Shopping & Learning: starting DIY on guitar amps

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 7:05 am

Cat-approved 1965 Vibro Champ:

Musicmaster Bass Amp in the background. It sounds great, but has developed a wicked, wicked bad hum. The plug is bad, which I hope is the problem, because that's easily fixed...

parry · Post by **parry** » Fri Jul 18, 2008 7:11 am

Okay, WOW!

That looks JUST like a cat we had when I was a kid...

Wow...
Thanks for the memory trip...

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 7:13 am

hansonparry wrote: Okay, WOW!
That looks JUST like a cat we had when I was a kid...
Wow...
Thanks for the memory trip...

Another Bob twin! Cool!

mjet · Post by **mjet** » Fri Jul 18, 2008 9:16 am

Maggieo wrote: Cat-approved 1965 Vibro Champ:

I know that Champs are small, but still - that's a massive piece of cat you've got there. It almost looks as if the photo was photoshopped!

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 9:24 am

mjet wrote:
Maggieo wrote: Cat-approved 1965 Vibro Champ:

I know that Champs are small, but still - that's a massive piece of cat you've got there. It almost looks as if the photo was photoshopped!

Bob is pretty massive.

All the Bob photos you could ever want.

mjet · Post by **mjet** » Fri Jul 18, 2008 12:07 pm

Maggieo wrote: All the Bob photos you could ever want.

Eek! Look what they did to that 2x10" cab!! Bad kitty! :P

øøøøøøø · Post by **øøøøøøø** » Fri Jul 18, 2008 12:37 pm

Maggieo wrote:
Musicmaster Bass Amp in the background. It sounds great, but has developed a wicked, wicked bad hum. The plug is bad, which I hope is the problem, because that's easily fixed...

In old Fender amps, everything is easily fixed.

that's why I like 'em so much.

It's most likely that you need a cap job. Which is more involved than replacing the plug, but is still not too bad.

mjet · Post by **mjet** » Fri Jul 18, 2008 12:44 pm

øøøøøøø wrote:
In old Fender amps, everything is easily fixed.

With one exception - the Musicmaster Bass output transformer is a real odd bird. I'm no techie but it was explained to be as almost a transformer within a transformer - totally unlike other Fender standard designs. I know because I had to buy a new one a few months back. Luckily Mercury Magnetics make a clone. Not cheap, but at least it's good quality.

Of course I was a bit peeved when I saw someone who had some MM Bass guts (including a working OT) on eBay that went for $15... [grrrr]

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 1:42 pm

mjet wrote:
Maggieo wrote: All the Bob photos you could ever want.
Eek! Look what they did to that 2x10" cab!! Bad kitty! :P

It would totally rock to have a wall of kitty tree cabs. Take THAT, Brian May!

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 1:44 pm

øøøøøøø wrote:
Maggieo wrote:
Musicmaster Bass Amp in the background. It sounds great, but has developed a wicked, wicked bad hum. The plug is bad, which I hope is the problem, because that's easily fixed...
In old Fender amps, everything is easily fixed.

that's why I like 'em so much.

It's most likely that you need a cap job. Which is more involved than replacing the plug, but is still not too bad.

That's easy for you to say!

I'm askeeert of the lectricity!

Everything I read about working on amps comes with a huge THIS WILL KILL YOU disclaimer.

BTW, the Weber sounds fantastic in the Vibro Champ!!!

BenHagerty · Post by **BenHagerty** » Fri Jul 18, 2008 2:03 pm

Maggieo wrote: Cat-approved 1965 Vibro Champ:

Musicmaster Bass Amp in the background. It sounds great, but has developed a wicked, wicked bad hum. The plug is bad, which I hope is the problem, because that's easily fixed...

Wow that thing is in excellent shape.

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 7:07 pm

Man, that's cool!!

I asked my dad about working on amps and all he had to say was "I failed electrical engineering! Wanna build a dam?" (retired civil engineer)

So, how would a girl go about taking up a new hobby and fixing her Musicmaster Bass amp?

BenHagerty · Post by **BenHagerty** » Fri Jul 18, 2008 7:27 pm

Wow that is nifty

Maggieo · Post by **Maggieo** » Fri Jul 18, 2008 7:39 pm

WooHoo! Shopping!

And learning.

Learning & Shopping.

Ohhhhh, yeah.

øøøøøøø · Post by **øøøøøøø** » Fri Jul 18, 2008 7:53 pm

Here is the schematic for your amp.

Look how simple! That's probably why the musicmaster bass sounds good and also why it would be a pretty good amp to do your first job on.

Near the bottom/right is the power supply. you'll see the plug, the power transformer (transformers look like two coily things with some lines in the middle on a schematic). You'll see three capacitors that are 20µf @ 300v. Those are the caps you'll be changing. The question is, are they "discrete" (individual) caps, or part of one of those multi-section "can-style" caps. You'd have to determine that by looking at the amp (or posting a pic of the underside of the chassis where the tubes are).

Before touching the caps you'll want to double-check that there's no voltage stored in them. If you posted a "gut shot" of your amp, I could tell you where to put the multimeter leads to test for residual voltage. You could also just set the meter to read volts (500VDC range) and put the red probe on the "positive" of the cap and the black probe on the chassis. It should read a very low number (<30 or so, probably zero) and then you're safe. If it reads a number that's high-ish, like 200 or more, definitely don't touch anything. :)

Good places to buy caps and such are Hoffman (www.hoffmanamps.com) and AES (www.tubesandmore.com). Hoffman has smaller selection but great service and near-instant delivery. AES has better selection and prices. Actually, they ship from AZ, so since you're in Nebraska it probably wouldn't be so bad. AZ to NYC metro is a long haul. :)

If you're new-ish to electronics, I'd recommend practicing first on your soldering skills. A good way is to build a Fuzz Face clone, which is about $20 in parts. And then you get to have a Fuzz Face, if you can find a use for one of those (I never could). You could also build a tube screamer, which is about $50 in parts-- a little more complex but still a good beginner project. www.generalguitargadgets.com sells PCB boards and even kits, maybe. Lots of distortion/overdrive pedals make good "get your feet wet" projects.

Good books to check out are Dave Funk's "tube amp workbook." Gerald Weber also has 3 books out, and while engineer-types often turn up their nose at them, they offer lots of great practical advice even if they're a bit short on theory, and I learned a lot from them when I started out.

What's interesting is that it looks from the schematic like the MM bass has a transformer phase inverter instead of a tube phase inverter. That's a pretty rare thing... A few Gibson amps from the 50s had that, but most manufacturers deemed that it was more cost-effective to use a tube there. Why they chose this low-priced amp to do that with, I don't know, but (at least in theory) it makes a better phase inverter than a tube, and allows for fewer parts in the circuit. The problem is that the transformer itself is expensive. The MM bass looks like a really interesting amp!