6SN7 6.3 V heater supply

[charliet]

Hi folks, and happy 2007!

Has anyone tired using a standard (and very common) + 5 VDC power supply (wall wart), for a 6SN7 heater supply - or any other 6xxx tube for that matter?

I currently have it powering the 6SN7 filament in my pre-amp, and it seems to sound decent (ok, maybe a bit, crunched, although this might be my wacky imagination!)

Has anyone else tried this?

Thanks,

Charlie

[ArtG]

What's the voltage that being supplied under load? If it's 5 volts, then you are underpowering the heater. This is a subject of debate, but I prefer to power heaters at, or slightly below the recommended voltage.
Another possible issue is noise, which can work into the signal, particularly in low level applications such as a preamp. I once looked at the output of a "wall wart" on my scope, and found it was anything but pure DC!

[zigzagflux]

There are a few types of wall warts, but the 3 significant ones are:

1. Linear with only filtering. This will have some amount of 120Hz ripple on the output.

2. Linear with regulation. Typical specs are around 1-5 mV ripple noise, high frequency. Higher cost.

3. Switching with regulation. Noise is typically a factor of 5-10 higher than linear, with even higher frequencies.

Bottom line, if you know what you have and what the noise is, you can determine if there is any detriment to your circuit. I would assume you know you are slightly underpowering your heater, but I hear many do this (though I think more so in output stages).

Additionally, all these wall warts will be isolated from your third prong (ground), so you will need to provide a reference to your circuit somewhere along the way.

[charliet]

Thanks for the reply, Art.

I should have added that the 5 VDC wall wart is actually regulated - (it is rated at 1w). I scoped it under load (connect to 6SN7 heater), and no ripple....real clean at stated 5 VDC voltage.

I am using a standard common cathode gain circuit, into cap coupled cathode follower driver. I think the sound is a bit, ...I can't find the right words....."confined"...."crunched"...."small", etc.).

But again this might be all in my head, because I know that poor tube, is "starved" for electons (my wife says it sounds fine).

Anway, I was wondering if anyone else have ever tried this (I have heard, that under-powering a heater, can be just as bad as over-powering, but this does make much sense).

Charlie

[wa2ise]

An underheated cathode will not be able to produce as much current than if operated with normal heating. The "crunch" you are hearing is likely to be the circuit wanting more curent than the cathode can provide with underheating. Your two choices: redesign the circuit to operate correctly with around 1/3 the current, or change the heater supply to one that can provide 6.3V under load. this 2nd choice is far easier. If you can open up the wall wart (big if!), you could try changing the regulator circuit to give you the 6.3V, but that would work only if the internal circuits can provide enough overhead at the current the tube heater will want. If it's a linear type supply, you probably could just remove the regulator and connect the heater to the unregulated regulator chip input node. If the voltage at the tube socket turns out to be too high, insert some dropping power resistors in series to get it down. Tube heaters are not that fussy about ripple on the DC.