One of those two 6x6550 amps I’ve been building had a weird buzz that took me the better part of a week to track down. One was dead silent, the other sounded like filter cap charging currents were being injected into the signal. Because of that characteristic “sound” it had me off chasing my tail looking in the wrong paces. Isolated to the front end - then finally the input tube itself. Swapping to another 5751 cured it. What I can’t figure out is why - I have no idea what mechanism could be in play. It wasn’t hum being induced, that would be mostly 60 Hz not 120 with a strong 3rd harmonic. Megohm measurements don’t show any h-k leakage, which would be 60 Hz anyway. It’s not a hum/ripple balance issue, or even just excessive ripple - that would have shown up with the input tube pulled. Not likely with a heavily filtered (cap multiplier) mosfet regulator for the front end. That was needed just to get the *hiss noise* down to an acceptable level. That input stage is set up as a gain stage/concertina with only 75 volts on the 2nd cathode followed by a short-tailed SN7 pair which by itself is well enough balanced to be hum free. Nothing exotic going on, so where would be wrong with that tube to cause the buzz? Would it be ok for me to use in something else, perhaps with a DC heater, or should it be considered potentially hazardous? What about the odds of this happening, on any given Sunday?
Could be "microphonics "--
internal vibrations of the tube elements which can modulate the signal undesirably --and/or sound or vibration pick up .
This isn't or was unknown at the height of tube history , the worst cases I came across were --the tube still worked but if you took it out it rattled or gave out a low vibration due to insecure welding internally of the elements.
Not common in the big names of UK tube (valve ) manufacturers but more common with some smaller companies.
internal vibrations of the tube elements which can modulate the signal undesirably --and/or sound or vibration pick up .
This isn't or was unknown at the height of tube history , the worst cases I came across were --the tube still worked but if you took it out it rattled or gave out a low vibration due to insecure welding internally of the elements.
Not common in the big names of UK tube (valve ) manufacturers but more common with some smaller companies.
A confusing buzz? Too bad it's not the fun kind of confusing buzz! Is there anything he can do to dampen those internal vibrations?
I didn’t notice that the tube was particularly sensitive to being “thwacked” while operating, nor is there noticeable physical vibration in the power trafos. I’ve heard AX7s in phono pre stages sound like reverb tanks. The biggest offenders I’ve seen for microphonics are horizontal sweep tubes. Most of them vibrate or physically rattle when tapped. I wonder if the annoying 15.75 kHz squeal coming off TV sets was as much the sweep tube as the flyback trafo.
I suppose it could be the *field* coming off the trafo causing it, but wouldn’t it affect any of them equally?
I suppose it could be the *field* coming off the trafo causing it, but wouldn’t it affect any of them equally?