Question on shared rectifier / signal tube filament windings

[bst]

I have several power transformers with fairly stout single filament windings - 6.3v at 4 amps.

I know that one cannot power a directly-heated rectifier tube, like a 5U4 or 5V4, from the same transformer tap as the other tubes in the circuit.

However, I've also seen claims that one should't run indirectly-heated rectifiers, like a 6X5, from the same transformer tap as the audio signal tubes. However, several damper-diode data sheets specifically state that one CAN run their heaters off the same supply as other tubes in the circuit.

I'd like to run a pair of damper diode heaters and my audio tube heaters off the same 6.3v filament winding. Can I do this?

[tubelab.com]

As long as you don't violate the heater to cathode voltage spec on the rectifier that you choose you are OK. For the 6AX4 damper diode that is 900 volts. If you are building a typical 350 to 450 volt supply this is no problem. Usually the 6X5 type rectifiers can also be used within their ratings in the same manner. The 6AX5 can take 450 volts H-K. There is tons of old equipment out there with only one filament winding and 6X4's or 6X5's in them.

If you do this you need to ground one side of the heater winding, or use two low value resistors (22 ohm 1 watt or larger) from both sides of the filament winding to ground. This is in case the rectifier ever does develop a H-K short. The resulting B+ on the filament supply will make your other tubes very unhappy! The low resistance path to ground should blow the fuse.

[bst]

Thanks for your clarification!

I have a couple sleeves of 6AX4GTs, so this will work out very nicely.

Time to plug in the soldering iron...