Any Problem with using Mains AC on the Heaters?

[Alexo]

I've got a bunch of old radio tubes floating around, 35L6, 35C5, etc... all designed to run their heaters in series off of 120VAC, or rather 115, whatever.

Assuming I can come up with a scheme to make the heaters add up to 120 (maybe 2 50C5's and 2 12AX7's), is there a safety risk to this practice? My thinking is that no one will ever come in physical contact with the heaters, so an iso transformer is not necessary.

However, some ground reference for the heaters does seem necessary to prevent noise, the 100 ohm R's tied to ground - would this even work using mains AC? Unlike a heater winding, you won't have half the voltage on either leg, you'd have 120 VAC on one leg and the other is just neutral.

This is a pretty archaic practice, anyone ever give it a shot? S'pose I could just rectify it to DC.... get about 84 volts to work with, 2 35L6's and 2 12ax7's at 6.3 volts.

EDIT: Er wait, wouldn't I be half-wave rectifying it?

[DonMoose]

Those old series-heater circuits also didn't isolate B+ from the line. Both of these practices risk making the chassis hot; especially if you want to gnd-reference the heaters.

Use an isolation transformer for the heaters (1:1) and you're free to ground one side of the secondary - I strongly recommend you ground the end nearest the input. To be more specific, line them up 120VAC - 50 - 50 - 12 - 12 (stage 1) - GND. Do make sure all the tubes in the line need the same heater current, or the lower-current heaters will starve the higher-current ones.

Hope this helps!

[Alexo]

Thanks Don, very helpful on the ordering and the current specs!

And yep - definitely going to use an iso for the B+, but it'd be nice not to have to worry about sourcing an extra 150ma for the heaters.

I'm starting to think if I just DC rectify the mains, I won't have hum to deal with and therefore won't have to ground-ref them. But then the heaters will just float relative to the cathodes et al, and that might cause problems.

[TheGimp]

Just remember you are tying your mains return to one side of the filament string. You are safest to keep it isolated from chassis and B+, but this may not be the best from an ac related hum standpoint.

[DonMoose]

Thanks Don, very helpful on the ordering and the current specs!

And yep - definitely going to use an iso for the B+, but it'd be nice not to have to worry about sourcing an extra 150ma for the heaters.

I'm starting to think if I just DC rectify the mains, I won't have hum to deal with and therefore won't have to ground-ref them. But then the heaters will just float relative to the cathodes et al, and that might cause problems.

Unless you transformer isolate the heater string, one end of the recto/filter circuit will be grounded and thus not float. DO NOT float the isolator's secondary.

Note that, even in the most cost-sensitive applications, supplies to devices that aren't 'double-insulated' are transformer isolated - I believe this is required by NEC and/or UL (and CSA, CE, VDE ... ).

[davent]

A few months ago at ax84 Doug Hammond (Firefly designer) posted his work on an update of a Magnatone amp that used a 50L6 and a 12ax7. Ran the heaters in series off the b+.

His schematic; http://www.aronnelson.com/gallery/main. ... e.GIF.html

ax84 thread; http://ax84.com/index.php/bbs/dm.php?thread=431919
dave