Tuesday, April 7, 2015

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

12 comments:

  1. Just re-reading this and post Musicmaster melt down! I have been wondering how to get this thing sounding better...thanks for the advice! All this looks good!

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  2. Your welcome! They are great little amps. First thing I would change is the speaker. Screaming for a nice Celstion!

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  3. Can you Please explain the connection of the 1 meg carbon resistor added to the jack, as the pic is a little hard to see... Thanks!

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    1. Hi John. Just added a photo above. Have a look! One side of resistor is on the hot, the other on the ground which is connected to the switch (center terminal) of the jack. Thanks!

      http://el34world.com/charts/images/jackwire2.jpg

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  4. The good news is once you spend the money you really won't have to worry about purchasing any other amps because this one will last you for a very long time. Get redirected here

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  5. Hi gohstwriter, thank you for this nice sharing.
    I have enjoyed mine (1972, "early" 6AQ5A version. Yea, you should correct the sentence about this, 6V6 is the "later" ver.) for a few months. It sounds great even with no mods and with the original CTS stock 12" speaker.
    The previous owner said that it's fully serviced and it worked with no issues until now. Now, I've found its tone control works funny; sound gets softer after "7".
    No other issues, the amp produces good sound with no hum or buzz.
    What do you think?
    And do you use variac to get the original 117 voltage for this one?
    I'd appreciate much if you could have time for me, thank you in advance.

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    1. This comment has been removed by the author.

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    2. Ah! Thanks for the correction! I'll update it.

      I don't bother with the Variac, no need really. And yes, these do sound good stock and I'm a fan of the CTS speaker.

      With regards to the tone control sounding funny, I'm assuming you are using this for a guitar amp?

      And the problem just developed or was it odd in the first place?

      If it's a recent problem the first thing I check is your preamp tube, next would be your signal caps. Make sure they aren't leaking.

      The tone stack in these does tend to roll a lot more highs off than your typical Fender amp. If you have a look at a Musicmaster Bass Amp schematic and a Deluxe 5E3 first thing to notice is the cap values. The rolloff cap is a .005 in the Deluxe, .02 in the Musicmaster. But like I said, if this is a recent phenomenon I would check your signal cap, the .01 connected to the anode of the 12ax7 (either pin 1 or 6). Chances are it's good but you never know.

      Oh, and now that I'm thinking about it, do check your pots as well, hit them with good quality De-oxit contact cleaner. I have had mystery issues be something as minor as this.

      Jef

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    3. Oh Jef, thanks for the quick and very specific explanations!!

      The funny issue has been worse, now the tone control seems to have almost quit functioning... almost no tone difference is made but subtle hiss noise is increased when dialing it clockwise... and the volume gets softer after passing "7" with not touch to the volume knob (mine has stack pot at volume to serve for on/off switch).

      I tried a few different new tubes and cleaned the pots, but no change.
      I'm new to the amp circuit thing, so no caps are touched by myself.

      Maybe I would need some professional job.
      Do you happen to know any good shop (in technique, in responsibility and in price) in Manhattan? If not, I’d go to the 30th Street Guitars.

      Once again, thank you very much for the input, I really appreciate!!

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    4. I have heard good things about 30th street guitars. Also Blackie Pagano in the East Village if he's around. He's supposed to be great.

      It does sound like it could be a bad pot. These are pretty simple machines so I'm sure when someone gets in there the solution will be pretty easy.

      Best of luck to you! Jef

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    5. Thanks Jef!

      Yeah, I too think it’s just the matter of a bad tone pot, caps seem all OK.

      I’ll do it myself, the pot should be a 1meg linear, right?
      I used to worked at a repair shop but had done finish & fret jobs mainly (done few things inside of amp chassis).
      Want to do it all by myself, but have forgot many things and knowledge is all based on useless Japanglish. lol

      So won’t go further than that for now, will ask an expert if the issue isn’t eliminated by changing the pot… Do you accept a customer?

      (Once posted this as a new comment below not as a reply on this thread, so re-posted here deleting the original whose trace is left.)

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