Presence Control Not Working?

[FourT6and2]

I have a Hiwatt Super-Hi 50 here with a few odd issues. It's basically a 4-gain stage DR504. Very modern metal sounding. This one had two factory wiring errors that I caught: V1a cathode bypass should be 1K8/1uF. But it had a 10uF cap by mistake. And the tone stack treble cap should be 470pF but it was 4700pF. After fixing these two things, I also discovered the presence control isn't functioning. Turning the control does nothing. I've played this same model of amp in the past and the presence control worked well on that one and wasn't subtle at all. So I do know how it should sound.

I checked the pot and it measures fine on lug 1 to wiper and lug 3 to wiper. 100K linear.

I checked every other connection I could think of and everything seems to be wired right. I know the Marshall presence circuit well, but I'm not knowledgable about the Hiwatt version. It seems to utilize NFB in a different way. So any troubleshooting ideas would be appreciated.

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[martin manning]

I checked the pot and it measures fine on lug 1 to wiper and lug 3 to wiper. 100K linear.
I checked every other connection I could think of and everything seems to be wired right.

The pot wiper should be a constant 100 ohms to ground. Does that check?

[FourT6and2]

I checked the pot and it measures fine on lug 1 to wiper and lug 3 to wiper. 100K linear.
I checked every other connection I could think of and everything seems to be wired right.

The pot wiper should be a constant 100 ohms to ground. Does that check?

Yes, I read a constant 98-100 ohms from wiper to ground.

EDIT:
One other discrepancy I found is at the junction of the 100K NFB resistor and the typical 470R, there is a cap in series to ground after the 470R. In the other Hiwatt of this model I've seen, the cap is 0.01uF. In this one, it is 22uF. I'm not sure what this cap does. Could it affect the presence control functionality? It's the smaller blue electrolytic in the second photo above.

[cdemike]

That 10nF cap to ground in the typical Hiwatt NFB network functions as a low pass filter. Tracing the signal from the output transformer to the grid where NFB gets injected into the phase inverter signal, the 10k NFB resistor and the 10nF cap form a low-pass network with the 470R resistor limiting the effect (very similar to half of the 6G2 tone stack in my mind, if the comparison helps). The effect is basically the same as turning up the presence control to 9/10 on Marshall if you were to put the "wrong" presence resistor in where most of the high end feedback signal gets shunted to ground, preventing cancellation of those high frequencies in the phase inverter, and resulting in a brighter signal reaching the power tubes.

The Hiwatt presence circuit is pretty cool since it can function like the typical Tweed Fender/Marshall presence circuit allowing more highs through, but it can also dump highs from the signal coming into the phase inverter via the connection to V3 through the 1nF cap when you turn it all the way down.

I noticed in your amp there's a 100k resistor where there's commonly a 10k resistor. A resistor that large would dramatically reduce the amount of NFB signal from the OT. I couldn't find a complete layout of the amp, so I might be misinterpreting that, but I'm referring to the resistor farthest to the left on the single-row turret strip on the bottom of the second picture.

[cdemike]

One other discrepancy I found is at the junction of the 100K NFB resistor and the typical 470R, there is a cap in series to ground after the 470R. In the other Hiwatt of this model I've seen, the cap is 0.01uF. In this one, it is 22uF. I'm not sure what this cap does. Could it affect the presence control functionality? It's the smaller blue electrolytic in the second photo above.

The larger capacitor will shunt more NFB signal to ground, making the amp brighter and also reducing the total amount of NFB signal reaching the phase inverter. I think it's unlikely the culprit for a non-functional presence control (think that might be the 100k NFB resistor I suspect was substituted for a 10k). Pretty strange in my opinion to use a polarized electrolytic capacitor there too...

[FourT6and2]

One other discrepancy I found is at the junction of the 100K NFB resistor and the typical 470R, there is a cap in series to ground after the 470R. In the other Hiwatt of this model I've seen, the cap is 0.01uF. In this one, it is 22uF. I'm not sure what this cap does. Could it affect the presence control functionality? It's the smaller blue electrolytic in the second photo above.

The larger capacitor will shunt more NFB signal to ground, making the amp brighter and also reducing the total amount of NFB signal reaching the phase inverter. I think it's unlikely the culprit for a non-functional presence control (think that might be the 100k NFB resistor I suspect was substituted for a 10k). Pretty strange in my opinion to use a polarized electrolytic capacitor there too...

100K NFB on the 8ohm tap is normal for this amp. At least from the two I've seen. This particular amp, with the non-functioning presence control, is MUCH brighter than the other one. And that other one does have a 0.01uF film cap where this one has 22uF electrolytic. I do have a schematic for the amp that is like 95% accurate to the amp. And where this cap is, it does say 22uF. But it's crossed out and says 0.01uF next to it. So there have been a few revisions it seems. However that first amp predates this current one. So I'm not sure why this newer one would have 22uF here.

Then again, this one did have a wrong cathode bypass cap and wrong tone stack treble cap. So who knows what's up with Hiwatt's QC.

I have a 0.01uF cap on order. But that still doesn't explain why the presence isn't even working.

[cdemike]

I misread your initial post and thought you said 22nF, not 22uF -- my apologies. 22uF is huge and would pass basically any NFB signal containing audible frequencies to ground (read: make the amp act like there was no NFB signal reaching it at all except for the tiny amount retained by the limiting 470R resistor). I haven't been able to find a schematic for a Super-Hi 50, but assuming the topology is the same as the typical Hiwatt arrangement but with value tweaks like the 10k NFB resistor being bumped to 100k, that would almost certainly be the culprit. You could test it by removing the capacitor altogether to see if that restores the presence control's authority.

[FourT6and2]

I misread your initial post and thought you said 22nF, not 22uF -- my apologies. 22uF is huge and would pass basically any NFB signal containing audible frequencies to ground (read: make the amp act like there was no NFB signal reaching it at all except for the tiny amount retained by the limiting 470R resistor). I haven't been able to find a schematic for a Super-Hi 50, but assuming the topology is the same as the typical Hiwatt arrangement but with value tweaks like the 10k NFB resistor being bumped to 100k, that would almost certainly be the culprit. You could test it by removing the capacitor altogether to see if that restores the presence control's authority.

Yeah, this cap is the only thing I can think of that might be causing it. Here's the schematic. Though I don't think it's 100% correct.

[cdemike]

Ok yeah that schematic shows a typical Hiwatt NFB setup but with the NFB resistor raised to 100k and the switch from the 16 ohm tap to the 8 ohm tap. 22uF at C9 would dump nearly all of the NFB signal before it made its way back to the phase inverter, so reducing that cap's value or eliminating it would restore the presence control's function assuming the remainder of the circuit is wired correctly.

[FourT6and2]

Ok yeah that schematic shows a typical Hiwatt NFB setup but with the NFB resistor raised to 100k and the switch from the 16 ohm tap to the 8 ohm tap. 22uF at C9 would dump nearly all of the NFB signal before it made its way back to the phase inverter, so reducing that cap's value or eliminating it would restore the presence control's function assuming the remainder of the circuit is wired correctly.

Fantastic. I have a cap on order. Unfortunately, I'm out of town from tomorrow until like mid April haha. I won't be able to actually get this solved until then

[FourT6and2]

So Hiwatt did confirm this cap should indeed be 0.01uF and not 22uF. I just did the swap and flipped the amp out of standby. Instant low-pitch horn/oscillation. Temporarily clipped the original 22uF cap back in and no noise. So that cap is definitely doing something to block some inherent oscillation in the circuit. But it's also disabling the presence control. I'm not sure what else to do here. Hiwatt isn't being helpful at all.

I'll also mention that replacing the 4700pF tone stack cap with the seemingly correct 470pF yields a very thin and harsh sound. It just sounds like this amp is wired wrong somewhere but I can't seem to figure this one out.

[FourT6and2]

OK I think I figured it out.

Hiwatt swapped the OT primaries. And that 22uF cap was providing positive feedback so the amp still worked?

[martin manning]

More like the 22u was large enough to dump essentially all frequencies to ground, eliminating the positive feedback. The smaller cap only eliminated the higher frequencies, so you had a low frequency oscillation.

[FourT6and2]

More like the 22u was large enough to dump essentially all frequencies to ground, eliminating the positive feedback. The smaller cap only eliminated the higher frequencies, so you had a low frequency oscillation.

Yes, thank you for the clarification!

I sent photos to Hiwatt and they told me it was impossible that the OT primaries were reversed. But I swapped them and yup... no more oscillation.

Amp still sounds quite thin and harsh though. Something else is also wired wrong because the tone stack cap should be 470pF but it sounds much too thin and harsh like that. There's a reason they stuck a 4700pF in there...

[FourT6and2]

Fixed it!

Bad cap in tone stack was causing thin/harsh sound. 470pF is the way to go. The 0.01uF NFB/low-pass works now that OT primaries swapped around. I did learn that Hiwatt does plan to implement the 22uF on a switch for future builds, as an option for people looking for more low end. That value does sound good now that the OT is wired properly, but it does disable the presence control.