Question about bias scheme on Wards/Danelectro/Airline Amp

[makinrose]

I have a customer who is curious about me building him amp similar to this one:dano_commando.pdf (el34world.com) It's pretty straight forward except for the biasing scheme for the power tubes which to me does not look like a great idea.  The cathode of the power tubes runs to the heaters of the 12AX7 which are wired in series to the ground along with a 10uf cap; a odd ball form of cathode bias.

I could be wrong but it seems to me if the you ever had a 12AX7 heater go bad you would have real problem and not develop voltage on the cathode and the power tubes could go in runaway.   I'm not sure if this type of bias system has any tonal properties.  Does anyone know?  Would I be better off making the amp a more standard form of cathode bias or fixed bias?  

Thanks for the input!

[K O'Connor]

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

[makinrose]

Kevin,
I really appreciate all the insight. It's actually worse than I thought! If end up building I'll be using a cathode bias with TUT style improvement and bias points. I find it interesting that so many different circuits were tried in older amps and so many seem to be chiefly centered around reducing price rather than increasing usability or reliability.