Transistor as variable DC resistor?

[diskoduck]

I'm looking for the simplest way to equip some of my pedals with a "tube rectifier power sag" feel,
or at least something witch is close.
I really like this liquid compressed feel as if the amp is reacting on every specific note played.
So it came to mind to use a transistor as a kind of "audio triggered" (dynamically changing) resistor!
Would this be possible?
By the way: what is exactly sagging with a rectifier tube: current or voltage?
Any hints welcome!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Gilles C]

An example of it

http://www.londonpower.com/kits/sck.htm

And more info here.

http://studentweb.eku.edu/justin_holton/sag.html

Gilles

[rockgardenlove]

He wants it for a pedal I think...

[Gilles C]

That's right... but he also wanted to know what it was...

Maybe a way to do it would be with an old battery simulator like this http://www.muzique.com/lab/patent2.htm

Gilles

[Nasse]

Valco Sustainer

[diskoduck]

So according to the "London power kit" it`s current limiting?

Regarding low-battery-sims:
Until now I thought of the power-sag artifact as a DYNAMICALLY changing voltage/current sag!?!?!?!?!
Isn't it: the harder I hit the more it sags???

Any suggestions welcome:-)

[diskoduck]

Hey Nasse,
any links?

[R.G.]

Regarding low-battery-sims:
Until now I thought of the power-sag artifact as a DYNAMICALLY changing voltage/current sag!?!?!?!?!
Isn't it: the harder I hit the more it sags???

Georg Ohm said "V=I*R".

So the more current through a resistor, the more voltage it drops. Put a between a power supply and the amp it runs, and the harder you hit it, the more it sags.

Looks dynamical, yes?

[Nasse]

http://www.diystompboxes.com/smfforum/index.php?topic=9402.msg56467#msg56467

Silly typo, I meant Walco Sustainer, I think you find lots of talk if you search this forum. I think 2sc644 is used as DC controlled variable AC resistor, did you mean that... I never built the circuit, just a wacky compressor and they who built it say it is too noisy to be useful for not much...

[zachomega]

There is a more recent Walco thread I started as well which deals a lot with this.  The thing is really noisey, but it has very little to do with the 2sc644.  The 3sk30 fet is pretty obnoxious. 

The real problem with this circuit is the unpredictability with regard to how it will function given an input signal. 

-Zach Omega

http://www.diystompboxes.com/smfforum/index.php?topic=9402.msg56467#msg56467

Silly typo, I meant Walco Sustainer, I think you find lots of talk if you search this forum. I think 2sc644 is used as DC controlled variable AC resistor, did you mean that... I never built the circuit, just a wacky compressor and they who built it say it is too noisy to be useful for not much...

[The Tone God]

Look for Punisher:

http://www.diystompboxes.com/smfforum/index.php?topic=49860.0

Andrew

[diskoduck]

R.G.:
This works on the VS+ of an op-amp? An op-amp draws different current if it's hit harder?
I tried scaling down pedal voltage down to 6V.
Especially the Guvnor behaves pretty well with it.
Doesn't it act the way that the rectifier tube is providing less current if the amp is hitten hard
or is it that the rectifier doesn't deliver enough current anyway but it's just noticed if the power-amp is getting loud?
I realize that there are things not real clear to me.....
Can the whole stuff be done by one resistor?
And why is it that Tone God (thanks for the punisher) providing a whole scheme on this item?

[brett]

Hi
I'm no RG, but I know that op-amps can draw plenty of current (ie many mA).
In fact, the popular TL071 and 072 have the capacity to power quite low impedances (output impedance = 200 to 300 ohms).
Therefore, I *think* that anything more than a few tens of ohms could cause a fair bit of sag in a TL07X op-amp power supply if the op-amp is driving a heavy load.

Also, some people have said they quite like the distortion from heavily loaded op-amps, if you want to throw that into the mix.  The lowest load I've used on a TL071 was 100 ohms, but I can't remember the voltages involved.
cheers