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

[Bill Layer]

Hi Jezz,

>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.

That would be fine, but get prepared to pay some cash. Xformers for valve
projects are not cheaply priced if bought new. Frankly, I like their quick
and dirty approach to the supply. Just do what you said 220V to 6.3 and
12.6 to 220. Another point: one of the worst mistakes a beginner can make
is to second-guess or modify an existing design. Make it work there way
first, then your way.

>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)

Considerably. Higher plate voltages tend to sound better, but there are
many other issues which come into play. Plate dissipation, for one. A
typical supply for a 12AX7 will be about 165VDC at the head, with the plate
of the tube seeing  about 100V. Typical plate loads for a non-specific
design are 470K-200K ohms.

>Could I hack it by varying the plate-load resistor?

Don't.

>How do I know what the current flowing in the plate resistor is and what
>drop to expect across it?

Well, ohms law will tell you that. If you know the resistance, and you know
the volatge, it is a simple matter to figure for current and then watts.

(E=IR)

Best thing to do with tube circuits is to build them. It will take you days
to guess what you will learn in a hour hands-on.

>
>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.

Well, it's perfectly feasible. I'm not sure why you would ask, though. The
venerable 12ax7 is being used in just the service it was meant for. Are you
concerned about the quality of the design?

>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.

Buy a copy of the RCA Receiving Tube Manual and read the engineering
sections. 

A few others on this list have expressed an interest in basic tube designs,
and it might be fun to build one as a group. I've got a pretty good
protyping system figured out, that I can share with everyone who is
interested. It's kind of a modular 'breadboard' system for tubes.


Bill Layer
Sales Technician
<b.layer at vikingelectronics.com>

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