"Gareth Magennis" wrote in message
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"klem kedidelhopper" wrote in message
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On May 19, 9:36 pm, "Arfa Daily" wrote:
snip











Keeping a safe B+ voltage to the electrolytic you could do a test to
see how much sine wave power the amplifier will deliver into a
resistive load. If it's a bit deficient perhaps you could raise the B+
slightly to try to boost it. This is of course assuming you happen to
know what the RMS output rating of the amplifier is.

I've found some oddities in some juke box amps over the years. I
remember working on an old Wurlitzer from the 1940's many many years
ago. They used two 6L6's for the outputs and two or three additional 4
and 5 pin tubes from the period as well. And they ran 950VAC to the
plates of the rectifier. When a record was selected and power on was
established, the 6.3V filaments were treated an initial dose of 9.5
volts until the B+ came up and dumped the relay. Warmup time was
approximately six seconds. Ingenious design for it's time but a scary
thing for a kid like me back then to work on. Lenny

Hi Lenny. Yeah - I can imagine ! It was scary for me too as a
fresh-out-of-school kid, to be diving in the back of TV sets -
especially
some early colour ones with a shunt stabilized voltage of close on 30 kV
to
the picture tube ... I had an apprentice mentor who was supposed to
keep a
continuous eye on me, but I'm sure you remember what a busy workshop was
like back in those days !

There's 9 or 10 tubes all told I think in this juke amp. It's a fairly
complex stereo design with one or two slightly unusual design features -
not
the least of which is this odd use of a voltage doubler for the HT
supply.
Considering the final HT is only 375 V, I'm at a bit of a loss as to why
they would have wanted to design it that way, given that the transformer
that supplies the input AC for the doubler, is a custom job with a
number of
other windings to feed other areas of the juke such as the mechanics.
About
the only reason that I can come up with is to keep the voltages on the
umbilical down. Even that doesn't really seem to make a lot of sense
though.
The connector is a heavily insulated and mechanically very sound
multi-pin
type. Very similar to an octal plug, but with only 5 pins fitted, and an
offset rather than a keyed spigot. Such a connector would easily handle
the
250 V AC that would be needed to derive 370 odd volts DC directly.
Still, I
suppose the designer must have thought it was a good idea for some
reason.

The amp claims to be 2 x 25 watts RMS max. I would think that it would
be on
the limits of a pair of 6973s though at that power. I would have
expected
them to be 'comfortable' at about 18 watts max, with that sort of plate
voltage on them. I had a look around the workshop today, and found a
split
primary Marshall transformer. The HT winding will be too high to feed
this
voltage doubler, but it occurs to me that I could use one primary half
fed
from my variac set at 120 V, and then have the full 6.3 V for my heater
supply, and 120 V from the other half of the primary for my doubler
feed. I
know that I shouldn't really use a primary like that and draw current
off
it, but I figure that it will do enough to bench test this thing and see
that it at least works.

Arfa

Arfa
That doubler does sound like a strange thing to employ when they
didn't have to. We used to find them often on 120V line operated
equipment here in the US where they of course needed the higher DC. I
can't imagine though why anyone would do it though if as you've said
if they really didn't have to. And with a 240V mains you certainly
wouldn't have to, (even if it was line operated). I'd be interested to
see where this thing starts clipping with a moderate B+ applied.

You'll appreciate this one.
I was 16 I think and had been fixing TV sets for several years. I
remember one of my first TV repair jobs, if not THE first, but who can
remember all that... Anyway the boss took me back into the shop to
show me around. It was very impressive, with test equipment like
generators and scopes, etc. that as a kid I could never have afforded
at the time. Well I was standing about five feet in front of the bench
and I noticed these odd lines drawn on the floor parallel with it
going out about four feet. I asked what they were and with all
seriousness he said, "well the first one there is 120VAC. The second
one is 240VAC. The third one is 450VDC. And after that it doesn't
matter how far you're thrown any more. We'll just drag your dead ass
out of here"... And after picking up a live chassis one time and
taking 350 right through the chest I learned to keep one hand behind
my back. Lenny


I had this experience on a Fender Twin - one hand on the chassis, the
other hand's finger on 500v DC.

I seriously thought I was going to die (whilst standing on tip-toes
spasming ......).
Quite a lot goes on in your head in those kind of brief moments, its
actually very interesting.


One hand in the pocket now, always.


Gareth.

Me and Lenny have been discussing this further off-group. It is indeed a
very frightening and sobering experience. I had full 240 volt mains across
me many years back, and I too had muscles in spasm, that held me on. I
remember very clearly, also thinking I was going to die. Very calm and
rational. I also remember making a super-human effort to break free, and
somehow, I did. Remarkably, I was no worse for the ordeal, except I was very
weak for a while and couldn't stand up, and my right arm hung limp for quite
a while, and ached for a long time afterward.

Arfa