made a small, magical change to my Bronco (aka Vibro Champ)

[tdbajus]

I was hoping for something that sounded like my old BF Deluxe. I've been fooling around with a Bronco (same thing, mostly, as a Vibro Champ- just with a cooler face plate) and today swapped the 16K resistor on the tone stack (the connects the tone stack to ground) to the Fender standard 6800 (which is on pretty much every single one of their other amps).

It's astounding. I've been poking around to see if anyone has posted this mod and haven't found anything. The tone controls on the Bronco/Champ are OK, but if you miss that nice bloom you get in that place between 6 and 8 on the treble pot, try this out.

Suddenly it acts like all the other Fenders. Which is nice.

[Jim Williams]

I use a 10 k audio pot for an adjustable mid control. That re-creates the same range as a Twin Reverb. It's good to have that adjustment as guitars need different amounts.

The mid cap value is typically .047 uf. That sets the mid cut at 400 hz. Use .022 uf and it moves it up to 700 hz, that value is used in the Super Reverb with 10" speakers.

[tdbajus]

Also installed a bypass for the tone control (but still left the negative feedback in) and it is a fire breathing distortion monster, especial when I plug it into my Marshall 1x12 with a Weber ceramic Blue Dog (Vox Bulldog-ish 12" speaker, 50 watts).

It's funny what a high gain sound it is, especially with a bridge humbucker- i'm getting a 'Strap It On' era Helmet crunch, and if i back off on the volume just a bit, it's 'Rust Never Sleeps'. Does a better job of it than my 5E3 clone does, and at a volume that makes it easier to record in my big open studio with no isolation at all.

[roygbiv]

hmm, I'm very curious how to do these mods - especially the tone-bypass and 10k pot switch.

I would like more distortion out of my 70's Vibro Champ that I got at a Goodwill 15 years ago.

Do you know of any good links with pictures?

thx

[KennyLusk]

hmm, I'm very curious how to do these mods - especially the tone-bypass and 10k pot switch.

I would like more distortion out of my 70's Vibro Champ that I got at a Goodwill 15 years ago.

Do you know of any good links with pictures?

thx

That's funny, because I have the opposite problem with my 70's vibro. I wish I had more clean room before break-up. Maybe we should trade, LOL.

[roygbiv]

fair enough, faulty language on my part.

Clean I don't care about so much, as I have an old Princeton and Pro Reverb.

I guess what I want is better tone to my distortion - maybe a better speaker would help, just was intrigued by the "Rust Never Sleeps" mention above (I jess love it when Neil Young gets that out-of-control distortion, to me that IS the sound of Rock and Roll)

[themagicmanmdt]

to me, tone bypass is *ok*, since all you hear is the straight in sound of your guitar - which could be awesome or meh, especially since the tone stack, to me, helps overcome the freq. deficiencies of any guitar speaker. it's definitely not a 'fender'-y sound.

i'd agree that one of the big fender sound components is a low value mid setting, which gives it that 'scoop' that makes the treble stand out and shine.


as far as gain goes, more or less, there are a few things to do, and it all depends on what KIND of distortion/gain coloration you're looking for.

[roygbiv]

hmm, good point. I guess what I want is for the distortion to not sound really Fendery.

As an aside, I must admit I am beginning to wonder if the last 35 years I've spent being a "Fender guy" were a mistake, at least regarding distortion. Other than an old blackface Bassman head I once had that had been "Marshallized", I don't usually like the distortion I get out of a Fender.

Don't get me wrong, I still loves me some clean Fender. But for distortion, I always seem to try to get rid of the Shrill, scooped tone with various pedals, etc.

For example, on the 1970 Pro Reverb I use for all my gigging, I set the Treble and Bass almost completely off to get a relatively flat EQ. For distortion, I then slam it with a Seymour Duncan Twin Tube, and/or a ACTone or Analogman King-o'Tone.

That almost gets me there.

(another aside - best distortion I ever had was an old Brown Gibson Amp from the late 50's/early 60's, that thing was great. Slight to heavy breakup, warm, etc. And it had an Accordian input too (or was it harmonica?)

So, to wrap it up, I'm interested in a mod for my Vibro champ that will let me get a warm, old Fender/Gibson/Voxy/Marshally cranked up distorted sound.

[Jim Williams]

On my 1973 Vibro Champ I removed the vibrato circuit and used that tube to redo a marshall-y 4 stage preamp into the treble/mid/bass tone stack.
The volume pot sets gain and I added a master volume pot. The mid and master volume fit into the holes for the vibrato so no drilling there.

Next, I removed the 470 ohm 2 watt plate resistor and put in a Deluxe Reverb choke. Then I removed the multi cap can and installed 2 2x100 uf 500 volt caps from JJ. Lastly I changed the 6V6 to a EH 6CA7 power tube, I get around 12 watts now. There is no residual hum like on all other Champs.

I got rid of the tinny Jensen speaker and installed an Eminence legend 4 ohm 8" speaker.

This amp SCREAMS! It also has enough poop to drive my 4x12 cab.

[roygbiv]

man, that sounds great! I am lusting.

How much is something like that?
(hypothetically and approximately speaking of course).

s this something that someone with basic soldering skills could accomplish, or would I need to take it to a technician.

I know how to melt solder. And could change out resisters. But not a lot more than that.

[Jim Williams]

If you have soldering and schematic reading chops you can just copy the marshall JCM800 preamp and do those other things I mentioned. Any amp tech can do this work.

[roygbiv]

cool man, thanks Jim!

I may give this a try, I got this thing so cheap so long ago that I am not too worried about tweaking it (besides, I've had both a Bassman head and a Princeton head tweaked years ago, and they both sounded fab! well worth the effort)