Heater voltage too high!

[thomasdj]

Hi,

I know that this is a know issue but I can't find any thread describing directly what to do..

I am building a 100w amp with 4xEL34 and 5x12AX7 tubes and I measure the heater voltage to 7.2VAC. The amp works fine so far but I wan't test it further to avoid frying the tubes.

How do I get the voltage down?

Thanks

Thomas

[MWJB]

Select the next highest voltage PT primary tap, e.g. if you have 120VAC connection wired up, switch to 125V. This will also have a knock on effect, dropping B+ & bias supply voltages.

I would really want to see <7VAC at the heaters.

If you don't have any scope to rewire PT primary taps, either live with it & see how you go (yes 7.2VAC is a little high but there are plenty of reliable amps out there running 7VAC), or you can add resistors (10W? Measure power dissipated by resistor & use double that rating) in series with the heater secondaries - be aware that due to the high current you may have to experiment with very small resistances like .1ohm?

Alternatively, you could experiment with 6550 in place of EL34 if you have a suitable 6.3VAC current rating on the PT (8-10A?), these draw a little more heater current (what PT & Current rating do you have?) and may pull the voltage under 7VAC...as long as you can live with the tone difference?

[tubeswell]

I've heard of putting a couple of diodes nose to tail in series with the heater rail to drop the voltage by 1/2 volt or so. (I think it was from Enzo?)

[thomasdj]

Hi MWJB/tubeswell

Sorry for hiding information

I use a mesa boogie roadking PT and don't know the power rating but I know it's well overrated (150w/three rectifier tubes and six EL34 power tubes). However, I didn't thougt about switching to "higher" primary voltage but I remember that mesa has a switch doing that..so guess that's why the heater are that high...

I think I'll also try the diode solution (I have now low ohm/high watt resistors in stock).. Just to be sure tubeswell, I've attached a picture of how I understand it, - is this correct?

Thank you very much

Thomas

[thomasdj]

Hi again,

Sorry for the trouble!! I just found this thread:

http://music-electronics-forum.com/s...t=heater+diode

With this pic illustrating the diode setup..

Thanks

Thomas

[MWJB]

Be aware that you amp is drawing 6.7A of heater current, twice what the amp in the example is drawing, see Tom's recomendations re current draw.

[HTH]

Hi again,

Sorry for the trouble!! I just found this thread:

http://music-electronics-forum.com/s...t=heater+diode

With this pic illustrating the diode setup..

Thanks

Thomas

I used this trick in a build of mine and it's been going strong for more than a year now.

[thomasdj]

Be aware that you amp is drawing 6.7A of heater current, twice what the amp in the example is drawing, see Tom's recomendations re current draw.

I also noticed that...

Which diodes should I use? ..the only ones I can find is rated 6amps.

Thanks

Thomas

btw..HTH, nice to know that someone has tried this before, and that it works!

[Rob Mercure]

Since I don't think that anyone else has asked (looked for it) - are you measuring the heater voltage with the tubes in circuit or out. If out of circuit it's going to measure high - just about what your report. Put the output tubes in and measure the voltage - I suspect that this is your problem.
Rob

[thomasdj]

Since I don't think that anyone else has asked (looked for it) - are you measuring the heater voltage with the tubes in circuit or out. If out of circuit it's going to measure high - just about what your report. Put the output tubes in and measure the voltage - I suspect that this is your problem.
Rob

Hi Rob,

sorry for that one...the 7.2 voltage AC is measured under load..

Best regards

Thomas