[LAD] simulating analog audio devices part II

porl sheean porl42 at gmail.com
Tue Jun 19 10:51:58 UTC 2007

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.


On 19/06/07, Giuseppe Zompatori <siliconjoe at gmail.com> wrote:
> 2007/6/19, porl sheean <porl42 at 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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> Linux-audio-dev at lists.linuxaudio.org
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