first DIY project - Musicmaster Bass Amp mods + rebuild

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

 

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.

 

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 ?

 

@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.

 

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

 

@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.

 

--- 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 .

--- no stupid questions, just better answers (Mick from That Pedal Show ) 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.

 

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 ...