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Author Topic: M80 bass amp tone stack  (Read 244 times)

Tennerondack

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M80 bass amp tone stack
« on: September 08, 2018, 02:30:01 PM »

Hey everyone. I introduced myself in the newbie section and glad to be here.

I had asked about altering the EQ section in my Fender m80 bass amp.
I is very honky thin midrangey in nature, and I would like to scoop out a lot of that mid and have a fatter more balanced tone capability.

Forgive my ignorance, in the meanwhile I have been reading about slope resistors in tone stacks. Does that apply in this case ? Only on tube amps ?

I  have good soldering skills and know how to find whatever wherever in an amp. Can I do a simple resistor change to change to a more fat natural response without going to extremes on the tone controls, currently have to crank bass and severely cut mids to get a more traditional, less "transparent" tone.

Are there other things I can do, such as filter caps etc. to beef this up some. I do have a 4 ohm speaker installed and it is really pretty powerful for it 160 watt rating. No damping in speaker box.

It's fairly compact and light, it would be nice if I could optimize the tone more.

Any and all help appreciated, many thanks !

EDIT : here is a link to details aboutthe amp including schematic.

http://www.rlinwood.com/family_webpage/linkpages/music/music_fender_m80_bass_amplifier.htm
« Last Edit: September 08, 2018, 04:08:47 PM by Tennerondack »
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Loudthud

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Re: M80 bass amp tone stack
« Reply #1 on: September 08, 2018, 10:10:14 PM »

The M80 is a complex circuit. There is no tone stack with a slope resistor like many tube amps. The Bass and Treble controls look to be of the Baxandall (google it) variety and the Mid control is some second cousin to a graphic equalizer with synthetic inductors and a dual pot. In addition, there is a bridged T network between the first and second stages.

Modifying this would be a nightmare or perhaps a train wreck. The most sane way to do it would be to use a modeling program, model the entire preamp and try to mold the EQ to something you like. Change components, test and try again.

Slightly lower tech approach would be to plot the EQ curve, change one component and plot again. Repeat as necessary.

Still lower tech, install some kind of graphic EQ in the effects loop and see if you can fix it that way. Or just build your own preamp and jam it into the effects return. You can't bypass the "Delta Comp" which seems to be some kind of compressor. It might have some influence on the tone you don't like.

FYI: The MN3007 and MN3101 chips are impossible to find replacements for. You might be able to remove them and sell for big bucks.

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Tennerondack

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Re: M80 bass amp tone stack
« Reply #2 on: September 08, 2018, 10:37:29 PM »

Thanks loudthud.

I kind of suspected this, it's no shock.
I pulled the amp out and ran it through my Peavey 1-15 and it did show a big improvement in tone.

I like to use it because my Peavey Nitrobass has a loud fan that drives me nuts and I don't really like the tone suck inherent in practice amps.

I have concluded that the small enclosure is a big part of the problem. I do put my Hartke bass attack into the FX loop and it makes a huge difference.

I did look up the power transistor specs and though it was largely over my head, they are rated at ~ 100 watts each and this thing is surprisingly loud, my guess it is under rated ?

Anyway, I think I will make it into a head and ditch the box, or I even wondered if it at ~ 2.75 volume would sound better with a good 12" speaker tuned properly ? Just a crazy thought, no sense in trying to something out of nothing.

Thanks for you help and the heads up about those chips !
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Enzo

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Re: M80 bass amp tone stack
« Reply #3 on: September 09, 2018, 01:06:51 AM »

As I posted in your other thread, the speaker is a MAJOR contributor to tone, try different ones.
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Tennerondack

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Re: M80 bass amp tone stack
« Reply #4 on: September 09, 2018, 09:20:28 AM »

Thanks Enzo, I very likely will.
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phatt

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Re: M80 bass amp tone stack
« Reply #5 on: September 09, 2018, 09:40:52 AM »

Yes try other speakers but if it's still bright,, then try this little mod.
Add another 47nF cap to ground between R9 and R10.
I can't promise perfect as that is not a great circuit for guitar.

Maybe a temporary mount on the track side existing solder pads might be doable.
up to you .
Phil.
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Tennerondack

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Re: M80 bass amp tone stack
« Reply #6 on: September 09, 2018, 01:35:39 PM »

Wow phatt this is getting interesting ! I recognize that cap value as a treble bleed such as on a passive
 bass tone, correct ?  That being the case, can I incorporate a pot so as to experiment and find a potential sweet spot ? What value would you recommend ?

BTW I am discovering that even as it stands, the amp is capable of much better tone than I had found before. being naturally reluctant to over boost bass frequencies, I hesitated to do that.

However, whatever the workings of this tone stack setup, I can crank the bass and drop both treble and mid and bring them up to taste and get a much better overall sound. Rather than overload low frequencies and mud and send the speaker into xmax as it would on some amps, cranking the bass seems to bring about a balance.

The tone through my 1x15 becomes fat punchy and clear w/o overpowering boomy lows, much like an SWR I once had. Today I will experiment with my Bass Attack and see what I get with that layer of tone shaping incorporated.

I'm going to experiment with different speakers too like enzo ssuggested to get better sound out of the small box as well. Many thanks to both for helpful suggestions so far.

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phatt

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Re: M80 bass amp tone stack
« Reply #7 on: September 09, 2018, 10:58:38 PM »

First up, Correction I said 47nF,, wrong  :duh
470nF is what I meant to say.
Here is a pic of the placement of that cap on the schematic.

Stock the treble has max 45dB boost while the bass is only max 25dB
at output of tone control.
The addition of the extra cap pulls down the treble so that both bass and treble are more equal in boost as can be seen in this screen shot.

I'm all for speakers but this is a lot easier and far cheaper than a new speaker and speaker cabinet. 8|

Try this first and see if it helps.
Phil.





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Tennerondack

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Re: M80 bass amp tone stack
« Reply #8 on: September 09, 2018, 11:30:48 PM »

 :dbtu:

Wow thanks can't wait to try this  woot !   :dbtu:
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