Gareth Magennis wrote:


"John Robertson" wrote in message
...
Gareth Magennis wrote:
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer
(shorted primary).
http://www.vintagehofner.co.uk/brita...cs/dommk3.html

As a possible alternative to a rewind, which I haven't investigated
yet ......

if I got 2 x 240v : 6v transformers (I am in the UK) and connected
both secondaries together to get 240v back out for the HT, could I
use the 6v as a heater supply, or is it best to use a separate
transformer for this (or a better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp,
but maybe loading the 6v tap with heaters might be just the thing to
bring it down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this
circuit)



Cheers,


Gareth.





Hi Gareth,

Have you no isolation transformers on your side of the pond?

Remember that the maximum current you can draw is the ratio of the
6.3V to its amperage compared to 240V. So if the 6.3VAC was rated at
10A, then the other side of your proposed pair would be 240V @ 262.5ma
(assuming 100% efficiency). I suspect you will be having trouble with
your idea if the 6.3V output current is even less!

John :-#)#




OK, so this amp, according to my calculations, requires a minimum of
just over 15 Watts of 6.3v heater power, having 3 x ECC83 @ 300mA and 2
x EL84 @ 760mA.

In the real world, how many ballpark Watts of power supply transformer
should you be thinking about for the rest of the amplifier?





Cheers,



Gareth.




If you have the original transformer you can get some sort of an idea of
the current requirements by checking the wire gauge for each winding.

That's about all I can think of other than figuring out what style of
output you have and then calculating (based on) the maximum plate
dissipation (usually in watts) and convert that to current as you
roughly know the DC voltage.

Beyond that, pull out some tube amp design books and see what they
recommend for transformers for your style outputs...

John :-#)#
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