5150 III Mod Changed Tone of the Amp

I got my 5150 III 50 watt combo modded awhile back with the official Fender kit to give the clean channel its on dedicated volume and gain controls. I can't help but feeling that this mod shifted some of the frequencies of the amp. It does not feel as full, seems to have less bass and a lot more treble. The clean channel itself is horribly bright.

I am not technical enough to know if the mod itself could have done this or maybe something else was done while modding it to make this happen. Just wondering if anyone has any insights into what may have happened or has experience with this mod going wrong or changing the tone of the amp.

Thanks,
Jeff

Ive seen it installed two ways - with the gain pots in parallel and the wiper/ouput switched, and with the gain pot input switched. The former way will double the bass cut off frequency if the coupling cap isnt doubled or the pot values doubled. So basically yes, quite possible to revoice those channels if care isnt paid to how the new parts affect cutoff frequencies

I agree with Warhed, with gain pots in parallel with the outputs switched, there is still a resistance referenced to ground by each potentiometer which is in parallel with one another. You could get some wonky RC responses based upon your gain potentiometer settings.

It's best to switch the inputs.

WarHed, would you have a similar problem attaching center wipers together and switching inputs between two master volumes? Would the un-used master cause an unintended short to ground? Seems like it would.

Yes it would..
Switching the output is the way to minimize required relays but as you've said the RC constant has to be corrected

Okay thanks guys. I know the tech got the kit from Fender but I'm not sure how he installed it. I'm not sure if I should go back to him or know how to ask the right questions =D. Maybe I can find another tech but for some reason in the NW burbs of Chicago here they are hard to come by. I knew I wasn't hearing things though and the amp does definitely have more treble than before and it is thinner.

WarHed":167if8q1 said:

Yes it would..
Switching the output is the way to minimize required relays but as you've said the RC constant has to be corrected

Click to expand...

My solution would be to switch the inputs and outputs both. Requires another relay but that’s the correct method in my mind. That way the unused potentiometer is correctly left referenced to ground at all times and not left floating.

jeff5":32eup2dt said:

Okay thanks guys. I know the tech got the kit from Fender but I'm not sure how he installed it. I'm not sure if I should go back to him or know how to ask the right questions =D. Maybe I can find another tech but for some reason in the NW burbs of Chicago here they are hard to come by. I knew I wasn't hearing things though and the amp does definitely have more treble than before and it is thinner.

Click to expand...

You should contact the tech before and ask this question:

“Does the mod switch the inputs of the potentiometers or the outputs of the potentiometers?”

“Was the installed mod adjusted to correct the shifted RC low cutoff frequency?”

The answer to those will tell you what you need to the next tech

If it used dual parallel pots and just switches the wipers, which is the easiest/cleanest way to do it, then the value of the pots must be doubled. If they didn't double the value, they didn't think it through. Shame on fender.