Filament Power, for larger builds

[aurt]

Why is that? In the distant past there were lots of battery radios that used directly heated tubes.

Cheers

Ian

I suppose you can, but you run into some interesting bias problems when one side of your cathode is x number of volts higher than the other. It can also cause uneven wear/stripping. I think there are reasons they stopped doing it in the distant past.

 

[RoadrunnerOZ]

As for distance of supplies from the console, how far is too far? I have 600V 16 AWG wiring i use for HT leads which seems to work great in other projects, and then will have the just mentioned 14 AWG wiring (shielded one end) for Meanwell. Is 6-8 feet too long for either of those runs?

6 - 8 feet is not a problem - I’ve run 30 - 40 feet using power multicore wiring consoles with the supplies in another room.
The MSP’s can tolerate current overload and recover automatically like the LRS just without hiccup mode - the use of an over-rated supply should preclude this but for normal every day use I’d be using the LRS anyway - had great success with these - same as seen in the preceding photo - the supplies I chose after doing a clamp meter test to read the turn-on surge and running current of the Neve - 350W for 15 and 24 and 500W for the +16 - the fast blow fuses in the original power supplies were a good indicator of what current rating to use anyway. Been running every day for 4 years and no issues. Drawer temperature 27°
I also have a couple of Meanwell multi-voltage supplies I use for quick testing, all sorts of mistreatment and they have been going for 5 years so far.
Before buying a supply you can always hook up a car battery and get a read on the start and run current for all filaments together.
These LRS supplies are cheap enough to be a painless replacement if they blow. I think $22 in the USA for what you need

 

[RoadrunnerOZ]

I suppose you can, but you run into some interesting bias problems when one side of your cathode is x number of volts higher than the other. It can also cause uneven wear/stripping. I think there are reasons they stopped doing it in the distant past.

Indirectly heated cathode doesn’t care about the filament polarity - if there was enough filament to cathode leakage to cause a problem with dc then with ac you’d have massive hum problems.

 

[Rusan]

You can’t run directly heated tubes that way, but otherwise it’s a fine idea with definite benefits. I did get one DHT amp in once that had the 300B heaters run from a 40kHz oscillator and SS power amp through an interesting handmade toroidal transformer to give them a center tap ground. I guess if you really really don’t want any 60Hz in there, that’s one approach.

Why is that? In the distant past there were lots of battery radios that used directly heated tubes.

Cheers

Ian

I suppose you can, but you run into some interesting bias problems when one side of your cathode is x number of volts higher than the other. It can also cause uneven wear/stripping. I think there are reasons they stopped doing it in the distant past.

The voltage gradient across the cathode also carries the caveat of reduced emission on the low voltage end. The Japanese DIYers who pioneered the single ended DHT amplifier renaissance back in the 80s found that directly heated power triodes sounded flat and dull when heated with DC, with the ubiquitous 300B being notorious for it. I've heard several DHT amps with AC filaments and even built a couple myself, but none with DC, so I can't comment from personal experience. But, countless reviews (including from some trusted friends) of DC-fired DHT amps have all agreed with this.

In the early 2000s, some of the more serious DIYers and then a few manufacturers in the U.S. started powering their filamentary cathodes with ultrasonic AC oscillators, like Aurt had mentioned. One particularly interesting circuit was based on a UC3872 push-pull resonant lamp ballast controller, which drove a small transformer with a capacitor placed across its primary to tank at about 100kHz. The idea was that the 100kHz "hum" couldn't couple across the low-bandwidth single ended output transformers. Therefore, no chance of ultrasonic voice coil heating in the crossover-less, single driver speakers commonly used with flea-power amps.

 

[aurt]

Indirectly heated cathode doesn’t care about the filament polarity - if there was enough filament to cathode leakage to cause a problem with dc then with ac you’d have massive hum problems.

Indirect is fine. I was speaking of directly heated tubes.