EasyDrive resistors question

[calpolyengineer]

I breadboarded an easydrive last night but I didn't have 680R or 68K resistors so instead used 2.2K and 220K (the lowest value I had that kept the 1:100 ratio). It worked and everything so I'm wondering how much these resistors actually change the effect. I'm pretty sure they are biasing resitors (thats why I kept the ratio). Basically I want to know that I'm not missing out on anything that the easydrive could be giving me. Thanks for any help. Heres the schem http://www.diystompboxes.com/analogalchemy/pedals/easydrive.html

-Joe

[petemoore]

  I breadboarded an easydrive last night but I didn't have 680R or 68K resistors so instead used 2.2K and 220K (the lowest value I had that kept the 1:100 ratio). 
  Check the math. 2k2 is what % of 68k? 
  With no schematic..and the respective high and low resistor pairs mentioned being listed non-respectively [big/small compared to small/big after scaling]...pretty darn hard to way, I guess the 680k is a base bias and 220k'll just shunt more input to ground...some difference, the 68k...hmm just have to guess that 2k2, being 30x smaller will make a big difference, to detriment of...who knows.
  Not me.
  Breadboard is perfect for demonstrating these questions in the form of experiments which provide action packed, better than sound clip information results from which texts can be written. One tonal difference may = 1000 words.
  The "DEBUGGING" page, as always, a great source.

[brett]

Hi
Interesting question.
It was a good idea to keep the ratio intact, because the ratio, and those 220k resistors, determine the DC gain and DC bias point.  However, the AC gain is determined by that 68k resistor (It would be the ratio, but the AC bypasses the 680R resistor through the bypass capacitor).  Anyway, the gain is potentially too high after you replaced it with that 220k resistor.

BUT

Lucky for you, common, garden-variety transistors will be working flat out in this circuit, so it will be the transistor, not the circuit, that sets thhe AC gain.  Find a transistor with 200 to 400 hFE (2N2222A, 2N3904, etc).  Do you have a multimeter with a hFE tester?  If so, test a few NPN transistors.  Avoid ones like MPSA18s and 2N5089s, which have hFEs of 500 to 1000.  They might sound harsh or cause feedback.  Of course, if you want harsh and feedback, they're ideal with your version of the circuit.

cheers

[calpolyengineer]

Well I did use a 2N5089 and it is indeed very harsh (no feedback though), but in a grungy kind of way. I'm glad I was semi-right in keeping the ratio, anything besides multiples of 10 and I never would have caught it.

I don't have any plans to keep this circuit the way it is, RatShack didn't have the resistors I needed so I made due. Thanks for the information though.

-Joe