Hi, Working on an RCA radio that used a 6H6 tube for the audio detector. While going through the Riders diagam on voltage measurements I see only a couple volts across the tube! How can it work with such a small voltage?

Corby Dawson

Its a detector diode it only takes a couple of volts to bias it on .

Think of that little workhorse as a dual diode. In fact you could substitute a 1N34 germanium diode for each section when in this service at lower frequencies. Higher frequencies, not.

Tube could also be used as a rectifier for very small amounts of current within it's boundries, sometimes found in old precision instruments.

The 6H6 (and its 7 pin minature clone the 6AL5), work pretty well with low voltages on them.
I seldom have had to replace one.
I have seen them used as rectifiers over the years also. Mostly in some VTVM's and signal tracers as diode detectors in probes and IIRC, some home brew projects from the radio projects magazines from the "good old days".
I have also seen projects using them as bridge rectifiers in metering circuits (2 tubes).

You would be amazed at how little voltage is really required in some projects to make the tubes work. I made a radio using a 6J5 in a regen circuit and got down to 6VDC on the plate with it still working!

You could try hooking only the heater voltage to the 6H6 and see if there is any voltage between
cathode and plate, with a DVM or VTVM on DC.

Here's a data sheet that includes the plate characteristics:
http://frank.pocnet.net/sheets/093/6/6H6.pdf

It seems that it will work even with a plate voltage less that 1v. Most of the data sheets just show the in-circuit performance.

The 6H6 was designed from the start to have high "perveance"; that is, it needs hardly any voltage to get some current moving. That's what you want in a detector. It so happens that they also have high enough emission and breakdown voltage that they can be used as a power rectifier if you don't need much power, but that's a secondary application.

Dave Wise

A 6H6 or 6AL5 contains very small plates which are very close to its cathodes. Consequently, they begin conducting at much lower voltages than a power rectifier. The trade-offs for this is that the tubes can only handle a few mA of current, and the peak inverse voltage is also pretty low.

Which means you always blow one out if you exceed max AC voltage in a VTVM. Same 6AL5.

I hooked up a 6AL5 in place of the crystal in a crystal set awhile back and it makes a fair detector.

-Dennis