This is part of a msg I sent last week regarding one possible tube swap.
--------- Begin forwarded message ----------
SNIPS
>
>The MLA-2500 is a solid general purpose amplifier but not in the
>"contest" class, particularly due to the high cost of replacement
>tubes. Treat it right and keep it to 1100 watts or so of output
>and it will be a reliable performer. Unfortunately, I haven't found
>a suitable replacement for the 8875.
IF the Svetlana 8874 becomes available with removable radiators that
would be one answer. That also assumes a realistic price structure.
Converting over to 8874's is fairly simple if you have a sabre saw.
Remove the sockets, etc and cut out the chassis for a plate made from
double sided PC board that will accept the sockets and the required air
flow cutouts. Screw that in place an build the below deck box structure
from the same material. Run the filament, etc, leads in thru 1000pf
feedthrus.
With care in construction the box will just make contact with the lower
cover. Cut out a hole and place the existing fan on the bottom cover and
seal off the perforation holes in that area. Cabinet feet will have to be
extended to allow at least 1" clearance below the fan. Do not forget the
fan safety cover to keep things from being sucked up. Also the tube fins
will require periodic vacuuming.
Surplus 4CX250 chimneys fit perfect and the exhaust can be the silicone
rubber material mentioned here many times.
It is not a pretty modification but if one has access to 8874 pulls, etc
it is cost effective. Air flow in the two amps that I modified appears to
be more than sufficient.
73 Carl KM1H
Added comments:
I suppose you could also just hack away air holes into the original steel
and solder the PC board box to the steel also. I did it the above way to
make it possible to change tube types at some later date.
Several have modified with a single 3CX800A7 but that requires another
filament xfmr, a swamping resistor, along with the air flow issue.
Svetlana tetrodes would entail even more work.
------ End forwarded message ----------
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