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My first amp modification story - success! Bogner Metropolis content.

RockStarNick

Gold Supporting Member
Messages
3,632
So last year, I scored a mint Bogner Metropolis 30W. I've been playing it for the past year, and while it's an amazing amp, there was something about the tone that I wasn't completely bonding with. Even after many tube swaps and EQ changes, I still couldn't dial it in exactly to my liking. It was pretty bright, and a little to bitey on top. So, I did what anyone would do. RIP IT OPEN and take a look under the hood. :D

Here's what I found:

Inside the Bogner Metropolis, there is an interesting juxtaposition of super-tidy, super sturdy PCB work (typical Bogner high quality), combined with some really wacky, old school, flying PTP components (like a Matchless). In particular, the 5 position "Schizo" switch. It's described as Bogner as a tone-shaping circuit. From what I can see, is that it selects one of 5 capacitors, or capacitor/resistor combinations, to alter the bass response and EQ curve. (I'm not an amp tech, by any means, but from my experience working on Randall/Egnater modules, I can assume that it's some sort of coupling cap).


The COOL part is that the flying PTP portion of the circuit is that it makes it very easy to work on it. No need to take out the circuit board, or work in a cramped space. The PTP portion is right up front, and wide open, with plenty of room to solder, remove old components, and get new ones in. (Not to mention use alligator clips for beta-testing).


The cap combinations of the Schizo switch have evolved over the years, according to Bogner. In the past, the capacitors just increased in size, adding more bass. The more recent Trops have cap/resistor combos, which do more EQ filtering. I appreciate the EQ filtering, but I felt that the amp was lacking a "normal" sound. Just a straight up, straight thru cap with a typical midrange and bass response. And my assumptions proved correct, when I realized there wasn't any .0022 "straight thru" cap. (a very common input cap on marshalls, Dr. Z's, etc). (Pardon my incomplete drawing - I'm no electrical engineer, I just drew out what was visible to me upon inspecting the inside.


Well, now, onto my testing and conclusions:

The cool part about having the 5-way switch in there, was that it provided a platform to do real-time, back-to-back comparisons between caps of different constructions and values. So for giggles, I soldered in three .0022 caps: a Silver mica, an Orange Drop 716, and a Mojo Dijon.

I know it sounds ridiculous, but there was a huge difference in sound between them. The Silver Mica (Bogner used Silver Mica's stock for the cap combos) was pretty decent. Smooth, but bright. The Orange Drop was harder, more hard trebles, bigger bass. But just kind of offensive sounding, and way too aggressive - at least in this amp.

Now, I hate to use all the typical Gear Page buzz words, but The Mojo was sweeter, less extreme EQ. Dare I say woodier, more vintage, more 3D. Sounds a lot closer to my friend's Dr Z Maz 18, which is precisely what I was going for.


The Mojo Dijon just fits this amp so much better, makes it more pleasing to the ear, and it's much more enjoyable to play now. Really cops the Brad Paisley tone (which I'm obsessed with right now.) Now instead of the useless Position 1, which was 100pf thin screetchy crap, and position 2 which was a super hollow scooped tone, I now just have my choice between a a .001uF and .0022uF Mojo Dijon cap, to tailor the bass response, but still sound similar. Much, much more useable tones now.

I showed my findings to a good friend who is a very respected amp builder and member here on TGP, and he said "It's important to stress, in your findings, that you were really hearing the difference in cap CONSTRUCTION, not brand. In this case, a Polypropylene OD vs a Polyester Mojo. Had you compared the PP OD's to another brand of PP caps, you would have heard much less of a difference between the two. Comparing composition & hearing a difference....valid & legit. Comparing "trade names" & hearing a difference...slippery slope." I think he's 100% spot on in this.

It's kinda crazy how one $1 component can change the character of an amp this much. It's also incredible how that first cap makes such a big difference. kinda like the first preamp tube in an amp.

The moral of the story: If an amp sounds 95% how you want it, but something's still missing, chances are a little bit of research, and $2-3 worth of capacitors, can transform an amplifier, and tweak it to your exact needs.
 

MT Buckaroo

Gold Supporting Member
Messages
952
Here we go... Cottage industry springing up for the RockStarN mods.

I might still have a Metropolis if you had figured this out sooner.

;-)
 

RSRD

Member
Messages
4,973
Bro, this is an awesome post. Really cool. Grats on the Mod! Wish I knew 1/2 of what I just read.
 

robertkoa

Member
Messages
4,189
Cool on the spot testing with the caps and congrats on the more usable tones.

Did you try one setting that bypasses all the caps just to check ?
 

Coolidge

Vendor
Messages
2,442
Are you sure Mojo Dijon's are not polypropylene film? I see them advertised as polypropylene film at The Tube Store, the "Mojo Tone" caps are polyester film.
 

RockStarNick

Gold Supporting Member
Messages
3,632
Are you sure Mojo Dijon's are not polypropylene film? I see them advertised as polypropylene film at The Tube Store, the "Mojo Tone" caps are polyester film.
Hmm. I'm going to have to do some more research now on this. I had no idea there were two different types.
 




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