28 Feb
2007
28 Feb
'07
4 p.m.
The type of those valves will give you a good idea as
to the voltage
required of course.
Yes. I need to look them up; they don't appear in the reference book I
have at ready hand. ("Had", actually; it's at home and I'm not.
That's also why I don't give the valve type number here.)
Almost certainly the EHT rectifier vavles are
directly-heated,
Yes. All the rectifier valves (including the B+ one) have filaments
rather than heaters.
(There are
some 12V-heater tubes, but they all have centre-tapped
heaters.)
I assume things like 12AX7s... My first concern is that the primary voltage is very
low -- in other
words this transformer draws excessive input current even when
unloaded.
Yes - as I mentioned back in the summer. This is just further
confirmation of it. I was using a 40W light bulb in series with the
primary, as a current limiter, which is why the primary voltage was 25V
instead of ~120V. The bulb was glowing (not quite full power, but
close).
This suggests to me shorted turns, probably in the HV
winding (which
would alos explain the very low outptu voltage there.
I agree; this is why I suspect the HV winding of being the culprit.
You might consider rewinding the original transformer,
although
getting the insulation to withstanda a few kV would be 'interesting.
Quite. It also would be "interesting" to get it apart far enough to
rewind. It's a laminated core made up largely of E-shaped pieces, and
getting it apart would mean either cutting them (= destructive),
solvents to unstick the tar/shellac/resin/whatever that's holding them
together (= likely damage to the other windings' insulation), or a
*lot* of patience working them apart. And unwinding a winding
nondestructively without taking the core apart...well, it better be the
outermost winding, and would require a lot of patience to feed the wire
through over, and over, and over again.
To just pull off the HV winding, if it's outermost, I could cut most of
the turns to simplify the work, since that winding I don't care about
preserving. (I should also check out the three filament windings,
mostly for completeness...and I really need to trace out a schematic of
at least the power-supply portions of the thing.)
I don't know how easy it would be to remove the
shorted HV winding
(is it near the outside of the windings?).
I don't remember.
If you can get it off and put the rest of the windings
back with the
right insulation, you could keep the exisitng transformer fo the
heaters, HV rectifiers filaments, B+ line, etc and only have to
provide the HV input. Maybe a seprate transformer (or a transformer
+ voltage multiplier) for that.
I'll look at that when I get home. Certainly there seems to be some
goodness happening between the primary and the heater winding, ditto
the B+ winding.
I imagine the HV winding is either innermost or outermost, simply
because it's easiest to insulate that way. But of course that's a
guess, and in this case it'll be easy to look and find out.
Actually, come to think of it, looking at your (natural enough) use of
"valve" where I'd write "tube", I'm curious - are CRTs called
CRTs on
your side of the pond, or something else (CRV?)?
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