Musicmaster Bass Amp - A Hum Troubleshooting Guide

Hi folks -

Longtime lurker, first-time poster and all that. Had a late 70s (6V6) MMB on the bench this week and I thought I'd post notes from working through some nasty hum issues with this weird little amp. I'm not a professional tube-amp tech by any stretch, but my day job is in electronics troubleshooting and repair and I've designed and built a couple dozen amps over the years. I've benefitted extensively from y'all's expertise and knowledge here, and I'd love to do what little I can to give back to that body of knowledge. Anyway, on to the meat...

THE VICTIM

I recently picked up a '78(ish) MMB from a buddy. Needed it like I need another hole in my head, but it's a fun oddball amp that seemed to present some interesting non-destructive mod possibilities. I have a regular gig where having a smaller amp with decent low-volume clean headroom that I could leave at the venue would be great. This little guy seemed to have the potential to fit the bill. One problem: the amp as received was an absolute hum factory at idle, even at moderate volumes. Noisy enough to be unusable mic'd for a low-volume gig.

I've read several threads over the years here and elsewhere about chasing noise in these MMBs. It's common knowledge that they were cheaply built out of spare parts. Most of the ones I've seen inside look like they were assembled on Friday afternoon by a methed-out raccoon with a soldering iron and the absolute minimum amount of wire necessary to get electrons from one end of the signal chain to the other. My current victim was no different. If you're (un)fortunate enough to be fighting with one of these, the following mods/changes attempt to correct some of the layout issues with the amp, at a total cost of a little time, a little solder, some wire, and a few caps and resistors.

[STANDARD DISCLAIMER: This is where we put the standard reminders about understanding tube amp safety, knowing how to safely discharge capacitors before licking components, working with extreme caution and one hand in your pocket whenever possible and at all times when the amp is plugged in/powered up, etc. If any of these words are unfamiliar, read up on safe practices before proceeding. It ain't rocket surgery, but these things can literally kill you.]

DIAGNOSIS

In powering up the amp for the first time, I was greeted by the lovely whine of amplified AC power. Listening carefully, there was a strong 60hz hum, but pronounced noise at 120hz and some of the higher multiples as well. This seemed to suggest that we were dealing with power filtering as well as grounding issues. Plugging in a guitar didn't fundamentally change the issue, which disappeared almost completely with the amp volume all the way down and increased to downright frightening levels as I turned up beyond 5 or so.

After studying the layout, I came up with the following plan of attack to get the noise under control. This is a small, simple amp - if you're reasonably quick with a soldering iron, all of this can be addressed in well under an hour, even working slowly and methodically.

STEP ONE - TUBES

I swapped the (factory original!) tubes for some fresh JJs, with no apparent change in behavior. Pulling the 12AX7 preamp tube and powering the amp back up with tube out did eliminate a significant portion of the noise, however, and all of the 60hz component. This confirmed my suspicion that there were a couple of different issues at play here - both grounding and filtering.

The original preamp tube was microphonic and cooked, but I ended up putting the original power tubes back in once troubleshooting was complete. They sound great, and given the low plate voltages at work here, probably have plenty of life left in them.

STEP TWO - CAPS

Pulling the chassis revealed the same kind of mess I've seen in others' gut shots of these. Fundamentally, these amps seem to be built on a "shortest path possible" principle, which leads to some weird and potentially detrimental signal and power routing.

The 20uf/20uf filter cap can in my particular amp had been replaced with a shoehorned 20uf/20uf/20uf/300v can that, if possible, fit even more oddly than the original. The 20uf filter cap on the board was the original Mallory and nothing else appeared to have been touched. Not loving the solder work or can placement on this partial recap, I opted to go ahead and deal with all the electrolytics first. The 20/20 can was replaced with a 500V 40uf/40uf JJ located in the empty space off the board. A 40uf/500V Sprague single was swapped in for the factory Mallory.

With the filtering a little more robust and consistent, I went ahead and replaced the rest of the caps while I was in there. If you're at this stage, this also a great time to lift resistor legs and check values on those just to make sure all is well. It's a cramped little board - I found that removing all old caps and then working from the input side toward the filters allowed me to lay everything in a little neater. Once the filter cap on the board goes in things get tight in a hurry, so save that one for last.

While I was at it, I took the opportunity to address a couple of functional issues as well, namely, adding a fuse holder and fuse, and clipping out the death cap.

STEP THREE - HT LEAD DRESS

With the new caps in and settled, the 120hz hum was significantly reduced. I still had grounding hum to deal with, but I wanted to touch up a couple of layout issues first. Namely, the HT AC from the power transformer was routed somewhat...carelessly across the top of the board to the rectifier diodes. Obviously, these diodes could be relocated to a separate board or terminal strip, but at the very least it's worth it to clean up this lead dress. Some prodding with a chopstick with the amp running revealed that moving these AC leads away from the tone stack cleaned up some hum and other extraneous noise (duh!).

In my case, I ended up disconnecting the leads from the rect diodes, untangling them from the overly-long center tap lead (which can be shortened and run tight to the chassis floor), twisting the pair, and running them over the top of the board and straight down to the diode solder points. I could see the possible advantage of extending these and flying them to the back of chassis and back across the board if absolute silence is the goal, but I opted for the simpler solution and ended up not feeling the need to go back and readdress it.

STEP FOUR - HEATER LEAD DRESS

Squaring away the HT leads continued to chip away at lowering the noise floor. My tube sockets needed a good tightening anyway (and could probably do with a replacement sooner rather than later), and I took the opportunity to deal with what may be the worst heater wiring scheme Fender ever devised. To call it a "scheme" is actually not fair - they just ran wires wherever they'd fit and called it a day. Thankfully, it's not hard to put it right.

Working from the pilot light, I disconnected the filament leads and gave them a good twist. Thankfully, there was just enough slack to get them wrapped nicely and still make it to the pins on V3 without straining these connections. The filament string is grounded to a solder pad on the chassis from the pin 4/5 jumper on V1. While I've built plenty of quiet amps with a grounded filament, the "interactive" nature of my noise issues in this amp suggested that an artificial center tap would be worthwhile. A couple of 100R-1/2W resistors from V3 pins 2 and 7 to the ground on pin 8 seemed like the logical place.

From here, I removed the sloppy heater wiring from V3 to V2 and replaced it with a new twisted pair running along the upper lip of the chassis and dropping down. The wiring from V2 to V1 got even worse, with the filament wires splitting the interstage transformer and running under signal wiring. Disconnect, fish out, and replace - there wasn't nearly enough wire to properly twist and route with what was there. ("We're saving pennies, I tell ya! Pennies!!")

The ground jumper from V1 pin 4 was clipped once the new filament string and artificial center tap was in place.

STEP FIVE - GROUND BUS

At this point, we were down to a strong 60hz grounding hum only in listening tests, so I turned my attention to the signal/input side of the chassis. Grounding for the entire circuit is pathed through a single ground bus on the board, making contact with the chassis via the Input 1 jack. Some wiggling and prodding suggested that this wasn't a solid connection. Removing the jacks, cleaning and scouring the bare metal contact surface, and reinstalling dropped my 60hz hum by nearly half again. We're zoning in on being mic-ready, but the noise was still enough present enough to be an annoyance to sensitive ears at low volumes.

Finally, my eye turned to the shared preamp/power amp ground bus. Clipping the death cap gave me a lovely unused ground terminal near the end of the board. I ran a chassis-floor wire from the PT center tap/filter cap ground junction point back to this terminal. Opting to leave the now-cleaned input jack connection as the ground point for the preamp side, I was able to simply clip back the jumper that crosses over the rectifier diodes from the cap can to the single 20uf filter cap.

AND...

Success! Firing it up after completing this work, this particular amp is now dead silent. By incrementally working away at a few obvious noise sources, I was able to get the idle noise down to "quiet modern tube amp" levels. As noted, the whole process took about half an hour from start to finish (including several work breaks to test/confirm and isolate trouble spots by poking/prodding/moving).

With the hum sorted, I did go back and tweak several sound-related things. Clipping the bypass cap at the input goes a long way toward making the amp sound thicker and less gutless/nasally. I'm still playing with coupling cap values (cf. the often-referenced Hasserl mods). Likewise, I modded the tone control to a Harvard-style setup a la Hasserl, and I think I'm going to leave that alone for awhile. The trashed stock Oxford speaker was swapped for an Eminence Fender "Special Design" blue-label speaker I had sitting around. All in all, I'm happy with how little it took in terms of parts cost and time to get a good-sounding rig that punches well above its weight out of this simple little circuit.

Sorry for the long info dump. If you're fighting with noise issues on an MMB, I hope some of this might be helpful in tracking it down. If I have time this week while it's on my mind, I try to add some photos showing end results (I was a dolt and, as usual, took exactly zero "before" photos of this one).

Cheers, and happy hum hunting!