the plug in coil
> is what prompted me to this question:
>
> Is there a difference in feeding each of the inside sockets on the
> jack-bar with B+ from the top of the RF plate choke, vs having the two
> coils tied together, in the middle, and feeding B+ there?
>
> I understand that the potential is the same, but for the sake of having
> less stray capacitance and a 'cleaner' RF environment, which would be
> better, and please... tell me why?
>
> Thanks!
>
> --
> 73 = Best Regards,
> -Geoff/W5OMR
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