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

National Model 22 Amplifier. RARE AND AWESOME!

This is a truly rare gem, a mid 60's National Model 22. This one wins the "I want one" award of the last 15 years. It's a whopping 5 watts with RIDICULOUS reverb and tremolo. I mean, Fender did a nice refined job with their reverb but other manufacturers such as Valco (this is associated with them) made that Wild Man reverb that I prefer.

This amp was sick when it was brought to me. Noisy, volume dropouts, tremolo caused this horrific noise through the speaker.

First thing to address was the obvious. It spent time in a barn. It needed major cleaning. Pots, jacks, sockets, tolex, the works! Next the power supply caps were toast. One had burst. Easy fix. I noticed the 330 ohm 3 watt cathode resistor on the 6V6 output tube broke in half. So I replaced that with a 10 watter. It will never die again. Then the tricky part: the tremolo. BUH BUH BUH BUH THUMP THUMP CRACK CRACK CRAAAAACK.... There are no schematics I could find for these but other National amps are close enough. I found a Gretsch that was really similar, single ended class A 6V6 loaded amp with reverb and tremolo. But if you can't find a schematic let your experience find the circuit:

Yes, these amps have the 'bowl o' Pad Thai' hand wired circuit. I know this pic sucks but can you find the tremolo circuit? I can!

When you see 3 caps in series like this with one going to ground you most likely found the tremolo circuit. I replaced the old 220k load resistor since it was way out of tolerance. I also added a 100 ohm buffer resistor between the tremolo output and the tube it feeds into. This amp lacks an 'intensity' control. This helped but the real cause was much sadder....

This amp has it's original Jensen speaker. Sad to say, this was the cause of much bad distortion and the tremolo nonsense. The voice coil was starting to separate. Too bad. Worthy of a re-cone but we're just going to drop in a replacement today....

I like these Mojo Speakers. They sound nice and are affordable. 25 Watts. The original was ceramic so this makes life easy.

Also added a line out off the speaker. Plugged into my amp it just sounds huge at any volume. Really warm and friendly. You want to never stop playing.

One other little thing I did: the cap after the first preamp stage was a .005. I upped that to a .022 for a fuller bodied tone. For a 5 watt amp this baby has nice low end.

All the tubes are good, nice old American glass. None finer!

How does this amp sound???????????? All I can say is wow. Surf is most definitely up! Reverb has that characteristic buzz but so what?! When you are blasting it's not an issue. It really has a punch to it with my trusty '58 Jazzmaster. Yeah, I'd play jazz through this. Cowboy music too. Psychedelic! I WANT ONE! It's not common to see a 5 watt amp with a 12" speaker, great combo that holds together better than one would think. I'll be sad to see this go. In the meantime I may just have to record with it......

JB

1965 Gibson Hawk amplifier, 7591 model

  Here's a rare, um, gem? A 1965 Gibson Hawk Amplifier. They made several versions of the Hawk and I believe this one to be the rarest of them all. It's loaded with that typical AMAZING Gibson reverb circuit and unlike it's smaller brothers and sisters, it uses a pair of 7591 output tubes with fixed bias and rather than using a phase inverter circuit it simply uses an interstage transformer driven by a 12AU7.

  I ain't gonna lie to you all here, yes, this is the amp used by the band Ultimate Spinach to cut those groovy psychedelic tracks*, and also, I'm no fan of this series of Gibson amps. This job was a nightmare....

  Amp was brought to me cause it had low output and was distorted. It also had in intermittent reverb short and weak tremolo. On top of that it hummed badly. It also lacked that Gibson high end, I mean, no treble. How can you capture that awesome tone on "Vision of your Reality" in this condition? Simple enough. Tube and cap job. NOT! Ok so I did replace some filter caps. They were re-done by an amateur in the 80's, terrible messy job. But once I opened it I noticed the interstage transformer business. Hmmm.

  Well after a re-cap of the power supply, it still hummed. Not as bad but not acceptable. This is where the job gets risky. 7591 tubes are expensive. This had a nice pair of Ampeg stamped ones but they read about 18ma off from one another. In a Fender amp you may not notice a hum unless it's one of the awful later Silver panel ones. But on this amp this could be the culprit. But there is that transformer too which was replaced. That's expensive as well. I ordered parts but you know if it's not the transformer, who am I going to sell it to?

  Turns out it needed both. Once I replaced the transformer (Mojo part made for Musicmaster Bass amp! VERY NICE!) which was a pain to do, the hum came down but not where I like it to be. What did happen was the volume went way up and the high end restored. Whatever jive part Dr. Amateur threw in there it wasn't doing this amp any favors. No wonder this amp is so clean, it wasn't exactly useable.

  So next I threw in a pair of fresh JJ 7591 tubes. I've spoken about JJ tubes before: risky. These worked fine but the bias was super duper hot. So I threw in an adjustable bias trim pot. Hum is passable but not to my liking. Some Gibson amps from this era are just like that. It may need a DC filament but for now I'm simply done. I don't care to spend any more time on this. I'm losing dough and that's no fun.

  I suspect that it's still the interstage transformer, not that this part isn't right but because you need to position them just right for maximum quiet. There is no room in this amp to play around with it and though I needed to drill some holes to make it fit, it's oriented the same way as the original once was. Before I bolted it in I rotated it and yes, noticeable change in hum. Lordy this is a poor design. Heck, maybe it just needs the exact part? I used to have a box of vintage interstage transformers, Triad ones. Bought them at a Hamfest years ago and sold about one a year till they were out. All to Gibson owners and they were always dead quiet. Oh I just don't know.....

  Yes, the schematic above is the best I could find for this amp. GIbson changed their stuff so much it can be difficult to find the right sheet of paper.

  Other bugs: Reverb cutting in and out: bad cap. Tremolo weak: Home made 'bug' I suspect but also previous tech put in the wrong value cathode resistor. Ugh. And this brings me to another point: If you don't know what you are doing and want to fix amps, don't mess with something like this. A guy like me or someone else will spend days un-doing a hack job. It's cool to do your own thing but start on something simple like a Champ. Better yet, build one from a kit and trouble shoot it when it starts smoking. And by all means, BE CAREFUL! Voltages in these amps can be lethal. It's no joke.

  So like I said, I'm not 100% happy with the result of this amp. It sounds much better and can play a gig but I like near silence. I don't feel too bad, I even solved some problems for Dave Friedman he burned out on. We all have our limits and maybe Dave will get this someday and make it silent!

JB



*April Fools! Ultimate Spinach probably used something worse! I don't know what they played and don't care!!!
  

Fender Musicmaster Bass amplifier

Today we have a Musicmaster Bass amp from the 70's. The customer complaint was it smelled burnt and the volume was rather low. Also had a faulty input jack.

I used to live with a cat that played 2 of these. He was way too loud! These little cheap hand wired tube amps can pump it out even stock. With mods they can really do some damage.

So to start I changed all the electrolytic capacitors. Typical stuff. The burnt smell was most likely the 1k 1/2 watt resistor:

It doesn't appear burnt but it was reading 2.5k. It's the first power stage resistor between the output transformer and the screen grids. I upgraded to a 1k 5 watt:

Yes, I like overkill in my power supplies. This baby will NEVER burn out. I also changed the first filter cap to an 80@450V cap. Deep, rich tones with no hum. Oh, may lose a little sag compression but really, it's a silicon diode rectifier so why not. Besides, the owner needs this amp to play loud and kind of hold it together so it's a wise choice.

These amps stock are meant to be a cheap bass practice amp. I've used them before with good results in the studio for, yes, bass and guitar. They are only about 12 watts so most bass players won't touch them. Yes, they are considered 'vintage' (wank wank wank) but this is an amp where for guitar the tone is kinda limp. So I say: If your tone is a' starvin', git on in that amp and start carvin'!

First thing: The first gain stage cap is a .01 ceramic disc:

Change it to a .022 cap of your choice:

Ok, missed a good photo op, but it's the yellow cap on the right. I also changed the 2.2uf bypass cap to a 22uf.

Now looking at the above pic you see a 470k resistor connected to a .0047 cap. That's the input going to the first stage. Not acceptable! Get rid of that cap and that resistor.

Since we are changing the input jack I'll put the load resistor right on that. I prefer a 1 meg carbon composition:

Should look like this:

Gah! Again, sorry about the poor pic! But notice that the 470k resistor is gone and the wire going to your first gain stage 12ax7 is connected right to the 68k input resistors.

Next, the very tone robbing tone stack:

Get rid of that 100k resistor and .022 cap. I simply copied a tweed Deluxe:

I'm using a 250pf cap for the highs and if you notice, I'm re-using that .0047 input cap as the rolloff. I could sell the customer a new cap here but why? This one is perfectly good and I'm a reduce reuse recycle kind of guy. Seriously. These are made of plastic. If I drop a new one in there this either sits around my tiny space or becomes sea turtle food eventually.

So how does it sound now?

In a word: British. Jangly top end with that nice euphonic mid. Think 18 Watt Marshall with a $300 pricetag. The customer will be replacing the speaker, I would go with a Weber English series or a real Celestion. Great amp for a jazz player who doesn't need fusion volume. Rich, warm and solid. These amps can be a real treat for those on a budget or those who just like a wonderful tone!


Another nice thing about these is they use the rather easy to find 6AQ5 tube. Plenty of cheap NOS examples out there. The later ones used 6V6 tubes, I think these sound more English than a 6V6 model. I kinda prefer these. And, you can just buy a pair of tubes and drop them in. No biasing required.


JB

Alamo Montclair Reverb

  Had a rare old amp on the bench today, this little single 12" Alamo Montclair Reverb. Nothing special about the job, just a power switch replacement. But the amp itself is really cool. It's an early hybrid amp, 1/2 tube and 1/2 transistorized. The power amp is loaded with a pair of 6V6 tubes with a single 12AX7 phase inverter. It's about 15 watts and packs a mean lil' punch! The preamp is all transistor and has a nice reverb and decent tremolo. This one was loaded with a Celestion Vintage 30, one of the finest speakers out there in my opinion. Great live speaker. I'd love to hear this with a trashy old Jensen though. That's more my taste....

1998 Fender Hot Rod Deville 3/13/15

Need a good, well made amp that is LOUD for cheap? You still can't beat these. This is an earlier version. Easy to work on, mod, bias and make music with. Tonally? Kinda harsh for my taste but I have had success with changing signal caps and resistors in the signal path. Heck. I once changed every signal cap and resistor plus the harsh, terrible modern Jensen speaker for a customer out west at his request. I thought he was nuts as it was a $350+ job. In the end though it sounded like an old amp. I was quite pleased with the results and so was he!

  This amp just needed tome new tubes and routine maintenance. Every few years a bunch of little solder joints inside need to be touched up, they simply come un-done and cause noise and dropouts. This was no exception. Also, I've replaced more load resistors on the phase inverter in these amps than any other. If yours sounds particularly harsh and lacks volume, there is a 50% chance that is the problem. Easy fix for the experienced tech. Don't try it if you don't know what you are doing!

  Great amps overall. They get my thumbs up. One of the only modern amps I enjoy working one. Many modern amps I simply won't look at cause the construction is so poor and they are not made to be worked on at all. These are no problem!

JB

All Test Devices Wah/volume pedal. 3/13/15

Here is a rare bird. An All Test Devices Wah Volume pedal. Yeah, it's really just a Maestro Boomerang in disguise. Nice wah, very nice indeed, but these pedals aren't my favorite. Why not? I'm just not a fan of the volume wah arrangement to begin with. And these really eat your dry signal alive. This one was purchased at a junk shop for $10 last week. The pot was broken, or at least the housing had come undone. I found the perfect replacement for it but still, these have a nice lil' flaw. When you adjust the volume for maximum sound (which also gives you the brightest top end for the wah) it has a 'bump'. In other words a pop happens. The old pot did this and so does the replacement. I needed to back it off a bit to get it to behave. Sounds like a parasite of some sort to me. I'll mess with it to find a solution but I think anything that will help may rob even more tone.

I wish this were mine cause the wah is really good. I would gut that stupid volume part and throw in a true bypass switch. Yup. That would make me happy.

JB