!!
Ok: first: what's the dilectric strength per mil for
a vacuum? And at how good of a vacuum?.
None of my books say.
That ought to give Duncan some idea what the
absolute limits are!
Maybe if I'm lucky Scott Frankland will weigh in
as well, re theory,and what really happens to them
'trons in a tube, he's way better than I.
postmaste--(at)--uffy.demon.co.uk (Duncan Munro) wrote:
>On tubes, what is the failure mode with over-voltage???
AFAIK, they arc over internally.
Arcing around the tube sockets might happen sooner, depending
on the situation...
I once had someone show me a cracked, blacktracked
ceramic socket out of a Marshall (a hairy one, not yer typical
100W lead head). Apparently (remember, I hadda wing this a bit)
the player liked to overdrive his amp, and the harmonics reached
out to way out past where the speakers unloaded, and a big AC
voltage was showing up on the primary of the OPT.
Enough to whack the socket.
And it blew up tubes, too. Of course that was under overdrive
conditions, I'd certainly like to know more as to what
exactly what happens to a guitar amp power tube when it is
clipped with a distorted signal at full power.
What kind of peak voltages show up at the plate?
I'm on the learning curve as far as this geetah stuff goes..
Heck, I learn something new every day, for that matter.
>I'm no tube
>expert, but I'm still waiting to see a tube croak from voltage
>overdose.
The way I get it, these main factors affect voltage ratings:
1. Plate dissipation & current. As I noted, people have used
6V6-GT in Marshalls, but the idle current had to have
been very low, the typical Marshall has a g1 voltage
of about -50 volts! (note the model used, I do not know)
2.Vacuum. Take a 4-65A. It's about the same size as a 6550,
but it has no "getter", like a 6550A. But they run 3KV
at 115 ma as a Class C RF amp. I was told they were pumped
for like 24 hours for a much better vacuum than what one would see
in a normal recieving tube.
This makes sense. Look in a power tube like a 5881, you'll see a
blue glow which to me looks very similar to the glow I see
with 2050 thyratrons, ie: the less inert gas there is left,
the less chance of an arcover.
3.Quality control. Compare a 4-65A to a 4PR65A.
They look the same, but 4PR65A is pulse rated
(from memory, specs no longer listed in the Eimac catalog)
for use with a 10KV supply. I know 4-65A won't work,
in place of 4PR65, I sold some (back when they were a
bit more plentiful) cheap to place that makes flight simulators,
where they promptly failed. (their idea, not mine!)
Since I couldn't discern the
differences by eyeballing, I'd assume even
more vacuum pumping & tighter tolerance control.
BTW: a cousin of the 4PR65, the 4PR60C, does
20KV, 1.25KV screen, and peak current of 18 amps,
peak power of 337KW. It's only 6 inches long, but
has a ..egads.. 54 watt filament!
4. Duty cycle. Hmmm, perusing my Eimac book
I see the Y-141 planar triode. 59 mm long, weighs
70 grams. Typical operating parameters: 5 KV anode,
plate current 4 amps. Output power, 11 KW.
Duty cycle, .001 %.
5.Distance between pins. Note the differences
between 807 & 6L6 re voltage rating. Again, back to
the socket-cracking problem, as noted above.
6.Plate material, at least on glass recieving tubes.
I have a T-40 Taylor here. Looks just like a '50,
but has carbon anode and plate cap.
Voltage rating, 1500 volts.
>I know they can take big K's. Has anyone actually got a case of tubes
>dying from too many K's?????
Well, it's lot easier to blow up gas rectifiers than vacuum tubes...
>Lacking the equipment to do proper tests, I wonder if anyone out there
>has done proper lab tests on 6L6's or EL34's to see what they really
>will take......
In terms of terms of plate dissipation and max cathode current,
you probably have a pretty good idea already.
Guys that build illegal CB kickers with tubes really go
nuts on that angle, one customer with a *mobile*
(musta had a double alternator) tube kicker reported
that he was running 40KG6's, at 1000V, plates glowing
with the mic keyed. Yow!
I'll bet it has been done. What's more, I'd say no tube mfr
in the old days, at least, would've shipped boxcars full of
tubes to OEM's without extensive testing.
Ned Carlson, Triode Electronics, Chicago, IL http://www.triodeel.com
Open 12:30-8 PM CT, 12:30-5 PM CT Sat Closed Wed
ph:773-871-7459 fax 773-871-7938 "where da tubes are"
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