I have an old sweep tube amplifier (G3SRW 'Loudenboomer') which I forced into
service last weekend following the failure of an MLA2500.
(Yes, I know that (1) I should invest in some newer equipment, (2) I should do
some preventative maintenance before the contest, and (3) there was also a USA
made amplifier called a Loudenboomer which I don't think had any relationship
to the G3SRW version at all.)
The 'SRW is an interesting design with no power transformer, with all the live
circuitry insulated from chassis, and with +/- 340V supply rails. It uses 4 x
triode connected PL519 tubes and puts out about 400W.
After I had been using it for an hour or so there was a lack of action caused
by the blowing of the fuses in the +ve and -ve lines. That kind of destroyed my
competitiveness in the contest....
Turns out the problem was a short between suppressor grid and anode on one of
the tubes.
I don't think I was particularly overcooking it - I was running about 300W out
as I was on CW - and the thing has a fairly hefty fan. Is that a common failure
mode on this type of tube? It's not one that I've come across before, but then
most real rf tubes are tetrodes or triodes.
(The fault on the MLA2500 was entirely predictable and I had known for a long
while that I was on borrowed time. It had the original filter capacitors and
carbon balancing resistors. That will get fixed now!)
Comments, particularly on the PL519 failure, would be welcome.
73 Roger
VE3ZI
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