Fender Musicmaster Bass Amp

[TIMBO]

Hi guys, A bit of an unknown Fender student level amp that looks to have some hidden talent.

The interstage transformer is also intriguing.
This is where it is a bit grey on the info.
I am unable to find any specs on the transformer and Evatco (local supplier)have a limited range.
The Stancor equivalent is 1:3 ratio, so I'm hoping that will work.
There are some modified versions of the original circuit to convert it from bass to guitar.



I'm not sure there is a version of the amp as per the bottom schematic, most of the amps only have two knobs.
I would think it would loose a bit of signal and a bit natural growl that is in the clip.

Any thoughts would be great.

[vampwizzard]

Pretty sure any additional circuits like that reverb would be on a modification. Theres two different versions of these amps the only difference really being the output tubes. They are pretty sweet cheap fender amps and a lot of folks use them unmodified, or with an upgraded speaker. I did look at possible mods to these amps but I didn't trust its available current to add another tube but I may be wrong. Are there examples on the net of someone actually playing a musicmaster bass with reverb?

[TIMBO]

Hey mate, The reverb is a mod I like to add to my builds, so it is not part of the original circuit.
I think 6AQ5 power valves were used then change to 6V6s.

[shooter]

I like inter-stage PI trannies,  did 2 amps with them, you get a nice secret sauce using them 
as for your reverb, you are the reverb add-in expert 

[Ritchie200]

So you don't need 16 cascaded gain stages for good distortion?!?!  Gasp!  Who woulda thought!!!!!!   That's cool, and what, about 10 parts?  Timbo, you need to build this!!!

Jim

[TIMBO]

Thanks guys.

Shooter, I'm not too schooled when it comes to the interstage transformer.

Love REVERB 

[shooter]

schooled

Current kills 'em
now you're schooled 

they usually have a very low spec, mine are 10mA.  so maybe measure vdc at V1B's cathode (470r) to calc I.

other than that, it's about 15 solder connections and a wad of parts less than an LTPI

[mresistor]

Timbo..  Classic Tone has a replacement and I'd bet they would give you specs if you asked..   also don't know if they'd work but there were interstage trannys in Hammond organs..


http://www.classictone.net/40-18093.html      they say it is 15 watt 


I also found this that has some specs  https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma

[TIMBO]

OK, here is where I'm at...
There has been some mixed reviews on this one, so a bit of head scratching has been needed to have a backup plan if the interstage transformer is a no goer.


I have laid out the circuit board to accommodate some extra components that will make it more like a 5E3


All components can be added even with the board in place.
A 6AV6 will be added for the phase inverter.

[PRR]

.

[gwtekman]

What power transformer is being used?  I see that you have modded yours  using a different one as you have switched to one that supports a 5V rectifier tube.

I built one last year, but mine was only slight modded from the original, So I used the solid state rectifier and the replacement transformer set  for the Musicmaster from Classictone (PT = 40-18100, OT = 40-18101, Driver = 40-18093).  I too have left plenty of room on the board for further modification but, in my case, because I started with a little  Acoustic G-20 as the donor amp, my chassis would be the more  limiting part for me to expand should I later decide to redo with added tubes (as well as the space around the speaker when mounted in the cabinet).   But to do that even, I would have to change the power transformer or add a small filament transformer somewhere to augment as the 40-18100 PT only delivers 1.1 amp (and it runs pretty hot even with the one 12AX7 and two 6AQ5's I used).

Here was mine as built:  http://gwtekman.x10host.com/MM-12/

[TIMBO]

Awesome stuff mate! How does it sound.
The PT is similar to a Hammond 290AEX.

 Got some time to continue....


10 guesses to the potential problem in this photo

[TIMBO]

Back on the job guys...




A few untidy wires, these have been left long to get the phasing right if needed.

[sluckey]

Those component and wire colors really pop inside that black chassis. 

[Ed_Chambley]

As usual, great work Timbo.  This amp was sort of looked over as Vibro Champs started fetching over $400.  These would sell here in Atlanta GA at the same time for $100 to $150, so I ended up with 6.  4 had 6v6 and 2 had (early brain fart) the little version.  I could look it up, but you get it.


The cabinets on the later ones were much more cheaply made.  Little sleepers.  BTW, they are bringing near $400 now.  They are actually a great small gigging amp if your drummer is a wimp!

[TIMBO]

Thanks guys.
Amp fired up well, voltages were a bit high so a dropping resistor adjustment has got it at a better place.


Sounds pretty good....

Just a low level hum that I have tracked down to the power amp.

[TIMBO]

Guys, A bit more progress....

[vampwizzard]

Lovely! That will be sick when complete. Hows the weight balance with that chassis mount?

[TIMBO]

Hi guys
This one has collected a mountain of dust.
So there was a hum that was annoying me, it was a ground loop with the reverb tank.
The two RCA's are grounded on the tank, so I had both plug shields grounded on the amp chassis.
Disconnecting one shield from the chassis did the trick.
There is still some room for tweaking....

[BrownIsound]

Any update, sound clips?

Thinking about building a MM based amp myself….

[TIMBO]

Hey mate, This one has been shelved for awhile, still deciding whether I like it or not