Heater voltages

[Nuuk]

I've just been checking the voltages in my DIY valve buffer. I measure 6.2v on one valve heater, and 6.5v on the other (I'm using one side each of two valves). The two heaters are wired in series and I wondered if this is a 'normal' difference, or it something is amiss?

 

[DF96]

No problem. A few percent difference in heater resistance is of no consequence.

 

[Rod Coleman]

6.2V is no problem, especially with small-signal valves.

6.5V is within specification, and is OK, but for long-time running, or if the valve is not easy to replace (eg some lovely old OSRAM rarity) it is worth avoiding overvoltage. 6.15 to 6.25V is worth aiming for. Even a few percent extra will reduce the cathode's lifetime, since it's running hotter.

If it were mine, I'd add 100-ohm resistor [1W] across the heater with 6.5V on it.

 

[Nuuk]

Many thanks guys. I'll add the resistor to the one running hotter (5687).

 

[Nuuk]

Rod, did you mean 100R or 1K? 1K2 actually brings the voltage down to spot on 6.3v.

 

[Rod Coleman]

Nuuk, I was only guessing!

If you find that 1,2K works, then that is fine. Forcing it down a little more may be worthwhile.. perhaps 6.2V would be a perfect perfect solution.

Maybe, you will find that the sound improves, somewhere between 6.05V to 6.25V - could be worth experimenting!

 

[Nuuk]

Thanks Rod.

I always measure stuff before closing the case but wondered if you had perhaps used a calculator and got the decimal point in the wrong place.

Not being pedantic but in another situation hazarding a guess for somebody less experienced in DIY could spell disaster. Better just to say something like try some resistance across the heater with the higher voltage.

 

[Rod Coleman]

OK! I should have said that this was a guessed value.

But one should never build a heater circuit without measuring the voltage, right at the pins of the base. The lifetime of the valve is VERY dependent on getting this right.

 

[Palustris]

Another solution to too high a heater voltage is to "adjust" the voltage by putting a pilot light either in series or parallel whichever is more appropriate for the application. The bulbs come in a variety of voltages and currents so you can adjust the voltage by changing the type of bulb.

 

[Nuuk]

Thanks again for the suggestions guys. It has all been taken on board.