I’m quite aggravated with this Heathkit EUW-15 power supply design

[MisterHeadache]

I’ve got a rather obscure Heathkit Heathbuilt EUW-15 power supply.  I would like to use it to power a vacuum tube device that needs +300VDC and 6.3VAC.  The EUW-15 is capable of providing these voltages at the current levels that the device needs – HOWEVER – the device has a common connection ("ground") between the 6.3VAC filament and +300VDC circuits.  I cannot modify that interconnection.

Looking at the schematic of the EUW-15, for some reason that I cannot understand, they tied the low side of the HV DC to a center tap on the 6.3VAC filament secondary on the transformer.  So that means I cannot connect the low side of the DC and one of the 6.3VAC together (the red arrow), that would short half of the secondary windings in the filament transformer!

The filament winding (the yellow highlighted X-X) also powers the filament of the 6BH6 driver tube.  I am guessing that they didn’t want the filament of the 6BH6 to float relative to the low side of the HV?  Otherwise I am befuddled why that connection (red circle) exists at all.

Can I just sever that connection from the center tap to the low side of the DC and make my connection with the red arrow?  Or am I missing something here?

[TimFox]

Yes, the heater of the 6BH6 cannot be allowed to float:  see its data sheet for maximum heater-cathode voltage.
The second winding is necessary since the cathodes of the 6L6s will go up to at least +300 V
It would be preferable to have a third heater winding, but the Heathkit was not expensive.
You need to add another power supply that can supply 6.3 V at the required current for your external device.
If you want 6.3 V near the low side, the center-tap connection is reasonable.

[Andy Chee]

How original do you want to retain the power supply circuitry?

For example, how about replacing the 6BH6 with a MOSFET/transistor?   This would free up the filament winding to use for your external vacuum tube device. 

If you still wanted to "light up" the 6BH6, you can leave the plate, cathode and grids disconnected but connect the filament in parallel to either the 6L6 filament, or your newly ground referenced filament for your external vacuum tube device.

[Ian.M]

OTOH the heater cathode leakage current should be pretty small, so adding a 330 ohm resistance between the transformer secondary center tap and DC- should keep the 6BH6 heater cathode voltage within its permitted limits and have negligible effect on normal operation, but still permit its use with loads that ground one side of the filament supply.  There may be some  slight increase in hum  on the regulated HV DC+ when the filament supply is asymmetrically grounded due to heater cathode coupling in the 6BH6.

[TimFox]

With the 6BH6  cathode at +105 V, it would be dangerous to use the 6L6 heater winding (connected to the DC output).  If you want to be clever, you could add a voltage divider to bias the 6L6 heater winding so that with extreme output values (0 to +350 V or so), both heater-cathode ratings are observed.  Heathkit’s engineers chose not to do that.  Physically small “filament transformers” are readily available to add inside the box for the 6.3 VAC output.

[MisterHeadache]

Thanks for all the quick and detailed replies.  I understand better how this design works now.

With the 6BH6  cathode at +105 V, it would be dangerous to use the 6L6 heater wining (connected to the DC output).  If you want to be clever, you could add a voltage divider to bias the 6L6 heater winding so that with extreme output values (0 to +350 V or so), both heater-cathode ratings are observed.  Heathkit’s engineers chose not to do that.  Physically small “filament transformers” are readily available to add inside the box for the 6.3 VAC output.

Yep, I think I'm gonna go that route, use a separate filament power supply.  I only need to power up my device for function checks and output verification, so its just a short test.

[Andy Chee]

With the 6BH6  cathode at +105 V, it would be dangerous to use the 6L6 heater wining (connected to the DC output). 

Please read my post carefully for context.

If you replace the 6BH6 with a MOSFET/transistor, the now disconnected, non-functional, and floating 6BH6 can be safely illuminated (for strictly vanity appearance) from the 6L6 filament wiring.

[TimFox]

With the 6BH6  cathode at +105 V, it would be dangerous to use the 6L6 heater wining (connected to the DC output). 

Please read my post carefully for context.

If you replace the 6BH6 with a MOSFET/transistor, the now disconnected, non-functional, and floating 6BH6 can be safely illuminated (for strictly vanity appearance) from the 6L6 filament wiring.

I was replying to the OP.

[Andy Chee]

With the 6BH6  cathode at +105 V, it would be dangerous to use the 6L6 heater wining (connected to the DC output). 

Please read my post carefully for context.

If you replace the 6BH6 with a MOSFET/transistor, the now disconnected, non-functional, and floating 6BH6 can be safely illuminated (for strictly vanity appearance) from the 6L6 filament wiring.

I was replying to the OP.

I don't think the OP had any intention of connecting the 6BH6 heater to the 6L6 heater.

[MisterHeadache]

I don't think the OP had any intention of connecting the 6BH6 heater to the 6L6 heater.

Correct, I just want to be able to power up my device with the HV DC and filament AC.  My device shares a common ground on those two circuits, and the way the EUW-15 is built, I apparently cannot do so without modifications.