variable heater voltage

[andrew]

I would like to be able to occasionally lower the voltage to the power tubes only ( 2-6v6 ) to act as a master volume. As little as I play I am not concerned about stripping cathodes and tube life.
My 12sl7 preamp tubes are running on 12 volts DC from a small 120 volt AC/DC regulated 12volt + - supply. I have a power transformer with 6 volts AC and a 6 volt center tap. I wired the 6 volts AC to a full wave bridge rectifier and a 4700 uf cap with the - side of the cap not grounded. With one probe of the the voltage meter on the + side and the other probe to chassis ground I get close to 3 volts. Same for the - side.
I wired this arrangement up to the power tubes with pins 7 and 7 to + and pins 2 and 2 to -. The heaters did not light up. Do I wire them in series? I could not find any clear instructions ( for me anyway) on how to wire the much less how to vary the voltage.
Other stuff I have is a VVR and an extra 3A 12volt regulated power supply.
Any help would really be appreciated.

[Kagliostro]

I don't understand exactly what you are asking for

Do you want to supply the 6V6 power tubes with DC or are you saying you want to supply the preamp tubes ?

I think you refer to the 6V6 tubes, if so, first of all, remember that there is no necessity to feed power tubes with DC (unless you want to avoid problems with other parts of the circuit, such as, as an example, when the circuit is on a PCB and you cannot twist the wires)

Other thing you must follow is that if you use a FWB and want this supply be floating (no connection between minus of the bridge and ground) you must connect the tubes heater pins to the minus and the plus of the bridge, no ground chassis connection

Same thing if you want to measure voltage, you must measure connecting your instrument to plus and minus of the bridge (when the Heater PS is floaring)

Note that if you have a 12v - 0 - 12v (12v with CT) transformer and you use a FWB if you connect the CT and the negative of the bridge to ground, your Bridge will blow, instead, if you don't connect the CT to ground and connect the AC extremities of the transformer to the AC connections of the FWB, you are rectifiyng 24V AC and obtain at the minus and plus of the FWB around 31V DC

--

About variation on voltage, don't vary the voltage on the heaters, this is near no sense if no sense at all

if you want to vary the power of the Power Section of the amp, use the VVR to control the B+ voltage that feeds (ONLY) the anode of the Poer Tubes (lot of people don't use VVR to feed also Phase Inverte or Phase Inverter & Preamp Section), or, alternatively, use the VVR only to feed the G2 of the power tubes (this is a different but effective way to control power using a VVR)

I hope I was able to explain what I mean in a comprehnsible way

K

[andrew]

Thanks for the reply K. The problem was a soldering error. (poor eyes and poor lighting in a poor work space) works just fine now. Thanks again for the helpful information K.

[pdf64]

If you search back through TAG I’m sure I remember a guy that reporting that he fed the heaters via their own variaced PT, he had great results with no tube issues.

[Kagliostro]

If you search back through TAG I’m sure I remember a guy that reporting that he fed the heaters via their own variaced PT, he had great results with no tube issues.

That seems odd, many tube datasheet give a max and min reference for heater voltage and, if I remember correctly, to be out of this tollerance isn't a good thing, referring to the tube life

K

[pdf64]

If you search back through TAG I’m sure I remember a guy that reporting that he fed the heaters via their own variaced PT, he had great results with no tube issues.

That seems odd, many tube datasheet give a max and min reference for heater voltage and, if I remember correctly, to be out of this tollerance isn't a good thing, referring to the tube life

K

My understanding is that exceeding the max heater voltage might shorten its life due to increasing the likelihood of burnout, below the minimum and the tube characteristics may diverge from its type bogie.
I seem to remember reading that if the heater voltage falls to such a point that the electron cloud around the cathode becomes depleted, then the surface of the cathode (and hence its future performance) may be degraded. I thought it was in Tomer but if so, can't find the passage.

The guy experimented in ignorance of that risk, and reported that he hadn't had a problem.

[R.G.]

Re tube heater voltages, from reading lots of articles and papers from the Golden Age:
Tubes are/were designed to run with a specific amount of emission from the cathode. The heater and cathode area are designed to provide that amount of emission. Too much heat and the heater life is reduced. In incandescent filaments, life goes down by the fourth power of the voltage over the design value. So running at a higher than nominal voltage eats filaments.

Running at a lower voltage cuts emission by the amount that temperature goes down. Lower emission below the amount that maintains the space charge cloud of electrons over the cathode, which lowers the transconductance and hence gain. Running tubes without a space charge lets stray gas ions hit the cathode, poisoning the surface. This lowers emission, and eventually makes the tube too weak to function right. The punishment of the surface of the cathode is called cathode stripping. It requires plate voltages over about 500V to really be significant, so cathode stripping is not really an issue in situations where the plate voltage is low. Another possible issue is that the plate voltage of the "off" tube on every half cycle is transformered up to twice the plate supply voltage by the OT in a push-pull amp, so that may make this worse in those amps.

The +/- 10% datasheet spec was largely set by the fact that heaters in the Golden Age were powered directly from an AC power transformer. The AC line during the Golden Age could not be relied to be any tighter than that, and was often worse, but they had to say something that AC line voltages might be able to do, so the tube designers did their best to design a reasonable tube heater that would stand that variation for a long enough time.

Running preamp tubes at significantly lowered heater voltage works better than lowering power tube heaters in terms of not affecting life. Lowering preamp tube heater voltage has been used to extend preamp tube life. Lowering power tube heater voltage may or may not make tube life shorter depending on other factors.

And lowering power tube heaters may or may not lower the overall gain you see. On power amp circuits with overall feedback, there is excess gain that the feedback uses to stabilize the circuit and make performance more predictable. Lowering the power tube gain inside a feedback loop that includes a phase inverter as well may have little effect.

It's probably simpler to put a master volume before the phase inverter.

[andrew]

Well guys I've taken your warnings into consideration and I just had to try. I wanted to vary the power tube voltage from 0v to around 6.3v. to see how it affected the amp's volume.With a FWB rectifier and a 4700uf cap I got about 7.3vt DC across pin 2 and 7. The 2 6v6s are wired pins 2 to 2 to the + side of the capacator and on the ground side pins 7 to 7 to the - side of the capacator.
I hooked up a tested and working VVR on the + side ( + side of the capacitor to pin 2) but the voltage across 2 and 7 was barely 1 volt and the pot rotation made no difference. Did I hook the VVR up correctly?
One thing I did notice is that on AC the 6v6 tubes have that nice blue glow but there is no blue glow on DC.
Things have shut down in town and I haven't been to work since Saturday. This experiment and Google helps me pass the time. Thanks for any help

[pdf64]

VVR? A design intended for high voltage low current tube amp HTs probably won’t do what you want here.