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

26 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. 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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  5. This comment has been removed by the author.

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  6. I'm about to do these same mods for a friend, and I noticed that you reused the .0047 cap from the input to the tone stack, but instead of reusing the .022 cap from the tone stack to stage 1 anode, you used a new cap. I was just wondering if you'd recommend using a certain type of .022 cap there, or would it be fine to reuse the old one from the tone stack. Thanks in advance.

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  7. Hi Unknown!
    I say experiment. Try that cap first then try others. I probably didn't use it cause I don't like that particular brand of cap that Fender was using at the time. The .0047 is merely rolling off highs to ground so it's not so critical there which is why I reused that one.

    I'm a fan of the Mojo Dijon caps myself, they sound not too unlike a good old Fender blue Ajax cap. They're physically bigger than a Mallory 150 series which is also a fine choice.

    I also like Sozo caps, they're nice and I typically use them when I build an amp, but not for general repairs as they're expensive. If a customer want's them I am happy to install.

    And finally, I'm also a fan of the Polyester Sprague Orange Drop, the 225P series. You can buy them at Antique Electronic Supply. I like them in certain amps, especially if it's a mushy sounding amp. They can tighten things up. But these are still warm and not harsh like a 715 or 716 Polypropylene cap. I've used them in hi fi circuits with pleasing results. In most guitar amps I tend to like them in the phase inverter and use the Mojo Dijon in the preamp stages. Warm and clear combination.

    Thanks for asking and hope this helps! J

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  8. Hello...I got one if these and it is a 1975.. though strangely the tube chart says 5y3gt in the first tube... it came w a 6v6 in it... and I replace w a pair of groove tube 6v6.... but I have not found another w the 5y3 in it... should I have a5y3 in that slot or is the 6v6 safe?....

    Also... the death cap.... can I just snip it off away the legs or is that something that has to be done by a pro... as I have zero talent at this stuff... it had a3 prong cord...n It's quiet w all stock guts... thanks

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    1. Hi.

      Definitely do not put a 5Y3 in there. It should be a 6V6.

      The 5Y3 is a common rectifier tube whereas the 6V6 is a power tube. These amps have a solid state rectifier.

      Most likely this is an error on the tube chart. Some folks were asleep at the wheel during the CBS era!

      That being said, even in the tweed era I've seen oddities on tube charts, like 6SL7 simply crossed out and replaced with 12AX7. But that was done on purpose cause they did switch to a different tube.

      Yes, if you feel comfortable enough, you can snip the lead on that death cap. If not get a tech to do so. Thanks for your questions!

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  9. Thanks for the response I appreciate it...

    So when it comes to that death cap... would I need to do that cap discharge process prior to snipping it at the base of the legs?

    Also... would I need to do any sort of jumper wire in its place or would the small bracket it is attached to already make it safe to plug in as it is? My concern lies in the story of if the cap fails I get the full 120 volts as it is not there so if I just snip it off at the base wild it not result in essentially the same as the cap failing and the 120v flowing through the chassis add if the cap failed?...I apologize if this is a dumb question...I just want to make sure my understanding is correct so I don't snip it n get the 120v...

    I appreciate your help...

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  10. First of all, no question is a dumb question. When it comes to safety every question is vital.

    The death cap doesn't store any energy. Caps pass AC and block DC. This cap comes right off one side of your AC line so one side is going right to ground. Should you clip it just make sure the amp isn't plugged in!

    I'm not sure what you mean by a jumper wire in it's place. It seems like you are asking to bypass it with a wire? If so absolutely not. Just clip both sides and get it out of there.

    If that's confusing I would just get a tech to take care of it. Electricity is no joke and I've had my share of jolts. They can do serious damage.

    J

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  11. Ha ha ha. .. no sir.... you cleared it up for me...I did crack a grin when you said to make sure the amp is not plugged in... when you read the threads they always back track n start talking about the wiring...n how if it's this way or that you die.... if it's as simple as cut the legs no die...I get it...I just wanted to make sure I was clear... because like you said electricity is no joke...I got a kid...n kinda wanna see him grow up so I try and ask as many questions just to make sure I don't miss a step.... ha ha ha...

    N yes you answered my jumper wire question exactly right...I was wondering if the absence of the wire was the same as a broken cap...

    I really do appreciate the help... and your patience w me... thank you very much...

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  12. Hey, thanks for this write-up. I just got one of these amps today, and it needs some work, so I am studying up!

    Could you clarify this quote from your post:
    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.

    I am reading it as if you are just removing them rather than replacing them, though removal doesn't seem right. If replacing, what are the values? I tried to sort out from what you wrote, but was having trouble. Thanks!

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    1. Ha! Well, I admit I was being a bit dramatic when I wrote that. The amp does work perfectly fine with that resistor and cap arrangement.

      But to answer your question, yes I just take them out. You don't need a cap there at all. For ground reference I prefer a 1 meg resistor.

      If you look at the photo above (not my photo but the one with the 2 input jacks, 2x 68k resistors and 1x 1 meg resistor) notice that it has a lead saying 'to tube input'. Simply copy that arrangement.

      In the end it's a matter of preference. Re-doing the input like this gives a more lively and open sound. Do keep the old parts just in case you don't like it and wish to put it back the way it was.

      Thanks! Jef

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  13. Thanks, Jef! I appreciate the clarification, and drama often adds flavor! ;)

    I am enjoying learning about mods for this amp, so appreciate the feedback.

    Cheers,

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  14. I wish you made a pre and after vid so we could hear the difference.
    I own one of these and am thinking to replace the speaker with a Celestion Ruby.
    The former owner is a amptech and brought it back to stock, serviced but I wonder if I would likes the mods he removed

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    1. The mods are pretty simple. And as a rule o' thumb, always keep the old parts in case you don't like it. Celestion is an excellent choice, almost always! I'm a lifelong fan.

      I've thought about making videos but honestly myself I just don't get a lot out of them. Better to wire it up and see for yourself. No matter how well I record something on my phone or whatever it gets clouded with digital chunder by the time it reaches the listener.

      If you do mod your amp feel free to post your impressions here! That will be helpful to whoever is reading. Thanks! J

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  15. Glad on finding your blog,grateful to you for sharing your knowledge.
    Did not realize one could pick what works or makes amp"x" sound like it does and transplant that component or components value to the amp under construction.Enlightening.Guess I thought of it independantly but was ascaret to deviate from the written word meaning the schematic of course having burned a few and raised the hair on my forearms.The photo of the input jack saved me great tribulations as I have been struggling with a Fender Deluxe no reverb or tremolo scratch build with no guitar sound out issue.The non complex pic you post may get me past this trouble and on to sound.Thank you for helping.

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  16. Hi, nice article. I have an old 6aq5a version of this amp with an on/off volume pot which I think needs replacing. Can you ( or anyone) tell me exactly what part I need to buy and where to get it? Also, what Celestion would you recommend? I have a brown Fender/Eminence in mine at the moment as I wanted more headroom than the original. Cheers.

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  17. The original circuit has a 250k switch pot in it. I wouldn't think twice about replacing it with a 1 Meg part like this:

    https://www.mojotone.com/1-Meg-On-Off-Potentiometer-Tweed-Amp

    It just gives the whole thing a more open sound. Finding a 250k pot with the same type of switch is a bit challenging surprisingly! You can sorta fake it with a 1Meg pot and put a 270k resistor across it. The taper might get a bit weird but in a pinch it's a solution. But me? I prefer 1Meg.

    Speaker..... Does it need to be a Celestion? My friend installs Vintage 30's in his stage amps for that purpose. They're actually a 70 watt speaker I believe, more headroom, very punchy. That's a good solution. It does add weight so do consider that. I happen to really like the Eminence Cannabis Rex. That's a 50 watter. I had one in a 1964 Deluxe with no Reverb years ago and it had a nice "bark" to the sound, very warm and clear. I hope this helps!

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