Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on December 29, 2021, 04:10:44 pm
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I was always under the impression that heater filament voltage didn't have to be an exact 6.3v? And that 5.7v to 6.8v range could work without a concern?
Does anyone have a link that addresses that issue? Or knows what range is "safe" for tube 6.3v heater filament voltages to properly operate? What's too high and what's too low in voltage?
Example: If I had a cathode biased push/pull KT66 amp with 400v on the plates and I changed it to using push/pull 6V6 amp with 400v on the plates, would this cause the voltages on the heater filament to raise and lower the life expectancy of the tube? The KT66 draws 1.3A and the 6V6 draws .45A
OR if I had a AC30 with 4 EL84's and simply pulled two of them & adjusted the power tube cathode resistor accordingly, would this increase the heater filament voltage and cause a problem?
With respect, Tubenit
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Tube data sheets say +/- 10% is fine.
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Unless using regulated supplies, no secondary voltage can be closer to its intended nominal than is the mains voltage fed to the primary.
Each region has an acceptable range around the mains nominal, but it’s rarely tighter than +/-5%. So to keep within +/-10%, variations due to components (eg those described in post #1) mustn’t be allowed to be worse than +/-5%. Otherwise, the additive effect with mains variance may take things over the +/-10% limit.
https://tubedata.altanatubes.com.br/sheets/093/6/6L6GC.pdf
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Does anyone have a link that addresses that issue? Or knows what range is "safe" for tube 6.3v heater filament voltages to properly operate? What's too high and what's too low in voltage?
Not sure about all of the tubes you mention, but the heater coils in the 12A(X,T,U)7 series are helical tungsten, and function much like a light bulb would. Not sure how high a voltage you would need to apply to burn out the coil, but you could probably test this in your lab . . . sounds like fun!
As for under-voltage, again not sure what that would be, but the result would be unavailability of electrons on the cathode, and the tube would not function properly as the required heat is not present. Another fun experiment, best to do this one first. How cold can you go?
If I had a cathode biased push/pull KT66 amp with 400v on the plates and I changed it to using push/pull 6V6 amp with 400v on the plates, would this cause the voltages on the heater filament to raise and lower the life expectancy of the tube?
No, with this caveat: The heater circuit voltage is bound at the upper range by the windings and the primary voltage. On the other hand, if you were to overload the PT heater windings (for example, by using too many tubes with high current draw referencing the spec of the PT), voltage would be lower, but you'd be burning up the PT.
OR if I had a AC30 with 4 EL84's and simply pulled two of them & adjusted the power tube cathode resistor accordingly, would this increase the heater filament voltage and cause a problem?
No problem with the heater circuit in this scenario.
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Thanks for all the responses! It is appreciated.
With respect, Tubenit
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Here's another source, confirming what the guys have already said about +/- 10%
The Valve Wizard (http://www.valvewizard.co.uk/heater.html)
Read under Dropping Heater Voltage
OR if I had a AC30 with 4 EL84's and simply pulled two of them & adjusted the power tube cathode resistor accordingly, would this increase the heater filament voltage and cause a problem?
The heater voltage will raise when you pull the 2 EL84s but "shouldn't" make enough difference to create a problem unless you were already on the high side with all 4 in.
Changing the output tubes and the current through those tubes will not make a big difference in the voltage on the heater winding. Unless you are extremely loading one winding it shouldn't have a great effect on the other.
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In some datasheet the specific voltages of the range are specified, however, as told + - 10% is generally fine
Franco
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Tomer addresses heater voltage early on http://tubebooks.org/Books/Atwood/Tomer%201960%20Getting%20the%20Most%20Out%20of%20Vacuum%20Tubes.pdf
p11-17.
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http://tubebooks.org/Books/Atwood/Tomer%201960%20Getting%20the%20Most%20Out%20of%20Vacuum%20Tubes.pdf
Also check out chapter 10 pages 145-151.
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I was always under the impression that heater filament voltage didn't have to be an exact 6.3v? And that 5.7v to 6.8v range could work without a concern?
Does anyone have a link that addresses that issue? Or knows what range is "safe" for tube 6.3v heater filament voltages to properly operate? What's too high and what's too low in voltage? ...
See Page 4 of this Hickok Tube Test Manual (https://bama.edebris.com/download/hickok/533a/533a.pdf).
Hickok's Life Test was to drop the heater voltage of the tube down to the next-lower voltage setting. So when testing 6.3v tubes, the Life Test dropped the heater voltage to 5v.
Tubes with a strong cathode will show almost no change of Gm when switching to the lowered heater voltage. However, marginal or weak tubes may initially measure "like new Gm" but then fall rapidly when the heater voltage is lowered.
What that means to you is you will likely experience a reduced useful lifespan if the heater voltage is too low. How much? No one knows for certain. Is it still an "Acceptable lifespan"? That's up to your judgment.
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Uki - the thing to do if worried about filament voltage affecting tube life is to regulate the filament voltage. Throwing this into the discussion.
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I personally have ZERO concern about this suggested concern of using tubes that draw less heater current increasing voltages on the heaters of two of my amps where I change out tube rectifiers and switch between 6K6, 6V6, 5881 and 6L6's. I've been doing this for years and have found no problems doing that.
Someone on another forum suggested it would cause tube failure for me. So, I posed the concern here to see what you guys would say and I took one of my own amps and checked voltages switching between 6L6 and 6V6. The results of experimenting with this is below:
Today the wall voltage was 122.9v. It varies considerably at my home and usually is around 121v. I've seen it at 117v before.
With Tung-Sol 6V6's, the heater filament voltage was 6.6v (which I understand to be acceptable) 6.6v is a 4.7% increase above 6.3v (which is still under 5% increase)
With Electro-Harmonix 6L6's, the heater filament voltage was 6.5v. 6.5v is a 3.17% increase above 6.3v (which is still under 5% increase)
There was 1.5% increase in voltage ( percentage increase NOT volts themselves) between 6L6 and 6V6 tubes and only .1v actual increase in voltage.Given these very 6V6's and 6L6's are over 10yrs old (probably closer to 14 yrs old), I am not going to consider this a concern for my amp.
With respect, Tubenit
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Who has had a tube failure due to heater failure??
(Since 1969 when the last daily-use series-string TVs were put to curb.)
I've had two. A 12V tube died in minutes at 80+ volts. And a cracked (I assume) heater in a sig-gen which had been thrown into a metal Dumpster after a hard life in academia.
Modern tubes die for many reasons but heater burn-up may be the least likely.
But if you are Furby: "Worried!"