Jtm 45 tone stack

[Snake5150]

Finally played the amp build which was a good feeling. That was quickly shattered by the fact it sounded like ass.

So the presence adjustment doesn’t do anything.

Bass kills audio when all the way turned up.

Middle adjustment is reversed

And the treble adjustment is very slight almost non existent.

 I have check the wiring per the layout and schematic and all seems correct. Ideas?

[shooter]

bypass the TS, use guitar tone pots, and/or EQ pedal, sound better??

[TitaniumValhalla]

What layout did you use to build it? You have what sounds like multiple wiring errors. I would print out a known good layout - not look at it on a computer or phone, physically print it - and then take a highlighter and compare every single thing crossing out as you go until you find the issues. You already know the mid pot is not wired correctly, so start by looking at that to see what you did wrong and work from there.

The presence control in that circuit is just a negative feedback control that is inserted in a totally different part of the circuit, so it would be a different issue if it is an issue. But just to set expectations, it is going to be subtle and really not very apparent to do much unless the amp is cranked. Even then it will be subtle. With your issues with the tone stack it might be working but you just can’t hear it yet, so fix that first.

[AlNewman]

Middle adjustment is reversed

That would be the 1st problem to rectify.  (should be pretty simple.)

Normally the treble has the most effect in a marshall/fender tonestack.  (mids maybe more on a fender TMB type variable resistor setup...)

The presence Is sometimes not very defined, based on capacitance and pot value, but if it isn't doing anything, something could be wrong, depending on the power section.

[Snake5150]

So I had some wiring reversed going to the middle and treble pot. That fix all the tone issues.

The presence control is still not functioning and the amp is very loud . Like can’t turn up to 2 or more

[SEL49]

The presence control is still not functioning and the amp is very loud . Like can’t turn up to 2 or more

Maybe the NFB phase is wrong. Swap the OT primary plate leads to verify correct phase.

[Snake5150]

The presence control is still not functioning and the amp is very loud . Like can’t turn up to 2 or more

Maybe the NFB phase is wrong. Swap the OT primary plate leads to verify correct phase.

I had a previous issue that lead me to believe flipping the leads was the solution. They are currently not per the layout and the previous issue was unrelated to the ot so I’ll re flip the ot leads and report back

[d95err]

So I had some wiring reversed going to the middle and treble pot. That fix all the tone issues.

The presence control is still not functioning and the amp is very loud . Like can’t turn up to 2 or more

Good to hear you found and fixed the main problems!

Regarding being loud - what did you expect? It's a non-master volume Marshall...

[pdf64]

A feedback loop decreases system gain if the loop is negative.
Whereas it increases gain if it's positive.
So to identify which it is put the loop on a switch, feed in a constant tone and check whether the system gain is higher or lower with the loop open and closed.

The JTM45/50 has a lot of negative feedback, so the difference between open and closed should be very noticeable.

With a non oscillating positive feedback loop, turning the presence control up would decrease the presence, make the amp a bit duller.

The symptom of the presence control doing nothing indicates that there's no feedback loop, or if there is, the presence control isn't connected to it.

[Willabe]

A feedback loop decreases system gain if the loop is negative.
Whereas it increases gain if it's negative.

[Snake5150]

A feedback loop decreases system gain if the loop is negative.
Whereas it increases gain if it's negative.
So to identify which it is put the loop on a switch, feed in a constant tone and check whether the system gain is higher or lower with the loop open and closed.

The JTM45/50 has a lot of negative feedback, so the difference between open and closed should be very noticeable.

With a non oscillating positive feedback loop, turning the presence control up would decrease the presence, make the amp a bit duller.

The symptom of the presence control doing nothing indicates that there's no feedback loop, or if there is, the presence control isn't connected to it.

Reversing the leads led to instant feed back.

I un reversed them. The volume thing isn’t as bad as I originally stated, having done some reading I realize it kinda nature of the beast. Literally the BEAST. This thing is louder than my 100w peavy. The only thing close is my 74 super reverb.

The presence still seems to do nothing.

Only thing I can think is the cap is bad.

I don’t have a selector switch, nfb from the resistor should go to tip on the output jack right?

I have it on 8 ohm, the Marshall schematic shows it in 16 ohm what the difference?

[AlNewman]

I have it on 8 ohm, the Marshall schematic shows it in 16 ohm what the difference?

The NFB?  That makes a significant difference.  If you're drawing from 8R in comparison to 16R on the output side, it would mean you have to adjust the voltage divider and probably capacitor of your NFB loop to accommodate.  Or just switch it to the 16R tap, and try that.

[Snake5150]

I have it on 8 ohm, the Marshall schematic shows it in 16 ohm what the difference?

The NFB?  That makes a significant difference.  If you're drawing from 8R in comparison to 16R on the output side, it would mean you have to adjust the voltage divider and probably capacitor of your NFB loop to accommodate.  Or just switch it to the 16R tap, and try that.

The layout I used has had a few minor things like this that were incorrect.

And nfb regardless of what ohm tap always goes to tip correct?

[AlNewman]

I don't see a layout or schematic in this thread, I just looked at the JTM45 schematic from this site, where it is drawn from the 16R tap on the output.  The NFB is what creates the howl if wired incorrectly, so if the NFB is drawn from the tip, and if incorrect, you'd have to reverse the plate wires at your power tubes. 

[Snake5150]

I don't see a layout or schematic in this thread, I just looked at the JTM45 schematic from this site, where it is drawn from the 16R tap on the output.  The NFB is what creates the howl if wired incorrectly, so if the NFB is drawn from the tip, and if incorrect, you'd have to reverse the plate wires at your power tubes.

Layout attached as built.

The ot legs are currently reversed of the layout.

So if I move the nfb, and the oscillation occurs that’s means that again the ot leads are reversed and that the nfb goes on the 16 ohm tap?

Whats the best course of action to avoid chasing my tail?

[Calboy]

Without an impedance switch the amp is not built as the layout shows.
How are the output jacks wired ?
How are the 8ohm and 16ohm taps wired?

[pdf64]

A feedback loop decreases system gain if the loop is negative.
Whereas it increases gain if it's negative.

Many thanks, I've now corrected it

[Snake5150]

Without an impedance switch the amp is not built as the layout shows.
How are the output jacks wired ?
How are the 8ohm and 16ohm taps wired?
Let's say the output jack(s) has only the 8ohm tap connected to the tip, then the nfb resistor should also be connected to the tip and a nfb resistor value of 47k-56k is more appropriate but is a matter of taste.

If the 16ohm tap is the only one connected to the output jack 27k is more appropriate.

The nfb coming from the 27k res goes to 8 ohm tip.

The other jack has the 16 ohm tap and both jacks get ot secondary common and ground.

[AlNewman]

The nfb coming from the 27k res goes to 8 ohm tip.

The other jack has the 16 ohm tap and both jacks get ot secondary common and ground.

I'm not sure I understand what you're saying here, but for testing, when using an 8R speaker at the 8R tap, unhook the 16R tap at the jack and hook it directly to the NFB loop. 

[Snake5150]

The nfb coming from the 27k res goes to 8 ohm tip.

The other jack has the 16 ohm tap and both jacks get ot secondary common and ground.

I'm not sure I understand what you're saying here, but for testing, when using an 8R speaker at the 8R tap, unhook the 16R tap at the jack and hook it directly to the NFB loop.

All the nfb stuff goes to the 8 ohm tap.
The other jack is the 16 ohm tap.
The common and ground go to both jack sleeves.

I’ll try connecting the 16 ohm tap to the nfb and see if it makes a difference.

[Calboy]

If the jacks are done the way shown in layout posted that is wrong, since no switch is used.

[Snake5150]

If the jacks are done the way shown in layout posted that is wrong, since no switch is used.
For the time being disconnect any wires connecting the 2 output jacks and disconnect the ground wire from one of them.


Basically, ground one output jack and connect 8ohm tap to the tip along with 47k nfb resistor.

The other output jack  is totally disconnected from circuit except the 16ohm tap connected to tip. Make sure this is not grounded.

The above assumes an 8ohm speaker/cabinet is being used.

If a 16ohm speaker /cabinet is used.
Connect 16ohm tap along with 27k to tip and make sure output jack is grounded.

Disconnect the 47k nfb from 8ohm tap and
Make sure it is not grounded.

Hopefully this "word map"makes sense.

I’ll remove the ground and common going to the 16 ohm jack. And leave the tip connected.

That will leave the 27k nfb and 8ohm tap to 8 ohm out tip and the common and ground to the 8 ohm jack only.

[AlNewman]

I'm not sure what you mean by common and ground.  At the secondary of your OT, common IS ground.
The 8 ohm tap should connect to your speaker, the 16 ohm tap should connect to your NFB.  The black wire from your OT secondary connects to the sleeve of the output jack.

[Snake5150]

I'm not sure what you mean by common and ground.  At the secondary of your OT, common IS ground.
The 8 ohm tap should connect to your speaker, the 16 ohm tap should connect to your NFB.  The black wire from your OT secondary connects to the sleeve of the output jack.

I was confused by the original layout and ran a ground direct from the common on the jack.

So it should be :
8 ohm tap to 8 ohm tip
Common to 8 ohm sleeve
Nfb to 16 ohm tip
Nothing else connect to 16 ohm sleeve