Electronic – Low power tube headphone amplifier design

Ok, if you want to be a little more meticulous.

Assuming this is a classic AB tube amp, it should run around 70% of its plate dissipation in quiescence and about 100% on average when the full signal is applied.

Preamp tubes usually draw constant average current of about 3mA per envelope (assuming 12AX7), and you can roughly estimate total plate+plate load resistors dissipation from there. Consult the anode curves for particular bias modes. I believe this figure should be around .75W per envelope.

The rectifier dissipation can be estimated from the maximum anode current (for safety) and the internal resistance of the rectifier (from the datasheet curves) or from observed voltage drop. I got about 10W which conforms with my experience that rectifier tubes are usually really hot.

Don't forget to add filament power as all of it is converted to heat.

Output transformers do not usually heat up if they are of any quality. Power transformers usually heat up to 80-90F above ambiance. You can estimate power figure if you know the type of the core used - usually it has a specific W/kg at given Tesla rating. You can assume the induction to be 1.5T (common for commercial trannies). Add resistive losses of the windigns as well.

P.S. Whatever results you get - do NOT mount tubes horizontally - stick to upright/upside-down variants only.

P.P.S. On a more general note, I have seen amps that fried their electrolytic caps which were situated way to close to output or rectifier tubes. I have never seen the heat do any other kind of damage.

Cheers.

One specific datapoint : some years ago I made some measurements on a 12 watt valve amplifier (Leak TL/12+) it met its rated distortion specification (0.1%) at 12W rms (at 1kHz).

However, by the time distortion reached 1% it was generating 20W rms, while most transistor amplifiers would have started clipping immediately beyond its rated power.

I stopped there and didn't find out where it reached hard clipping or even 10% thd.

(At high levels, very little of that distortion was 2nd harmonic, thanks to the push-pull output stage)

As for why : with transistors, it is easy to add gain and thus employ more negative feedback, and the emitter followers typically used in the output stage are relatively linear; so it is easy to maintain linearity right up to the point where the output hits the supply rails.

With a valve amplifier, additional gain stages are expensive, so you have to meet your linearity target with relatively little negative feedback. That means designing the hardest bit - the output stage - to be reasonably linear across as much of its range as possible, and restricting the rated power to that range. There are tricks to increase that linear range somewhat, such as connecting the screen grid to an intermediate tap on the output transformer, somewhere between V+ (pentode connection) and anode (triode connection) - tap at 43% for the so-called "ultra-linear" connection.

Beyond that range it will still amplify, and produce additional power, but no longer meet its distortion specification, up to some limit where real clipping will occur. Triode-connected gives the smallest linear range (therefore the lowest rated power) but, by the same token, offers the largest reserve of power in overload.