"Broken" Musicmaster Bass amp question...

[AaronP.]

Hi all,
I'm Aaron and I'm still a noob around here but trying to learn. I recently picked up a "broken" Fender Musicmaster bass amp. 1x12 12watts 2 6v6s volume and tone knobs. It arrived w/ only 1 tube and it was a 6L6, which I'm guessing was part of an attempt to get the amp working by someone who knew nothing about tubes or they were running it w/ the wrong tubes. The seller was selling it for parts/repair and said that the transformers were good and that the unit powered up so I took a chance and bought it. Well, I got it home and put in some 6v6s I had around and it fired right up, sounds pretty good turned up about half way. Nice and juicy tone in this little guy. No power supply hum as far as I can tell. I little bit of hiss but not bad at all for being "broken". Need a good speaker for this one after all the checkup procedures.
My (humble) question is about the tiny little PT that comes in these amps. After running the amp for a couple of hours the PT gets pretty warm and the preamp tube gets hotter than I've previously encountered.

Are these issues normal for this amp? I didn't note any funny smells or burning scents.

The power tubes (now 6v6s as god intended) do not red plate (as far as I can tell). The jewel light IS burned out, not sure if that's a big deal or not.

I don't have my multi meter right now but I intend to get some voltage readings and trace out the circuit to see if any "improvements" have been made. It has been modded previously. It has two extra 1/4 jacks on the back (for what?) of the chassis and two plugs on the front faceplate where it appears additional controls once lived and may live again if I choose.

Thanks for taking some time to read this, Ampagers.

Appreciative,
Aaron

[hasserl]

I've got one of these I use as my practice amp with my band. Actually, it's just barely loud enough when the drummer is there, but if it's just the bass player and even if the keyboard player joins us it's more than enough. It's just when the drummer is also there that the volume floor raises high enough that it barely keeps up.

It's mostly stock, exc ept the speaker is a Weber Alnico Signature 12, and I did rewire the tone control to work like the 5E3. Other than that I haven't done a thing to it. These are darn good sounding amps. I haven't checked the PT to see whether it gets hot or not. I just plug in and play and enjoy it.

[AaronP.]

The scheme: http://www.schematicheaven.com/fende...bass_schem.pdf


Hey there, Hasserl. Thanks for the input. Yeah, I DO need to get the tone control wired up to be more guitar friendly and get a new speaker for sure. Probably will omit the "bass suck" cap on the input to see how it changes the sound.

If you get a chance maybe you could check the heat levels on your PT and preamp tube, mine get hot but not burning hot (i hope). Just want to get an idea if I'm going to fry my new toy/family member.

In my reading today I came across one bit of info where a guy said that the second half of the 12ax7 was running as a low output power tube or something like that (gotta plead ignorance on this one). Could this explain why it gets hotter than the 12ax7s in my other amps?

Really can't wait to get this thing in solid condition and take her out on the town... Her name is "Rusty". Can you guess why?

[Steve Conner]

These amps have transformer drive to the power tubes. One section of the preamp tube drives the interstage transformer, so yes, I guess you could say that it's acting as a "power tube". Kind of like the reverb driver tube in a Fender amp with reverb, and those get pretty hot.

As for heat in the PT, I once heard that if you can stand to hold your hand on a transformer for 10 seconds, it's probably not too hot.

I once saw one of these Musicmasters in a junk shop with EL34s fitted, and big gobs of wax dripping out of the PT. I passed on that one

[hasserl]

My amp is at our practice studio, and I won't be there till next week as our bass player is out of town on vacation this week. But I'll check the temp after a couple hours of practice. It should be the full band too, so it'll be cranked up (darn drummers,,,,,).

[AaronP.]

Thanks guys! The "10 second heat test" for the PT should give me some idea if the heat is beyond normal ranges. I don't think it's dripping wax from the PT (yet) so that's good.

Thanks for offering to check out the operation on your own MMB, Hasserl. Really appreciate that.

best,
AP

[hasserl]

Used the amp at practice last might, full band in attendance, the amp was cranked up to 7 or 8 for a couple of hours, slamming the front end part of the time with either a TS9 or a clean boost. The PT was hot, I could hold my hand on it for 10 seconds, but it hurt.

But man, that little amp sounds good!

[AaronP.]

Hasserl,
Thanks for the heat check on the PT. It seams everything is normal as far as I can tell so far. Oddly enough, the jewel light started working again by itself. Weird...

Aaron