David
I was designing a VHF amplifier for broadcast FM,
up to 108 MHz, using the then new 4CX3500A from
Eimac. The SK300A was about as worthless as you
know what, for low inductance. We needed a nice
low inductance base, with very significant VHF
bypassing of both filament connections, as it was
to be a grid driven/grounded cathode amplifier
with high gain and efficiency. The folks at Eimac
had developed an improved socket to be called the
SK350, which was in their own VHF amplifiers for
that tube (CV2225 and CV2226). It had integral
Pyralux* film capacitors to the base plate, which
helped immensely for filament grounding at RF.
But it also had inadequate cooling spaces in the
four corners for 2-3 kW of plate dissipation with
the tube (or with the 4CX5000A in the same). So i
took a punch and cut holes, to get adequate
annular airflow around the base and up through
the anode cooler, and made careful RF
measurements before and after. Hot spots around
the ceramic seals were reduced. The engineers at
Eimac were happy with the mod, and made the same
for all SK300 and 360 sockets.
The SK300 is more of a solid cup with slots
around the base while the 300A is mostly open
space. It is made for LF and Medium wave
circuits, but not recommended higher. Even the
300 has a lot of openness which shows up as
increased inductance at VHF. The SK360 is for
4CX5000A and larger (except for the 4CX7500A),
and when you get to 8171/4CX10,000D the SK-1307
is the socket.
I have drawings of the SK350/360 base I believe
if you want, I can scan or copy them and mail to
you, better yet, email to you.
73
John
K5PRO
>I have used both and I can not see why the SK300 should have less
inductance than a SK-300A.
Firstly I must say that Iíve never actually had a SK300 in my hands ñ itís
only my assumptions based on visually comparing the Svetlana SK300As (which
I use) with the datasheet photos of the Eimac SK300.
(This is really where photos are required to accompany this text ñ what I am
trying to describe would then become self evident).
Looking at the filament connection side, the SK300 has 8 radial supports
whereas the SK300A only has four. Also the SK300 skirt goes the full
vertical length of the base, whereas on the SK300A this is reduced to zero
except where the four radials are located, all of which are no doubt to
improve airflow. Now my background is really VHF/UHF where all attempts are
made to try and provide the shortest connections to ground to minimise
inductance. To my way of thinking, the radial supports, which on the SK300A
are about 2-1/2 inches long from the chassis to the outer filament ring, are
less than ideal in assuring a good RF ground.
What I would like is a good quality photo of a SK360 base ñ I have a scan of
the Eimac datasheet but the detail in the photo isnít sufficient to work out
how itís better at VHF than the SK300/300A.
Regards
David G4FTC
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