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