The TMB Tone Stack Discussion

[Platefire]

I've done several Bassman 5F6A type of conversions and a Plexi clone with this type of tone stack.
So I've been studying the history and workings of this tone stack a little bit because sometimes they seem a bit bright to me. So I've been looking at my schematics for these amps and have learned that some of the components of my builds are not the same as the original Bassman. All the technical discussion
or frequency, low and high pass filters is more or less over my head even though I do comprehend in part. 

The original bassman that has become so famous and sparked the original Marshal JTM 45 had a 100K slope resistor and a bass cap of .1. My builds have a 33K slope resistor and a .02 bass cap. I have read that these values are very important in maintaining that original sound some call "Fender Shimmer"? So it seems the more modern versions of this tone stack have changed these values to be more like I have in my amps.

So I'd like to hear some discussion from you guys with more amp tech knowledge than me. If the original recipe was so great and instrumental in getting that famous tone, why have these newer versions being changed??? 

The reason I'm asking is I'm wondering if that might be part of the reason I have a hard time dialing down the bright tone in this tone stack?

[Williamblake]

I am guessing a lot of people back in the day had bad hearing but didn't know.

[PRR]

Have you tried 0.1uFd?

Instead of saying "a bit bright" say "no bass" and think what a fatter cap could do for bass.

[Platefire]

Thanks PRR
Yes I am considering changing to those original values in at least one of those builds just to see if it helps? Willamblake might have something, my 75 year old ears "is not what they use to"

We had a discussion several years back on my Kalart Victor 70-15 that has an active Baxandall tone stack.I love that tone stack because when you dial up/or down the bass and treble you can actually hear it doing exactly that. I could easily dial in what I want.


I did an experiment on my Allen Accomplice with a TMB tone stack yesterday. The closest I could getto the tone I wanted was T=2, M=2, B=2 and RAW "0".. I decided to just go for what I was looking for and disregard the label on the knob and that's what I ended up with.

[dwinstonwood]

Here's the 5F6-A calculator. It's fun to edit various component values to see how the frequency curve changes:

https://www.guitarscience.net/tsc/bassman5f6a.htm#RIN=1300&R1=56k&RT=250k&RB=1M&RM=25k&RL=1M&C1=250p&C2=20n&C3=20n&RB_pot=LogA&RM_pot=Linear&RT_pot=Linear

[Platefire]

Thanks dwinstonwood for the 5F6A calculator. I intend to play with that and try to understand more what the graft chart means.

I just located my Accomplice schematic and it does have the 100K slope resistor and the .1 bass cap.So I do know now that it has the bass values that I was wondering about. So 2/2/2 it is:>)

[sluckey]

The original bassman that has become so famous and sparked the original Marshal JTM 45 had a 100K slope resistor and a bass cap of .1.

Not true. The 5F6A has a 56K slope and a .02µF bass cap.

     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf

Believe it or not, the 5F6A was originally built to be a ***BASS*** amplifier.   

I want to mention the Twin Reverb TMB circuit. It's a bit different from the 5F6A TMB. It does have a 100K slope and a .1µF bass cap. I think this is an excellent tonestack. It behaves like a three band equalizer. Look at the attached schematic. Now visualize all three tone knobs set to zero. You should be able to see that this puts a ground at the top of the volume pot. Absolutely no sound gets through. The amp behaves as if the volume control has been turned to zero. Now turn up the treble knob and you will hear only the treble sound coming through. Set the treble back to zero and turn up the middle knob and you will only hear the mids coming through. Set the middle back to zero and turn up the bass knob and you will hear only the bass coming through.

The action of the TR TMB provides a very versatile tone control and the component values are just right, at least for my ears. 

[Platefire]

Thanks tubenit
I ran a copy of the schematic so I could check it out. Looks like a lot of options there.

I put all my Accomplice tone stack values in the 5F6A calculator including my tone stack settingsand attached is a scan of the results. Looks like my Hz and Db is too small to be heard:>)

[Platefire]

sluckey
Don't even try to tell me that a Bassman is suppose to be for Bass Players!!! I thought it was for musicians
fishing for Bass. ><> Reminds me of a song I heard on PBS Blues Radio Program "Fishing Musician"


Anyway, You nailed it with that TR tone stack. That is the exact tone stack I have in the Accomplice save the RAW control and with that tuned to "0", it's off. Attached is the Accomplice schematic so you can check it out. I tried the controls just like you described and it worked just like you said. I played around with it a few minuets and ended up with a setting with mids turned completely off, T=2 and B=2.5. Thanks a bunch for pointing this out to me. Starting from a blank slate and adding what you want makes it a little more simpler. Platefire

[sluckey]

That is the exact tone stack I have in the Accomplice

Guess where David got it from...   

[Platefire]

Well in my studies on this I kept reading about using the 100K slope resistor and the .1 Bass cap but when I got to looking on the old Fender schematics on the Bassman, I couldn't find it
I guess there was a good reason for that because it was on the Twin Reverb

I tried it some more today and it sounds better to me with the middle off or almost off. So after a time and I'm still happy with this, I may make this change to my Dukane 5F6A. It's got the 56K slope resistor, .02 bass and .02 Mid cap. The Pignose G40V, I will probably leave as is because I don't want to fool with the PCB
 

[PRR]

....visualize all three tone knobs set to zero. ....turn up the middle knob and you will only hear the mids coming through....

Actually, B=0 M=10 T=0 gives "everything at reduced level". It sounds like mostly-mids because raw guitar is strong middles and these tone-stacks taught us to expect bass/treb boost all the time.

And this design goes back before 5F6 or Marshall, but I've misplaced my notes.

[pdf64]

Regarding the 100k slope with 0.1uF bass cap, my understanding is that’s the final version of the tweed bassman.
If documented, perhaps it’d be the 5F6B, but the changes may only have been hand amendments to the shop floor copy of the 5F6A drawing.
That’s the tonestack Fender settled on for the ‘59 Bassman reissue (the 6k8 LTP upper tail resistor being another change from the 5F6A).
https://el34world.com/charts/Schematics/files/Fender/Fender_59_bassman_manual.pdf
Apparently Clapton likes all the knobs on his amps set to ‘7’ 
Dunno if that applies to 1-10 as well as 1-12 type knobs 

I think Gibson might have been the originator of the FMV tonestack   
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga77_1953.pdf

[PRR]

I think Gibson might have been the originator of the FMV tonestack   

I'm waiting to learn who Gibson got it from. (If they ever had an electronic wizard, it was a mistake.)

This is the thread I was thinking of:
https://el34world.com/Forum/index.php?topic=27456.msg279790#msg279790

[HotBluePlates]

I've done several Bassman 5F6A type of conversions and a Plexi clone with this type of tone stack.
... learned that some of the components of my builds are not the same as the original Bassman. ...

The original bassman that has become so famous and sparked the original Marshal JTM 45 had a 100K slope resistor and a bass cap of .1. ... I have read that these values ...

The original bassman that has become so famous and sparked the original Marshal JTM 45 had a 100K slope resistor and a bass cap of .1.

Not true. The 5F6A has a 56K slope and a .02µF bass cap.

     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf

You're both pretty!!  (I mean you're both right.  Or wrong...)

I am not rich enough to own vintage tweed Bassman amps.  But I've been on forums with people who have owned many.  They say the actual amps do not match the schematics, that Fender often used Linear taper pots where you'd think an Audio taper should go, and that there were at least 3 different sets of values in the "original Bassman."

    Version 1:  56kΩ slope, 0.02µF, 0.02µF
    Version 2:  100kΩ(?) slope, 0.02µF, 0.1µF  (I might have the slope resistor wrong on this version)
    Version 3:  100kΩ slope, 0.05µF, 0.1µF

They say the 0.02µF & 0.1µF one was the most common to leave the factory.  Another nugget:  apparently 1954 Strats left the factory with 100kΩ Linear Volume pots.  This according to the guy that owns Komet amps, who repairs vintage guitars & owns several 1954 Strats.

In light of that, swap values, play & listen.  You'll get much more out of that than any technical discussion.

One bit of tech that is valuable: 
PRR pointed out to me once that the Fender/Marshall tone stack is sort of a "Bridged-T Filter" mangled a bit, with some parts made variable so you could adjust Lows, Highs, Mids.  And a "Bridged-T Filter" is basically a low-pass and a high-pass mixed, so that they scoop in the middle.  Below is a graph of the Marshall tone stack from the Tone Stack Calculator; you can see the low-pass part & how it crosses over the high-pass part to scoop the mids:



Don't like how the low-pass & the high-pass work?  Then jigger a cap-value or a pot-value or a resistor-value until you do like the result.

[sluckey]

Version 1:  56kΩ slope, 0.02µF, 0.02µF

That's the one Marshall copied...

     https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf

[PRR]

It's a shame we can't change these parts without a lift and an air-wrench.

[Platefire]

Well I was wanting to hear from what I consider the Heavy Hitters of this Guitar tube amp forum
on this subject and I surely did. I've learned a lot more than I did on this tone stack and was really shocked to learn that maybe Gibson had the first version of this tone stack. Of course the water seems to be a little murky on that subject now looking back to the 50's documentation. I read the link PRR posted on that earlier discussion. Very Interesting.

So I certainly got your message, tweak away until you hear and get what you like. But first, now that I have a better understanding of how it works, I want to do some more experimenting with different settings.

The first amp I had that had this type of tone stack was a Silverface Bandmaster Reverb in the early 70's which IMHO was a great sounding amp that kind of set the standard of how I judged amps from then on. I got in a habit of setting everything kind of in the middle. I think my taste and maybe hearing has both changed so-----I may need overcome some of my pre-fixed ideas or amp setting habits. Thank you so much, this is a great conversation. Platefire 

[dwinstonwood]

Another thing that looks like it would have affected the tone stack is the change in the NFB/Presence circuit from the 5F6 to the 5F6-A.

[PRR]

I wonder if the 5F6 (no a) ever really existed. It sure was not common.

[Platefire]

The presence control, might be another thing I mis-understand how to use. Its effect to me is ever so slight to my ears but seems to make for more brighter overall sound as you turn it up.  I'm not sure if it effects touch responsiveness or not?  If you think of "Presents" meaning you would think by turning it up your sound would be ever more obvious and noticeable. On the other hand, by turning it down,less outstanding and obvious. 

So if the overall sound is a little to bright overall, I tend to turn it down. Usually I run it about 12:00. I do
that because I can't really hear what it's doing when I change the dial. I could use some help on that to. Maybe I'm approaching it all wrong. Maybe I need to go off to a Presence Seminar:>)

[sluckey]

Maybe Santa will bring you some presence.   

[Platefire]

Tell Santa I haven't been using the Presence I got. He can give it to those who's Presence goes up to 12

[bmccowan]

Platefire - I'm with you on the effect being so slight that I can hardly detect a change. I think that is partly because I do not play my amps very loud, and because I have tinnitus and other hearing issues. As part of the feedback loop it changes the frequency content of what is cancelled. One of the first amps I built was a Tweed Super and I kept checking to see if I had hooked it up wrong, as it seemed to be doing nothing. This Fender explanation is interesting https://www.fender.com/articles/tech-talk/be-in-the-moment-the-presence-control-explained
Although there are likely better explanations here on the Forum.

[Platefire]

bmccowan
Believe or not I just read that about an hour or so ago. I've been reading up on it a bit to tryand understand it better. According to that article when playing clean it enhances your upper-mids and high frequencies, not the volume of it. When you amp is driven into distortion it effects the response of the high notes in a "wild" raspy way.

I cranked up my Mesa Boogie Rocket 440 while ago and played around with the tone controls and the presence. I can tell a tad bit of response from the presence, but not much. If I was lacking highs, I would turn it up, but I'm not. So I set it low or about half up.

[HotBluePlates]

For reference, here are links to some of the comments on tweed Bassman tone as well as photos of original Bassman amps.

"... the very best ones I've heard were all 1960 models with P10Qs."

Photos of original tweed Bassman amps with original caps are fairly uncommon, because Yellow Astron caps are notorious for leaking DC.  However, here are a few I found (right-click image and "open in new tab" so see full-size):

1959 Bassman:  0.02µF, 0.1µF, 100kΩ resistor




1960 Bassman:  0.02µF, 0.1µF, 100kΩ resistor




1959 Bassman:  0.02µF, 0.1µF, 100kΩ resistor




1960 Bassman:  0.02µF, 0.1µF (judged by size); resistor value not visible




1958 Bassman:  0.02µF, 0.1µF, 100kΩ



1957 5F6 Bassman:  0.02µF, 0.02µF, 56kΩ




1957 5F6 Bassman:  0.02µF, 0.02µF, 56kΩ

[SILVERGUN]

I have found this to be one of the best sounding demos I have ever heard. It might have something to do with this guys playing, but just wow.

[Platefire]

HotBluesPlates

That is some fine Bassmans. The 57 5F6 for $19K is super clean and original but one of those raggedy ones would be fine with me:>). I haven't seen a true tweed Bassman in person since the 80's. I played at an outdoor event and heard another group playing and the lead guitar was using a tele through a 59 Bassman and it sounded Awesome. Before then I had no idea that a Bassman could put out such sound.

Every since that day I always thought there was something special about a tele pared with a tweedBassman and that video that SILVERGUN posted reinforces that again because that a tele sounds mightyfine going through that Bassman. I reckon part of that Bassman sound it having that 4/10 cab.

I do have a Mesa Boogie 4-10 cab with Jenson Special Design Alnico speakers. That may get me part of the way there hooked up to my Plexi or Dukane Bassman Heads:>)  Thanks! I enjoyed the tour. Platefire