[Amps] Eimac SK350/360 VHF sockets [TSPA]

[John T. M. Lyles]

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