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.

porl


On 19/06/07, Giuseppe Zompatori < siliconjoe@gmail.com> wrote:
2007/6/19, porl sheean < porl42@gmail.com>:
> thanks guys, i'll look into those models and links when i get a chance. i'm
> hoping to be able to learn a lot more about valve amps this way. apart from
> a few odds and ends it is hard to find out how they *really* work off the
> net.

The "AX84 P1 Theory Document":

http://annex.ax84.com/media/ax84_m35.pdf

Noob-oriented introductory document to their simplest SE amp, the P1.
( http://ax84.com/index.php?pg=legacyprojects&project_id=p1 )

The Radiotron Designer's Handbook 4th Edition is online here:

http://geek.scorpiorising.ca/RDH4.html

The RDH is also pretty noob-friendly (everything you need to learn is
there), it's a much longer read though.
It starts by the describing the way a valve is made and works up to
complex schematics. It's considered by many the "bible for thermionic
valves(tubes)".

> ps. (offtopic): anyone know the approximate b+ voltage i should using for a
> SE 6v6 amp through a 4k primary? i built one from odds and ends (i seem to

You need an OT with an higher primary impedance IMHO, 6V6 tubes need a
higher load to operate correctly. Having a valve load mismatch might
have bad consequences for your amp.

At an anode voltage of 250V it should be a 5k5 ohm load and at 315V
8k5 ohm according to the RCA receiving tube manual.

4k is a bit low, you'd have to run the tube "starved plate" (at low
plate voltages).
The sound is usually considered poor when a valve is starved plate. it
might be your kind of thing though.
If it really is a 4k primary transformer I'd run it at a ~320V B+ and
mis match the speaker wiring (old trick) to double the impedance to 8k
(much closer to the ideal load)...
i.e.: plug an 8 ohm speaker to the 16 ohms secondary tap (or 4 ohms
speaker into the 8 ohms tap), that will effectively double the primary
to secondary reflected impedance. And this configuration would give
you around 5 Watts of power. Thats much cheaper then buying a new OT
;)

Also a quick link to a well designed SE 6V6 based PA:

http://ax84.com/static/corepoweramps/5W_SE/AX84_5W_SE_Poweramp_Schematic.pdf

> like that phrase) and my valves started glowing blue... looked cool though
> ;) hehe

Some blue glowing is usually pretty normal, I have an 6L6GC based
small combo sitting next to me with a blue glowing power tube, and its
pretty fine. If I swap this Electro Harmonix tube for a chinese made
one the blue glow disappears.
You should watch out for any other part than the filament in a tube
that are red plating though, now that would be dangerous.
Watch out for those big power supply caps! always make sure they're
completely discharged before you operate on your amp... potentially
lethal voltages inside, as you probably already know.

Cheers,

-Giuseppe
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