For sale, audited trios of NOS Mullard AC-128K germanium PNP transistors for Tonebender Professional MKII circuit (OC75 version). I'm asking $17.00 per set postage paid in the USA, paypal accepted. Overseas I'll need to add a little more to cover postage. First come first serve on which set you want. Contact me first (modellurker at yahoo dot com) to reserve a set, then I'll give you the payment info. PM is also fine, but I check my e-mail more regularly. I'm lowbrow on pretty much all the ideal/guitar and amp forums, and Indychamps on ebay. I've got a good trading history on Harmony Central and the gear page and 100% positive feedback on ebay.
I've breadboarded every set of these to test for sound, get an idea of bias voltage and trimmer resistance should be. I also cut one open just to have a peek inside...see photo above.
In the table below the sets are, more or less, arranged by gain buckets...highest is set #1, lowest is set #12. In the table below I list gain/leakage (microamps) then in the last column the Q2C voltage at full attack and the trimmer resistance I used to get there.
I use a 100K trimmer on the collector of Q2 and bias by ear. Basically, I put the transistors in their respective slots, crank the "Attack" knob all the way up and increase the resistance on the trimmer till the oscillation and noise go away. As such, my voltages tend to be a bit higher than many recommend, but when adjusted for a true 9 volt source (I was using 9.62 volts when I did this), they really aren't much more than the 7-7.5 a lot of folks like. You can bias these AC-128Ks lower, but it renders the very last bit of Attack knob too noisy to use.
I also tend to like less gain than a lot of people recommend. The old 65/75/110 guideline is fine, but you can get a sweet sounding MkII with less gain that that too. I've recorded a short clip of the set#12 to give an idea what I'm talking about. I don't think you give up a lot in tone, but you do sacrifice some of the overall level or volume. Even with gains in the 50s, though, you still have way more than unity on tap with this three gain stage circuit.
The clip was recorded with a Edirol handheld digital recorder placed about one foot in front of a stock late 80's Fender Champ 12, I was running from my start with DiMarzio Super Distortion in the bridge direct into my breadboard and then the amp. The amp is set very low in volume so I don't have to turn on the clipping and compression on the Edirol. At stage volume (which I also tested) the whole set-up is a darker and more complex. Some of the noise you hear is because it's all hanging out there on the breadboard.
At the start you can hear me dial in the bias by dialing out the oscillation and noise, then I murder a Pinstriped Clips riff at full Attack. From there I back the Attack off to about 50% and continue to launch clams for a couple minutes.
http://lowbroweffects.com/wp-content/up ... K_clip.mp3
Set Q1 Q2 Q3 Q2C/TRIM (source -9.62V)
1 73/336 83/344 142/429 -8.9/56k
2 67/427 82/368 110/328 -8.8/47k
3 64/469 81/412 104/441 -8.5/45k
4 64/291 80/279 98/328 -8.4/56k
5 62/271 80/251 94/388 -8.2/43k
6 62/275 77/279 89/730 -8.8/58k
7 60/243 77/283 88/518 -8.3/51k
8 60/283 74/437 87/400 -8.7/46k
9 60/255 74/384 88/416 -8.8/49k
10 53/311 73/279 84/315 -8.7/56k
11 50/303 57/259 70/291 -8.4/56k
12 53/311 54/255 55/251 -8.7/56k
