Drop DC heater voltage by 1 volt, what resistor

[daz]

Well...then we must consider some form of measurement or setup error or some other parameter that changed at the same time. Otherwise, a couple of participants in this thread seem to be on the verge of creating free energy.

It was my screwup. I spaced and did it wrong where they attach to the pilot bulb. I'll let you figure it out as to what i did...i'm too embarrased to say but it should be obvious Not sure i'll even bother now tho.

[g1]

Free energy, that's like those power amps that can deliver 3KW per side running off a 1800W wall outlet right? Now why can't I just sell all that "found" power back to the utility company?

[daz]

I decided to try it again since i realized my mistake. 1 ohm drops it about 2 VAC. I dunno if the value will drop it in a linear fashion as it relates to R value, but if so then i suppose i need a .4 ohm since i want it to drop about .1-.2 less than a full volt. I'll have to dig thru a lot of junk to see what i have.

[big_teee]

According to the Valve Wizard you can drop heater voltage .7v by adding a pair of 60S1 Diodes.
The Valve Wizard
T

Did you try the 2 diodes like in the Valve Wizard Diagram?
It will drop your voltage by 0.7 Volts.

[olddawg]

According to the Valve Wizard you can drop heater voltage .7v by adding a pair of 60S1 Diodes.
The Valve Wizard
T

If it is a DC filament (and I think it is), then it just needs one forward biased diode. It will drop the voltage .6-.7v.

[Chuck H]

Did you try the 2 diodes like in the Valve Wizard Diagram?
It will drop your voltage by 0.7 Volts.

Well he sure should have! We all have PILES of 6A diodes layin' around. I'm just glad I finally found a use for them! Am I right guys!?!

[J M Fahey]

No, at a preamp tube. I'm dumb, but not THAT dumb.

It's impossible to add a resistor in series with a filament and not have filament voltage (at the tube socket itself) drop.
Impossible.

[Chuck H]

Daz mentioned in a following post that he'd made an embarrassing mistake in his implementation of the circuit and that it's working correctly for him now. But we should keep bringing it up until he has a complex

[Tom Phillips]

Daz mentioned in a following post that he'd made an embarrassing mistake in his implementation of the circuit and that it's working correctly for him now. But we should keep bringing it up until he has a complex

Daz,
I think you should explain what happened for the record and for those who read this thread in the future. We all make goofy mistakes and they become a learning experiences.
Tom

[daz]

Daz,
I think you should explain what happened for the record and for those who read this thread in the future. We all make goofy mistakes and they become a learning experiences.
Tom

Stupid mistake, but honestly it was just a matter of not thinking while i was doing it. Not something that i would expect to work if i actually saw what i did. (as i did when i looked at it later) I have the heater wires going to a pilot lamp, then the heater wires going to the tubes also paralleled to the pilot terminals. I removed the 2 wires from one side of the pilot and soldered a resistor from the pilot to the 2 wires instead of leaving one on the pilot and soldering the resistor there too them the other end on to the tubes. I noticed the bulb wasn't lit but i figured that was just bad contact because for the last month thats been intermittently happening on and off and i haven't gotten around to replacing the socket or bulb to see whats going on. Later as i was thinking theres no way this could be happening (voltage going up) i went to take a long look at it but it didn't take but a second to see what i did. I just spaced...

[R.G.]

Lower heater voltage shortens tube life. It does not prolong it.
It applies to broadcast tubes and audio tubes, alike.

Do you have any external facts to support this contention? I'd love to see it if you do. I can go dig out my reference eventually. It was a large-numbers test of tubes, conductance, and usable life. I want to say "GE", but I'm not sure on that fact.

Broadcast has known this for a long time, simply because, they pay a lot for tubes.
They pay $395 for one output tube, they sure as heck want it to last longer.
But, we pay $9 for 12AX7, and the same rules apply. Still has heater and cathode, just like broadcast.

While you and I are still looking up our reference materials, let's reason about the problem.

(1) Why do tubes have heaters?
The best I understand it, they have heaters to force electrons out of the surface of the cathode over the work-function threshold of the material. The heat raises the random motion velocity of the electron sea in the material so that the high end of the electron-speed (= temperature) distribution exceeds the work function and they pop out into the vacuum outside the cathode material. Barium and strontium oxides are used for coatings because they have much smaller work function thresholds than metals, so electrons boil off at lower temperatures. This is why oxide coated cathodes are only orange, while directely heated cathodes glow yellow white, even if they ARE thoriated tungsten. So - higher temps, more electrons at the cathode for the tube to work.

(2) What do the electrons do out there?
If there's no biased plate for them to run to, they wander around and fall back into the cathode. If there's a biased plate, some of them run for it. If less than the total the heater can boil off the cathode are sucked away by the plate, there is an equilbrium reached where the electrons falling back into the cathode equal the amount boiling out, making an "electron cloud" over the cathode. This is the reservoir for tube current conduction, and it also provides some help in **extending** tube life as it gives some shielding from gas ion bombardment from residual gas, as well as other reasons. So it's GOOD for there to be an electron cloud.

(3) How do you kill the electron cloud?
Two ways. You can reduce the thickness of the cloud by dropping the temp of the cathode, or by sucking more electrons away than the cathode can provide at that temperature.

(4) That would be bad, right?
Yes.

(5) What tubes need lots of electrons for their electron cloud most?
High current and high power tubes. The broadcast industry in particular has very, very high power and current tubes, so they really, really need lots of electrons, and tube life goes down if the heater current-voltage-power-temperature drops. So they are very protective of their electron clouds and hence filament temps.

(6)What tubes need lots of electrons for their electron clouds least?
Small current tubes. Hmmm. Much like the archetypal 12AX7. If a receiving tube is run well below its max current, and most of them are, then you can still have a functional electron cloud at lower temps from lower heater power, and hence lower heater voltage and current. This has the advantage of lowering the thermal stress on the filament at each power on-off cycle, less metal boil-off from heating, and in general longer life kind of things. If you can use that.

If 12AX7 ran acceptably on 5-6.3 volts, it would say so in the spec sheet.

I don't think they would. During the "Golden Age", it was better to be average than best. Besides, getting to +/-10% on ANYTHING was really good at that time. I suspect that if they could have specified special high-precision heater voltages for lower heat and longer life and made money from the military at it, they would have.


I'll go see if I can find my back up data. You do too, OK?

[big_teee]

TAD data sheets show 6.3v + or - .5v
That would be 5.8 to 6.8v.
http://www.tubeampdoctor.com/images/...sion%201_0.pdf
Valve Wizard says " Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V"
The Valve Wizard

[Jazz P Bass]

I have a Hartke HA5500 here.

The Vdc on the heaters is 7.2.

Go figure.

[g1]

I have a Hartke HA5500 here.
The Vdc on the heaters is 7.2.

For fun, see what you need to drop the incoming AC to (variac) in order to get 6.3V heaters.

[Wombaticus]

Do you have any external facts to support this contention? I'd love to see it if you do. I can go dig out my reference eventually. It was a large-numbers test of tubes, conductance, and usable life. I want to say "GE", but I'm not sure on that fact.


While you and I are still looking up our reference materials, let's reason about the problem.

RG, your thoughtful comments notwithstanding, I do seem to recall a figure reproduced in my old copy of "The Audio 'Cyclopedia" that purports to show just this detrimental effect of low heater voltage on tube life. If memory serves, low voltages were actually worse than high. On the other hand, the AC itself is something of a not-entirely-reliable second-source, but perhaps it's a place to begin digging for the primary reference.

EDIT: I'm away from home presently, or I'd fish it out myself