I am getting two six packs to build one of these for a friend. I was going to buy the bits myself but my friend is going to buy them so that he can pay as much or as little as he wants (I think he wants to try a five finger discount). So I worked out readily available Australian substitutes for all the parts but was wondering if the gain pot should be log or linear?
P.S. I am going to paste a label on the inside of the box showing the original schematic crediting Tim for the design, plus a schematic showing my mods. Anything else I should do to keep it on the up 'n' up?
I used a linear taper. Most of the "gain" happens near the full clockwise position of the pot, but it's not a bother. You could use a antilog taper pot for "proper" response, or a audio taper wired "backwards".
I think I'm going to revise the designe again, using a 2.2M resistor in place of the 5M resistor. It seems to offer better performance when the MPSA18 is subbed with a lower gain transistor.
QuoteP.S. I am going to paste a label on the inside of the box showing the original schematic crediting Tim for the design, plus a schematic showing my mods. Anything else I should do to keep it on the up 'n' up?
Have a beer for me. :)
Thanks for the update Tim. I trust you used log for the volume. Do you think a 2.2m will work better for the 5088/5089, or did you have lower gain in mind? Sounds like that resistor is a definate candidate for a socket.
Cheers Tim! Thanks for the quick reply and also for your page.
Quote from: Fret WireThanks for the update Tim. I trust you used log for the volume. Do you think a 2.2m will work better for the 5088/5089, or did you have lower gain in mind? Sounds like that resistor is a definate candidate for a socket.
Playing with the circuit, a 2.2M will work OK. However, I think there's some kind of biasing point/collector voltage where it sounds best. And it varies depending on the setting of the Gain control. With a high hfe transistor, the value of that resistor places the collector voltage at a point that sounds better to my ears. Dropping in a lower hfe transistor changes the collector voltage range. The simple fix is changing the resistor to compensate.
Also, keep in mind, the lower hfe transistor also exhibits slightly less drive capability to the NPN/PNP pair.
Thanks again Tim. I think I'll give it a try with both 5089's and MPSA18's, and also try different resistor value's.