[linux-audio-dev] [ann] unmatched - a LADSPA amp tone

[Lamar Owen]

[Note: I've only been loosely following this thread, but this message caught 
my broadcast engineer eye, being that I work with tube gear nearly daily.]

On Monday 28 October 2002 01:55 pm, Tim Goetze wrote:
> Steve Harris wrote:
> >That is a side effect of class B amps IIRC. I would have though that
> >preamps would be class A, but maybe not.

> they are, afaik. you're right, it's a property of class B.
> still my amp's preamp stage shapes the lower half differently
> (less) than the top.

It would depend upon where in the transconductance curves the tube is biased, 
as well as plate voltage.  The behavior you desribe sounds like an underbias 
condition, where the tube isn't biased approximately in the middle of its 
linear range (for class A).  Either that or the plate voltage has sagged (old 
dried out electrolytic caps?).  And if it's a tetrode or pentode amp, the 
screen and suppressor bias level and type figure into it dramatically.

> >> i'm still trying to model the hard-driven valve in the time
> >> domain. the more i look at it, the more i think the 'building of
> >> potentials' i've tried to describe comes near it. though it would
> >> be nice to base it on hard facts about valve amp behaviour, too.

> >'building of potentials'? Are you refering to your sinc summation idea?
> >Its a nice idea, especially as you get anitaliasing for free, but I think
> >it will be hard to turn a hardish clipping shape into a sum of sincs.

> no, this is the other idea that would try to model a simple
> wave shaper (pushing energies and so on). the sinc idea is
> reserved for dire emergency situations, it's not really gentle
> on the cpu and tough to get right.

Ok, from an engineer's perspective:

It seems to me that modeling the transfer function of a tube would involve 
running high level triangles at various fundamental frequencies.  This data 
would then be analyzed to see the necessary time and frequency domain 
artifacts of tube operation.  You need to see the group delay, and the 
frequency response for the fine details -- but the big issue is the simple 
'if I put in X volts I get out Y volts' transfer function.  This can be 
modeled with finite element techniques if you have the exact parameters of 
the tube (plate size and shape, interelectrode spacings, grid dimensions, 
filament/cathode emission and dimensions, etc).  

I have seen this done for large tubes (4CX15,000A's as used in broadcast 
transmitters), but not small ones (6146's, 2E26's, as well as the even 
smaller ones like the 12AX7 or AU7, or the venerable (and expensive) 5763, or 
even the ubiquitous 6L6 and variants like the 807).  

The purpose in the transmitter case is to see what frequencies need what 
compensation and potential neutralization.  Since the 4CX15,000A is used at 
frequencies well over 100MHz, knowing this sort of thing (particularly 
interelectrode spacing and coupling) is essential is designing the tuned 
cavity output and input networks required at FM broadcast frequencies.  But 
the old '15,000 would make a hoss of an audio amp -- the 15,000 number is the 
rated plate dissipation in watts.  A pair in class B push-pull can make 50kW 
of output at audio frequencies easily.

Certain frequencies must be avoided for these tubes -- if the frequency 
response has an area where the output is 180 degrees from the input 
potentially destructive positive feedback can occur.  If not properly 
neutralized and bypassed, these tubes can self-destruct in a pyrotechnic 
manner.

A 4CX15,000A takes around 160A at 12 volts AC for filament, and 10KV for the 
plate @ 2A or so, with several hundred CFM of air through its fins... A fun 
tube -- particularly when it blows.....  I can take a picture of one here 
(paperweight dud) if desired.

For what it's worth I dug out my RCA Receiving Tube Handbook and its cousin, 
the RCA Transmitting Tube Handbook, from the mid sixties.  Some curves are 
reproduced there, and I can probably get the curves you need from manuals I 
have.
-- 
Lamar Owen
WGCR Internet Radio
1 Peter 4:11