[LAD] simulating analog audio devices part II

Giuseppe Zompatori siliconjoe at gmail.com
Tue Jun 19 07:52:13 UTC 2007

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":


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:


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

> 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:


> 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.



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