On May 18, 12:44*pm, "Arfa Daily" wrote:
"Arfa Daily" wrote in message

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"tm" wrote in message
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"Arfa Daily" wrote in message
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Anybody out there got any original Rockola service manuals ? I need a
decent copy of the schematic for a 1478-85 stereo tube amp. There seems
to be plenty of copies on line, but all sourced from an original poor
resolution scan-to-jpg. I particularly need to know what the AC voltage
is from the transformer winding that feeds the HT supply for the amp.
You can read the text at the heater supply winding as 6.3 V as you would
expect, but the HT winding's voltage is illegible on the scan, no matter
how much you zoom it.

Arfa

What voltage rating on the filter caps? That will give you a maximum
point.

What tubes are used? That would give you a design window.

Is it center tapped?

tm

The main filter caps are rated at 400 V so I'm guessing that the HT is
probably around 350 V. Output tubes are 6973 beam tetrodes, which are
rated for a max Va of 400 V, so that would seem to go along quite well
with a rail of 350 V. The winding is not tapped. It is a single winding
applied to a bridge of four metal top hat diodes. My best guess is that
the winding is going to be about 250 V RMS, which was a common value 'back
in the day', and would rectify nicely up to about 350 V DC.

It's not really a problem. Just too lazy to sit down and do the workings
out, if there was a good copy of the schematic anywhere. Still, it's done
now. I'm not anticipating anything being particularly wrong with it - the
owner just wants it checking over. The 'real' tranny sits in the *juke's
main chassis, and has a bunch of other windings to power the motors and
lights and so on. This amp plugs into that chassis with a 5 pin plug, of
which just four are used - that's two for the 6.3 VAC heater supply, and
two for the ??? VAC HT supply.

Thanks for your input

Arfa

Update. Someone on a Jukebox group that I'm also posted on, has just given
me a heads up on an odd design quirk of this amp in that it uses a voltage
doubler to obtain the HT rail. Now that I look again at the very poor copy
of the schematic that I have, that does indeed appear to be the case. This
guy reckons that the input from the tranny winding is around 150 V RMS to
produce about 360 V DC at the output of the doubler.

Arfa

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