Heater voltage mismatch

[m6tt]

So imagine I have an 8-volt tube, and a socket wired for 6 (parallel wired, naturally). Imagine, being ornery, I plugged such an 8-volt tube into that 6-volt socket, and that it sounded pretty good. I have read conflicting reports of why this may be either a great way to preserve tube life (although certainly not with such a big voltage difference) or the work of the devil, stripping cathodes and all that.
So my question to the knowledgeable individuals on this forum is this: A) Will this damage the amplifier B) Will this damage the tube C) How does a given manufacturer actually know how many volts are ideal for a given tube D) How does undervoltage affect bias point and distortion?
I would like to add that the 8-volt tube above-mentioned was intended for series operation, where voltages are often highly screwy.

[Tom Bavis]

The tube will have lower emission, thus lower transconductance, and may not last as long as at rated voltage... though if you could turn the heater voltage up as it weakens, you might get longer than normal life. You won't get as much output if it's a power tube.

[m6tt]

I may have noticed some lowered output, them bein' power tubes and all...it's an instrument amplifier, and still sounds great although quieter. It's probably alright though, great for recording now, I'll have to consider changing the voltage but the transformer only provides a 6volt winding, and I do not want DC on my heaters at all. It's quiet enough for a harmonica amp and I don't want to shorten tube life.
Here's the interesting part though, Volts * Amps = Watts, and Watts = heat, in the case of heaters. So...if it just so happened that the data for the 6 volt version and the 8 volt version seem to end up at the same wattage, is it possible that the 8 volt is just a tube tested to have higher voltage on the heater and draws different current at 8 volts to arrive at the same wattage? Or is that crazy? (I am assuming I am wrong but it sure sounded good).

[kevinkr]

Originally posted by m6tt
I may have noticed some lowered output, them bein' power tubes and all...it's an instrument amplifier, and still sounds great although quieter. It's probably alright though, great for recording now, I'll have to consider changing the voltage but the transformer only provides a 6volt winding, and I do not want DC on my heaters at all. It's quiet enough for a harmonica amp and I don't want to shorten tube life.
Here's the interesting part though, Volts * Amps = Watts, and Watts = heat, in the case of heaters. So...if it just so happened that the data for the 6 volt version and the 8 volt version seem to end up at the same wattage, is it possible that the 8 volt is just a tube tested to have higher voltage on the heater and draws different current at 8 volts to arrive at the same wattage? Or is that crazy? (I am assuming I am wrong but it sure sounded good).

Yeah that is wrong.. 8V filament tubes draw about 75% of the filament current of a 6V tube for the same overall heater power, and are designed to have the higher resistance required to do so on an 8V filament supply - a 6.3V filament will quickly fail at the much higher overall heater power that results from operating it at the higher voltage. And of course the manufacturer knew exactly how to design a filament of a given heating power for 1, 2, 3, 4, 5, 6.3, 8, 10, 11, 12.6, 13, 15, 19, 22 volts or any other voltage they cared to choose if you see the point I'm trying to make. Tubes were selected for a lot of parameters in critical applications, much as semiconductors are today. These guys were pretty rigorous in their approach and by the 1930's there was little if any guess work or luck involved in the design of most tubes.

Operating an 8V tube on a 6.3V filament supply will result in significantly shorter life, and in the case of a power tube is likely to result in lower emission and possible cathode poisoning. Shifts in operating point with output tubes may result in higher than expected plated voltages due to reduced idle currents. Better I think to replace with the right tubes, although in most cases I think the amplifier will be fine, the tubes probably not.

[diy4]

What tube is this?

For Indirectly-heated tubes (IDH), lowering heater voltage can
be a good thing, depending on the particular tube type.
Some designers do this to lower noise; it can also enhance
linearity in a few types. This link has some info:

http://www2.famille.ne.jp/~teddy/datalib/heater.htm
(Thank you Teddy!)

For your purposes, if it works alright now, that's fine.
The decrease in life only happens because the cathode
is 'under-emitting'... increasing Vh later will extend its
life past what you would expect with correct Vh.

This is, however, a variable effect, depending on the
particular tube.

You *could* add a 2 volt transformer (with sufficient current
rating)in series with the 6 volt winding you already have.

[Boris_The_Blade]

Note that a famous example of this is the first amplifier stage in a HK citation II -- a 12BY7 with 2.2ohm resistor is series with the filament.

[m6tt]

diy4-
yes, it's an indirectly heated output tube, and thanks for the link, although I speak poor japanese I did get something from the pictures Wouldn't adding a 2v transformer in series with the tube's heaters also affect other heater voltages on that winding?

[diy4]

Glad you appreciate Teddy's work. I speak no Japanese at all,
but a babelfish helps somewhat. You're right, the pictures say
it all.
Yes, you would have to wire the transformer after the other
tubes but before the outputs. A picture would be better than
this explanation, but in series with one lead after the preamp
tubes, wired in phase as to boost, not buck the voltage.
Again, current rating must be adequate.

OK, I made a picture; it's ugly, but I have an excuse;
I used a microsoft program to make it. (Can I say
microsoft here, or will it be filtered out, like s**t, etc.?)

To adjust the voltage, you could add a resistor in series
with the primary.

BTW, what tube is this you're using?

[Geek]

Not needed. There's a simple way of powering 8V heaters from the 6V line using a single electrolytic and rectifier diode (on each 8V tube).

I just have to dig up the configuration, but I believe it was feeding the tube in series with the cap, then the diode was parallel with the filament.

[cerrem]

I take it we are refering to a small pre-amp tube????
Not a power tube???
If the tube is in-expensive and easyily available...then don't waste your time raising the voltage to 8v.... Run the thing at 6.3V if you like what it sounds like....and so what if the life expectancy is shorter, which you still have yet to prove, when that time comes you just replace the darn thing...
Most reports of cathode poisoing, stripping and other ill affects don't always apply here...Many of those papers from the 1940's and 1950's were refering to Transmitting tubes of thoriated tungsten...ect.... and power tubes....
Many small signal tubes will last much longer when operating at lower filament voltage....depends on the circuit and how much current is flowing.... the lower heater voltage will reduce the space charge, essentially a reservoir of electrons ready to be used.... Now if the average AC current flow is small enough, it will not overwelm and dry up this smaller space charge and your in the clear....but if the signal current is to strong, then durring peaks you can dry-up the space charge and strip the cathode...which is next to impossible to do if you are running a small signal tube at very modest operating levels....
For example....the Nueman U-47 mic uses the famous VF-14 Telefunken tube... This tube is a small pentode that was original intended for 60V filament...... Well the U-47 runs the VF-14 filament at about 32 volts and wires it in triode mode....all to keep the grid current down to nill....those tubes tend to last a very long time , and then some....

Chris