Power Transformer Windings

[Justin Thomas]

Hi All,

I have a 1962/63 Thomas Model 300 Tone Cabinet chassis here, along with trannies & sockets. Powered 2 pairs of 6973 & 5 12A_7 tubes. 2 output trannies, 1 PT.

Trying to figure out a "mystery winding" on thePT. Using measuring & various charts, I've got the HV w. a 32V bias tap, 5V, & 6V w CT figured out. But I have a leftover winding.

By all I can find, in RDH4 & others, extra windings are listed as "Additional Filament Windings." They list 2 possible colors, Brown & Slate. No voltage is given for them, so I imagine they could be anything.

I do not have a brown winding. I DO have a Blue winding with a blu/ylw CT, at 46VAC (across the winding). The wires COULD be slate, as they are different from the blue on my OTs, so I was going to assume it's a filament winding. But for WHAT? Anyone know?

It's loaded with Schumachers so I don't mind some "tranny abuse" in testing, loading down & measuring, etc. Just figured I'd ask first in case anyone knows. I honestly could use some added 6.3V capacity since I don't feel like shelling out for new 6973s...
Maybe wiring some stuff in series, whatever.

Thanks!

Justin

[Justin Thomas]

After quick testing on the 6.3V heater winding, I wired up 3.6A worth of heater draw. It dropped to 5.8V but rose again to 6.5V after a few seconds & stayed there. I left it on for maybe 15 more seconds before shutting down. I'll wire up a couple more 12A_7 sockets & try it out progressively, and keep eyeballs & fingers on it. I'm hoping to use 4 preamp tubes, which may include EF86s so that'll save a miniscule amount...

Still wondering about that 46V winding too. The leads seem to be of a slightly smaller gauge than the 6.3V winding so I'm thinking that was for some non-heater purpose. I've scoured the web for a schematic but no luck.

Justin

[The Dude]

Not sure about the Model 300, but I know that some of the 300 series had some SS circuits. My bet is that the extra winding connected to a supply for the solid state part of the unit.

[mozz]

I bought a thomas chassis for parts once, i think there was a 12v winding and i used it for the output tubes in series. The was also a small Leslie that went with it too.

[Justin Thomas]

Thanks for the input. This one says on the chassis (& is backed up by the internet) that the Model 300 was specifically made for Thomas; I also couldn't find much similarity to the rest of the 300 series.

I just ran it with 4A draw, the 6.3V winding dropped to 6.15 & seemed to be stable for the 30 seconds I left it on. Looks like I might be investing in at least one pair of 6973s for the lower heater draw. They also seem to take higher plate voltages than 6BQ5s... Or Maybe I can manage to find that little 1A 6.3V transformer I have simewhere... Anyway, it's a long way from completion so the experimenting will continue.

Justin

[The Dude]

I don't know the current rating on your 46V winding, but it's possible you could either knock it down via resistors or build a DC supply for additional filaments. Something to consider.

[Justin Thomas]

It's on the burner. I was thinking 4 12AX7s in series, just to see how low it drops. Kinda wish I'd kept a few wacky-heater tubes... I'd love to use it but the current rating is indeed a mystery.

If I burn it out I guess I'll slap that old Ampeg GU-12 tranny on it.

Justin

[trobbins]

The initial dip to 5.8V is due to the much higher cold-start heater current - so not a concern to the heater winding itself. If you can measure the heater voltage without load, and then with load, then that is more informative as it indicates the regulation level, and may indicate if you have too much heater load, or could add some more heater load at low risk. If you were really well set up for testing, you could do a heater winding temperature rise test based on measuring heater winding resistance when cold, and immediately after being on load for say an hour - that would also indicate how stressed the heater winding is.

[Justin Thomas]

Thanks for the insight & checkout process, trobbins. I'm not well set-up for anything much beyond plug&chug. I'm gonna hafta do the "smoke test" most likely, just make sure it doesn't get crazy-hot on the PT while monitoring the voltage. That resistance check is a good tip too. I don't have anything else hooked up besides the heaters (& the EL84 cathodes are cathode-biased pretty cold, but there's no plate or screen voltage or signal) so I figure it's kinda like havung the amp jn Standby. I can check my meter regularly & make sure it doesn't drop any farther. If I got down to 5.9 I'd start to worry.

Justin