Heater wiring Conn stereo organ

[cbass]

The taps off the transformers are 6.3-0-6.3

I tried to make a drawing.

Where the 47k joins the CT is on the socket of a tube thats not being used now.I assume it was dc elevated.

Should take the CT straight to ground now?Since that tube won't be installed?

[Phil_S]

From your drawing it looks like you have 4x 7868 + 2x 12AX7.
The two 12AX7's are provided with 12V in parallel. You can wire each pair of 7868's in series, making them 12V per pair. Like this picture. I assume all tubes are filaments 4&5. I didn't look up the 7868.

If half the winding 0-6.3 provides enough current, then you can tape off the other 6.3 and just wire normally. For those tubes you will need 3.8A. That means you can do it if the 12V rating is at least 7.6A. Many times organ amps powered a large battery of 12A?7 tubes, so it might be possible.

[cbass]

yes pins 4 and 5 are heaters on the 7868's

I was not sure about the CT being conected to ground through the 47k resistor.

[Phil_S]

I don't know what purpose the 47K on the CT might have. I don't see why it's needed. Maybe someone else can chime in.

If the filaments hum, you can elevate the CT to a DC reference. If that doesn't work, lift the CT and see if you have any luck with the usual artificial CT.

[cbass]

No there is no hum.

I wasn't sure because .When i measure pins 4 and 5 on the 12ax7 I get 6.5v but on pin nine I get almost 0v.

[cbass]

Updated schematic

[Phil_S]

I wasn't sure because .When i measure pins 4 and 5 on the 12ax7 I get 6.5v but on pin nine I get almost 0v.

I'm confused. If you take 6.3V per leg and hook one leg to pin 4 and the other leg to pin 5, you should see 12.6V across 4 and 5.

Pin 9 isn't used for 12.6V. You shouldn't have any voltage there. It shouldn't be wired.

I don't see a new schematic.

[cbass]

It's still hooked up as it came from the factory.Pins 4 and 5 on the ax7's are tied together and the other leg is hooked to pin 9.

I'm trying to figure out if the 47k is part of another circuit and they just tied the CT there to float DC on the heaters.

Maybe I should hook it to the power tube cathodes?

I fixed a mistake and showed where the DC was being injected on the new schematic.

Thanks for looking

[Phil_S]

It's still hooked up as it came from the factory.Pins 4 and 5 on the ax7's are tied together and the other leg is hooked to pin 9.

I see. 6.3 at one connection and 0 at the other. I believe that means 9 is grounded. We don't usually see it done this way, but I suppose it works. You could just leave it alone unless the amp gives you 60Hz hum.

[cbass]

I attached the CT and one leg of the heater circuit to the Cathode of one of the 7868's.No hum so I guess its ok.

[Phil_S]

If you've got the tubes and the sockets all ready to go, try running the CT and one leg to all the heaters. If you get 6.3V, then it has the supply capacity for the number of amps. If you get low voltage, it doesn't and then you will need to go to another arrangement.

I think, if you are careful, you can work this out so all of them can run on 6.3V if that's what you want to do. Some might use one leg and some the other leg. All will use the CT, which you don't ground for 6.3V.

When I build with used iron, the first thing I like to do is wire the filament supply, put the tubes in and see if the voltage is adequate. That saves a lot of trouble later on.

[cbass]

THis is what I have now no idea if it's right but everything works

[martin manning]

Looks good to me. I'd draw it like this:
Edit: I guess you have pin 9 of the 12AX7's going to the center tap...

[cbass]

Thanks Martin.

[cbass]

Edit: I guess you have pin 9 of the 12AX7's going to the center tap...

Yes I belive thats the way it's wired.I get 6.3VAC on pins 4,5 and 0V on pin 9.

Is that ok?