SOT - Firefly power supply - Help

[brett]

Hi.
I'm thinking about building Doug Hammond's firefly.
The rectifier requires 207 VAC.  Mains supply here is 240 VAC.
Can I feed it that?  In terms of voltage, it's just more than 10% over spec.  In a solid state amp, 10% extra on a supply rail is nothing, but I don't know if tubes are a bit more sensitive with their B+ voltages.
IF I reduce the voltage, I've considered doing it by putting a small 240V:12V transformer back to back with another 12V:240V transformer.  I assume the losses in the transformers would knock a few % off the output voltage (also, the transformers would only costs a few $$).  If the efficiency is too high, I guess I could knock a volt or so off the 12V line between the transformers with a small value, high wattage resistor (??) or even regulate the voltage down (??).

PS I'll power the tube filaments with a separate 6.3V 2A transformer.

Thanks for any help

[Paul Marossy]

You could do the back-to-back transformer thing. I would figure on probably 20% of voltage/current drop doing that. So, in other words, get transformers rated for 20% more than what you need. That should get you in the ballpark. Three preamp tubes running at 300mA each means that you'll have 1 amp of heater current. A fudge factor of 20% means that you'll need somewhere in the neighborhood of 1.2 amps for it have the right voltage and current for that circuit.

Of course, you could just go to www.tubesandmore.com and buy a Hammond transformer that would fit the bill...  :wink:

EDIT: Sorry, I didn't see the thing about a seperate heater transformer.  :oops:
That's kind of tricky to calc out since there is so little current involved for the plates of those preamp tubes...

[brett]

Thanks Paul.  
Any reasons againt using mains (240V) power (ie no transformer/s) ?

[Paul Marossy]

Any reasons againt using mains (240V) power (ie no transformer/s) ?

Yes. Your rectified voltage will be quite a bit higher than what the schematic specifies and more importantly, it's a safety issue - you want the transformer to isolate your circuit from the mains. If you could find a 1:1 isolation transformer, you could use that and knock down the B+ voltage any number of ways.