Tubes (or valves as we like to call them in the UK)

[Bissell, Harry]

You can probably use the higher voltage, with a higher value plate resistor
to hanndle the larger drop. It would probably work better than the 110VAC
unit. You can burn up some of the extra voltage with a RC "pi" filter and
reduce a lot of power supply noise. I'll check some preamp circuits I've got
laying around and see what typical voltage / plate resistor values are. The
original design probably just used two transformers (6.3V filiment) because
two of them are probably much cheaper than one 120VAC isolation transformer.
Mind a question? What do you want from the tube preamp, high performance
(audiophile?) or high distortion (musician?). Or maybe you want a space
heater ? ;-) Harry Bissell

> -----Original Message-----
> From:	Jezz Brookes [SMTP:jbrookes at bluebear.freeserve.co.uk]
> Sent:	Tuesday, February 09, 1999 1:49 PM
> To:	Synth DIY
> Subject:	Re: Tubes (or valves as we like to call them in the UK)
> 
> I got a few responses (thanks!) that seemed to suggest it was okay to ask
> in
> here, so here goes...
> 
> I got a photocopy of an article in February's "Electronics Now" (a US
> magazine) that had a simple tube pre-amp based around a single 12AX7 (in
> Europe, an ECC83). I've got a problem with the power supply side of things
> as US is 110V and UK 220V. The circuit used two transformers 110V to 6.3V
> back to back so that the mains was stepped down to 6.3V for the heater
> then
> stepped back up to 110V for rectifying for the supply to the plate. This
> provides isolation from the mains. Now I know I could use a 220V to 6.3V
> followed by a 12V to 220V to give me a step back up to approx. 110V but I
> was hoping that I could use purpose built valve/tube transformer that has
> got a 6.3V output and a 240V secondary for the high voltage.
> 
> My questions are:
> How much effect does varying the plate voltage have on the circuit design?
> (The ECC83 is rated to a plate voltage of 250V)
> Could I hack it by varying the plate-load resistor?
> How do I know what the current flowing in the plate resistor is and what
> drop to expect across it?
> 
> Due to all the high voltages, I'm a little scared to start probing around
> too much and I want to know how feasible the circuit is before I spend
> some
> money on it.
> 
> I've completely forgotten what valve stuff I knew, years ago, despite it
> still being taught on a degree course just over ten years ago! I could do
> with a quick primer on this.
> 
> Thanks
> Jezz.