[Re: Tubes (or valves as we like to call them in the UK)]

[Harry Bissell]

Here's part 2 of my anwser: The 12AX7 is good to 300 plate volts. At 250 volts
it should run about 1.2mA plate current in class A mode... Most circuits I saw
had 100K ohm plate resistor, and 1.5K in the cathode, bypassed with some cap
25uF or so. When you rectify 220VAC you will get about 310V or so, which is
real hot... your supply could knock that down a little bit by using RC filter
sections in series (good for no hum). Some schematics I saw ran this tube as
hot as 400 plate volts, but that would really be pushing it. Your idea of 220
to 6.3 volts feeding a 12 volt to 220 volt transformer (backwards) won't work
as well as you'd like because the impedance mismatch between the winding
designed for 12 volts (but run on 6) will cause the power to be much lower.
This might not be a problem, but if you are going to buy a transformer, why
not get one with a lower plate voltage anyway (about 180VAC would give 250
VDC). If you want distortion, lower the plate voltage way down, the lower the
better. Good luck :-) Harry Bissell

owner-synth-diy at mailhost.bpa.nl wrote:
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.


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