[Amps] Good amp to buy

[Rob Atkinson]

Re the 8877 with Ep of 4 KV -- you might want to look at the 3rd order
IMD specs. for that tube at increased voltage.  I am unfamiliar with
the 8877 operating conditions but I think IMD gets worse with a triode
when Ep increases.

Centurion:  Yes getting to and replacing the fan is somewhat involved
but not that awful once you figure out the procedure.  You have to
remove the tubes, then remove the bottom panel that is covering the
tube sockets and remove the socket platform h.v. choke assembly and
tilt it up out of the way.  one or both of the filament supply leads
has to be removed and the silver strap between the top of the choke
and the tune cap has to be disconnected and moved out of the way.

For my  purposes (using the Centurion to run 250 to 300 w. AM) the AC
fans were not powerful enough.  I went with a 12 v. 2 or 3 A DC drop
in rated at 180 CFM powered by an external supply.  I also cut out the
vent holes in the top cover and added expansion steel so there is no
restriction on the exhaust of air from the RF deck.  I realize this is
all unorthodox but it will now sit there and put out 300 w. modulated
AM for 15 to 20 minutes with no problem, with about 20 w. drive.

It is important to point out for those considering the 120 CFM AC fan
that the cathode choke sits under the socket platform directly across
the entrance in front of the fan, blocking the lower arc of the fan.
You will see this when you remove the socket / plate choke assembly.
If you put in a 120 CFM fan to run 100 % duty cycle for 10 or more
minutes you might not get enough air to the pins because the choke is
in the way.  I do not know this for sure -- just speculation.  I
considered moving the choke out of the way but then you get into
v.drop issue from the choke to the cathode when you lengthen the wire
leads betweeen the choke and the pins.

I believe the stock cooling and CCS current spec for the plate
transfomer are why Ten Tec derates the power to 650 watts for 100 %
duty cycle.  It doesn't have to be that way.

Note:  FWIW mine has the Dahl replacement B+ transformer.

73

Rob
K5UJ