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05-07-2021, 11:14 PM
Hi Guys
Using tube heaters to bias output tubes is an old idea that was convenient but not exactly beneficial. You might think that the tube heaters used this way might save needing a heater winding, if the heaters doing the biasing were the only ones present AND there is a synchronicity between their draw and the current need for the output stage. I've never seen that happy coincidence.
The only advantage is that the heaters used to bias are DC powered,and it is usually the case that the related tubes are at the front-end of the circuit, or some other position where a noise benefit may be helpful.
Okay... there's a second "only one" advantage
The heaters are almost like a constant current source - not a real good one but pretty close if you've never seen a solid-state CCS.
The downside is that you have no control over the idle condition and because heater is current is so high the current must be shared by all the tubes in the output stage. I saw an old amp that had two power amps sharing the heater bias current, with push-pull pairs driving their own OT. The amp was not actually stereo but I suppose the manufacturer was able to get a good price on a specific wattage OT in quantity.
Better to use Rs and Cs and consider splitting them as individual bias. This lets you pull tubes or let's the amp survive a tube failure.
As far a tube heater failing here, they usually fail open. The worse possibility is if the cap shorts, then you melt down the output stage and power supply.
Have fun
Using tube heaters to bias output tubes is an old idea that was convenient but not exactly beneficial. You might think that the tube heaters used this way might save needing a heater winding, if the heaters doing the biasing were the only ones present AND there is a synchronicity between their draw and the current need for the output stage. I've never seen that happy coincidence.
The only advantage is that the heaters used to bias are DC powered,and it is usually the case that the related tubes are at the front-end of the circuit, or some other position where a noise benefit may be helpful.
Okay... there's a second "only one" advantage
The heaters are almost like a constant current source - not a real good one but pretty close if you've never seen a solid-state CCS.The downside is that you have no control over the idle condition and because heater is current is so high the current must be shared by all the tubes in the output stage. I saw an old amp that had two power amps sharing the heater bias current, with push-pull pairs driving their own OT. The amp was not actually stereo but I suppose the manufacturer was able to get a good price on a specific wattage OT in quantity.
Better to use Rs and Cs and consider splitting them as individual bias. This lets you pull tubes or let's the amp survive a tube failure.
As far a tube heater failing here, they usually fail open. The worse possibility is if the cap shorts, then you melt down the output stage and power supply.
Have fun
