Wall warts as bias supply?

[Austin]

I have lots of wall warts collected over the years and after looking at all the elaborate bias schemes for fixed bias amps have been wondeing why people just don't use these things for steady fixed bias source. Why not I say! We could filter the dc with more filter caps to make good smooth dc and then use this as the bias, am I right? I just found a 25 volt 500ma one last night and thought it perfect for 6L6 power tube bias.. A pedal supply of 9 volts could be used as 12au7 or el84 or 6g6 etc.. Why not? I mean after all the cathode bias method requires caps and high power resistors that waste energy and introduce color to the sound, zener diodes have there own set backs and voltage dividers change with load so why not just milk these tubes for all they are worth with a nice low impedance voltage referance such as this?

[printer2]

The bias supply is delivering next to no current, you do not need a 500mA supply. It is easy enough just taking a tap off a transformer if you have it of just use a dropping resistor off the HV winding. It takes less space than a wall wart and you do not have to worry about a cheap transformer inducing hum into the amp. That being said I have used wall warts as heater supplies and have collected a number of them. Now I have to find another use for them as I see wall warts with switching supplies instead of transformers being more useful.

[R.G.]

Wall warts were invented to solve the problems of encapsulating (1) all the safety isolation issues and (2) all of the space/mechanical issues of an AC power line supply. The "sins" were shoved into that little box and put in an out of the way place to be ignored.

There's no technical reason you can't use a wall wart for a bias supply, or for that matter, any other transformer. The issues are all practical ones. Wall warts are essentially free if you're willing to go on a treasure hunt at Goodwill, garage sales, etc., but that doesn't solve the issues of them being clumsy to mount anywhere but in a wall socket and the one PT2 just mentioned: they're not the best, quietest transformers and they may introduce hum.

Any extra filtering could be done on the bias supply already in the amp, just as it could be from a wall wart.

Then too there's the issue of getting the right voltage. Wall warts run about 12-15% high at very light loads. That "25V" supply you got is probably only 25V at full load of 500ma unless it is internally regulated, which is unusual in wall warts. If it's a DC output, it's probably 32-37Vdc with little or no load on it, and maybe not filtered at all. If it's 25Vac, it's likely to be about 32-37Vac, which would rectify to 45-55Vdc under no load. And then the output voltage would sag that same 12-15% as you loaded it.

Last time I looked, you need about 35-50Vdc for bias on 6L6s, more for EL34s. And you don't want the bias to sag much, so a 25Vac wart is marginal for supplying a set of 6L6s reliably unless you do some kind of doubler, and then the complexity approaches that of the funny bias circuits you mentioned.

Easy concept. Tougher to execute.

However, a separate trannie is not all that bad an idea. Mouser will sell you a Triad VPL36-140, which puts out 36Vac at 0.14A and 43.2Vac at no load. That rectifies to about 60Vdc on loads much less than 140ma, as bias supplies probably will be. It's roughly a 1.5" cube, chassis mount (that is, easy to tuck under a chassis), and costs under $10.00 in ones. It's probably about the size of the filter caps you'd use for making a bias supply.

Just sayin'...

[printer2]

Wall wart transformer on the left supplies the heater while the one on the right was a 120 to 240 adapter that I rewound half of the winding in order to isolate the input and output (the adapters are normally autotransformers). Had some aluminum flashing that I bent up in a vice in order to make the bracket holding down the transformers. Most adapters are dc, ac ones are harder to come by.

[Austin]

Wall wart transformer on the left supplies the heater while the one on the right was a 120 to 240 adapter that I rewound half of the winding in order to isolate the input and output (the adapters are normally autotransformers). Had some aluminum flashing that I bent up in a vice in order to make the bracket holding down the transformers. Most adapters are dc, ac ones are harder to come by.

If you take them apart, they can be ac with no diodes and filter caps but isn't it cool that it is already dc in a nice neat package?

[printer2]

Hard to put them back together into a nice neat package if converting them to AC means cutting them in half. Most times the filtering is also substandard.