[AMPS] Alpha 76A tube change.

[Colin Lamb]

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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