Soundtronics of Chicago

[martin manning]

My question is "Would it be problematic for heater voltage with something like a four 6L6 amp to run two 6L6's or four 6V6's? Is that going to raise the heater voltage in a manner that hurts the tubes?"

So not loaded vs. unloaded (which obviously would have a significant current draw difference) but instead loaded with 6L6 vs loaded with 6V6 (which would have less of a significant difference in current draw than loaded vs. unloaded)

In some of the scenarios being discussed the filament current could drop by half or more (0.45A for 6V6 vs. 1.6A for big audio tubes), which might result in a 5% increase in voltage based on the unloaded-loaded difference. Here's a graph showing the effects of varying heater voltage on several parameters, where a 5% increase is shown to cut life by ~45%. Unfortunately, the site (https://www.ozvalveamps.org/heaters.html) does not provide a reference.

[10thTx]

Voltage drops in the wiring are important as the graph below shows and the voltage at the valve pins must be well within 5% of the rated value. The rule of thumb which also applies to light globes is “five percent up or down halves or doubles life”.

I appreciate the response and the information! Thank you!

With respect, 10thtx

[10thTx]

Some more resources .................... from Valve Wizard http://www.valvewizard.co.uk/heater.html

The heater voltage should be kept within +/-10% of its nominal value for optimum valve performance. For 6.3V heaters that means 5.7V to 6.9V, though it is even better to stay within +/-5% if you can (6V to 6.6V). Many guitar amps suffer from rather excessive heater voltages. This is sometimes because mains voltages are higher today than they were when the transformer was orignally designed (this is common in the US where wall voltages have risen from around 110V to 117V today)

So the question becomes if we replaced an amp with two 6L6's and installed 6V6's instead ................ what change in voltage in heater filament would take place? 2% 5% 10% ????

[Stevem]

It would depend on the quality of the power transformer!

Common ones used in non medical gear or top shelf audio test gear have no better then 10 to 12% regulation ability to them, so do the math!

If you had a typical amp with let’s say 4 preamp tubes and 4 output tubes, and you remove one of four output tubes that was pulling 1.6 amps, then that would remove more then 1/5 of the Heater load.
This is then at least a 25% change, so the heater voltage if the transformer was good for 10% regulation would go up 15%.
A 15% increase in heater voltage puts you well into the danger area of 7.2 volts!

[martin manning]

Common ones used in non medical gear or top shelf audio test gear have no better then 10 to 12% regulation ability to them, so do the math!
If you had a typical amp with let’s say 4 preamp tubes and 4 output tubes, and you remove one of four output tubes that was pulling 1.6 amps, then that would remove more then 1/5 of the Heater load.

4 x 0.3 + 4 x 1.6 = 7.6A removing 1 x 1.6A is a 6/7.6-1 = 21% reduction

This is then at least a 25% change, so the heater voltage if the transformer was good for 10% regulation would go up 15%.
A 15% increase in heater voltage puts you well into the danger area of 7.2 volts!

Here's the approximate* math: If the voltage at the original heater load was 6.3 and the regulation is 10%, unloaded voltage is 7.0, and the voltage change is 0.7/7.6 = -0.092 Volts/Amp. Pulling one output tube should result in an increase of 1.6 x 0.092 = 0.147V or 2.3%, to 6.45V

If all four output tubes were switched to 6V6, that would be a reduction of 4.6A, and an increase of 0.423V or 6.7%, to 6.72V

*this is an approximation since heater current is a function of heater voltage and the transformer voltage is assumed to be a linear function of current

[10thTx]

I have an amp here that I can experiment with. It currently has 6V6's in it. I can change that to 6L6's and measure the change in the filament heater voltage and report back. I've played it more years with 6L6's in there than the 6V6's

This is one of my two amps where I use 6K6, 6V6, 5881 or 6L6's and simply change the rectifiers from 5Y3,5V4, GZ34 and solid state plug in. I've been doing these swaps for 10yrs (or more?) with no issues and never had a tube go bad. Been using the same output tubes since I built the amp. Occasionally swap out preamp tubes for different gain levels, but I honestly don't remember one ever dying on me or going bad?

To experiment with this, I'll only change the output tubes and nothing else and let you guys know what the change is in heater voltage. I'll try to get this done in the next 3 days and report back. You guys have got me curious about this. My current notes on voltages show 6.5v for the heater.

With respect, 10thtx

[statorvane]

10th,

Not meaning to go off topic, but I have to say that is a really nice cab!

[10thTx]

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.

So ............................

IF changing four 6L6's to four 6V6's yielded an increase of 3.2% (1.6% x 2), then the voltage would be 6.81v on the heaters. 6.3v X .05 (percent increase in voltage) would be 6.61v 6
6.3v x .10 = 6.93v [This assumption assumes an amp giving similar increases to the amp I've experimented with which may not be the case at all changing between 6V6 and 6L6 tubes]

6.81v is still under the 6.9v (10% tolerance range) So, I am understanding this to still be within a reasonable range per the following comment below.

The heater voltage should be kept within +/-10% of its nominal value for optimum valve performance. For 6.3V heaters that means 5.7V to 6.9V, though it is even better to stay within +/-5%

So, if new production power tubes "typically" lasted for around 500hrs and a person played those tubes 2hrs a week, then they'd last about 5yrs. If the tubes were pushed to where they lasted only 55% of their "typical" life expectancy, then you'd have tubes last about 2.75 yrs,

I realize someone else's amp may yield different results that might concern someone.

With respect, 10thtx

[martin manning]

Did you happen to get the unloaded heater voltage? What other tubes are in the circuit?

[10thTx]

I did not get the unloaded heater voltage. See the schematic a couple posts up for the other tubes.

With respect, 10thtx