Advantage of AC heater DC B+

[iomegaman]

So I picked up an old General radio psu for $20.00 (theres a new "surplus electronics" store that has a bunch of old electronics crap opened next to my regular not-mouser place)

I bring it home put it on the bench to test it works fine giving me + 300 VDC and around 7  AC ...I figure i can use it for one of my tube projects (another REDD 47 or something else)...

And it got me to wondering if there is an advantage to run your heater AC while your B+ is DC...?

Is it easier on the filament or is it because DC is a bit more kludgy to control, or was it because of the applications these old PSU's were made for (I think General Radio made test gear back in the day) ?

 

[emrr]

There's plenty to read about AC versus DC filaments, I won't touch it.  Seems to get religious/political in nature. 

If it's giving you 300V on the bench with no load as you say, there's something wrong with it. 

If you look at the manual you'll realize the voltage changes with load, so no load approaches 400VDC and 65mA load is 285.  Plan on making circuit adjustments depending on your load.

 

[iomegaman]

There's plenty to read about AC versus DC filaments, I won't touch it.  Seems to get religious/political in nature. 

If it's giving you 300V on the bench with no load as you say, there's something wrong with it. 

If you look at the manual you'll realize the voltage changes with load, so no load approaches 400VDC and 65mA load is 285.  Plan on making circuit adjustments depending on your load.

My bad, I was spitballing appx...when I let it ramp all the way up under no load its about 401 VDC...so I think its solid...

I know the PC vs Mac/AC vs DC/Transmutation v Symbolic arguments get out of hand and was not really looking into that just wondering if the older gear was designed this way because of limitations of the day or did they know stuff we've forgotten...

 

[emrr]

DC heaters take big caps, they didn't exist reliably back then.  RDH4 says hum with a well balanced AC filament is typically lower than DC filament.  That's all about available cap size.

I have a lot of very high gain old tube preamps, all with AC heaters, none with practical hum problems. 

 

[iomegaman]

DC heaters take big caps, they didn't exist reliably back then.  RDH4 says hum with a well balanced AC filament is typically lower than DC filament.  That's all about available cap size.

I have a lot of very high gain old tube preamps, all with AC heaters, none with practical hum problems.

Thanks for that...all my builds have used DC and I have fought hum like crazy because I get tunnel vision on a project and don't look at other approaches...this might change that. Thanks!

 

[ruffrecords]

I think it is a question of horses for courses. Back in the day, for consumer equipment (Radio,TV guitar, PA) it did not matter if there was a little hum in the loudspeaker. For pro recording it did matter so much more attention was paid to lead dress and high gain preamps were containerised and had no real external controls (they were just isolated gain blocks). Unfortunately, pro audio tube design never got much more complex than that because they were overtaken by transistors so complex tube designs with ac heaters never existed. There was the occasional tube mixer with dc heaters ( one of the Altec mixers used them IIRC).

These days I am designing relatively complex tube mixers with mic pres with built in EQ and routing. It soon becomes impossible to maintain strict ac heater lead dress in complex modules so I soon moved to dc heaters and upped the supply to 12V to lower current. Even with regulated linear dc heater supplies there are problems in sensitive mic pres with electromagnetic interference from chunky mains transformers so I generally use external supplies. Then again, as the mixers become more complex, the amount of current required for the heaters rises dramatically and soon exceeds that of readily available regulator ICs and also requires massive heat sinks to dissipate waste energy. So I have moved to SMPS for dc heater supplies. This was not without problems in the early days, not from interference but with the difficulty of an SMPS to provide the necessary inrush current for cold heaters without going into a current limit and never powering up the heaters. However, the advent of hiccup mode SMPS successfully circumvented this problem and I use SMPS dc heaters on all my mixer designs. They provide the lowest noise figures I have been able to measure - better than any linear supply I have used.

Cheers

ian

 

[Whoops]

Also a good read as a start:

http://www.valvewizard.co.uk/heater.html

 

[PRR]

> wondering if there is an advantage to run your heater AC while your B+ is DC...?

Have you tried DC heat and AC for "B+"?

It's AC because who the heck can afford well-filtered high current DC??

High current low-drop rectifiers come in in the late 1950s. Even then you would ONLY do that for LOW-level stages. (Which seems to be what you do...) High level stages can always eat AC. (OK, a 2A3 will hum, but that's how you know it's alive.)

 

[CJ]

yes there are circuits with AC heaters and no hum,  but it takes some experimentation to get it right.

DC will get you there without all hours of layout investigation.

 

[andyfromdenver]

Tightly twisted heaters have "zero" hum in my experience (lots of tube gear) and are a less parts count (less chance of failure) solution.  Hum from misaligned transformers will swamp any ac heater hum.

If a kit or pcb had dc heaters, id probably do it just cause the pathway is there, but for a from-scratch build, I would skip it.