Help with a battery powered tube preamp

Can you post the schematic or tell me where to find it?

 

The 1M resistor is called a plate loader resistor.

Tubes amplify current, not voltage. A potential (voltage) at the grid changes flow in the electron current from cathode to plate depending on the charge present. A resistor "loading" the plate is used to transform the current change to a voltage modulation. Higher values increase the voltage swing for a given current, which allows for feeding higher-drive-requiring stages. Downside is, you get a higher output impedance which needs to be compensated for at the next stage's grid.

The reason the schematic says it requires more like a 4A transformer is that the heaters are a significant sap source of power. Your heater requires 150mA. The actual amplification circuit will only consume less than 1mA.

The tube will not draw more power than is needed, and its main limiting factor is the emission ceiling of its plate. That is, the plate (anode) has finite surface area and will not be able to draw more electrons past a certain point. That is the range at which you enter 'overdrive' distortion. This is most often done intentionally to achieve the harmonic effects which tubes display when they cutoff on the waveform.

 

The circuit refered to does not specify B+ voltage.

The tube (USA) needs 12V on the heater, some will run on a B+ as low as 12V.

1M resistor with 100V across it will pass 0.1mA. This is the maximum that could be passed. If the B+ is lower and there is voltage across the tube, then the current will be less.

 

Hi Cernnunos5 and welcome to the Electronics Point forums

I'm surprised you chose the 12AJ6. It contains two diodes, which would not be useful, and one triode, and is not a common component. Valve (tube) audio preamps usually use the ECC83/12AX7, which has two triodes, which can be cascaded or used for left and right channels in a stereo application.

As Solidus pointed out, the 150 mA specification applies to the heater. You need to apply 12.6V (nominal) (AC or DC) between pins 3 and 4 to make the filament glow; the valve will draw nominally 150 mA through these pins.

Small-signal valve-based circuitry generally runs with a B+ voltage around 200V. Valves can be used at lower voltages, but running B+ at 12V is pretty unusual - at least, for standard valves like these, and I would expect the amplifier's characteristics to change quite a lot at that voltage.

A lot of the famous "valve sound" is due to distortion in the output stage, which occurs when an output transformer is used; the transformer is often driven by two or more valves in push-pull configuration. I think you would be hard pressed to find anyone who could tell the difference between valve-based and semiconductor-based preamplifiers in a double-blind listening test, unless they were hearing limitations caused by the low B+ voltage!

If you really want valves in your signal path, you can generate a 200V B+ supply voltage from 12V using a boost or flyback converter. The anode current will typically only be in the milliamp range so the total power will only be a few watts.

Switching power supply circuits do generate some interference but careful mounting and use of series inductors and decoupling capacitors should prevent any problems with noise getting into the signal path.

If your concern is aesthetic, you could consider powering the filaments of the valves but not connecting them in the signal path. I imagine a lot of budget audio gear uses this trick, because "toobs are cool, don't you know", though I've never taken any apart to verify this.

If your customers are slightly savvy, you might need to add circuitry to simulate the gradual warm-up of the valves and/or to detect when a valve is removed and kill the signal. In that case, adding a voltage booster and actually using the valves might work out simpler.

The "load line" is not an actual part of the circuit; it is a graph that plots the anode voltage against the anode current. You need to study common cathode amplifiers to understand how the valve is used to amplify a signal, then you should be able to understand the purpose and meaning of the load line.

Valves operate at relatively high impedances - at least, when compared to typical semiconductor circuitry. The circuitry the output of your valve stage feeds must have a relatively high input impedance (preferably higher, or significantly higher, than the anode load resistor value).

Please do some more reading and research and get back to us with your ideas and questions.

Edit:

Oops I just looked more closely at the data sheet for the 12AJ6 and realised why you chose it - it's designed to operate with a 12V B+ rail. I guess if you can still obtain enough of them, they might be a reasonable option. I doubt anyone still manufactures them nowadays... What's the availability situation?