pcb

Deisel401 · Jan 4, 2012

Looking at the Plexi and SL+ , both black faceplate, one of the differences I see on the pcb is Plexi Gain knob is a 1M pot. The SL+ Gain knob is a 500K pot. Could anyone explain to me what difference this makes please ? Thanks.

SacredGroove · Jan 4, 2012

Hardly a technical explanation here, but I could give you one if you want...LOL. The difference in values of a gain pot will effect how much signal is retained from leaking to ground. The higher the value, the more signal is allowed to pass through to the rest of the circuit. But, retaining too much signal could result in crappy or no sound.

Gain pots can have a major role in overall tone and available gain.

Hope this helps. :wink:

Deisel401 · Jan 4, 2012

Hi Scott. Good to see you're still with us and thanks for the response. And that def helped but I guess my understanding was backwards. I thought the 1M gain pot in Plexi is of a higher value than the 500k pot in SL+, and therefore was shunting more signal to ground. So, just changing the Plexi 1M gain pot to a 500k gain pot probably won't give me the gain of the SL+ because I'd have to change a lot of other things too, right? Like the obvious resistor values in about 7 or 8 locations on the pcb ? I'm still trying to change my Plexi into an SL+, or at least kinda' close. I broke off the MIDDLE knob on the SL+ and made poop of a couple solder contacts. To be specific, on the SL+, C7 and C13. A couple weeks ago you explained how to fix those by using "jumpers". I think I did good , but won't know till I get a replacement pot for the broken one. Tried taking the MIDDLE pot off the Plexi but realized I'd probably ruin it and stopped. Figured it would be easier to turn the Plexi into an SL+. Anyway, if I can get the Plexi to have some more gain without killing it, I'm NEVER gonna mess with it again. Damn. . . sorry about my rambling. . . .thanks again Scott and hope things are going better for you.

SacredGroove · Jan 4, 2012

Deisel401 said:
Hi Scott. Good to see you're still with us and thanks for the response. And that def helped but I guess my understanding was backwards. I thought the 1M gain pot in Plexi is of a higher value than the 500k pot in SL+, and therefore was shunting more signal to ground. So, just changing the Plexi 1M gain pot to a 500k gain pot probably won't give me the gain of the SL+ because I'd have to change a lot of other things too, right?

I think you did understand it right. High gain mods (4 gain stages) are more likely to have a 500k pot, and everything else is likely to have a 1M pot. 1M is good for retaining as much signal as possible, and 500k is good leaking off some of the signal so not to "overload" the circuit. Some Mesa Rectos have so much gain that they've used 250k gain pots to help keep the circuit stable.

Like the obvious resistor values in about 7 or 8 locations on the pcb ? I'm still trying to change my Plexi into an SL+, or at least kinda' close. I broke off the MIDDLE knob on the SL+ and made poop of a couple solder contacts. To be specific, on the SL+, C7 and C13.

The Plexi uses one less gain stage than the SL+, so some values would need to be added, rather than changed. If you need help with that, let me know.

A couple weeks ago you explained how to fix those by using "jumpers". I think I did good , but won't know till I get a replacement pot for the broken one. Tried taking the MIDDLE pot off the Plexi but realized I'd probably ruin it and stopped. Figured it would be easier to turn the Plexi into an SL+. Anyway, if I can get the Plexi to have some more gain without killing it, I'm NEVER gonna mess with it again. Damn. . . sorry about my rambling. . . .thanks again Scott and hope things are going better for you.

The 25k mids pot is fairly standard across most amps. Unless you're experimenting, I would just leave the mids pot alone...LOL.

Thanks for the words. I'm slowly making my way back to making mods. I'll probably know more by the end of the month. FWIW, I haven't actually stopped working, but rather only stopped taking orders....LOL.

Deisel401 · Jan 5, 2012

Thanks again Scott. If you wouldn't mind, I would really appreciate any help you're willing to offer. I'm guessing I should change and add any resistors and caps that are on the SL+ that are not on the Plexi. One example would be adding an .022 cap in C8 on the Plexi. I see several things like that I could do without ruining the pcb. And adding another gain stage I have no idea. If you are willing to help me change my Plexi into an SL+ Scott , we need to figure out how I can repay you. Possibly sending you my non-working SL+ module as a donor with some cash?