[Ardour-Users] New tube amps

[Will J Godfrey]

<some snippage>

>> http://gjcp.net/solidstatevalve.jpg

<and some more>

First, let's be clear. I am not suggesting a valve amp can't be emulated, but
your drawing is, to put it mildly, 'basic'.

A friend of mine retired recently (well into his 70s) from a company that does
indeed make amps with extremely good emulation, and if it's good enough for
him, having cut his teeth on valve stuff, it's certainly good enough for me -
a mere refugee from the valve TV days.

Your drawing looks similar to one I've heard in operation, and where it failed
was on long slowly decaying single notes. As the volume dropped you could
clearly hear the steps in the harmonic content change. This of course doesn't
happen in a real amp where the non-linearity is a continuous sweep until actual
hard limiting is reached (quite difficult to get to).

A silicon diode is the worst possible device for doing this. It starts to
conduct at about 0.4V and flattens off at 0.6. I've heard much better results
from the now totally obsolete OA81. Being germanium, these start to conduct at
0.1V and level off around 0.5V (by which time you're at their disipation
limit). Therefore a chain of these will give a closer analog to the real thing.

You'll get even better results using an FET, but by then you might as well use
a JFET wired in a similar way to a triode! Wire two in cascode configuration
and you'll get somewhere near pentode behaviour.

Talking about pentodes, why on earth emulate an EF86? They were horrible
valves. Fragile, microphonic and frequently suffering from heater-cathode
leakage - everything you *don't* want in an AC heated input valve... in kit
that's going to be rattling around in the back of an old Bedford van. The only
ones I saw were in amps people were desperately trying to get rid of!

Back to subject, your drawing seems to try to combine the features of the input
stage (creating mostly even order distortion) with the output stage (mostly odd
order). This takes away control from the guitarist who may want to overdrive
the input for consonant harmonics with just a little compresssion, then with a
quick flick of the wrist change to clean input and heavy overdrive of the
output.

Added to this your need for EQ, if it was adequate to tame the sound for open
strings, would completely flatten the 'edge' when working high up the
fretboard. Watch the best Rock guitarists. They are all over the fretboard!

-- 
It wasn't me! (Well actually, it probably was)

... the hard part is not dodging what life throws at you,
but trying to catch the good bits.