[Amps] PL519

[Roger Parsons]

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