Dropping heater voltage

[gmoon]

I've been experimenting with some oddball projects. My current build uses a fairly large power transformer. The filament secondary tested about 6.7 V, no load. I just assumed the voltage would drop to a safe level when lighting the tubes. Heaters are wired as AC, no rectification, etc...

However, during testing, the preamp tube heater (12AX7) got very bright--and burned out. When I tested the voltage with a different VOM, I got a 7V reading. I should mention that the PT once powered 11 or 12 tubes, so no way would three tubes drop the voltage far enough..... Doh! Should have thought of that before.

So my thinking goes something like this:

--The 3 tubes in parallel draw a spec 2.7 amps @ 6.3

--So total resistance is about 2.3 ohms

--The way I figure, dropping the 7 V to ~ 6.3 would require a resistor around .25 to .3 ohms...(or maybe .4 to be safe.)

Am I way off here, or close? And am I missing a simpler solution?

[Bruce / Mission Amps]

I've had to add .5 ohm @5watt power resistors in each filament lead lower the voltage a bit.
That would be after the PT and before it goes to any tube sockets etc...
You can also use some 6a, low voltage diodes... I think it might be better to see if you can find some higher current Ge diodes for that instead of Si.

[Don Moose]

3 12Ax7 tubes in 6.3V mode drawing 300mA each -> 900mA.

7V - 6.3V --> 0.6V

0.6V/0.9A --> 0.66666 ohms
0.6V * 0.9A --> 0.54W

Some current-sensing resistors in series with the heater leads should to it --- or:

6.3V/300mA --> 21 ohms per dummy heater load. Line up a few and keep adding dummy loads until the voltage is where you want it.

6.3V * 300mA --> 1.89W so buy 3W or 5W parts.

Hope this helps!

[gmoon]

3 12Ax7 tubes in 6.3V mode drawing 300mA each -> 900mA.
.....-----.....
Hope this helps!

@Don--only one tube is a 12AX7, the other two each draw 1.2A per...(6DG6GT's , I did say it was an oddball project. )

@Bruce-- Would one .5 ohm resistor in series with the three paralleled tubes work? Otherwise, for three separate resistors it looks like different values for at least one. Not really a huge problem, but inserting one is easier than three....

(edit: but with a single resistor, if one tube filament fails then the others might follow, eh?)

[Enzo]

Take a pair of diodes and parallel them. Make them face opposite directions so the pair will conduct both ways. Insert them in series with the heaters. That will drop the voltage by one semi junction drop - about half a volt. Need more? Two in series.

They won't dissipate heat like a resistor, won't use any power. VOltage won't be an issue, but use appropriate current rated diodes.

[daz]

Mine reads 6.7v. Is mine high enough that I should i be thinking about doing this, and does it affect sound?

[Don Moose]

Tube heater voltages are supposed to have a +/-10% tolerance - 6.3 * 1.1 = 6.93

I wouldn't worry - you have some headroom.

[Don Moose]

That's why I show my math - if I've boogered it up in some way it will get spotted and complained about.

[gmoon]

That's why I show my math - if I've boogered it up in some way it will get spotted and complained about.

Naw, it was a reasonable assumption--I wasn't very specific there...

@Enzo: Great idea, thanks! Use diode voltage-drop. Now to find some diodes in the 3A to 5A range....

(oh, and thanks Bruce--you were making the same point.)

[black_labb]

That's why I show my math - if I've boogered it up in some way it will get spotted and complained about.

that \s what you think, but noone spotted the 7v-6.3v=0.6

[Enzo]

Hey, some of us put 2 and 2 together and get 22.

I think 7v-6.3v=0.6 works in metric...

3 and 5 amp diodes are common enough, aren't they? I used to carry MR751's in my tool box (in another industry) Those are 6 amp. (Allied sells MR754 for 66 cents) The common 1N5400 series are three amps. I carry FR301s for switching supply rectifiers. Those are 3 amp fast recovey. MAke sure to use conventional diodes, not low drop types like Shottky.

ANd if you have some of those bridges like Marshall heaters use, half of one of those could be used.

[Earl]

"...the preamp tube heater (12AX7) got very bright--and burned out..."


Here's my question - If, according to tube specs. heater voltage is +/- 10%, and the upper limit would be 6.93VRMS, is it likely that 7VRMS would light up a tube like that?

Not saying that one shouldn't get it to center of spec, just wouldn't think an additional 70mVRMS would push a heater into super nova territory.. :-)

[Enzo]

I would agree, there is not likely a causal relationship, but still not a bad idea to drop a 7v heater supply a half a volt.

[gmoon]

"...the preamp tube heater (12AX7) got very bright--and burned out..."

Here's my question - If, according to tube specs. heater voltage is +/- 10%, and the upper limit would be 6.93VRMS, is it likely that 7VRMS would light up a tube like that?

I dunno. Both the 6DG6GT's survived fine.... But the heater elements were bright orange when on.

[gmoon]

Added a couple diodes (chubby bridge diodes I had handy) and the heaters are happy now, @~6.3V.

Beats me why the "International" 12AX7 flared out; replaced with a Raytheon. The first tube was used, maybe it was "it's time" and the higher voltage just ushered it out...