I am wanting to experiment with the small signal characteristics of a few vacuum tubes I have laying about (and I have many) sort of in a guitar amp or home audio type of role, and am wondering about what the lowest voltage one typically can operate well at. Both my input and output signals are expected to be "line level" (-10dBm, and less than a volt
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peak to peak) and with near unity to double gain, although gain may be left unchecked at times.

...or to put it another way: can I operate a tube off of 12V DC plate to cathode, or will it require nearly a full 120V DC to work well?

The purpose of this circuit will be to experiment with things like how adjusting bias and even heater temp will affect the "sound" of a tube. May even aid in tube selection at times. It is intentionally quick and dirty.

 

Most valves (Vacume tube) need a higher voltage on the anode, I think around the early 1960s there was a range of valves designed for use in car radios that worked with 12 volts on the anode. I can't remember the numbers of any of these valves. The output stage used a single power transistor such as an OC28.

Les.

 

For some light reading on a sleepless night...

 

...or to put it another way: can I operate a tube off of 12V DC plate to cathode, or will it require nearly a full 120V DC to work well?

Depends on the tube or valve in question. I have an old military R392 receiver which uses a 28 VDC plate voltage. I also remember old 6V and 12V car radios of the 50s which used a vibrator & transformer to make HV AC to be rectified to HVDC as the plate voltage for the tubes. Tubes, like transistors have data sheets and many of those data sheets can be found online. For example, a popular guitar audio amp was the 6L6 and here is the data sheet,

Ron

 

As well as a DC supply, you also require a low voltage supply for the cathode heater.
Many of the low voltage (12v) anode types were preceded by 12 as in 12AF6.
Max.

 

Vacuum tubes require an AC supply for the heater or filament. Common AC voltages for this purpose were 6.3 and 12.6 VAC. Most of the tube circuits I learned in college were based on the 12AT7 twin triode, which used a 12.6 VAC filament and a +300VDC supply for the plate. We built amplifiers, oscillators, one-shots, and flip-flops from them.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AT7.pdf

 

Vacuum tubes require an AC supply for the heater or filament.

Bit difficult on a portable application!
They also take DC.
Max.

 

Bit difficult on a portable application!
They also take DC.
Max.

Yes, they do. If you have a power supply available you can take this voltage right from the transformer secondary without any additional circuitry or regulation.

 

In fact the origin for the odd ball voltages of 6.3 & 12.6 originates from the original tube equipment filaments being fed from lead-acid 'accumulators' who's fully charged voltage resulted in these values.
Max.

 

Hello,

Have a look at Technical books online site:
http://www.tubebooks.org/technical_books_online.htm

There is a section called:
Vacuum Tube theory & circuit design

Bertus

 

I have a 1966 version of 'Radio Valve Data' for over 7000 valves and semi conductors from valve manufacturers of the time.
These have all the detailed characteristics listed for each valve.
Max.

 

You might be able to find some old 'space charge' tubes (or use some regular types) for low voltage applications.
http://www.junkbox.com/electronics/lowvoltagetubes.shtml

http://www.tubebooks.org/tube_data.htm

 

Hello,

@MaxHeadRoom , The internet archive has some more valve data books:
https://archive.org/details/vacuumtubemanuals

Bertus

 

Hello,

@MaxHeadRoom , The internet archive has some more valve data books:
Bertus

Thanks, mine is kept on the shelf more out of nostalgia than necessity!
Max.