If I have a 12.6VAC winding, is there a sensible way to heat two of the same power tubes with 6.3V rated heaters? Not sure if heating two power tubes in series is safe/recommended.
Last edited:
... except if they are direct-heated types. Its hard enough getting a decent +- balance on a pair; adding series to that, well, balance becomes nary impossible.
No,
Now that you have two heaters in series you already have a natural center tap.
I would just ground the common node.
Humdinger pot not possible, I think.
If the transformer has a center tap on the 12V winding, connect that to one end of each of the tube heaters so that each tube has an independent part of the winding. If there is no center tap, tubes of the same type but different production runs or different manufacturers may settle at different operating voltages. Not all tubes are made for series heater operation.🙂
Don't do this with DH tubes
Even with all the bias problems it creates, I once considered running two 45 DHT in series with additional resistors from a 6.3VAC winding.
But one 45 tube was at spec, 1.5A @ 2.5V, and the other 45 tube was not at spec, instead it was 1.25A @ 2.5V.
I expect there are some Indirectly Heated tubes out there that do not meet their specified current when driven by their specified voltage.
Take a calibrated DC power supply, set it to the specified filament voltage for a tube type, and measure the current on a few of those tubes.
Only then decide if you will use series string.
Some tube types were Specifically made for series string filaments, they were either carefully built, or they were made to be tolerant over a range of filament voltages. But many tube types were not made for series string filaments.
Your mileage may vary.
Just sayin'
But one 45 tube was at spec, 1.5A @ 2.5V, and the other 45 tube was not at spec, instead it was 1.25A @ 2.5V.
I expect there are some Indirectly Heated tubes out there that do not meet their specified current when driven by their specified voltage.
Take a calibrated DC power supply, set it to the specified filament voltage for a tube type, and measure the current on a few of those tubes.
Only then decide if you will use series string.
Some tube types were Specifically made for series string filaments, they were either carefully built, or they were made to be tolerant over a range of filament voltages. But many tube types were not made for series string filaments.
Your mileage may vary.
Just sayin'
Last edited:
I like series stringing tubes that are wired in parallel, that way if one tube is pulled out or a heater fails, they all turn off. If paralleled tubes have their heaters in parallel as well, I'm not sure what the failure mode would be when someone pulls out a tube and the remaining paralleled tubes are still operating. I suspect the DC operating point of the remaining tubes would swing into a danger zone.
Another reason for series strings is to simply pull less current or a versatility choice based on the parts you have.
Another reason for series strings is to simply pull less current or a versatility choice based on the parts you have.