So why doesn't my presence control do anything?

[vintagekiki]

If the value for the presence pot is 5k (older era Marshall amps), there is still a minimal increase in treble frequency, even though the pot of presence is zero.
At the newer Marshall amps, the value of the presence pot is 25k, the increase of the treble frequency if the presence pot at zero is practically not noticed.

[greengriff]

I'm reticent to mention, but... If the .1uf cap on the pot were broken that would mean no presence and a duller sounding amp compared to another with the presence pot adjusted above zero. All things being equal otherwise. So your extra clarity and chime are coming from something other than the presence circuit and the broken cap making the control non functional made no tonal difference to the amp (other than no presence control function). As things are described you should be able to maintain your old tone (and background noise level) by simply leaving the presence pot on zero.

That's very interesting. I shall report back once I've had a chance to play test the amp at a decent volume.

[greengriff]

With this circuit there will be some presence effect even with the pot on "zero" (actually max resistance).

So that means that just reconnecting the cap cannot account for the increase in electronic background hum that I can hear now?

[greengriff]

Ah, right. I was thinking of the older style presence circuit where the pot was the feedback shunt resistor.

EDIT: So, just thinking out loud now, why couldn't the presence circuit in this amp be wired like the old style so that the original 'busted amp' tone could be preserved just by turning the control down? That would allow the OP to keep the tone he's used to without disabling the presence control or adding a switch. Makes sense to me.

I don't want to rewire an unmolested vintage amp, but I was thinking that if I didn't like the change in tone then I would just desolder the repair to put it back to how it was. Surely though, with no signal going through the cap to ground, the 'broken' setting was just the same as having the presence all the way off?

[Dave H]

Surely though, with no signal going through the cap to ground, the 'broken' setting was just the same as having the presence all the way off?

It's not the same. With the presence pot all the way off its resistance is still 5k so you have 5k in series with the 0.1 cap to ground not an open circuit as with the 'broken' setting. See post #31

[Helmholtz]

So that means that just reconnecting the cap cannot account for the increase in electronic background hum that I can hear now?

Reconnecting the cap should boost mid and hi frequencies by up to almost 6dB at min presence setting.

[Chuck H]

What Dave and Helmholtz said^^^^^^^^^^^^^^^^^^^^^^^^^^^

Rewiring the presence circuit need not be molesting the circuit. It's a pretty minor change and no less of a molestation than making it intentionally broken. See the diagram below. I used the Ceriatone layout to show the changes. Top would be the stock circuit and below would be the modified circuit. You would basically just lift the ground connection for the 4.7k resistor, move the cap connection to the center lug of the pot and ground the left lug of the pot. leave the 4.7k resistor hanging there just so the amp can be made stock again if you ever wanted to do that. This would be the older circuit and would allow you to effectively remove the .1uf cap from the circuit with the presence set to zero.

[greengriff]

Thank you! Yes indeed that is a simple change. Time to have a play test then get my soldering iron out, as this seems to be a 'no lose' change. I'm also now curious about what the amp might sound like with no NFB at all (a la Mesa's Extreme Mode). I presume there's no harm in simply lifting the purple wire from the speaker impedance selector in order to try it out?

[Chuck H]

... seems to be a 'no lose' change.

Well, maybe not entirely. This is supposed to be the "scratchy" circuit. Where the pot makes static noises when being adjusted. Though in my experience none of my builds ever have that problem. And I don't suppose it's a problem unless you're turning the knob in the middle of a recording

[greengriff]

Well I've just been playing it with the presence circuit reconnected, back to back with the reissue, and it (repairing the presence control) has bought the tone much, much closer to that of the reissue. Harder and less 'bouncy'. I'm going to have a cup of tea and make Chuck H's suggested change and we'll see what that does.

[vintagekiki]

If you are not sure what you are doing, the advice is not to experiment.

When disconnecting the NFB wire from the speaker selector, work carefully so that the amplifier does not remain without speakers.
Disconnected the wire that goes from the PCB (NFB resistor R20 100k) to the speaker selector terminal 4 Ohm.
Attention. Speaker selector terminal 4 Ohm has 2 wires on it (from OT and for PCB)
With the disconnected NFB the amplifier will work "wild" like the old-timers Vox from vintage AC series.

1) Do not disconect R106 (4.7k), it is a cathode resistor for a phase splitter.

[Chuck H]

1) Do not disconect R106 (4.7k), it is a cathode resistor for a phase splitter.

It is not a cathode resistor. In fact it's below the tail resistor in the ground path. It's a shunt resistor for the NFB circuit. But it's true enough that simply removing it would be the wrong way to disable the NFB circuit and the PI wouldn't function. But in this case things are not as simple as this. I just demonstrated a working scenario where the 4.7k resistor is lifted and then in your following post you said "Do not" do this...

I'm getting a little tired of you blithely posting things that undermine my posts without looking at thread history. In the modification I posted above the resistance across the 5k pot outer lugs replaces the 4.7k resistor.

And where are you getting these R numbers. They don't correspond to any schematic I can find for a 1977 2203.

[greengriff]

OK, so this is turning from a simple repair thread into something much more interesting: I carried out Chuck's suggested mod and here's what happened:

1. The amp's greater dynamic response is back, which is great. Strangely though it's not just with the presence turned fully off. Even sweeping through the range playing back to back with the reissue doing the same thing there's a little bit more depth and 'give' rather than the 'hardness' that was there before with the stock presence circuit restored.

2. The presence works more strongly than it did in the stock circuit - the amp just about slices the top off your head with the presence fully up.

3. Turning the standby switch off with the presence control set above zero make a very loud noise. Set on zero it is silent. It was also silent with the stock circuit.

4. There is substantial noise when turning the presence control with the amp on. It's as loud as playing a note on the guitar.

5. Once, and once only, the noise stayed when I'd stopped turning the presence knob. It was the same sort of sound I've had before when a valve is failing, but turning the presence control down and back up again made it go away. Some dirt in the pot maybe????

I know further back up the thread someone said that with the final version of the wiring (25k pot) turning the presence down to zero makes it out of circuit. My ears would say that's not true, as even with the presence on 0 the reissue does not have the same dynamic response. Whilst I would have previously assumed that something else was the cause, playing around today, and hearing how my old amp changed to sound like the reissue and then back again with just a few small changes, has made me think that the 'hardness' of the reissue's tone is down to the way the presence circuit is wired.

[vintagekiki]

I'm getting a little tired of you blithely posting things that undermine my posts without looking at thread history.

If your vanity is hurt, sorry. The ignore button exists.
If I may notice, I haven't quote any your post anywhere.

In the modification I posted above the resistance across the 5k pot outer lugs replaces the 4.7k resistor.

When replace the 4.7k resistor with 5k pot, the presence control works the same or similar as before the modification, except that the cathode current flows unnecessarily through the 5k pot, which over time can produce unwanted sound sensations when use the presence control.

And where are you getting these R numbers. They don't correspond to any schematic I can find for a 1977 2203.

JMP Lead Series Standard and Master Volume, 50W/100W
https://drtube.com/schematics/marshall/78331-3.gif
https://drtube.com/schematics/marshall/78331-2.gif
https://drtube.com/schematics/marshall/78331-1.gif

[Chuck H]

You have a hell of a way about you. Ok. I'm going to do this once. Then I'll let it go beecause I don't want to turn the thread.

If your vanity is hurt, sorry. The ignore button exists.

What does vanity have to do with it? I find it distasteful that jumping to personal accusations of a personality flaw is your first reaction. It's a matter of respect for other members. I'm not the only one you step on now and then. It's a matter of paying attention to others. Like a decent person. And ignoring you isn't my intent. You're valuable to the forum and I'm a moderator. So it's impractical.

When replace the 4.7k resistor with 5k pot, the presence control works the same or similar as before the modification, except that the cathode current flows unnecessarily through the 5k pot, which over time can produce unwanted sound sensations when use the presence control.

Wrong. The 2203 circuit using the 5k pot (as the OP has indicated is the case here) has the presence control permanently set about halfway up compared to the older 'scratchy pot' circuit. The only way to eliminate the effect of the bypass cap is to effectively remove it from the circuit. Which the older style presence circuit does. So if that is the goal, and intuitive adjustment of the control is desired, the older type circuit is the simplest and most eloquent solution. Therefore, if the stated aspects are desired, running the PI tail through the pot is not unnecessary. I agree about the unwanted sound sensations. But this has been manageable for classic amp users so far.

And thank you for the reference schematics.