Good 813s are often hard to find on the market since most used ones have been
abused.
Other possibilities are 4 parallel 4CX250Bs with 50 ohm load resistor on the
untuned input or
a 200 ohm with a 1:2 turns ratio transformer.
I really like the 4-1000A in GG but it is a bit bigger and uses much higher
plate voltage.
Instead of 813's a good possibility and rather cheap are 4-125As or 6155. Same
plate dissipation
and work well in gg as well. On Ebay usually very cheap new because
practically on radios use them
and even better yet the same sockets will allow an easy upgrade to 4-250A,
4-400A or even 3-500Z tubes.
If you use over rated parts you can migrate to higher power very quickly using
these tubes.
Also, some are not aware that 4-250A and 4-400A tubes have the same electrical
characteristics except the
4-400A has higher plate dissipation due to the fins added to the plate.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Diane & Edward Swynar [deswynar@xplornet.ca]
Sent: Monday, March 28, 2011 4:34 PM
To: Roger (sub1)
Cc: amps@contesting.com
Subject: Re: [Amps] Homebrew is back..
Hi Roger,
Nice note---thanks!
Yes, the 813 has cost advantages, alright---but you have to jump through
some hoops in order to make them "play" on 15- and 10-meters efficiently,
because of the high interelectrode output capacitance of them (ex. on
10-meters, a paralleled pair display a total of, I believe, 28-pfd., which
is actually more than the plate tuning capacitor value needed on that
band!).
Makes you wonder how that fellow you mentioned got away with FOUR in
parallel! (maybe his was strictly a low-band design...?).
What I did here with mine is employ a 25-uh. 1/4" edge-wound copper ribbon
variable inductor as a pi tank coil...and I incorporated a rugged ceramic
Centralab "bandswitch" that allows me to REMOVE the plate tuning cap out of
the circuit entirely. I then RESONATE the final tank on 15- and 10-meters by
varying the "L", and using the high interelectorode "C" as a "built-in"
capacitor.
For 10-meters, though, there's another step that needs to be taken: I switch
in a 0.3-uh. silver-plated 1/8" copper coil in series with the paralleled
plates, to act as an RF transformer of sorts. It's all detailed in my 1985
ARRL HANDBOOK, which references the issue of QST that the idea came out
of...
It's a bit of an exercise, to be sure, but the work-out entails one's GRAY
matter, and NOT one's pocketbook! Hi Hi. I think it's worth it.
~73~ de Eddy VE3CUI - VE3XZ
*******************************************
----- Original Message -----
From: "Roger (sub1)" <sub1@rogerhalstead.com>
To: "Diane & Edward Swynar" <deswynar@xplornet.ca>
Cc: <amps@contesting.com>
Sent: Monday, March 28, 2011 2:04 PM
Subject: Re: [Amps] Homebrew is back..
> On 3/28/2011 3:38 PM, Diane & Edward Swynar wrote:
> > Hi Roger et al,
> >
> > I use a pair of 813s in parallel grounded-grid service here in my
homebrewed
> > KW amplifier, and---I'm blushing as I type this!---but I have NEVER EVER
> > purchased any of the jugs that are in it, or the spares that I have
here,
> > brand new...
> >
>
> I forgot the old 813. I have 3 of them up on the shelf and keep looking
> for more.
> I used to run the "old legal limit" with a pair of them as you
> describe. I ran into a guy on 40 a while back who claimed he could get
> 2400 PEP out of 4 of them. I don't know if his amp was a monoband or
> covered all of the HF bands.
>
> I see the GS-31 (1KW), GI-46 (1KW), and little GI-7B are at reasonable
> prices and seem popular. With the removable anode coolers on this
> series and the popularity of converting the old Dentron MLA2500B to a
> pair of GI-7Bs that no one has machined a transverse version of the
> cooler to make them even more compatible with the old 8875s I think they
> used.
>
> > They are all either Hamfest finds, or freebies from friends.
> >
> > Life's far too short to pay-off a retiree's ransom for new& exotic
> > tubes---
>
> AMEN!
>
> > and I have yet to hear anyone at the DX end of my QSOs asking me (or
> > expressing any care whatsoever) which tube(s) I might be using! Hi Hi
>
> There does seem to be a lot of interest in what tube(s) are in homebrew
> amps though.
>
> 73
>
> Roger (K8RI)
>
>
> > ~73~ de Eddy VE3CUI - VE3XZ
> >
> >
> > **********************************************
> >
> > ----- Original Message -----
> > From: "Roger (sub1)"<sub1@rogerhalstead.com>
> > To:<amps@contesting.com>
> > Sent: Monday, March 28, 2011 1:11 PM
> > Subject: Re: [Amps] Homebrew is back..
> >
>
>
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