|[AMRadio] Home-brewing construction considerations|
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
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.
----- 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?
> 73 = Best Regards,
> Our Main Website: http://www.amfone.net
> AMRadio mailing list
> List Rules (must read!): http://w5ami.net/amradiofaq.html
> List Home: http://mailman.qth.net/mailman/listinfo/amradio
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:AMRadio at mailman.qth.net
> To unsubscribe, send an email to amradio-request at mailman.qth.net with
> the word unsubscribe in the message body.
More information about the AMRadio mailing list
This page last updated 22 Oct 2017.