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first DIY project - Musicmaster Bass Amp mods + rebuild

Discussion in 'Shock Brother's DIY Amps' started by cottontails1959, Jan 24, 2021.

  1. cottontails1959

    cottontails1959 Tele-Meister

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    This is my first amp R&R/mod project. I have a 1973 Fender Musicmaster Bass amp that was very noisy – I’ve taken it on as a rebuild and mod platform, with some add-ons that are getting some talk on this forum. The amp came with a replacement speaker and had some updates already done to it, including drilling the chassis to add a fuse holder – so I am not worried about keeping it stock with original components. Reducing noise and making it useable as a bass practice amp are the top priorities. Mods include …

    1. New metal film resistors
    2. 33uf input grid stoppers
    3. Increasing coupling cap from .01uf to .022uf
    4. Changing tone control to bass frequency weighted tone tilt stack ala Blencowe
    5. Increasing first filter cap from 20mf to 30mf
    6. Adding bleeder resistors
    7. Adding artificial center tap and heater elevation
    8. Splitting pre-amp section and power amp section grounding

    Attached is the layout and schematic, plus a pic of the eyelet board with pre-install components soldered on; schematic of original amp included, too. I am not happy with where the fuse holder was installed, so I may mount a block fuse holder where the power cord comes in. The original amp had a 20uf-20uf firecracker-type combo cap for B+1&2 filtering. I’ve not found a replacement that is dimensionally suitable … excepting there are Richey GOLDS at justradios.com that might work; they’re expensive to risk them not fitting, and the being vertical mounts, the all the connections are on the same end. Also, one of the original 7-pin wafer sockets was replaced with a ceramic socket – ugly … I have found wafer sockets on eBay, but the mounting hole centers are way off. Regarding the signal wire run from the volume pot back to V1B, I am going to run it under the eyelet backer board – of which I am going to cover the underside with copper tape and mount it with metal washer standoffs, so the shielding is grounded to the chassis. If noise is still a problem, I’ll have to get some shielded wire. FWIW, I’m using 22 AWG topcoat wire from AES. Thanks @King Fan ... it’s worked the trick.

    I am waiting for a few more components to come in from AES, so it’ll be a week before I can start putting things back into the chassis. I’d appreciate a go-over of the schematic and layout to make sure I haven’t missed anything; corrections and suggestions always welcome.

    Cheers, Peter
    PKS MM 6AQ5 schem tone tilt 2021_0118.png PKS MM layout tone tilt 2021_0118.png
     

    Attached Files:

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  2. cottontails1959

    cottontails1959 Tele-Meister

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    Needed parts arrived; so I am starting to mount and solder stuff in place. I think I've found a reasonable way to mount tag strips while insulating them from the chassis without having to use nylon sleeves and washers - I'm giving shrink tube a try.

    PKS MM mod tone chassis b 2021_0128.png PKS MM mod tone control backside 2021_0128.png PKS MM insulated tag strip mount 2021_0118.png
     
  3. drew1d

    drew1d Tele-Meister Silver Supporter

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    Interesting. I Completely butchered my Musicmaster bass amp, but it was in very rough shape when I got it. It will be nice to see one brought back to life.
     
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  4. cottontails1959

    cottontails1959 Tele-Meister

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    I finished soldering and installed the power cord this morning. Went through @robrob start up procedure. Everything went pretty smooth. Still fair amount of 120 cycle hum, but not as bad as before rebuild. Chopped stick-ed around a little bit - no obvious loose connections, no smoke or pops ... tubes heated okay ... voltages look okay as well ... I'm just not sure if I am measuring correctly ... whether they should be pin to ground, or pin to pin in some cases. This is what I have right now with tubes in ... I'd appreciate someone chiming in to confirm I've got the V-reference correct.

    V-switch 123 VAC
    V-heater 6.55 VAC

    B+1 300 VDC to ground
    B+2 293
    B+3 258

    12AX7 - V1A
    pin 6 -162.6 VDC to ground
    pin 8 -1.38
    pin 6 to 8 -162.2
    pin 7 -0.22

    12AX7 - V1B
    pin 1 -254 VDC to ground
    pin 3 +1.25
    pin 1 to 3 -252
    pin 2 0.00

    6AQ5A - V2
    pin 1 0 grid to ground
    pin 2 0 cathode
    pin 5 296 plate
    pin 6 293 screen
    pin 6 to 1 -276 VDC / 59.5 mA = 16.4W?

    6AQ5A - V3
    pin 1 0 grid to ground
    pin 2 0 cathode
    pin 5 295 plate
    pin 6 293 screen
    pin 6 to 1 -276 VDC / 59.7 mA = 16.5W?

    Also, I have it wired with an artificial center tap and elevated heater reference voltage. How do I measure heater elevation ... it should be 300V * (33K/(470K+33K) = 300V * 0.0656 = 19.7V ?
     
  5. cottontails1959

    cottontails1959 Tele-Meister

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    :D

    MM fin top.png MM fin close.png
     
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  6. Whatizitman

    Whatizitman Friend of Leo's

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    You will not find a direct replacement tube socket. They were larger than standard hole size, and were prone to failure. I had one that arced. I drilled out the center of the socket and mounted a NOS Bakelite 7 pin inside it, soldering it to the mounting tabs of the old wafer socket. Probably not the most sturdy. But it worked fine for my purposes.

    If the replacement is working, I would leave as is.
     
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  7. zook

    zook Friend of Leo's

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    It will be better to use nylon washers and nylon machine screws to insulate the tag from the chassis,
     
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  8. cottontails1959

    cottontails1959 Tele-Meister

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    @zook I looked at doing exactly that - I found nylon washers at our local hardware, but not workable nylon screws ... I'll look again.

    I am getting more 120 cycle hum than I think I should, so I am redoing/rerouting the power wiring. And I am not happy with the tone control ... I wired it backwards ... not a big deal because it's a linear pot ... and I get a nice sound with humbuckers, but there's way too much treble on tap, and it gets shrill pretty quick. So I am going to replace the 1M linear pot with an audio pot, use two 3nf caps so there's no scoop in the mids, and correct the wiring.
     
  9. Whatizitman

    Whatizitman Friend of Leo's

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    Did you mention what you are mounting to the tabs? If grounding is a concern, can you get by without the extra tabs? Or can they be secured to something else buffered from the chassis?

    Not a pro myself (obviously), but I've mounted tabs to chassis before, and just made sure to isolate connections from ground. Maybe cut off the top of the center tab. Or say cover it with heatshrink or whatever. Just make sure nothing touches it. Seems like the nylon washers/screws would be more trouble than it's worth. But there's a reason I don't do this for a living.

    EDIT: I would try to get a handle on that hum before making any other circuit mods, though.
     
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  10. cottontails1959

    cottontails1959 Tele-Meister

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    In this case, for space purpose I need to use the tab, and it must be isolated from ground. It's the one to the left of the B+1 filter cap in the pic above ... it's where the artificial center tap connects to the voltage divider for heater elevation ... so that ground needs to be on the other side of the 33K resistor. I am going to re-route the PT secondaries tucked up against the front of the chassis and twist the PT center tap with them as far as possible. I'm going to put a new tab on the left so I can mount all the mains, then run the hot/neutral as a twisted pair up to the combo vol-switch ... and wrap the PT hot wire in with them. All of this need to go past under the tone pot, so it's gotta come out. The return from the vol pot to V1b grid should probably be a shielded wire, since it spans the chassis. I vaguely remember muchxs saying that having to run power across the chassis to the combo switch was not good for noise.

    P.S. I did see on a Fender schematic that the voltages are referenced to ground :rolleyes:
     
  11. drew1d

    drew1d Tele-Meister Silver Supporter

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    Just an observation, but maybe move the ground for the interstage transformer to the Power tubes filter cap ground. The Blue and Red wire take it from the 12ax7, and the Yellow and Green do the phase inversion to the power tubes with the black wire as the center tap. I haven't tested this yet, it may not make a difference.
     
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  12. cottontails1959

    cottontails1959 Tele-Meister

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    @Whatizitman - I used a 5/8" washer to mount the 7-pin aftermarket tube socket - looks way better than the tin-snipped metal plate that was there before.

    @drew1d - I installed a (new) star * ground point for the power amp side, and I don't think I have enough length on the inter-stage center tap wire to reach (there) ... I'd have to tie it in at the end of the cathode resistor. I'll leave it for now ... you can see I'm in process of twisting wire to the on/off + volume switch; mebbe that will help.

    MM mod + text as of 2021_0207.png
     
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  13. drew1d

    drew1d Tele-Meister Silver Supporter

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    So this is the grid leak bias amp I ended up making with my MMB. One knob. Still using that PI transformer. This is like the 4th amp I built in the enclosure. Sadly the only thing original is the OT and the chassis. 958DA381-1FE6-4F88-AB28-23447D0B9F38.jpeg
     
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  14. drew1d

    drew1d Tele-Meister Silver Supporter

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    Another thought I always had about the PI transformer on these amps, I'm not sure if it's a 1:1 or 1:3 winding ratio. I wonder if a tube like a 12dw7 that is half a 12ax7 and 12au7 wouldn't work better. I would imagine that the transformer trades current for voltage in some way like an output transformer.

    I really like the PI transformer, it requires so few parts to make a push pull amp.
     
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  15. timfred

    timfred Tele-Meister

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    I have a stupid question...I’ve never spent any time looking at bass amp schematics but if I were designing one, my naive approach would be to choose bypass and coupling capacitor values that were larger than typical for guitar amps to extend the frequency response lower.

    But the MM schematic does the opposite. Anybody know why?
     
  16. Les Gear

    Les Gear TDPRI Member

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    I rescued a MMB a few years ago -- it came to me in a Champ cabinet as the leftovers after somebody put their Champ in a 12" MMB box. The writeup is here.

    In short, I found that just twisting the heaters and shielding the two preamp signal runs made a perfectly decent little amp out of a noisemaker.

    @cottontails1959 you did a lot more work and wound up with a lovely-looking result. I hope your build brings a grin every time you turn it on.
     
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  17. cottontails1959

    cottontails1959 Tele-Meister

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    --- when I get back to working on mine, I'll measure the voltage to see if I can determine what it is ... I vaguely remember reading somewhere it was 1:1.2 ... I like the ease of the PI transformer too. I do wonder what the amp would sound like with an ECF-82 pentode wired as an LTP PI; although I'm not sure there is enough PT juice to run another tube.
    --- Thank you for your kind words. The first go-round got rid of most of the buzz-noise that was associated with the tone pot, and some of the 120Hz hum. Hopefully this (post #12) will do the rest ... my work area is in an unheated garage, which has delayed the effort :eek:.

    --- no stupid questions, just better answers (Mick from That Pedal Show :D) A read around the internet would say it is NOT a good bass circuit ... so take it being a bass amp in name only ... also, the stock speaker does not handle low frequency well. You are correct in your approach ... the .01 coupling cap can be increased to .02 to reduce the bass cut; and things can be done to the tone/volume pot stack to move the bass roll-off down as well. See ... post # 27 ... https://www.tdpri.com/threads/if-i-...l-to-a-5f1-champ.1062475/page-2#post-10440863 IMO the existing circuit retains way too much treble frequencies.
     
    Last edited: Feb 18, 2021
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  18. timfred

    timfred Tele-Meister

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    So yeah, looking at the circuit more closely:

    • the 0.0047uF cap at the input is not a low pass/RF filter. It’s a high pass filter with a cutoff frequency of 72Hz.
    • The 2uF bypass cap on the first triode forms another high pass filter with a 53Hz cutoff.
    These together make some pretty serious bass attenuation at the front of the amp. I assume that these were bandaids applied because otherwise low bass frequencies would overdrive the rest of the circuit or the speaker too early. Fender cheated. More apparent clean head room for most of the bass fingerboard at the expense of low note thump.

    If you look at a classic “real” bass amp like an Ampeg SVT, there are few bypass caps at all and big 0.1uF coupling caps. They were trying to keep the freq response flat with very soft overdrive characteristics. They did add an input cap on the bright channel but in parallel with the grid stopper so that it acts more like a treble boost.

    If it were me using this amp for bass I’d get rid of the 0.0047 cap, increase all the coupling caps to 0.1, increase the bypass caps to 22uF, and then if the distortion is too much....turn the volume down. For guitar it’s probably great as is.
     
  19. cottontails1959

    cottontails1959 Tele-Meister

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    Even with no changes to the circuit, it'd be more bass friendly with a higher watt, stiffer speaker ... that is, a bass speaker. Yeah, start cutting/shuffling stuff before the first triode takes away what make a MM what it is, lol. You could change the .0047 cap to .0068 to move the cutoff well into the bass frequencies; or a .01, which would start the roll off at low-low B. As I said previously - IMO, even as a guitar amp, it let's too much treble through, so I think the tone control can be wired to lower the start frequency of the treble cut (not bass roll-off - my mistake). I've even wondered if there's someplace to hard-wire in a low-pass filter that attenuates >16K, let's says, right past the input.

    Regardless, making the MM a better guitar amp with minor tweaks will get a passible bass amp good for bedroom level practice.

    Reworking the treble tilt control ...

    tilt.jpg
     
  20. timfred

    timfred Tele-Meister

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    Easy. Put a 250K-1M grid stopper on the second triode.
     
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