ez-250

Joe · July 25, 2013, 11:51:37 PM

Sort of a simplified class-A version of a DOD250, sounds similar but is a little softer.

mistahead · July 25, 2013, 11:59:23 PM

Why the MPSA06 in particular? Just curious - haven't seen them used much around here that I can think of right now...

nordine · July 26, 2013, 04:04:11 AM

Quote from: Joe on July 25, 2013, 11:51:37 PM
Sort of a simplified class-A version of a DOD250, sounds similar but is a little softer.

Are you THE joe davisson? RESPECT

Welcome back

Bill Mountain · July 26, 2013, 06:41:17 AM

This looks perfect for a new project of mine. I'll breadboard it tonight and post my results.

I don't have that tranny though.

smallbearelec · July 26, 2013, 07:00:23 AM

Hi Joe--

I came up with a similar idea for my Ursa Minor:

https://www.smallbearelec.com/HowTos/BreadboardUrsaMinor/BreadboardUrsaMinor.htm

I agree...the pot in the emitter circuit gives good control of fuzz, and lining up the bias is important. If you have any, try a high-gain NPN germanium device in the Q2 position.

SD

Mustachio · July 26, 2013, 08:36:33 AM

This Looks pretty cool Joe! I've been practicing drawing pcb layouts from schematics lately, so I just started throwing this together tonight. I got it all laid out nice and small about 1"x1"(I think). Still have to label components , and look over it a few times before I etch it. I'll test it out soon! If it works and you want it, ill post the layout.

I sold my original 250 so its time I replace it. My original was a bit harsh until I changed a few components , so this one being softer sounds like what I want. Thanks for the Schematic!

I have a few AC176's Ill give em a try. Thinking about replacing the ~12k at the bias with a 10k or 15k trimmer. I know I have 10K on hand.

Joe · July 26, 2013, 04:06:17 PM

Not much reason to use the MPSA06 specifically, it's a good audio transistor though. Any NPN will do. Just something I was messing around with that seemed promising.

Thanks for offering the layout, I don't really make pcb's anymore but I'm sure someone trying to make a real "mini" would like a 1 sq inch (!) board.

Small Bear: yeah I usually do that for gain control on single-stage transistor jobs, seems to work as good as anything. Nice on the Electra improvements, will try it out.

mistahead · July 27, 2013, 05:17:17 AM

Ta - thought they may offer a special property that made them better than the rest... I'll have to get a few at any rate.

Variety, spice, etc

Mustachio · July 27, 2013, 07:25:47 AM

So here's what I threw together. Not sure if its right or not yet. If anyone wants to give it a look over, spot any mistakes that would be great! I have some mpsa06's on the way , gonna look over it a few more times and if its a go ill etch it soon maybe in a few days.

Let me know if you guys spot any problems this is unverified at the moment.

Oh yeah that breadboard tut you have there Steve, is awesome!

Joe · July 27, 2013, 07:50:24 PM

nice job

Gus · July 28, 2013, 10:11:22 AM

Note what Joe did with the input emitter follower. I like simple min parts designs
The EF is biased more to ground allowing the emitter DC voltage to be coupled by the 10K resistor to bias the next stage.

The EF base should be about 2VDC and The emitter about 1.5VDC from LT spice sims of different transistors. Good for guitar signal input level

PRR · July 28, 2013, 08:13:41 PM

> EF is biased more to ground allowing the emitter DC voltage to be coupled by the 10K resistor to bias the next stage.

Or... you could see that this 10K does very little. Q1Q2 reduce to a Darlington. The 9V 4.5V 12K and 2K reduce to 0.75V at Q2 emitter. Q1 base is 1.2V higher, need 1.95V at Q1 base. To get 1.95V from 9V we need a 1/4.6 ratio divider. Or 3.6 on top, 1 on bottom. 1Meg may do for bottom. 3.3Meg is close-enough on top.

Joe · July 29, 2013, 09:02:20 PM

The rational behind the 10K resistor is to help keep Q2 out of saturation. The following examples are from the basic stage with a 250mV input, and no clipping diodes. Despite having similar output voltages the waveforms are very different.

No resistor, lots of base current draw and enough saturation to cause the "bad transistor sound":

10K resistor, not enough base current available to saturate Q2, resulting in a 50/50 duty cycle:

(Note that with clipping diodes the situation is different and the resistor may not matter then, I'm not sure...)

Renegadrian · July 30, 2013, 10:14:43 AM

hope I'm gonna post my vero in a couple of days (not in my actual home right now...)

bluebunny · July 30, 2013, 03:37:52 PM

I just knocked this up:

(click to enlarge)

Not verified it yet, but looks OK to these tired eyes? (Watch out for four track cuts "under" resistors; one out in the open.) I'll give it a try when I get the parts.

P.S. @Adriano - don't let me stop you doing one of your famous tiny Veros!

Joe · August 01, 2013, 08:37:44 PM

Looked over the layouts. The PCB layout has the first 10K (R1) after the 3.3M/1M bias divider, although I really doubt it matters. Vero layout looks ok to me.

bluebunny · August 02, 2013, 03:14:36 AM

Quote from: Joe on August 01, 2013, 08:37:44 PM
Vero layout looks ok to me.

Cool. Just waiting on some 9mm pots and then I'll be building this and squeezing it into a 1590A.

Mustachio · August 03, 2013, 07:14:18 AM

Haha I hope its ok how I laid it out, I etched a board a few days ago , getting ready to populate it now. I'll let ya guys know how it goes, The board is tinny!

bluebunny · August 03, 2013, 01:20:02 PM

Built my board this afternoon. Slightly different layout than the one above - managed to trim a little off the original size, since I knew I had some sub-miniature electros to hand. Now just waiting on those 9mm pots...

Edit: added link to updated layout.

Mustachio · August 03, 2013, 03:41:20 PM

I populated it and tested it out. With 2 mpsa06's I had to use 144K to bias q2 at 4.5v. Then I tried to pop in an AC176 and got 4.5v using 55K . The only thing other then that was I used a 5KB pot for the drive I didn't have any 2K on hand figured it wouldn't make a big difference.

It sounded really nice, I socketed the diodes and tested out a few different diodes. The silicon sound nice though and might just stick with em. When I pushed an orange squeezer in front of it, It really opened up and was a very responsive OD! will have to play with it more. I wonder if the way I routed the first 3 resistors changed anything, or caused me to use a larger resistor to bias Q2? Maybe ill etch another one with it fixed and see if it makes a difference. Pretty cool overall and simple!

Nice layout bunny! looks nice and small on vero! Let me know how yours bias's and sounds.

Here's mine with long legs, gotta trim em when I settle on what I'm gonna keep in there.

Its just a lil bigger then a quarter. I'm wondering what a buffer would do to this.

News:

ez-250