K&K Audio / Lundahl Transformers

Reconfig ST 70 output tube heaters?

76.5.27.74

Hi All,

I'd like to use the VT-101 (807) in my K&K ST70 which would require reconfiguring heater supplies to 12.6v. It appears there's plenty of current on-board (only 4x 0.7A drawn for the VT-101 quad.)

I feel I should know this but I'm rusty: Referring to the K&K ST 70 schematic, Can I re-rig just the power tube's heaters as follows?

1. Connect one end of all VT-101 heaters to the "outside" of one 6.3v secondary;
2. Connect the other ends of the VT-101's heaters to the "outside" of the other 6.3v secondary, thereby deriving 12.6v for the output tubes;
3. Connect the "insides" of those secondaries together, lift and float the original pair of heater secondary center taps, and connect this new derived center tap to the ground arrangement provided;
4. Leave the driver tubes wired as they are.

The end result is I get 12.6v for the outputs, create a new, single center tap and reestablish it at original heater system potential via C6 and R2, and now have the driver tube's heaters connected end-to-end.

Something doesn't feel quite right with the driver tubes being tied at the heaters -- electrically each one is connected across half the "new" 12.6v heater secondary and neither would be technically centered around ground potential any longer. What am I missing?

Follow Ups:

Correction - Jon Lane 19:00:03 01/04/09 (0)
RE: Reconfig ST 70 output tube heaters? - KevinC 13:20:16 01/03/09 (1)
- RE: Reconfig ST 70 output tube heaters? - Jon Lane 13:59:50 01/03/09 (0)

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