From: dam@dcs.glasgow.ac.uk (David Morning)
Newsgroups: alt.guitar
Subject: Variac Madness (Was: van halen's variac
Date: 12 Nov 92 10:58:09 GMT
Organization: Glasgow University Computing Science Dept.


The thought occured to me as I slunk off home last night that maybe we had
a variance of terms along the lines of 'you say tube, I say valve' etc.
There was also an incorrect assertion by a previous poster that a variac
had no core.

At the risk of flogging this to death, maybe it'd be better if I defined
the terms. I've also included what I believe to be Eddie van Halen's variac
set up. It came from some comments Jim Marshall made a year or so ago in
a mag but I can't remember precisely the detail as I don't have the mag
any more. It makes sense though.

Isolating Transformer: Consists of two or more seperate and isolated windings.
                       Power is fed to the primary winding and voltage(s)
                       are taken from the secondary windings. Traditional
                       'E and I' core although torroidal wound ones are
                       becoming more common. Fully isolated. Can be step up,
                       step down or both.
                       Windings are wound on a bobbin usually impregnated 
                       before 'E and I' core is fitted.
                       Moderately expensive.
                       Most common type of transformer. Used internally for
                       Hi-Fi systems, guitar amps and many other pieces of
                       domestic and professional equipment

Auto-transformer:      Single coil with various fixed voltage taps along its
                       length. No secondary winding. Traditional 'E and I'
                       core. NOT isolated. Can be step-up or step down or
                       both (if you know what you're doing).
                       Construction similar to isolating transformer.
                       Cheapest of all transformers to make as only one winding
                       is needed.
                       Commonly used a voltage converters to match locally
                       non-standard equipment to the local line supply.

Variac:                Single coil with exposed windings. A wiper moves around
                       the windings to provide an infinite number of voltage
                       possibilities from a SINGLE tap.
                       Specially built toroidal core (*note*) permits the
                       wiper actuator assembly to be mounted through the centre.
                       NOT isolated. Can be step up or step down but not both
                       at the same time.
                       Construction requires very expensive machinery due to
                       the shape of the core (Try winding a coil around a
                       doughnut and you'll get the idea)
                       VERY expensive.
                       Commonly used in laboratories for non-standard supply
                       values.

Note that the variac *does* have a core. A transformer without a core would
be, well just a big lump of copper shunted across the supply, unless you're
talking UHF, which we're not.
With no magnetic core, the impedance of the tranformer's primary winding,
R+jWL (sorry, no Greek symbols :-) ) would resolve to just R, the winding
resistance of the copper wire since the L component (inductance) would be
virtually zero. Can you say 'welding current'?

Eddi van Halen's Variac.
From what I understand, Eddie van Halen's variac set up involves a heavily
modified Marshall amp. The valve heaters (tube filaments) are fed from a
seperate fixed supply to maintain the thermionic effect. The other supplies
(HT, bias etc) come from the variac. It may well be that the Marshall has
had it's power amp bias adjusted to cope with the changing voltages.
As I said before, killing the voltage to the valve heaters doesn't make sense
and would just sound like playing with dud valves.

Does that clear it up now?

Dave

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