Tube heater voltage

[beasleybodyshop]

How much heater voltage can, say a 12AX7 or 6L6 handle before it causes performance issues?

I was working on my Sovtek MIG 50 and noticed there are two heater taps. one set measures out the normal 6VAC, while the other set going to the power tubes and PI are 18VAC

Anyone run into this before? Will it burn up tubes faster? Any suggestions for taming this? Big ol Zener?

[tubeswell]

Most 6.3VAC heaters can take a certain amount of voltage change and still function (say +/-10%, so thats like 5.7 to 6.9 VAC). Too-low-a-voltage and there's not enough energy to create a space charge at the cathode, too high and you may start to get thermal runaway as the control grid heats up too much.

[beasleybodyshop]

after getting a diagram of the trafo from mercury magnetics i see that one set of heater taps is 4amp while the other set is a whopping 8amps....

I think im just going to use one set of heater taps and heat shrink the other ones.

Update

after disconnecting the "other" 18V heater winding and using the 4amp 6VAC tap as the sole heater source, the amp sounds MUCH better.


I have no clue why they even used the other heater tap. I've heard Martin mention that the transformers in these things are most likely meant for some other purpose and just adapted to the MIG. Live and learn.

[Milkmansound]

possible that those other taps are intended for a DC heater circuit