So why doesn't my presence control do anything?

[greengriff]

Amp is a stock 1977 2103. It sounds stellar - far more 3 dimensional and 'alive' than my friend's 2203x through any one of my cabs. The presence circuit appears to be wired correctly (although I didn't lift the board) to the 4ohm tap, just like the reissue. Presence pot measures 4.1k in circuit (I presume it's a 5k pot?) and sweeps cleanly down to 0k. However the presence knob does absolutely nothing. The amp is very quiet in terms of background noise and hiss (quieter than the reissue), but even with all the controls on 10, turning the presence knob makes no discernible difference to the tone of what little noise and hiss there is. I'm really curious as to why that might be, and so would welcome any input.

[Leo_Gnardo]

The cap in the presence circuit seldom goes bad - but it's worth a few minutes to check. Typically it's a 0.68 uF if I'm not mistaken. Try clipping or tack-soldering a similar value cap in parallel with the one in the amp. Anything from 0.22 to 2 uF should work. If you notice the presence control works with the extra cap added, then you know the one that's there is your culprit.

[Enzo]

The presence control is part of the feedback circuit. If you are just looking for a change in background sound, I wouldn't expect much. Crank a good strong guitar signal through it and see if it works. In other words, give it a good strong signal to feed back to itself.

[greengriff]

OK will do. It's a little late hear to be turning on a 100w Marshall but I'll try tomorrow. On my other amps though I can hear a change in the tone of the background hiss when turning the presence knob.

[Helmholtz]

The presence cap is 0.1µF, typically wired directly to the pot. And yes, background hiss should increase when tuning up presence as high frequency gain increases.

[greengriff]

So if it isn't working (or is having no apparent effect) but the components in the presence circuit are functioning correctly, could that suggest a problem with the phase inverter? Are there any simple tests that I can carry out with a multimeter?

[Helmholtz]

So if it isn't working (or is having no apparent effect) but the components in the presence circuit are functioning correctly, could that suggest a problem with the phase inverter? Are there any simple tests that I can carry out with a multimeter?

Step by step. Is the cap ok and correctly connected? Please post a closeup pic of the presence pot.
We've seen Marshalls where the presence circuit wasn't correctly wired.

[Leo_Gnardo]

The presence cap is 0.1µF, typically wired directly to the pot.

By coincidence today a 100W Marshall head of similar vintage appeared on my workbench, and the presence cap is indeed 0.1 uF . It's a rectangular red one rated 250V, possibly a WIMA. Thanks Helmholtz for the correction !

[Chuck H]

Agree that the presence control should make an audible difference in hiss and even some buzz background noise even with the amp turned up to clipping. Though it may not make a big difference in the actual guitar tone when clipping hard. Still, the point is that if you perceive no difference with pot adjustment there is likely something wrong. I don't know of any way the phase inverter would be responsible for that. The phase inverter is the circuit being 'fed back to' but if the amp is otherwise working correctly then it's doing it's job. Failure of the presence circuit will be within the feedback loop as Helmholtz implied.

OTOH you mention that the amp sounds good as it is. Tone is subjective and if an error exists in the circuit, but you like the amp, I might be reticent to correct it I think the problem should be identified to determine that there are no dangerous operating conditions. Otherwise I would leave the amp as you like it. If you don't use the presence control now (and it would seem you don't since it doesn't do anything) and the amp sounds better to you than other amps of it's type then why "fix" it to perform like those lesser sounding amps?

Then again... If the culprit can be positively identified you may have the option of making it switchable. But I've seen cases here where a guy "fixed" his amp only to chase the broken amps tone for ever after without satisfaction. Something to consider I think.

[greengriff]

[greengriff]

Here's a photo that I happen to already have that you can zoom in closer to see the presence pot wiring. If that's not good enough then I can open up the amp and take a true close up photo. As you look at the photo you can see the centre and left tags of the pot are connected to the cap which then goes to ground via the bus wire, and the right tag is connected to the resistor which then goes to ground via the bus wire.

[greengriff]

Agree that the presence control should make an audible difference in hiss and even some buzz background noise even with the amp turned up to clipping. Though it may not make a big difference in the actual guitar tone when clipping hard. Still, the point is that if you perceive no difference with pot adjustment there is likely something wrong. I don't know of any way the phase inverter would be responsible for that. The phase inverter is the circuit being 'fed back to' but if the amp is otherwise working correctly then it's doing it's job. Failure of the presence circuit will be within the feedback loop as Helmholtz implied.

OTOH you mention that the amp sounds good as it is. Tone is subjective and if an error exists in the circuit, but you like the amp, I might be reticent to correct it I think the problem should be identified to determine that there are no dangerous operating conditions. Otherwise I would leave the amp as you like it. If you don't use the presence control now (and it would seem you don't since it doesn't do anything) and the amp sounds better to you than other amps of it's type then why "fix" it to perform like those lesser sounding amps?

Then again... If the culprit can be positively identified you may have the option of making it switchable. But I've seen cases here where a guy "fixed" his amp only to chase the broken amps tone for ever after without satisfaction. Something to consider I think.

Yes you're quite right. I don't intend to change anything (unless it's dangerous, but if it is then it's survived 40+ years so far), but I do want to know why it is the way it is, and if that is making any contribution to the particularly outstanding tone of this amp.

[pdf64]

From the symptoms (eg livelier sound), my guess is that the ‘series‘ side of the feedback circuit is open, rather than the presence boosting aspect of the ‘shunt’ part. ie it’s running open loop, like an AC30, AC50.
So perhaps no feedback wire at the output impedance selector, 100k series resistor bad etc.

[greengriff]

From the symptoms (eg livelier sound), my guess is that the ‘series‘ side of the feedback circuit is open, rather than the presence boosting aspect of the ‘shunt’ part. ie it’s running open loop, like an AC30, AC50.
So perhaps no feedback wire at the output impedance selector, 100k series resistor bad etc.

Interesting. The wire is definitely connected to the impedance selector (it's on the 4 ohm tap) but it then disappears under the board out of sight and then emerges again to go to the presence pot. I guess my only option is to open up the amp and lift the board.

[Jon Snell]

The 2203 has the FB coming from the 8 Ohm tap, see attached schematic of 2203.
There is nothing much to go wrong, check the control is connected to the 100n at one end and the 4k7 at the other to make a variable resistor formation, check the 100n cap is intact and has a good ground. There is nothing else there.
The 2204 uses the 4 Ohm tapping but is the same circuit. Don't forget to check the 22n from the grid to the 4k7.