So why doesn't my presence control do anything?

[Chuck H]

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.

I think there may be something unique about your amp. Which my also account for some of it's difference in tone. It could be the lead dress, overall component tolerance variance, something a little different from the factory than usual or some combination of these things. It sounds like you may be hearing phase anomalies when adjusting the control or switching the standby. This isn't unheard of but it's not usually so much of an annoyance. If the amp is operating fine just playing through it I don't believe there's a problem, other than the inconvenience of having to remember to turn down the presence control when switching to standby.

[vintagekiki]

And thank you for the reference schematics.

I am still at your disposal.

https://drtube.com/en/library/schematics/69-marshall-schemas
Marshall Schematics

[Chuck H]

That is an excellent resource that I've used before. Don't know how I missed it, but we can always count on you for schematics.

[vintagekiki]

Thank you very much.

[greengriff]

I think there may be something unique about your amp. Which my also account for some of it's difference in tone. It could be the lead dress, overall component tolerance variance, something a little different from the factory than usual or some combination of these things. It sounds like you may be hearing phase anomalies when adjusting the control or switching the standby. This isn't unheard of but it's not usually so much of an annoyance. If the amp is operating fine just playing through it I don't believe there's a problem, other than the inconvenience of having to remember to turn down the presence control when switching to standby.

I obviously don't know that, but I can say that with the stock 1977 era presence circuit reinstalled (broken capacitor leg soldered back) the amp sounded almost identical to the reissue. If you weren't hearing them directly back to back you wouldn't have known there was a difference. Yet switching to the 'scratchy' circuit did change the overall tone slightly, regardless of the position of the presence knob. Now how much of that was to do with the scratchy circuit and how much was to do with other factors is impossible for me to say, and without the other amp there as a comparator I might have been convinced that the difference was in my head. Of course it could be that the change in the effect of the presence control (it's a much more powerful tone shaping tool with the 'scratchy' circuit), and it's more aggressive sweep means it now harder to compare to the more gentle sweep of the reissue. Either way, I'm very, very happy with the result, so thank you!

Out of curiosity I disconnected NFB completely on the reissue by desoldering the purple wire from the 4 ohm tag on the impedance selector. The change was quite noticeable - the amp became looser, spikier, and believe it or not, even louder. It was an interesting tone, but again it was audibly not quite the same thing as the scratchy circuit on the 77 amp. Stupidly I returned it back to stock without trying a clean tone on the low sensitivity input. I bet it would have been lovely. Nevertheless it's an interesting change in tone and one that might be useful sometimes. If the reissue was my amp I'd be interested in making it a switchable option (maybe by sacrificing one of the speaker output jacks to use the hole for a switch).

As one final test I would like to modify the reissue to the 'scratchy' presence circuit. If anyone is curious, I got the schematic from here:
https://www.thetubestore.com/guitar-amp-schematics

There's loads of different manufacturer's circuit diagrams on the site.

Anyway, I've (hopefully) attached the pdf to this post. It looks like to modify the reissue presence circuit to the 'scratchy' spec, one would jumper C12, snip and lift R18, change VR8 to 5k, add a .1uf cap between the centre tag of VR8 and ground, and ground the remaining tag. Can someone confirm?

[Chuck H]

FWIW the older style presence circuit is electronically the same as the stock circuit when adjusted to 10 on the knob. So the perception that the amp sounds different even at 10 with each circuit is actually just perception. Probably due to the more dramatic sweep of the older circuit. The stock circuit for your amp basically adjusts the presence from 5 to 10. The older type circuit actually adjusts it from 0 to 10. So it's a dramatic difference in range. But at ten they are both essentially the same.

[Helmholtz]

and believe it or not, even louder

No surprise. As NFB reduces power stage gain, removing it must increase gain. Gain meaning amplification factor. Available output power will not increase, though.

[greengriff]

FWIW the older style presence circuit is electronically the same as the stock circuit when adjusted to 10 on the knob. So the perception that the amp sounds different even at 10 with each circuit is actually just perception. Probably due to the more dramatic sweep of the older circuit. The stock circuit for your amp basically adjusts the presence from 5 to 10. The older type circuit actually adjusts it from 0 to 10. So it's a dramatic difference in range. But at ten they are both essentially the same.

Quite possibly. I'd certainly like to change the presence circuit in the reissue amp to the 'scratchy' circuit for a final test. However I couldn't be more pleased with the result on my 77, so thank you all again!

[Chuck H]

Well... Since there's always room for improvement

On the hunch that the odd noise when switching to standby is a phase anomaly you might be able to minimize it by adding a 22pf cap across (parallel with) the 100k feedback resistor (R20). I don't think this would make an audible difference to the amps tone and it may keep the extreme HF phase more stable when the standby switch is flipped. If you notice a tonal change or it doesn't help the issue you can always remove it.

[greengriff]

It's a loud 'pop'. An easy workaround would be to turn the master volume back to zero before switching to standby - something that I used to do on old amps years ago because that was the way I was shown to switch a valve amp off. I can't remember why, as it was many many years ago, but maybe these switching noises were the reason?

[Chuck H]

It's a loud 'pop'. An easy workaround would be to turn the master volume back to zero before switching to standby - something that I used to do on old amps years ago because that was the way I was shown to switch a valve amp off. I can't remember why, as it was many many years ago, but maybe these switching noises were the reason?

So you're saying there's a POP on the standby switch, but only when the presence is adjusted above zero?

[greengriff]

So you're saying there's a POP on the standby switch, but only when the presence is adjusted above zero?

Yes, exactly right.

[Chuck H]

Huh? I gots nuthin'

Could still be phase thing, maybe? Are the noises when adjusting the presence control a scratchy/static sound?

Have you blown out the pot with compressed air? Or used any pot cleaner? (do blow out pots when possible before spraying pot cleaner btw).

[greengriff]

Yeah, the noises when turning the presence knob are static/scratchy noises. I've previously attempted to clean all the pots with contact cleaner. I've also got some cans of compressed air for cleaning computers. I'll give them a going over with one of those at some point. Any thoughts on what the popping is? I would swear than in the past I've had amps that popped loudly when being switched off, I just can't recall what they were.

[Chuck H]

It's not uncommon for standby switches to pop. Though it's usually not very loud. I've never diagnosed why though because I don't do repairs and I haven't had a problem with it in my builds. Though I think it's obvious that cutting the HV causes a voltage shift and if that voltage shift finds a path to ground that bleeds onto the signal chain it could be seen as an abrupt AC swing. What seems odd to me is that in your case it's associated with the presence control setting and nothing about the ground point for that circuit was changed. Hopefully someone else has a theory that can be investigated.