Pulling my hair out trying to track down elusive distortion in a Music Man 410 Sixty-Five

johnners

Hey everyone! Hopefully this is the right board for this. This seems to be the only one that might be active right now. I'm going to lay out my repair journey with this Music Man 410 Sixty-Five (2275-65) and hopefully someone can help me get unblocked. I've been very stuck trying to get rid of some very nasty distortion when running an overdrive pedal through the amp. 

Earlier this year, I picked up an extremely worse for wear Music Man 410 Sixty-Five (2275-65) for $150 off Facebook Marketplace. All the wires had been cut out (speaker and reverb tank wires), it was extremely dirty, and had a blown out speaker. The guts looked pretty good, though! Really clean inside and all original. It even had the original tubes (Sylvania 6CA7's and 12AX7 phase inverter). According to the seller, it sat in an old lady's basement for decades. Bringing it up on a variac though, it sounded awful. Blown out, awful distortion, and crackly. It obviously needed a ton of work.

And for a description of the distortion, basically every note sounded flubby and farty. Especially low notes.

Here are the first things I did:

  • Replaced all the electrolytic capacitors
  • Replaced a completely cracked R-56. The 10Ω resistor going to ground from the power tube cathodes
  • Replaced two of the speakers, including the blown out one, with 10" greenbacks
  • Replaced missing power cord with grounded cord
     

Replacing these parts didn't really seem to help a whole lot. I was still getting the same awful distortion. I thought I should rule out the tubes so:

  • Bought a pair of JJ EL34's and a JJ 12AX7 and biased the power tubes.
  • Check every cap on the filter board (the 0.047uF 600V ones) with a megger and they all seem to be okay.
     

No real changes. Still dealing with nasty distortion. After reading a few threads on this forum, I got the idea to check the +16v and -16v rails for the op-amps. These were way off! I tested the zener diodes (D5 and D6) and they were bad. So:

  • Replaced zener diodes with new 16v 1W diodes
     

Amp now sounded WAY better! Clean clean clean. I thought I had fixed the issue and put the chassis back into the amp. I plugged my pedal board in and turned on my overdrive pedal and damn, it still gets flubby on the low notes. It just sounds like a fart and the amp volume makes no difference. The preamp volume can be at any level and you still hear the flubb. The bright switch makes things WAY worse and having the treble turned up also increases it. Clean, though, it sounds great.

This is where I start pulling my hair out. I decide to swap out some other parts to check it off the list:

  • Replaced op-amps in the "vibrato" channel
  • Replaced bright cap
     

No change at all. As I'm tracing the signal with the oscilloscope, I notice that I'm hearing the distortion basically any time the preamp clips. The way the preamp clips is very harsh. I have no idea if this is how it should be clipping so:

  • I check D3 and D4 and they test fine. I have a whole bunch of 1N914's though so I decide to replace them.


No change. And this basically brings me back to where I am now. I decided to put the original tubes back in because they were still good and again no change. I've noticed a couple interesting things though:

  • I can't seem to get the amp to reach the bias voltage anymore. Turning the bias pot all the way and only get up to around 0.425v measured across R56. The pot tests fine-ish. 8 or 9k across the whole thing. It's supposed to be a 10k pot so that puts it within 20%. I honestly can't remember if I checked but I'm pretty sure I did: R45 also tested good close to 18K
  • Voltage at H is around 680V, not the 725V I should be getting according to the schematic
  • F and G voltages are also low
  • A and C are both measuring good at +46 and -46
     

I'm kind of at a loss now. I'm wondering if maybe I need to swap out some more diodes. Maybe the 5kv flyback diodes on power tube plates? I don't know if 680V is too low and if that could be causing my biasing issue now. Is the biasing issue even what's causing me to get this distortion? Oh I should also mention I've tried it with another cab as well. No dice.

Okay, that was a longwinded post. Here's the TL;DR:

I've been fixing a Music Man 410 Sixty-Five. I have gotten it to sound good clean but if I clip the preamp with an input signal, like from an overdrive pedal, I get a really nasty distortion. Especially in the lower notes. Happens at any volume on the preamp. Distortion is different feeling than the natural overdrive you get from playing with the master volume knob. The bright switch/treble pot seem to make it worse/more noticeable. All this has been happening on the high power setting and both channels seem to be affected. I've been exclusively working in the vibrato channel though.

Hopefully someone here has some ideas because I'm pulling my hair out. Help me get this thing off my bench finally! Thank you!

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