So why doesn't my presence control do anything?

[greengriff]

Hi Jon. American spec 2203s with 6550s use the 8ohm tap. UK spec amps with EL34s use the 4ohm. It looks like taking the amp apart is going to be the way forward.

[Jon Snell]

If the wires are connected and the feed resistor is in tact, there is no reason why it won't control the contour of the audio response, it is so simple, the way it works. Sometimes the simpler it is, the more baffling we get.
The bias circuit will be different as well if using 6550s and the output transformer anode load impedance.

[greengriff]

Could the resistor be broken? An idea what sort of voltage I should be getting at the presence pot with the amp idling?

[Helmholtz]

Could the resistor be broken? An idea what sort of voltage I should be getting at the presence pot with the amp idling?

Measure its resistance (power off!). Should be around 4.5k in circuit. You can use your Ohmmeter + schematic to verify connections. Good connection = 0 Ohm.

[greengriff]

It's 4.4k measured from the purple wire at the impedance selector to the purple wire at the presence pot.

[vintagekiki]

Rewire presence potentiometer pins.
Check and (or) replace the presence capacitor .1uF

[greengriff]

The solder joints to the pot are good, and the cap tests fine too! It's a mystery.

[Helmholtz]

Did you check the .022µ (22nF) cap and its connections as Jon Snell suggested? This cap provides the NFB path to the right/lower side PI grid input.

[vintagekiki]

There are no mystery in electronics.
What looks “good” it doesn’t have to mean it’s good.
What is the voltage on the R106 (4.7k) and does it change with the change of VR6 (25k).
It's not hard, re-solder VR6 pot joints and replace cap .1uF

1)
Presence pot is 25k lin. When sweep VR6 (25k), the resistance at point R106 (4.7k) does not change.
When measuring resistance disconnect the amp from the ac mains.

[greengriff]

The presence pot is a 5k on my amp, which is correct for the era I think. Anyway I've now found the problem - there was a tiny break in the leg of the .1uf cap. It had a blob of solder on it which made it hard to see (maybe it had been snipped and repaired previously during the amp's life? Can't think why else it would be there.) When I pressed the probes of my multimeter on there to test the cap it must have closed the gap and remade the circuit. However I accidentally knocked my finger into the cap when putting the nuts back onto the PCB and it moved! I resoldered it and now the presence control works.

I've not been able to play test the amp because it's late evening here and I like my neighbours, but turning the amp on there seems to be quite a bit more electrical hum (not hiss) with the master set beyond 5. I would swear it was quieter yesterday. Is this a likely result of the presence circuit being active again? What would the effect on the level of NFB have been with the cap disconnected, but the rest of the circuit still in place?

[vintagekiki]

Increased hum or hiss when the master set beyond 5 occurs due to the presence of control activity, because there is an increase in gain at higher (treble) frequency.


https://www.marshallforum.com/threads/what-is-a-presence-control.61427/

[greengriff]

Thanks! At least that reassures me that I haven't knocked anything. The hum is not gain noise though, it's more like power supply hum.

[Chuck H]

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.

[Helmholtz]

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

[Chuck H]

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

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.