Insturmenting the amplifier for sampling (Was:Re: [linux-audio-dev] Re: Cheby amp code?)

Mark Rages markrages at
Wed Nov 6 14:52:00 UTC 2002

On Wed, Nov 06, 2002 at 02:09:50PM -0500, Lamar Owen wrote:
> On Wednesday 06 November 2002 12:50, Steve Harris wrote:

I'm an electrical engineer and I've been following this discussion with 

> A series 1 microfarad capacitor in the 1kV breakdown range in series with your 
> line input should work, but you might want to use an old soundcard and 
> machine to do it... :-)  Paper 1 mic caps in that voltage range are available 
> -- in a pinch 0.47 or even 0.2 would work, but the lower you go on 
> capacitance the more the probe will pollute the signal.  Assuming the 
> soundcard's line input doesn't already have a low-voltage series coupling cap 
> already, in which case you have real problems, because the effective 
> capacitance or two series caps is equal to the reciprocal of the sums of the 
> reciprocals of the individual caps (the same as parallel resistors).  

Ahh, but like parallel resistors, if one is "big" the other resistance will set 
the value.  So choose a "big" balue for the external cap and you'll be fine.

> I have a couple of 50 microfarad oil-filled caps in the 12kV range...

I'd like to see those reviewed in an audiophile "capacitor review!" Heh.

> Photflash electrolytics would be the next best thing, configured in a nonpolar 
> back-to-back arrangement, as they have extremely low ESR and inductance, 
> meaning they should be pretty linear.

But you don't need nonpolar caps... Just get the polarity right.  Assuming your
soundcard is going to be grounded with the amp (grounding in tube amps is
itself... interesting), you will, in almost every circumstance, want the + side 
to go to the amp.

> I have a mostly working Fender Bassman available for testing.  Does anyone 
> here have a schematic for that beast?  I can get the particulars.  It has a 
> nasty hum, and the bias on the 6L6's is quite bad (they run bright orange 
> instead of just barely red).  There are probably many resistors that have 
> changed values in it, along with several dried out electolytics.

I've seen schematics for the Bassman on the web.  In fact, I used the Bassman for
my computer "wallpaper" for a while.  Look around a bit. If you don't find it I
may have a hard copy I can scan in.  AFAICR, My Bassman 100 has a pot internally
to adjust the bias current and a sticker inside that gives the right current
value. (Clip an ammeter across the output transformer primary, adjust for
recommended bias).

> I also have a Dynaco three-tube kit special (A450 output with twin 8417's) 
> available for testing.
> And I'm still wondering whether a SPICE model of the amplifier in question 
> would tell us anything about the waveforms.... :-)

Probably not much more than careful thought and intuitive mathematical 
modeling... besides, SPICE probably doesn't consider things like capacitor 
soakage and transformer hysteresis, which can give an amp some 'flavor'.

You can probably do this kind of testing without taking the amp apart... just 
hook a wire around the appropriate tube pin.  I hesistate to give instructions, 
because you can end up seriously dead if you aren't careful with tube equipment.

> -- 
> Lamar Owen
> WGCR Internet Radio
> 1 Peter 4:11

markrages at

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