Harold B. Mandel wrote:
>Not letting a tube amplifier go through a minimum
>warm-up cycle is like getting in your car on a brisk
>winter morning and instantly driving off on the
>interstate.
>
>I bet the guys at Richardson's and RF Parts just love
>shipping those "Instant-On" tubes.
>
>Hal
>W4HBM
>
I built a twin 4CX250B amp for 2m. As a test one day I decided to apply
RF power to the the grids before the heaters were on. Then I switched
the amp on (i.e. HT, followed by grid, heaters and screen). Nothing
happened for about 15 seconds, then there was an almighty bang as the HT
supply blew the fuse. (There was no glitch resistor, and the HT fuse was
a 20mm type, so not exactly designed for HV use. But I was young then -
about 16 or 17 when I built it, so don't knock me too hard.) The screen
supply was current limited, using the G4FRX (now GW4FRX) design in Short
Wave Magazine.
After replacing the HT fuse the amp worked fine, and still had the
original pair of tubes when I sold it. It produced about 600W with no
grid current drawn, on 2kV anode, and I pushed it to 1kW once or twice
as a test in class C.
Of course I knew you were supposed to leave it 30s (if my memory from
the data sheet is correct), but this was the 'mad scientist' in me.
Does anyone know what *specifically* would have caused the fuse to blow?
Obviously I can understand (now anyway) it would not do the cathodes a
lot of good, but why did the HT fuse blow? Remember this was a twin tube
amp, so you might guess one tube of the amp starting to draw anode
current before the other, but I can't see why that changes the
situation. Perhaps the amp went unstable?
I can't recall the exact design, but it was quite a common one, with a
U-shaped anode line so the tubes were in push-pull. Two pieces of tube
about 6-8mm OD for the grid lines. And a cross-over pair of 'caps' for
neutrilisation. In the end I actually removed these cross-over wires, as
I found I could only get more RF feedthough with them when measured on a
diode detector with cold tubes, not less. It was stable without them -
even after following the advice of G4FRX in removing the bits of metal
from the cross over wires, and just leaving the wires, I could only get
more RF feedthough with them, not less.
Any explanation for why the HT fuse blew when I essentially tried using
4CX250B's as instant warmup?
--
Dr. David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|