On 20/06/07, Dominique Michel <dominique.michel@citycable.ch> wrote:

Le Tue, 19 Jun 2007 20:51:58 +1000,
"porl sheean" < porl42@gmail.com> a écrit :

> thanks for the links!
> the ax84 one is really good, pretty much exactly what i was after - short
> and to the point (i don't mind a bit of complex math as long as it is
> explained why i am doing it :)
> i've looked through a few old valve books (
> http://www.pmillett.com/technical_books_online.htm is an awesome site) but
> it was hard to find certain info.
>
> i should probably have told you that i am making a guitar amp that i want to
> sound dirty (not heavy metal distortion, but old overdriven blues sound). i
> was only talking about a 4k primary because i'm pretty sure that is what the
> OT that i gutted from my fender hotrod (a bit high rated for this project
> but i can't afford to buy much at the moment). i have tried the same tactic
> with PTs of old amps off ebay (the fender one doesn't have a 5V output and i
> want to use a tube rectifier), but even the small one i realised is
> 375-0-375 for the primary! i think this is a *little* over what i need, so i
> am going to have to buy a new PT. i can't figure out how the old amp used
> this voltage: it was only a p/p el34 amp (2 output tubes). how would it have
> brought the voltage down enough to not kill the valves? maybe el34s can
> handle a lot but i didn't think the preamp tubes could.
>
> anyway, sorry about how off topic this is. i should probably take my
> questions elsewhere.
>
You can try the diyaudio forum. It is a thread with a lot of links on vacuum
tubes theory:
http://www.diyaudio.com/forums/showthread.php?s=5fc6a5fdf8f4d9101e3f07260bc6c498&threadid=38278

You can find some with amule too. Just search for vacuum, valves or tubes.

About the EL34, 375V for Ua is a very low voltage for such a tube. (You can
find the datasheet from http://www.nj7p.org/Tube.php or with google).
You can go up to 600 or even 700 V without problem. But you are right when you
said at 375 is too much for the preamp tubes. Only some old japanese preamp
tubes can handle such voltages, and they are very hard to find. The best is to
made a separate and low-noise alimentation for the preamp. A simple R-C can be
enough.

For the tubes, I generally prefer new tubes over old ones. Some new tubes are
even better as the best old ones. The engineers of today know what they are
doing when they manufacture a new tube.

Dominique

> porl
>
>
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