Music Man Amps

Ok-let's start at the top. First off, that's NOT a bass amp. You will ruin the speakers playing bass through it. First look and see if it has a 12ax7 tube in the little metal shield. If it does, it's say 75-78. If it has no tube it's later. Are the badges silver on black or black on silver. Casters, any pop in caster will work. Chrome, hard work, steel wool and if all else fails, just replace them. If it REALLY has 6v6's in it, they won't last. They are NOT designed for the voltages in that amp. It should be set on 8 ohms. THat's the only amp that should, with 4 speakers. Any speaker plugged into it will ADD to the impedance as the jacks are in series. Oh, I see later you DO have the 12ax7 so it's 75-78 or so. Tone is ALWAYS subjective and the early MM's always sounded better to me but for guitar. I am NOT really a tube guy for bass. Anything I missed? Mike.

Now THAT will work. Some guys used the 130 heads for bass. They were designed for that. Wouldn't suggest using until you get the tubes changed though. JJ 6V6's will take a lot bot not even THEY will take 700 volts! Mike.

..if you ask me. JJ's 6V6S tube is limited at 500V, so unless the amplifier has been modified substantially the tubes should not be 6V6. The older MM amps were designed for 6CA7 which is very similar to the European EL-34. EL-34 is limited to 800V (not sure what the old Sylvania 6CA7 was limited at, but probably something similar) and so is able to work with the ~ 720V plate voltage in the MM amps. Anyway - I suppose the amp could have been rewired to use 6L6 at some point but I'd rather not speculate and merely suggest you look at what may be printed on the existing tubes.

Personally I wouldn't chase distortion in a 130W MM amp - at the very most you can achieve a nice compression from the 12AX7 on the the low power setting. The advise in the old brochures about how the master volume should be operated at '10' unless special circumstances (like, oh I don't know, fear of going deaf) dictate otherwise is a bit out of date by now when we're dealing with the 130/150 (Heavy Duty as MM called them) amps.
For crunch, use a pedal - they have come a long way since MM amps were new. I agree with Mike - excellent amp for bass too - with a closed back cabinet of course.

Cheers,
Lars Verholt

The EL series are USUALLY a little taller and skinnier. Here's one. The E34L version is just the supposed heavier version. Kind of concerned about having mixed tubes in there from different manufacturers. That's USUALLY not a good sign. Even JJ 6V6's won't take the strain cap'n! Mike.

I recently had a tube failure on my HD130 212, it blew fuse and when I replaced it it sparked and smoked a little bit so I unplugged it. This happened during last set of a 3 hour gig. I ordered 4 new JJ EL34L's and am wondering if should have tech install all of them. The amp was only running 2 at the time. I have the SS version preamp. I only play it on 3 for preamp volume. and 3 on master volume and run it at full power. Will it run too hot with all 4 tubes? Thank you. I am new to this forum.

Somebody PLEASE try the new JJ 6CA7's and tell us how they are! (hint hint hint) Mike.

Per schematics, the wiring difference between 6CA7 and 6L6 is minimal. A 6CA7 should have 1.5 kOhm resistors to bias the accelerator grid (pin 4) in each tube (R63 and 64). A 6L6 tube would use a 470 Ohm resistor to bias the accelerator grid (pin 4). Take a look inside and see what value resistor (R63 and 64) is connected to pin 4 on the tubes and that will tell you which tube to use. 1.5 kOhm for 6CA7 tubes. 470 Ohm for 6L6 tubes.
http://pacair.com/mmamps3/sites/default/files/docs/2100-130.pdf
http://pacair.com/mmamps3/sites/default/files/docs/2475-150_and_2275-150...
Good Luck. -mgriffin

OK-let's get back to basics. Did you ever determine if it's a 12ax7 phase inverter amp? That make ALL the difference in the world. Let's start there. Mike.

Dunno what to tell you - if your new set of tubes had one or two bad ones in there, it could cause the amp to fail (hopefully without irreparable harm). If your tube set is OK, but the amp has a weakness somewhere, you need to find out where it is before you either put yet another set of tubes in, or you put the amp back in service with the same tubes. Using a variable transformer would allow you (or your tech) to find out if there is a badly failed component somewhere by running the amp at, say, 50% line input power for a while. Of course sometimes this test yields no results - the failing part only misbehaves at close to full operating voltage. Also, bias adjustments should be done at or very close to actual running conditions because the tube amp circuit is not entirely linear except for at a relatively narrow operating range.

Sorry to sound like a politician here - it´s hard to get really into the details without the amp in front of you.

To follow up on Mike "210"¨s question: The older 12AX7 phase splitter circuit had an inherent risk of catastrophic failure due to the design of especially the feedback loop. A high frequency oscillation (frequencies well beyond the human ear's capabilities) in the output section could cause output tube destruction or OT meltdown - or both. Mind you, I am not saying you had a situation like that going on, but a problematic 12AX7 tube could indeed cause unpredictable issues in this type of MM amp, just like the driver transistors and related bias components are often the culprit when the later type MM amps display instability.

Put the amp on the bench, hook up a load resistor in place of a speaker, feed a test signal to one of the inputs and hook up an oscilloscope - preferably with two channels so you can monitor both goezinna and goezoutta at the same time.

Hope this helps,

Cheers,
Lars Verholt

Yeah Lars, THAT'S exactly where I was heading. I've had one pf those. I even converted it to SS driver but didn't like the sounds afterwards. Went back. THAT'S the amp I dropped the pliars in while it was running and smoked it! Anyway, I have always wondered if there was a better way to tame it. I have a 2-10-65 that runs just fine and have never (knock on wood) smoked the tubes. The 130 head I had DID. I have always wondered if just biasing them a LOT cooler would help with the problem but never fixed my 130 so I never had a chance to try it. OR maybe putting a variable feedback control in it to reduce the feedback. I don't know, like I have said, I'm more of a mechanic than an engineer. I have always been under the impression that the PI tube WAS the cause though and a GOOD quality tube was essential to the tube burning problem. Don't know. Some seem to do it and others are fine. I never fixed my 130 because geez, who need 130 watts these days!
Mike.

HHMMMM? Just re-read the post a little bit. from what I remember, when the PI tube models went chernobyl, ALL the power tubes lit up. If you had just two do it, which two did it? Maybe it was loosing bias voltage? Just guessing here. Also, I never had good luck with EH tubes in MM's. They just won't take the voltage to me. Mike.