Can't Hear Presence Control. '73 Circuit Help (Really Probably Solved This TIme!)

[dtier]

It is possible still that your 100k is open and your just seeing the 4k7 and output transformer. Its hard to test without opening the circuit at any point. Then everything should read accurately.

 

[XTRXTR]

I believe your chain was being yanked.
Due to the extremely low resistance of the secondary winding what you are actually measuring is the parallel resistance of the 100k//4.7k ~ roughly 4.4k to common. The cap at the presence pot and the cap at the grids block DC so the only path is the winding to common and the 4.7k R to common. I would be curious if the same value would be across the 4.7k resistor?

 

[Mjh36]

Yes, and while you have your meter out check around the entire feedback circuit including the chassis ground return for cold solder joints. If the feedback circuit is open the presence control will be unnoticeable and your amp will sound looser and more distorted. Since your not sure if it initially worked when you got it, you may have not heard your amp with feedback yet!

Dave

Your reading the 100k resistor and everything that is in parallel with it, which is the 4k7+the output transformer in series. That actually sounds right and somewhat verifies the feedback circuit is intact.

Thanks and I will double check connectivity today. Everything beeped at me last night so I think it's ok.

I'll also probably try it as a stock non-master today and see what that does.

And I'm right that assuming the core nfb/presence circuit I hightlighted in blue on the schematic earlier, this is the area of focus? If presence is affected it's along that route and nowhere else? Just making sure I'm not missing something...

 

[Mjh36]

I believe your chain was being yanked.
Due to the extremely low resistance of the secondary winding what you are actually measuring is the parallel resistance of the 100k//4.7k ~ roughly 4.4k to common. The cap at the presence pot and the cap at the grids block DC so the only path is the winding to common and the 4.7k R to common. I would be curious if the same value would be across the 4.7k resistor?

I just measured across the 4k7 on the pot and it's the same = 4.3k

I sure am learning a lot! My head might explode today we'll see...

I was trying not to desolder factory joints unless necessary, but I could disconnect a purple wire to the presence pot, just take the nfb circuit out. Either will hear a difference or not.

 

[Mjh36]

I thought that everyone with the LarMar mod complained that the presence pot doesn’t work as it should.

Yeah that's what I heard too, but I didn't think it would be this subtle. I am doing these tests with the master on 10 which from what I've read that should be close enough to stock that the presence should be working to a reasonable level.

 

[XTRXTR]

I was trying not to desolder factory joints unless necessary, but I could disconnect a purple wire to the presence pot, just take the nfb circuit out. Either will hear a difference or not.

There is no need other than curiousity to do that. You have proven the NFB is in circuit with that 100k and 4.7k measurement.

But if you do, be prepared for squealing feedback or howling.

As @Jon Snell has suggested the only thing to do is to remove the LarMar mod.

 

[XTRXTR]

Try a Type 3 Trainwreck PPIMV and see if the same thing happens.

 

[Mjh36]

There is no need other than curiousity to do that. You have proven the NFB is in circuit with that 100k and 4.7k measurement.

But if you do, be prepared for squealing feedback or howling.

As @Jon Snell has suggested the only thing to do is to remove the LarMar mod.

Try a Type 3 Trainwreck PPIMV and see if the same thing happens.

I'll get on that here this afternoon. I'll try stock first as I don't have a 1M pot. I have to make a Valvestorm order.

So far I'm mostly a pedal builder, I have 1M potentiometers for pedals but those are rated at 0.1 watts. Or I have the dual 100k and 250k pots that I've used with this amp. Need to stock up on amp parts.

 

[Pete Farrington]

Type 3 master volume is fine for stage use, taking the overdriven volume down a few dB.
But not good / useless for low volume use; they can’t even achieve a -20dB reduction in signal level before the audio mutes.
The mute isn’t due to there being zero signal, but rather to the LTP outputs flipping to being common mode when the resistance between them drops much below about 10k.

 

[XTRXTR]

I remember it differently on my 2204.

The 2204 has the preamp overdrive though so maybe I was able to achieve some control there. I did not use the amp on the stage. In my case I had an oscillation issue that caused a harsh 3rd order harmonic as I went higher in volume and my ears fatigued quickly. This is why I removed it and then hunted down the oscillation. I just haven't tried it since.

 

[Gblev]

Yep, if your added Master volume is a post phase inverter master volume (PPIMV), then it will render your Presence control useless until you get it up about halfway on the dial. Even then, you won't get much response out of it until it's up all the way.

 

[Mjh36]

Yep, if your added Master volume is a post phase inverter master volume (PPIMV), then it will render your Presence control useless until you get it up about halfway on the dial. Even then, you won't get much response out of it until it's up all the way.

Yeah that's my understanding as well, so I'm doing all these tests with the master on 10.

I'm soldering in the bias resistors right now to go non-MV, but I'm unsure if I'll here a difference if I couldn't hear presence with the ppmiv dimed.

 

[Mjh36]

Ok I went non-master, 220k NOS Piher resistors, and I still can't here any presence shift I don't think. Sounded the same as the master cranked. Probably put the LarMar back in eventually.

Dumb question but to be clear, presence should get brighter as you turn it up, correct?

Because if I'm quiet I can hear the background hiss slightly change with the turning of the presence pot. But it's almost clearer hiss at 0, darker/muted hiss at 10. Very hard to hear but there is that.

I don't know if it's eyes and ears playing tricks, but I almost wanted to say there could be the slightest shift in presence, but darker at 10, and brighter at 0. But take that with a grain of salt I think my ears just want to hear something as I'm turning the knob. I'm not sure. If it's there it's an incredibly small of a change

 

[Mjh36]

Here's my half permanent non-MV. Everything is completely disconnected from the pot. Sounds good just don't hear any presence shift.

 

[FleshOnGear]

@Mjh36, this thread is getting long, so my apologies if this has been covered. Have you measured the resistance of the presence pot? Try turning the knob fully CCW (to zero) and measure the resistance between the CW and CCW tabs of the pot. I’m curious.

 

[Mjh36]

@Mjh36, this thread is getting long, so my apologies if this has been covered. Have you measured the resistance of the presence pot? Try turning the knob fully CCW (to zero) and measure the resistance between the CW and CCW tabs of the pot. I’m curious.

Thanks for diving into this with me. I feel like I'm in a rabbit hole for sure.

Amp off, I get a pot resistance reading of:

7 ohms presence at 10
5.8k ohms presence at 0

I can't tell if mine is 5k/.68uf or 25k/.68uf, I can't read any of the print it's on the other side. But it is wired like Mark II presence with the wiper and ccw connected.

Does it make a difference that those two tabs are connected for this reading?
Is that an expected value to get?
Is there a way to verify the capacitor while in circuit?
What would be the result of a non-functional capacitor here? Would there be tonal shift with the dial still?

 

[dtier]

Thanks for diving into this with me. I feel like I'm in a rabbit hole for sure.

Amp off, I get a pot resistance reading of:

7 ohms presence at 10
5.8k ohms presence at 0

I can't tell if mine is 5k/.68uf or 25k/.68uf, I can't read any of the print it's on the other side. But it is wired like Mark II presence with the wiper and ccw connected.

Does it make a difference that those two tabs are connected for this reading?
Is that an expected value to get?
Is there a way to verify the capacitor while in circuit?
What would be the result of a non-functional capacitor here? Would there be tonal shift with the dial still?

View attachment 131541

Your readings on the pres pot look normal for a 5k pot. You are right that if the cap failed the presence pot would have no effect. Its not likely but if it is failed open you can put a known good cap across it and it would work. If it failed shorted it will read 0 ohms with your meter.

I have LARMAR type PPI's on three 70s heads and the pres controls are all noticeable even at lower volumes.

 

[Mjh36]

Your readings on the pres pot look normal for a 5k pot. You are right that if the cap failed the presence pot would have no effect. Its not likely but if it is failed open you can put a known good cap across it and it would work. If it failed shorted it will read 0 ohms with your meter.

I have LARMAR type PPI's on three 70s heads and the pres controls are all noticeable even at lower volumes.

Thank you very much. I like the LarMar I'm putting it back in as we speak. Having attentuation and master volume is a good combo I feel.

Resistance across capacitor is:

4.34k presence at 10
10.2k presence at 0

Trying to read capacitance as-is in circuit, I just get 0.00uf with flashing zeroes? And my other meter says overload. Is it because it's in circuit?

 

[Mjh36]

It is possible still that your 100k is open and your just seeing the 4k7 and output transformer. Its hard to test without opening the circuit at any point. Then everything should read accurately.

Well I detached the purple wire leading from the nfb resistor to the presence pot. Hooked it up to a couple ghetto pot arrangements, a small C5k and B10k for pedals and one half of the dual gang 100k I have.

The nfb resistor reads 100k out of circuit. I did get the howl when disconnected.

Hooked up to the small .1watt B10k and Synergy .68uf, I do get an audible shift in tone. It isn't particularly the best sounding presence sweep, kind gets a little wild and ratty at 0, but something is happening that I can hear.

I'll revisit this in the morning when I can play loud and compare it to the stock presence again.

 

[mickeydg5]

I believe your chain was being yanked.
Due to the extremely low resistance of the secondary winding what you are actually measuring is the parallel resistance of the 100k//4.7k ~ roughly 4.4k to common. The cap at the presence pot and the cap at the grids block DC so the only path is the winding to common and the 4.7k R to common. I would be curious if the same value would be across the 4.7k resistor?

No. Reading resistance there as mentioned by @dtier earlier places the 100k in parallel with everything in the presence circuit which should give a lower value than the 4.7k resistor, something lower than about 4.4k in measurement. It was just a quick test to see if a proper value came up.

@Mjh36
Honestly the problem with the circuit which is what I thought from the get go is that the .68uF capacitor is far too large for the 5k potentiometer value.
I just wanted to see if all was intact by performing that quick feedback resistor resistance measurement in circuit.
You should replace that .68uF with a .1uF value which will give you about eight times more useable range for that circuit.
That big ole capacitor is passing too much frequency even when the PRESENCE is to zero.

I have no idea why Marshall people used a .68uF right there. It makes no sense and that value was quickly dropped from their lineup.