Once the filter cap on the board goes in things get tight in a hurry, so save that one for last. 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. 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. With the filtering a little more robust and consistent, I went ahead and replaced the rest of the caps while I was in there. A 40uf/500V Sprague single was swapped in for the factory Mallory. The 20/20 can was replaced with a 500V 40uf/40uf JJ located in the empty space off the board. 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 20uf filter cap on the board was the original Mallory and nothing else appeared to have been touched. 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. 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. Pulling the chassis revealed the same kind of mess I've seen in others' gut shots of these. They sound great, and given the low plate voltages at work here, probably have plenty of life left in them. The original preamp tube was microphonic and cooked, but I ended up putting the original power tubes back in once troubleshooting was complete. This confirmed my suspicion that there were a couple of different issues at play here - both grounding and filtering. 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. I swapped the (factory original!) tubes for some fresh JJs, with no apparent change in behavior. 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. 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.Īfter studying the layout, I came up with the following plan of attack to get the noise under control. This seemed to suggest that we were dealing with power filtering as well as grounding issues. Listening carefully, there was a strong 60hz hum, but pronounced noise at 120hz and some of the higher multiples as well. In powering up the amp for the first time, I was greeted by the lovely whine of amplified AC power. 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. 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. It's common knowledge that they were cheaply built out of spare parts. I've read several threads over the years here and elsewhere about chasing noise in these MMBs. Noisy enough to be unusable mic'd for a low-volume gig. One problem: the amp as received was an absolute hum factory at idle, even at moderate volumes. This little guy seemed to have the potential to fit the bill. 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.
#Fender musicmaster bass amp mods mod
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 recently picked up a '78(ish) MMB from a buddy. 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.
#Fender musicmaster bass amp mods professional
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. 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. Longtime lurker, first-time poster and all that.
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