Marshall 4203 c-13 mod

[Mars Amp Repair]

Say folks,
Just saw an ad on Craigslist for a Marshall 4203 that claimed to have "the c-13 mod" done to give it a warmer sound. I see the cap on the normal channel volume pot after the tone stack. It's the cap that allows more hi's to bypass the vol pot when it's not all the way up.
Anyone know what this mod is all about. I'm guessing they change or possibly just remove the cap. I always like to file this stuff with the schemo for future use. Esp is someone comes in & says 'you of course know about the c-13 mod, right?' Thanx, glen

[Enzo]

I call that a brightness cap myself. On some amps, it is the only difference between bright and normal channels. On other amps it is what is switched in and out by the brite switch.

Removing it or changing its value will affect the warmth of the channel sure.

But why limit yourself to C13 in 4203s? A little hf bypass cap on most any volume control will brighten things up, and removing one will mellow the tone. To me it is more important that I understand how the curcuits work, rather than remember that C13 - whatever that might be - needs to change from 120pf to 180pf or something.

That is a complaint I have about Gerald Weber books. There are some interesting things in them, but all his mods are of the variety, "Move this part to here and change this part to that..." He never explains WHAT he was doing electrically so you never learn anything.

So for example adding a cathode bypass cap to some stage will affect the gain of the stage and the bottom end response in most any amp, not ONLY the Fender so and so amp. Same idea for C13.

Think generically, amps are all alike under the skin.

[Mars Amp Repair]

Well Enzo,
I also know that cap is for briteness, in fact when I was 17 (& really didn't know much about much) my amp was a 200watt Marshall Major stack, I intentionally added that cap across the wiper/hot lead to the 2nd channel vol pot to give me some chunk sound with my 65 non-reverse Thunderbird bass...what a difference that made. Nope, don't still have it...the usual musician's crying in his beer saga...but I digress..

Since I am fairly new to the formal music amp field, I always try to glean as much info as I can owing to the fact that I have not been around as long as many of you-all in the amp repair biz.

I like yourself dont' just take the info at face value, but seek the underlying design reasons for why a certain change will bring about a certain effect. As I mentioned in my e-mail I actually learned tube design way back in '72 at RCA Institute in NYC & then transistor & opamp design knowledge at Metro State Univ here in Denver...tho admittedly I was repairing more vcr's & camcorders back in the 80's & didn't have much call to do design work, however it did put me way ahead of the usual techs who were merely 'method' repair guys.

Anyway, I am always greatful for all the wonderful info from all who have already spent the time experimenting & are so willing to impart their knowledge & experience on this & other forums, & as well as when it is info that debunks certain mods some hack has stumbled onto by trial & mostly error. It makes for a better pool of techs out there. glen

[Mars Amp Repair]

Well Enzo,
I also know that cap is for briteness, in fact when I was 17 (& really didn't know much about much) my amp was a 200watt Marshall Major stack, I intentionally added that cap across the wiper/hot lead to the 2nd channel vol pot to give me some chunk sound with my 65 non-reverse Thunderbird bass...what a difference that made. At that point I only knew I'd seen it in other amps & figured out what it must do by its placement, as well as experimentation.

Nope, don't still have the bass or the amp...the usual musician's crying in his beer about what stuff he used to have saga...but I digress..

Since I am fairly new to the formal music amp field, I always try to glean as much info as I can owing to the fact that I have not been around as long as many of you-all in the music amp repair field. I have no qualms about asking what might be considered 'dumb' questions.

I like yourself dont' just take the info at face value, but seek the underlying design reasons for why a certain change will bring about a certain effect. As I mentioned in my e-mail I actually learned tube design way back in '72 at RCA Institute in NYC & then transistor & opamp design knowledge at Metro State Univ here in Denver...tho admittedly I was repairing more vcr's & camcorders back in the 80's & didn't have much call to do design work, however the knowledge of how things actually worked did put me way ahead of the usual techs who were merely 'method' repair guys.

Anyway, I am always greatful for all the wonderful info from all who have already spent the time experimenting & are so willing to impart their knowledge & experience on this & other forums & as well as when it is info that debunks certain mods some hack has stumbled onto by trial & mostly error. We don't all have the time to experiment on every amp that comes in.

These forums vastly improve the pool of techs out there for consumers to use as well as us to hire from, as well as hopefully encouraging the recreation of what has been referred to as a dying breed. glen

[Enzo]

Hey, turn down the reverb, reverb, reverb...

[Mars Amp Repair]

In stereo, eo, eo eo......

[petemoore]

Same trick' works for guitar volume knob, for a bit of treble 'boost' [a little more bass than treble attenuation at guitar volume], brighter tone when rolled off some. I love that one, try that first even.