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

[Tim Goetze]

Lamar Owen wrote:

>[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.

it's a 12AY7, afaik, or a 12AX7 (double triodes, right?). the contacts
looked fairly clean the last time i had them out. remembering what
the circuit board and the resistors and capacitors look like, i
rather wonder at everything in there that is biased properly than the
other way around. ;)

>Ok, from an engineer's perspective:
...
>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).  

would you say that a 6L6 (output stage in my fender amp) behaves
vastly different from a 12AX7 when overdriven? it seems to me that
most of the actual distortion, at least in this amp here, is 
generated in the preamp stage.

>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.

jimi would have loved them. :)

>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.

i'm sure frequency responses and Vin/Vout mappings for common
amp tubes (EL34 comes to mind, too) would be very helpful, but 
can i ask you to take a look at
http://quitte.de/driven.gif 
first?

it shows my badly-driven fender amp's line output (top) vs. the sine
wave it was fed at the same time. i'm wondering how come the output 
overshoots every time the input crosses zero. trying to model this 
behaviour into a waveshaper, but i'm a bit lost on the physical laws
that rule here.

there's a schematic of a very similar amp at
http://quitte.de/super_5f4_schem.gif 
fwiw. the line-out should plug in right after the last tone
control i reckon.

(the waveshaper will probably alias badly, but i'm hoping it
 will give us a good start into the heavy sort of distortion.)

tim