Low voltage tubes

[cozza]

Someone have practical experience in using ECC88, 12AX7 or similar tubes with low cathode voltage (40-50V) ?.

Thanks

[Shoog]

Hi there,
The ECC81 works really well at very low voltages in the Musical Fidelity X10D valve buffer. There are copies of the circuit on the net somewhere. The wall socket supply is just 12VAC. Must use some sort of voltage doubling.

Joe Rasmussens Valve buffer for the Gainclones works reasonably well at +/-35V. My experience is that it improves with increased voltages. I personnal am running the ECC85 at 85V and am quite satisfied with the results.

[Shoog]

Correction to the above, the X10D uses ECC88's, as does my own preamp with 85V rails.
Sorry for the mistake, my brain must have been on low voltage.


Shoog

[PRR]

> tubes with low cathode voltage (40-50V)?

Surely you mean low anode voltage?

To a rough approximation, a tube is a resistor. If you can work it at 100V 1mA, you can work it at 10V 0.1mA. The problem is that tubes are all high resistance so the available power gets very small at low voltages.

To a better approximation, many tubes have a "bend" at around 50V on the plate, and at lower voltages the plate resistance doubles. So 10V might only allow 0.05mA.

At very low voltages, 1 or 2 volts, you get into "contact potential" effects. But in any practical audio use, you just can't get any useful output at such low voltages.

To run 35V-50V, just try it. Be prepared to increase the plate resistor, and accept that the output will be weak. If self-biased with a cathode resistor, you probably won't have to change the cathode resistor much or at all. If fixed-biased, change the grid voltage in proportion to anode voltage.

For best results, look for tubes with high current at low anode voltage. Many of the "TV Tuner" tubes are very good, since they had to run VERY high current for high gain and low noise, yet could not waste a lot of heat so they were worked with low anode voltage. Such tubes were often run 100V 10mA, and will often run 50V 4mA or 30V 2mA.

> My experience is that it improves with increased voltages.

This is my experience too. While any tube "can" be run at low voltage, and might be "OK", they almost always "sound" better if you crank them up almost to melting. Both voltage and current (being sure you have plenty of swing as well as bias). I don't understand the fashion for starving tubes. You may as well use a 741 chip for sound and an underpowered light bulb for glow.

[Giaime]

Originally posted by PRR
You may as well use a 741 chip for sound and an underpowered light bulb for glow.

Ah! That would make sense... At least you don't have to search for NOS or military-grade bulbs!

At what about fluorescent lights? I read somewere that they give a brighter tone...