ShortScale.org

Ok. So I've got a pair of guitars listed here for sale or trade. I've also got them listed on the local classifieds.

I just got offered a Mesa Boogie Subway Blues 20 my DiPinto Belvedere... this one in fact - http://www.clasificadosonline.com/UDMis ... er=5238140

To me it feels like I'm getting a really good deal Until he told me what's wrong with it.

He says that after 50 minutes or so, it starts to hum. It hums like a single coil no matter what guitar he's playing through it. He says that the previous owner had swapped out the output transformer to get a little more output (22 watts vs 20 watts), so that's concerning me. And that the tubes were swapped out, but the amp wasn't rebiased. Obviously without having the amp physically here to check out, I can't elaborate any further.

Any thoughts?

Sounds like preamp tubes to me, to be honest. I'd try swapping some out from an amp that definitely works.

The amp should definitely be rebiased asap, fwiw, but I don't think that's the problem.

i'm guessing that's a cathode bias amp, most mesas seem to be, unless someone has modded it to be fixed bias. so you don't have to worry bout biasing. if tubes were swapped for good ones then don't worry about that yet. swapping the output transformer is kind of dumb, but whatever. any time there's a super duper hum all the time, i suspect filter caps. change them and it's like playing a new amp!

If the hum comes on after a certain time of it being turned on, I would definitely suspect electrolytics.
50 minutes sounds kinda WTF to me, but yeah, you get the idea.

Could be a thermal issue, too. Check the caps first, though.

I finally saw the amp... brought a coworker of mine along who not only plays guitar in a band, but is an electrical engineer has done maintenance and repairs on his amps, and has even built a few oddball amps/pedals before. He's the guy who helped me decide that repairing my Deluxe Reverb wasn't worth it, and my discussions with the guy I sold it to afterward confirmed everything he told me.

Anyway: the hum is a design flaw with that amp, and it can be treated. In fact, it's a pretty cheap fix, but due to the mods which have to be undone, and the fact that the original parts were thrown away, it's not even close to being a cheap fix anymore.

My friend's reply: "you're looking at close to $400 in parts and repairs, and for another $100, you can probably find a used one stateside"

So a polite, thanks, but no thanks, and back to work.

If I'm gonna have to drop $400, I'd rather get a J Mascis Jazzy.

I found that interesting, so Googled for it
http://forum.grailtone.com/viewtopic.ph ... highlight=

The hum mod was released on paper in April of 1998. It changes the .01 cap off the potted module/PCB in the power supply to a .68 cap

I found & replaced it No more hum

Here it is

If so, I'm not quite sure how that comes to be as expensive as you friend says.

NickS wrote:I found that interesting, so Googled for it
http://forum.grailtone.com/viewtopic.ph ... highlight=

The hum mod was released on paper in April of 1998. It changes the .01 cap off the potted module/PCB in the power supply to a .68 cap

I found & replaced it No more hum

Here it is

If so, I'm not quite sure how that comes to be as expensive as you friend says.

It needs more than just that... needs a recap, new tubes, and a new output transformer.

mkt3000 wrote: needs a new output transformer.

That's the killer, if you're going to go for a factory replacement OPT. Could a local rewind be better value?


[edit: eliminate redundant "would"]

NickS wrote:

mkt3000 wrote: needs a new output transformer.

That's the killer, if you're going to go for a factory replacement OPT. Could a local rewind would be better value?

I don't know who would handle that kind of work down here

Small update. So I ended up getting the amp in late November. It was a two part trade - my Kustom Defender full stack for a Peavey Classic 30 2x12 cab, and the cab for the amp.

i kept it around, and used it a few buts, but just before I left for San Diego in December, I decided to take a risk with it, and left it with my amp guy.

It's ready today.

The mojo module was what was causing the hum and it needed to be replaced ($40 direct from Mesa). Basically it was causing the amp to overheat, and that's what was causing the hum. It also needed new power tubes and biasing. He also cleaned up the pots, and replaced any components that needed replacing (high salinity in the northern part of PR can kill gear).

The output transformer is an upgraded one, but for a Fender Deluxe Reverb Reissue, however, it didn't affect anything, and according to him, it looks to have been professionally done.

I'm off to go pick up the amp now

I use a mesa subway rocket and you couldn't make me change. I love it!

Gonna bump this for an update.

I got the amp back, yet again, this week. It was humming again like a bitch. One of the capacitors had started to leak, so he went ahead and recapped it. He did it at no charge, since his repair work was still under warranty. On top of this, he made a call to the local Mesa "expert", who saw the amp, what had been tone to it, and agreed with everything that's been done so far.

It's sounding beautifully.

mkt3000 wrote:Gonna bump this for an update.

I got the amp back, yet again, this week. It was humming again like a bitch. One of the capacitors had started to leak, so he went ahead and recapped it. He did it at no charge, since his repair work was still under warranty. On top of this, he made a call to the local Mesa "expert", who saw the amp, what had been tone to it, and agreed with everything that's been done so far.

It's sounding beautifully.

Great news.