Music Man 2100-65

I got this amp. It blows fuses … so need to be serviced !

Music Man 2100-65

Music Man 2100-65 schematic (unfortunately not very well readable, specially for the model with 12AX7 that I have ! )

MM Service Bulletin #4 (PT Wiring – from / Pacair)

Music Man 2100-65 components

Thermal breaker : I was wondering what the hell was this kind of small “bulb” in series with main fuse … and I found what was a thermal breaker (thanks to google !)

Music Man 2100-65 stuffMusic Man 2100-65 power supply

music man 2100-65 thermal breaker

Short circuit 6CA7 plate to ground capacitor

Music Man 2100-65 plate capacitorThis capacitor is dead (plate is directly to the ground through this short circuit capacitor). I must replace both 6CA7 plate to ground capacitors … Diodes seem to be OK …

Hence, the output transformer is not in very good health :-(

Music Man 2100-65 6CA7 socket (1)Music Man 2100-65 6CA7 socket (2)6CA7 socket

RB1 rectifier board

Unsoldering the dead plate capacitor is not sufficient. Fuse still blowing. Before testing the power transformer, I’d like to understand the rectifier and filter boards layout …

Music Man 2100-65 RB1 rectifier boardMusic Man 2100-65 RB1 rectifier board component view

Music Man 2100-65 RB1 and Filter BoardRB1 layout (copper view, components on the other side !)

Power transformer check (secondary VAC, without tubes, PT wires desoldered)

Music Man 2100-65 EX PT Secondary VACNot so bad, right ?

Bad 100uF filter capacitors

Music Man 2100-65 filter capacitorsWhen 100uF are unsoldered, fuse no longer blows … to be replaced !

OK guys ! electrochimic capacitors changed, output transformer changed, the beast is all right and sounds loud ! It’s a 3-65 OT from Mojotone

Mojotone 3-65 output transformer for musicman amp

How to bias this thing : see there (pacair forum)

Earth and ground

Music Man 2100-65 ground 1Note the 0.001 capacitor between earth and ground on the loudspeaker side of the amp

Music Man 2100-65 ground 2… and the direct connection earth to ground on the input side of the amp

February 2014

I got a nice Musicman 212 HD 130 (2475) to service. Bad power transformer and red glowing plate EL34. Works fine now !

Where is the bias pot ? OK, got it !

Links on the web :

Pretty cool Music Man forum (“Pacair”), plenty of information to service these amps !


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23 Responses to Music Man 2100-65

  1. Good information here. I am trying to understand how a tube amp operates. Starting with input transformer to rectifier board. thanks

  2. t-geek says:

    Hi, you are welcome !
    I had a look on the facebook page related to MM, great !
    If you are trying to understand how it works, here is a nice site :

  3. Ed Goforth says:

    Hi there, Those .001 caps across the plate to ground need to be higher than 1kv (2kv or more), as they tend to go bad, I have seen that happen often, also the diodes across the plate, as you know is to suppress the hi voltage spikes when first turning on voltage to the plates from standby. The idea is good, but there has been several times these diodes shorted out and took out the output tranny. Mesa Boogie had my Mark III years ago for an upgrade they were doing, or was it a degrade (Retrofit with the IIC+ preamp tweaks), anyway I had installed the spike diodes and they took them out, when asking them why they did that, they told me this story about blowing trannys and it’s cheaper to replace tubes than trannys. The best way to save your power tubes when powering up, is to start off in Low Power mode for a few moments, and when the tubes have had a chance to warm up a bit, you can then switch to full power if you want the headroom. This has saved my tubes time and time again, and letting the tubes warm up a few minutes of course before powering on from standby.

    I hope this helps!

    Ed Goforth

  4. t-geek says:

    Thanks for the advice, Ed !

  5. Dennis says:

    Could you give me the value of the large 5 watt resistor across pins 8 and 9 on the 6CA7 of the 2100-65 chassis? I have an amp like this, the resistor at that location is destroyed, burned far beyond recognition to determine the value. It appears to be only a 1 watt or so? Please help!

  6. t-geek says:

    Hello Dennis,

    Could you please be more specific ? There is no pin#9 on a EL34 / 6CA7 tube ;-)

    Maybe you mean the diode(s) between anode (pin 3) and cathode / ground (pin 8 ) ?

    I updated the pictures on this post, do you see the resistor you are talking about ?

  7. Dennis says:

    Hello Mr. T-Geek,
    Yes, it is the large 5 watt looking resistor in pics 4 and 5.

  8. Dennis says:

    Hi Mr. T-Geek,
    I suspect I miscounted the pins, maybe it is between pins 7 & 8? It has an ID on it that looks like 216-8 and also BR 2. The one in this amp is broken in half, heat has destroyed the color bands and I seem to be unable to find it on the schematic for some reason? The one in the amp is a much small resistor, maybe a 1 watt? I suspect this larger sized is needed for a reason. Thanks, Dennis

    • t-geek says:

      You mean the big square white cement resistor ?
      It’s not BR2 but 8R2 (8.2Ω – The value is not critical and could be 10Ω … It looks like a 10W resistor : 216-8 : see for instance ). This resistor is placed between pin 8 (cathodes) and ground (see schematic).
      I don’t know exactly why 10W, as I suppose only 50mA (bias) should cross it …

  9. Dennis says:

    Yes, that is the resistor I am talking about. The one in this amp is burned and broken. I suspect maybe that is why someone has gone to such a large resistor as a replacement? Anyway, you have been of trememdous help and I so appreciate it. Thank you again for all your expertise! I wish I could return the favor in some way!



    • t-geek says:

      Let me know if it works after this resistor replacement.

      I don’t know why Music Man put such a large (power) resistor here … if yours is burnt, maybe is there another problem with your amp ? Or simply broken and a kind of flash when it died ;-)

      Verify the bias after having replaced the resistor, just in case …

  10. Alessandro Costantino says:

    Dear ToneGeek,

    Great website! I am battling with a 2465 chassis (212-65 Model) very similar to this one. I have serviced recently the amp (new electrolytic capacitors) and bought new power transformer (had 120 VDC / Us version). All the DCV are in range with what the schematics says (hi/low etc) now. I have only one strange problem: when playing the guitar, if I hit the strings a little bit harder I hear a crackling sound, like saturation issue. It happens more and more if I raise up the master volume >3 .
    Any suggestion/help will be greatly appreciated! Many thanks in advance from Italy.

    • t-geek says:

      Hello Alessandro,
      Unfortunately, this kind of noise/crackling sound hunting is definitely pain in the a*
      I have had once with a Marshall 2203 : 2 plate resistors changed and all became right !
      It would be great to see if the problem is at the preamp level or later in the amp. Maybe you could check plate resistor of the PI (btw : tube or solid state PI ?) and / or screen grid resistors ?
      See also the forum pacair (, they know a lot MM amps there !
      Hope you will succeed to completely fix this !

  11. AmpDude says:

    I’m an amp tech in Toronto and I currently have two Music Man’s on my bench. This site has been super helpful so thanks for that. The first one is 2475-65, 212, and it came in with a pair of GE 6L6′s in it, they look original. The 10ohm resister on pins 8 and 1 was also blown in half, replaced it, works fine now. I’m going to go through the bias procedure now and I’ll keep you posted. Should the Low/Hi power make as big a difference as 8watt max out on Lo and 65watt max out on Hi? I’ve had a hell of a time finding new tubes that can handle 725v on the plates. The old ones seem to be holding up though so I’ll leave them for now. Apparently this thing worked for years with the 6L6′s in it, how can I tell if it’s supposed to have them or EL34′s? I’m going to make another post about the other one, it’s giving me more problems…

    • t-geek says:

      Hello AmpDude,

      Yes, many guys use 6L6GC on their MM, I don’t know how these tubes can handle 700V or more … but it’s a fact, there are a lot of MM equipped w/ 6L6 GC !

  12. AmpDude says:

    Amp #2 is a 1275-13-UL, 212, 4 EL34′s. I replaced two cooked grid resisters (pin4, 1.5k) and had to clean up the sockets and replace a lot of wires (they had melted when the grid resisters died). There was also a shorted cap I replaced going from pin 1 to ground (on that terminal strip where the grid resisters are). It’s working now but super fuzzy, not big loud and clean like it should be. What are the symptoms of a bad diode or cap (the ones from the grid to ground are apparently problematic)? I’ve got a decently clean waveform coming off the 12AX7 (though the top clips way before the bottom when turned up) but a super ugly crossover notch on the output. The resisters from pin 8 to 1 both read 6ohms instead of 10, going to replace those, then bias it, see what happens. I’ll try those little caps too but I don’t currently have 1000v 0.001 caps on hand (I have 630v but I don’t think they’ll cut it) so hopefully they’re ok.. Any advice would be greatly appreciated. Thanks folks.

    • t-geek says:

      In fact, capacitors that can be very problematic are between EL34 anode and ground (pin 3 -||- pin 1). If one of these cap is in short circuit, that means that the high voltage is connected to the ground via one half of the primary winding of the OT ! (I had one of these caps in short circuit on my MM and my output transformer had different DC resistance between one the two halves of the primary winding and the other => OT dead :-( )
      Check the diodes too … or simply remove them (see comment from Ed Goforth on this same page)
      So I hope your OT is still good …

      Let us know !

  13. eloy says:

    I have a 65 with two 6l6s a preamp tube and yet another socket with no tube….what goes in there?

    • t-geek says:

      Hello !
      Do you mean : 2 x 6L6 power tubes + a noval (= 9 pins) socket ? That should be for a 12AX7 as a phase inverter (= driving the 2 x 6L6 tubes)

  14. Alan says:

    I have an early release Music Man HD130 Reverb with the tube inverter. Chassis 2275-130. My tech told me that the op amp that drives the reverb is bad. I found a replacement, but how do I know which one it is on the circuit board? (There are seven of them).

  15. h.yoshida says:

    Do you have the driver board layout?? I need to see the back side of it to figure out where blue and white lines go from 12AX7.

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