I'm building my own guitar tube amp, and now I have a dilemma. The speaker magnet is so close to the preamp tubes that I'm worry about interference between both.
Should I be worried or that is not a problem at all?
Interference between speaker and vacuum tubes
amplifiervacuum-tube
Related Solutions
The power is the maximum electrical power the speaker can handle without damage. It has only a rough correlation with acoustic power output: some (perhaps even most) of the electrical power also goes into losses in the speaker coil, magnetic losses, friction losses, and so on.
If the speaker is rated for a higher power than your amplifier, that's good. It means it can take all the amplifier can dish out without damage. If the speaker is rated for less, then your amplifier at full volume may damage the speaker, and usually that's bad.
Beware that cheap speakers tend to use meaningless methods to arrive at their power rating, like the peak power they can take at the very peak of a cycle, instead of the average power of the AC being fed to them. When you see those $5 "100W" computer speakers that sound too good (the price, not the quality) to be true, that's what's going on. Your speaker sounds legit, however.
Further reading: audio power at Wikipedia.
Recommended configurations for the 12AX7 are below:
Since the cathode resistor is bypassed for audio frequencies by the 100uF capacitors, gain will be set by the transconductance of the tube under the operating conditions, as shown in the above diagram. Lowering the plate resistance, especially on V2, will allow you drive a lower input impedance input.
Below is your original circuit with your voltage measurements and calculated plate voltages shown.
Best Answer
Since tubes make electrons traveling in vacuum between electrodes they are sensitive to magnetic fields. But in preamp tubes, their grids are so close and the electrons are so fast, a magnet can hardly disturb their path. Nevertheless, some power tubes like beam tetrodes use electrostatic deflectors to concentrate electrons into beams. These kind of tubes must be kept in some distance of transformers and speaker magnets.