On 4/12/2010 3:35 AM, Jim Thomson wrote:
> Somebody asked a while back abt the need for parasitic suppressors' on a
> GG- 813 amp. A buddy
> had one blow up on his 2 x 813 amp last yr.. across town. he tossed both
> the blown one.. and also
> the remaining one. Amp runs just fine with NO suppressors. Another fellow
> runs a 4 x 813 amp,
> again with no suppressors. The tubes all balance out too..equal glow. I'd
> try it with it with no suppressors
> 1st.... then if you do have stability problems.. then add them.
>
> If the screen and control grids are well grnded [via strap] to chassis... you
> won't have stability problems.
> I have had 813's with as high as 3 kv on em, no load... yrs ago, with no
> stability problems, but that amp
> did have suppressors in it [2 x 813's in GG] [80-10m]. That was back in
> 1975. It's a pretty stable tube.
I was the one that asked. I have to admit that I haven't made a decision as to
whether to
use GG or grounded cathode. Since I'm planning to run it class C, I will have
to lift the
grids from DC ground to bias them even if it is GG. However I'm planning to
bypass the RF
grounded elements with paralleled 0.001 and 0.01 uf capacitors with leads as
close to zero
length as possible, grounded at a spot common to the pi input capacitor frame
connection.
Also I'm going to use a method of feeding DC to the plates suggested by WC6W in
which part
of the coil is wound from rigid coax or tubing with a hv-insulated wire running
through
it. The outside of the tubing is connected through the usual blocking capacitor
but the
wire goes directly to the plate. At the cold end I'll use a simple pi-wound RF
choke of 1
or 2 mh. This takes the stress off of the choke on the high bands, provides
better choking
on 160, and reduces the stray capacity across the tubes. Whether it will be
better or
worse from the point of view of VHF parasitics remains to be seen.
One of my goals is to have an amplifier for CW which is efficient from 160
through 10
meters (another goal is to spend as little money as possible on it). Getting
rid of the
suppressors will improve 10 meter efficiency. I'm also going to have a small
inductance
before the input capacitor to transform the tube's Rl to a lower value to keep
the Q of
the pi network reasonable on the higher bands. Who says 813's don't work on 10
meters!
For the other end of the frequency range I think the plate feed system
described above
will keep a lossy choke out of the tank circuit on 160. And -- I intend to
avoid the
mistake made by Heath in the SB-1000 and keep the tank coil away from the
cabinet. I
improved output on 160 in the SB-1000 by moving the coil as much as I could
away from the
steel cover. In my HB amp I plan to mount the coils well clear of anything
metal (and
there won't be any ferrous metals around!).
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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