1974 Musicman HD130 Bass amplifier

  Many moons ago I played in a couple bands in Baltimore with my dear friend Jack Denning. He's a bassist who currently plays with local funk legends All Mighty Senators. One day he want to an auction and picked up a Musicman HD130 head, not the model pictured here but the guitar version with reverb and tremolo. It was my first experience working on a Musicman. I think he paid about $70 for it. Great thing is they still can be had for cheap. The model pictured was bought 4 years ago for $400 and the owner toured with it, gigged with it reliably until it fried a tube. It was also time to change the 41 year old filter caps as well.

  Jack still rocks that amp with my old Jazz Bass and he has one helluva sound.....

  You know, this is a hybrid amp, and I'm a tube snob but so what. The great thing about these is the preamp is transistor and in this model the phase inverter is a 12AX7 driving a quad of EL-34s.

  One thing to know about these is if you are going to work on one, use extra caution.

  Take a look at this number here. That's 596 volts DC. And this is with the amp on a variac idling at 100V ac. We still had 20 more to go to get the full 700 volts but my meter doesn't go past 600. Long story short, this will put a hurt on you. It may even end your life. If you are not experienced with this kind of thing, take your amp to someone who is.

  So how the heck do modern tubes last in these amps? That's the kind of voltage a first series Mullard can handle, but not even later versions could really do that! Well, the screen voltage is about 350 volts. And the bias runs pretty conservatively.

  What tubes do I like in these? I like the JJ EL-34 tubes, not the E34L version, just the regular ones. If you get a good set they last. That's what was in here when I took this one in. The Russian and Chinese glass can't take it. Think of them like firecrackers and your Black Flag cover is simply lighting the fuse. I haven't tried the JJ 6CA7 tubes yet. Gotta admit I'm skeptical but would like to give them a go some day. The Russian 6CA7s die hard and fast in my experience. These amps came stock with Phillips/Sylvania 6CA7 tubes. The choice of Van Halen apparently in his old Marshall plexi amps. You can't get them anymore without second mortgaging your home so we'll just need to go with the best we can.

Here is a silly little video of testing that bad EL34 for shorts. This is how it's done:

https://www.youtube.com/watch?v=yCjEG60KsNo

  This amp was nice and easy to work on. Why? Leo Fender designed it. That dude was a genius. Really knew his business. If you have a bad transistor you don't need to solder anything, simply pull it out of the socket and drop a new one in. They're kinda like miniature tubes. I've had some before that went noisy. So nice to be able to just swap them out without any headaches. I wish more modern amps were built this well with the service person in mind! And that was Fenders genius. He was a mediocre sax player at once who never really learned guitar, but made a living renting pa stuff and fixing busted amps. He built things with service in mind. His guitars and amps can be fixed easily. Heck, I can overhaul a couple Fender amps in the time it takes me to find a problem in a new amp. Good stuff!

J