Hi Ed:
You are on the right track to change the 3CX800A7 tubes in for the 8874
tubes. I did the same thing. You will need a new pair of chimneys. Two of
the 3CX800A7 tubes act like 3 of the 8874 tubes, so it is an easy trade.
Perhaps the biggest challenge is finding a place for the new filament
transformer to fit - since the old one was on the main transformer. I used
the existing 6 volts and used 2 small stacked 8 volt transformers with a
series resistor to get the required voltage.
I think that is about all you need to do. I added a pep wattmeter reading
to one of my amps and a diode matrix between the meters (no holes) to
monitor peak grid current. It was a lot of work and probably not worth the
effort. The amp just runs and runs.
I did not change out the parasitic resistor in mine.
Grid current is a bit more important with 2 3CX800A7 tubes than with 3 8874
tubes. Watch it and increase the loading to reduce the current.
I increased the timing on the warm-up delay using standard components. It
ended up 3 1/2 minutes. It works fine, but the last 30 seconds seem like an
hour. Sometimes I get ready for a nap while the filaments warm up. So, as
a suggestion, check the timing at 3 minutes rather than 3.5 minutes. Better
to work the dx than be overly conservative.
I usually have my amp off and am now planning on putting an instant on amp
on the other station just in case I want to work something without a wait.
May your amp help you work everything you think you can hear.
73, Colin K7FM
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