Heater voltage drop under load

[nsubulysses]

So, maybe this is very incosequential but just figured I'd ask if anyone has a thought.

Using Hammond transformers that don't actually give you the correct heater voltage because they have a 115V primary and 125V primary for PT, I was wondering if it's actually a bit of a blessing. I am asking because now I will order a similar transformer from heyboer but was considering just getting 120V primary for correct 6.3V heaters. When you crank the amp all the way up to full power, the heater voltages drop slightly, but then they are closer to what they should be.

Is this good, inconsequential, or actually bad because 6.9VAC heater voltage is pushing it.

I used a variac to get 120VAC from the wall so if wall voltage at some place is 125V, heater voltage would definitely exceed 7V I at tube closest to PT with least voltage drop.

In an amp that has 6 KT88 and 4 12ax7 and using 115V primary on PT and 120VAC from wall I get

KT88 closest to PT
6.9VAC at idle
6.7V at full ouput

V1 12AX7, last tube in heater chain, farthest away from PT
6.6V at idle
6.3V at full ouput

[nsubulysses]

Cranking variac up to 125VAC wall voltage I get

KT88 closest to PT
7.1V at idle
6.8V cranked

12AX7 farthest from PT
6.7V at idle
6.4V cranked

It seems like my heater voltage is too high and has potential to exceed the 10% max deviation from 6.3VAC

[olddawg]

Honestly... there are many people here wiser than me.. but I wouldn't worry to much about it unless the PT is cooking hot... especially if it's a one off build. Maybe it's my area but I have a volt meter built into my pedal board. My worry is low line voltage. I don't think I've ever seen 125vac, its usually 115-119. I guess you could put a couple of diodes in the filament string in parallel with each other with opposit orientation. That would drop it by .6 v if you are really worried. I put a PT for a 50 watt amp in a 20 watt amp a while back because it was what I had. Voltages were a little high. Works, sounds fine even with Chinese tubes.

[Helmholtz]

So, maybe this is very incosequential but just figured I'd ask if anyone has a thought.

Using Hammond transformers that don't actually give you the correct heater voltage because they have a 115V primary and 125V primary for PT, I was wondering if it's actually a bit of a blessing. I am asking because now I will order a similar transformer from heyboer but was considering just getting 120V primary for correct 6.3V heaters. When you crank the amp all the way up to full power, the heater voltages drop slightly, but then they are closer to what they should be.

Is this good, inconsequential, or actually bad because 6.9VAC heater voltage is pushing it.

I used a variac to get 120VAC from the wall so if wall voltage at some place is 125V, heater voltage would definitely exceed 7V I at tube closest to PT with least voltage drop.

In an amp that has 6 KT88 and 4 12ax7 and using 115V primary on PT and 120VAC from wall I get

KT88 closest to PT
6.9VAC at idle
6.7V at full ouput

V1 12AX7, last tube in heater chain, farthest away from PT
6.6V at idle
6.3V at full ouput

If the PT has a 125V primary, why don't you use this? What is your actual mains voltage?

[J M Fahey]

Don´t overthink it.

IF you will have a custom made transformer, custom means custom, whatever you want, so order it for 120VAC primary (or whatever you regularly have ar your outlets) , correct 6.3VAC filaments and call it a day.

If you use an off the shelf Hammond, no actual 120V tap but near enough 115/125, actually measure what you have (I would be surprised if it were *exactly* 120V) and pick the closest tap.

Your amp will work fine.

[nsubulysses]

If the PT has a 125V primary, why don't you use this? What is your actual mains voltage?

using 125V rather than 115V primary drops B+ from about 580V to 530V so I can't get as much power.

Actual mains is probalby about 121-123V. I have measured as high as 128V at an old apartment. I have measured as low as 115V at an old practice space in a dingy basement.

[nsubulysses]

Don´t overthink it.

IF you will have a custom made transformer, custom means custom, whatever you want, so order it for 120VAC primary (or whatever you regularly have ar your outlets) , correct 6.3VAC filaments and call it a day.

If you use an off the shelf Hammond, no actual 120V tap but near enough 115/125, actually measure what you have (I would be surprised if it were *exactly* 120V) and pick the closest tap.

Your amp will work fine.

Using 115V primary tap with a HT secondary that is 800V CT I get about 580V

so I guess I will order a transformer with 120V primary and 840V CT so I can get 6.3V filaments and 580V B+ all day.

However, the real crux of my question is heater voltage drop under load, and wondering if having ~7VAC or so on heaters (at idle) would possibly be bad for tubes. When you crank the amp loudly, it drops to 6.3-6.5V or so. If I had a true 6.3V it would dip to about 6V or so when amp is cranked. I guess it doesn't matter much, other than the lore of going outside of 10% margin for 6.3V wearing out tubes faster.

This will be for a production amp not a one off so I would like to do everything I can to make it work good.

[Helmholtz]

7V heater voltage is above +10% and thus is critical for tube life. Did you consider that the variac increases sag?

[nsubulysses]

You are correct about the variac dropping more, something I never thought about

Wall voltage is 119V. Plugging amp into wall it does 59V into 16 ohms for 217.5W. Putting variac at 119 and plugging amp into that it can only do 57.5V, 206W.


With 119V directly from wall, heater voltage at power tube closest to PT is 6.9V. damn

[trobbins]

what is heater volts when using 125v primary tap.
your results already show that transformer regulation is tight, and that your heater wiring is not too good.

[J M Fahey]

what is heater volts when using 125v primary tap.
your results already show that transformer regulation is tight, and that your heater wiring is not too good.

Fully agree.
Consider using *two* filament lines, a short and thicker one feeding power tubes, a separate normal (not "thin") one feeding preamp tubes.
Flip a coin for the PI

And order your future transformers with 6.4V or 6.5V nominal filament secondaries, so you start slightly high but safe and drop under load either gives you perfect voltage or slighgtly low but still acceptable.

As of +V

Wall voltage is 119V. Plugging amp into wall it does 59V into 16 ohms for 217.5W. Putting variac at 119 and plugging amp into that it can only do 57.5V, 206W.

forget all about it, that "difference" is **inaudible** ... serious.

[g1]

Wall voltages will always trend upwards. I'd go with the 125V. We are always around 124V here.
We all want to build something that will last, like the old Fenders or Marshalls. Some of them are well over 50 years old. Consider how much the wall voltages have increased in their lifetime.
Power distribution is about minimizing losses which means higher voltage is preferred as less current is required. This allows for thinner wires running to the transformer at your house/neighbourhood.
Even automotive is trending toward 24V from the traditional 12V (or 6V even before that).