ts9 mod question - tantalum caps

[jms2009]

I want to replace the two .22uf tantalum caps (C105 and C106) on my Ibanez TS9 tube screamer with slightly lower ones since I heard it affects the frequency response of the tone knob, but am curious about what would happen if I replaced the two caps with different values (do they have to be the same cap values?). For example, if I replaced C105 with a .18uf poly film cap and left C106 as the .22uf tantalum cap, what is supposed to happen (I did just this since I only have one .18uf cap, but I can't determine how this is affecting the sound)? Does one cap affect certain frequencies differently that the other one? Does maybe one cap bleed off more high frequencies and the other cap bleed low frequencies (if so, then which cap affects which frequencies)? BYOC says you can use a .15uf cap, while I think Keeley uses .18uf. What is the difference between using .15uf and .18uf?

[duhvoodooman]

Rather than offer a poor explanation of my own, let me refer you to the definitive article on the workings of the classic Tube Screamer circuit by electronics guru R.G. Keen:


http://www.geofex.com/article_folders/tstech/tsxtech.htm
The function of those two 0.22 uf tants is covered under the section entitled "Tone and Volume Control Stages". Their values definitely relate closely to their individual functions. They don't both have to be the same value, though this is how the circuit usually seems to be set up. You can always install compression sockets in the eyelets for these two cap positions and experiment a bit.

Dropping the values of the caps from 0.22 will brighten the sound of the pedal, since it shifts the rolloff frequency of the RC low pass filter upward. For example, going to 0.15 uf like the BYOC kit option will move that frequency "knee" from 723 Hz to 1061 Hz. So if you find the stock TS circuit a bit on the dark side, this would be a good mod to try. But if you prefer it to be a bit warmer/more mellow in tone, I'd leave them at 0.22 uf. I've built them both ways--there's no right or wrong here.

Also, there's nothing "magic" about using tants there--they're small and don't take up much real estate on the board, which is prolly the main reason they were used in the original TS-808. But I've built them with box metal film caps there, and it sounds just as good.

[jms2009]

Thanks a bunch for that info. I'll likely experiment with different values. BTW, I used your post in this forum (http://www.thefret.net/showthread.php?t=2540) which listed different diode combinations using your Zonkin' Yellow Screamer as the test bed when I modded my TS9 this past weekend - very helpful. I ended up using 1N4001 pair, 1N4148/1N34A, & Red LED/1N34A. Cheers!