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.
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.
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.
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 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.
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.
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