Re: In need of a vacuum tube

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Shad <shenderson@xxxxxxxxxxxxxxxx>

Hi Matt, all,

The 572B is indeed rated for more plate dissipation than the 811A.  It
has different bias settings and self-oscillates at a different frequency
than the 811A, but that only really matters in RF amplifier
situations.

As for the center cap coming off, if the seal of the tube hasn't been
broken, you should be able to re-solder the cap on carefully and secure
it with epoxy suitable for use on glass.  Keep in mind that most 811A's
are usually fairly old tubes, and the epoxy used to hold the top cap on
can degrade with heat.

Be sure to bypass the non-grounded filament lead to ground with a few
pF, 1kv cap.  The beefy blue disc ceramics should do just fine.  Also,
remember to use a choke on the plate and grid leads.  10-15 turns of
magnet wire around the body of a non-inductive resistor will work just
fine, and locate them right at the plate and grid connections.  These
help curb the tube's tendency to oscillate.

The 811A should barely show *ANY* perceptible plate color under full
load.  I've found that when they get even dull red, they tend to go
gassy rather quickly.

Because the 811A is a zero bias tube with a very high Mu, it won't
require as much grid drive as an 833A or other large, low mu tube.
Won't need as many grid turns, and they can be physically further away
from the primary.

Because old tubes can be slightly gassy right off the shelf, run it
gently for an hour or two before throwing the coals to it.  Use a glitch
resistor in series with the B+ lead.  Something that'll limit the
current to a handfull of amps at full B+.  This will help protect the
tube in case it arcs over internally, and a fast-acting fuse on the
plate xformer primary will take over.

You'll love tube coils.  Less crash and bang, more hiss and hum.  But
watch out for RF burns. They're easy to get with a VTTC.

Shad H

On Sun, 2006-03-19 at 10:17 -0700, Tesla list wrote:
> Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
>
> Hi,
>
> My coil uses two 811a tubes.  It came with two "cheap" China
> types,  the top caps fell off and one fell out of the socket (new in
> the box...).  After re-gluing them one just arced over immediately...
>
> I replaced them with Svetlana from RFparts.com and the new ones have
> worked perfectly.
>
> The 811a is actually obsolete and may no longer be made.  The 572B is
> pretty much a drop in replacement for it though and can run higher power.
>
> http://www.rfparts.com/tubesvet.html
>
> So "I" wound get Svetlana so they work and get the 572B which is an
> improved version.
>
> http://www.rfparts.com/572bprices.html
>
> Cheers,
>
>          Terry
>
>
>
>
> At 12:31 PM 3/18/2006, you wrote:
> >Hello all,
> >originally i had planned to use an 811A tube for my first ever VTTC.
> >I got a cetron on ebay for 17$ and when i attempted to assemble it
> >into the circuit, the metal cap on top just popped right off :-( I
> >contacted the seller and he agreed to send another. But apparently
> >he never sent it as its been weeks now and my apartment complex says
> >still nothing from him. I guess it could have been lost in the
> >mail(although it was shipped priority mail). So now im in need of a tube.
> >
> >I was wondering if anyone here has a used one they would be willing
> >to sell?Id have to st ick with an 811A tube or similar because my
> >filament xfmr is only  rated for 4amps at 12.6volts(I use the center
> >tap for 6.3).
> >
> >Thanks again!
> >Matt
>
>