On Fri, 6 Oct 2000 16:49:54 -0700 "Lane Zeitler" <Ku7i@worldnet.att.net>
writes:
>
>Gents,
>The new amp project is four 4-400s in parallel. I would like to talk
with
>current or past builders of 4-400 rf decks for any advice. I pretty much
>have all of the data I need btwn the West Coast Handbook, AG6K's site,
old
>magazine articles, and previous amp projects. The biggest hurdle is what
to
>use as a regulated screen voltage source. Many options here. The deck
will
>cover 160 to 10 unless the combined Cout of all of the tubes in parallel
>makes it too hard to get to load on 10/12, etc.
>
>Lane
>Ku7i
>San Diego
>
Hi Lane,
The tubes Cout will present no problem since the impedance of the
combined tubes will be ¼ of an individual tube -- probably somewhere
around 1200 ohms which means you'll need almost 1000pF to tune 1.8MHz.
You will need a low minimum C for 28 MHz which suggests a vacuum cap
unless you plan on switching in a padder in addition to a smaller
variable unit.
No old 1000 VCT transformers laying around?
Here's a modern idea. Get a surplus 50W switching power supply and
wind a new output transformer for it. Update the feedback network and
viola, a regulated screen source Heck, in the old days that would have
been worthy of being published as a magazine article!
I've done the trick myself. I prefer the older low frequency (20
KHz) supplies and winding the new transformer using a "tape" toroid core.
4-400's stay pretty much in positive screen grid current but, you'll
still need a bleed, either passive or active, just in case.
While you are out looking for switchers... pick up a 5V 75A unit and
run the filaments off it; regulated! The output transformers in those
are usually in center tap configuration so, internally it is about an
11-12 volt CT transformer. If you tap off on the AC side of the
rectifiers and run the tubes in series parallel you'll be able to better
utilize the transformer and reduce the required wire size for the
filament connection. A small bleed on the existing 5V output (may
already be internal to the supply) will be necessary to keep the
regulator happy. Then, just adjust the supply for whatever voltage you
want on the tubes -- there should be sufficient range in the existing
Vout adjust trimpot.
You could even add another little transformer, or perhaps voltage
multiply a 15V winding, to generate grid bias. Or just delete this
message and go find an old piece of iron. :-)
I'll stop here before I venture into a switching plate supply. I've
never seen any suitable really big switchers surplus and doing a supply
like that from scratch is lot more work than the rest of the amp!
Incidently, if your cabinet shielding is good enough to keep all the RF
strays inside the amplifier it will likewise stop any harmonic garbage
from the switcher(s) from escaping too!
73,
Marv WC6W
*
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