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It's an excellent transplant candidate.....

Posted by Jim Doyle (A) on November 17, 2011 at 13:07:44

In Reply to: 600V B+... Modern 6550s might not like that..... posted by Jim Doyle on November 16, 2011 at 14:07:39:

I'm assuming you have two chassis...

What you've shown in pictures indicates they are in great cosmetic
condition. Now that you understand that the 600V B+ situation is a problem
here's what you can do about it:

1. Find another beam power tube to put in there that can take 600V and
work with the transformer... Remember when people were forced to do this
with 8417s a decade ago? Well - now the same thing for the 6550. You could go with 807s which are obtainable as NOS from WW-2 era. They do
the 600V well. You'd have to remove the 6550 sockets, insert the 7-pin
sockets needed for the 807. Above all - drill two holes for wires to the
plate caps... Oh did I mention the plate caps? Yeah.. No kids or pets
in the house if you go this route. The plate caps put 600V within fingers reach. Lethal. More than likely it will throw you across the
room and you'll piss your pants than it will kill you - unpleasant. This
modification is only for single or divorced men.

2. 450V downgrade. FWIW, the best thing to do in his case is to actually gut all the wiring, preserving the chassis, sockets, output
transformer. You'd go get an Edcor or Hammond Plate Transformer and
setup a power supply for 450V. You'd only need one rectifier tube socket
and could leave the other empty... Next, you'd rewire the entire amp to
use any number of acceptable 6550/KT88 topologies. You'd have plenty of
tube sockets to work with to do that. Ontop of that, the sockets for the
gas regulator tubes would be very handy - either to continue to use a regulated screen supply, or anything else. What you'd end up with is a very nice amp much like a tricked-out Dynaco Mk III or a Quicksilver. No new holes in the chassis except for the new 450V supply transformer... Even then, you might avoid drilling holes for the new power transformer if you built an adaptor plate to mate the new transformer footprint to the old chassis footprint.

Such a project would be cheap - since you have a nice chassis, and sockets. You'd end up with flexible amps that would last a long time, and,
could take anything from KT88 to EL34s from Russia or China. Because of
the large number of sockets available (2 octals if you decide against
screen regulators for ultralinear, 1 additional octal for the excess rectifier), and need be - the 9-pinner for the 12AU7 - you have alot of latitude for this amp that a Dynaco MkIII modifier would not have.

It involves giving up the 90W and taking 60W as the upper bound. Only a
thin freakish minority of people have whacko speakers that can't be
adequately fired from 60W of 6550/kt88.


-- Jim

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