Paleo-Electronics

The Real McTube II was designed with rectified filament voltages because Fred found that the original McTube could be noisy if not done impeccably well.    The original McTube (appearantly) had the 12 V AC from the transformers going directly to the heaters on the tube.

That is the way it’s always done on the old amps and I’m surprised in retrospect that he didn’t remember the simpler trick.   Better amps would place a 100 Ω potentiometer between the two sides of the 12 V tap from the PT.   Of course, I’ve just remembered that too now that my rectified filament is done.    Oh well.

So the way to determine quality with rectification is to ask how close you got to the desired voltages and how much ripple (AC in your DC) remains in the signal.   The only way you are gonna see this is on a scope or a dedicated tool for this.   You might get it measured with a DMM, but I never got that to work.  If the ripple is complex, the DMM will not settle on a value so you will be seeing snapshots that make no real sense.   Since the DMM is reading RMS,  the highest number you see will be about 2/3 rds the total ripple.

Unfortunately, I didn’t take a picture of the ripple in the Skylark when I had it open.   Suffice it to say that it was plainly visible.   With a scope, it’s easy to see.   You set the scope to AC (and you don’t have to worry about the DC voltage spec within reason) and you slowly dial down the volts/div.

For the McTube, on both the B+ and Filament voltages, there was practically no ripple.   At 0.01 volts/div at the slowest sweep, I saw about 0.005 swing that took a bit over a second.  My scope wouldn’t trigger on it but it was plain enough.   Normally you expect to see something at 60 Hz at least and I saw nothing.

So I guess it cost me some extra parts, but that is a clean power supply.

Next time I get an amp opened, I’ll shoot a pic of the ripple.