Humdingers RULE!!

[g1]

No info on the SIDAC aside from the Crate part #, 73-401-01

[dmeek]

There is some discussion about it in this thread https://el34world.com/Forum/index.php?topic=12201.0 A shorted SIDAC will cause hum.

Here is a SIDAC datasheet

https://music-electronics-forum.com/...1&d=1577567417

[Mars Amp Repair]

Great information. I have added virtual grounds in older amps that did not have a CT or virtual ground & solved a lot of hum/buzz issues and I also noticed that some amps with the 100ohm hum pot are not always adjusted in the center for minimum Hum. I've also had amps with the actual CT ground that hum/buzz but never could do anything but change preamp tubes until I stumbled on ones that reduced the hum.
This is a great idea to try in the cases where you cannot eliminate the annoying hum/buzz seemingly no matter what you do!
Thanx for posting. glen

[drewl]

I've found adding a humdinger helps noisy old Ampeg amps.
Remove center tap add a pot and I can dial out most of the hum.

[pdf64]

I’ve found that as the heater circuit reference voltage is elevated above 0V common, any buzzy hum (resulting from imperfect heater to cathode insulation) fades out and silence ensues. It may be especially beneficial for designs where any early stage cathodes aren’t fully bypassed.
And no pot to twiddle when, eg, a valve is replaced.

[Chuck H]

I’ve found that as the heater circuit reference voltage is elevated above 0V common, any buzzy hum (resulting from imperfect heater to cathode insulation) fades out and silence ensues. It may be especially beneficial for designs where any early stage cathodes aren’t fully bypassed.
And no pot to twiddle when, eg, a valve is replaced.

I've actually noticed another affect from early stage partially bypassed cathodes. They can have what I have to assume is emission noises from the cathode itself. This is especially prevalent with newer production Rusky preamp tubes. It's not hum, it's not buzz and it's still a lower frequency than white noise/gain/thermal hiss. It sounds more like a constant WOOOSH. I've found that full bypass on the first gain stage reduces it dramatically so I've tried to make a fully bypassed first gain stage cathode a practice for any designs since this discovery.