[Heathkit] SB-230 retrofit plans

[Kurt Fitzner]

Hi All, 

I was recently gifted with a great SB-230 (including a working 8873!) by
a friend who knew I had always wanted to complete my SB-104 set. And
while it causes me a certain kind of pain to be using a well and truly
irreplaceable tube, I'm not one to bubble wrap my equipment. So, I have
to plan for a future when it finally gives out. 

In planning for the 8873's eventual replacement, I want to keep to the
original design philosophy as much as I can. Seeing as the single most
unique aspect of the 230 is its conduction cooling, I intend to keep
that. The usual tube to replace an 8873 with is the GI-7B(T). The GS-9B
seems a little more appropriate for an amplifier, but the GI-7BT will
operate at a higher temperature. I have both to experiment with. Those
with more experience than I, though, suggest conduction cooling for
these tubes is inadvisable. The experiment most often cited for this is
http://gi7b.com/sb230/cooler/index.html [1]. However the large aluminum
block used in that experiment seems ungainly to me, and a needless
impediment. While it will offer a little bit of a thermal reservoir in
the short term, over the long term it acts simply as added thermal
resistance. I think I've come up with an idea to make conductive cooling
work better. 

My idea is to remove not just the heatsink from the tube, but to modify
the tube itself a little and cut off the heatsink's mounting thread.
Once done I can then mount the tube horizontally along the amplifier's
fore-and-aft axis and have the top of the tube mate directly with the
BeO block on the amp's heatsink. There is room to do this without moving
the coils, though I will have to make clamps rather than using a socket
for the electrical connections. Upon removing the heatsink from the tube
you see that the mating surface area is actually rather small (only
4.4cm²). I was surprised to see that it's not lapped, and is a bit rough
on both sides, which can't do much good for the thermal conductivity
there. If this is the way the normal heatsink is attached, I can't help
thinking that once lapped and with a thin layer of a good modern diamond
thermal conductive paste, that having it connected in this manner to the
SB-230's giant heatsink might be, even without a fan, equivalent to the
original heatsink and a blower. 

I'd love feedback on this, especially if you have a retrofitted SB-230. 
-- 

KURT FITZNER (VA1DER)
YOU DON'T KNOW THE QRO OF THE DARK SIDE! 

Links:
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[1] http://gi7b.com/sb230/cooler/index.html