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

[Mjh36]

Sorry that your query was kinda overlooked.
The feedback network is a potential divider. The input to the potential divider is a signal from the OT secondary, eg at the 4 ohm tap.
The feedback network consists of a series resistor, 100k in this case, and a shunt resistor, 4k7.
In parallel with the shunt is the presence cap and the variable resistance of the presence pot.
The output of the feedback network goes, via a 0.1uF coupling cap, to the grid of the triode of long tail pair that’s operating in common grid mode.
The signal at that grid is the (main) negative feedback signal.
If the negative feedback signal is 0, eg presence on full and the test frequency is high, the gain of the power amp will be its open loop gain.
As the presence control setting is reduced, as the frequency is lowered, the power amp gain will reduced to its closed loop level, ie we can ignore any further shunt effect across the 4k7 feedback shunt resistor due to the presence cap / control.

With respect to the closed loop gain, with presence on max, the 3dB lower corner frequency of the presence boost will be roughly about the frequency at which the presence cap reactive impedance = 4k7.

So about 340Hz for a 100nF cap, 50Hz for 680nF.

See the simulations for further detail.

Thanks Pete, I have a lot to learn. I can't view those simulations on ampgarage, I've been waiting on registration account approval but it hasn't happened.

 

[Mjh36]

I think the .68uF makes for a much darker circuit in this case especially with the feedback resistor at 100k blocking a lot of the feedback signal while the connection to the 4-ohm tap of the output transformer provides the least amount of feedback.

Is it possible that the 100k and 4 ohm tap are causing the problem? Just the values chosen. I'm still getting not much response from any presence combinations.

 

[FourT6and2]

Sorry for the delay on getting back to some results. Got the parts in and well... I don't know what to say but I'm hearing a whole lot of "not much" with any combination of presence circuits.
I rigged it up to try a each pot with both caps on a switch. Both old school with DC on the pot and the other way with the shunt resistor.

No shunt, old school way
5k / .1uF
5k / .68uF
25k / .1uF
25k / .68uF

With shunt and pot going in series to capacitor
5k / .1uF
5k / .68uF
25k / .1uF
25k/ .68uF
Some main points I took:

I hear almost no difference in switching the caps. Something is happening but not much. Blindfolded I couldn't tell what pot/cap arrangement I'm using.
Never heard any scratchy DC when turning the pots when wired to do so.
Hardly any change in tone from 0 to 10.
In fact, the background hiss is slightly louder at 0. Adding to that, I would say there's almost a touch more "hairiness" when the presence is at 0.
When using the 25k / no shunt, there's loud squeal and oscillation when turning it down towards 0 on the knob.

I would hit chords, then go and turn the pot and hit the capacitor switch. Tried all combinations. Low master volume, High master volume. With and without an attenuator. I don't know what to say.
This was my setup, I triple checked all wiring. Just really nothing, besides the initial "boost" of connecting the NFB in general. No dark to bright - 0 to 10 - at all.

Could there be anything else at all circuit-wise that's impeding the presence circuit from working correctly? I circled an area in the schematic, the phase inverter stuff that's connected.

View attachment 132010 View attachment 132011

It's hard to offer any advice without being able to see exactly how you've wired everything up. But the presence control should have a drastic effect, in any of the configurations, if you've wired it properly. Here are my best guesses:

1. You could have made a mistake in your wiring. I would literally remove the stock presence control and replace it with a dedicated pot, cap, and resistor/no resistor instead of using alligator clips and switches. Keep it simple to eliminate unknowns. It shouldn't take more than 5 minutes to do it.
2. The PPIMV could be affecting things because it might not be wired correctly. You might want to remove it completely and put the amp back to stock.
3. How do you have the EQ set during these tests?

 

[FourT6and2]

Is it possible that the 100k and 4 ohm tap are causing the problem? Just the values chosen. I'm still getting not much response from any presence combinations.

Not likely. Seems weird that none of the presence arrangements yield any results. Are you sure you have it wired properly? Can you record an audio clip of you playing while turning the presence control? Remove all your alligator clips and play the amp as it is so we can hear it.

 

[Mjh36]

It's hard to offer any advice without being able to see exactly how you've wired everything up. But the presence control should have a drastic effect, in any of the configurations, if you've wired it properly. Here are my best guesses:

1. You could have made a mistake in your wiring. I would literally remove the stock presence control and replace it with a dedicated pot, cap, and resistor/no resistor instead of using alligator clips and switches. Keep it simple to eliminate unknowns. It shouldn't take more than 5 minutes to do it.
2. The PPIMV could be affecting things because it might not be wired correctly. You might want to remove it completely and put the amp back to stock.
3. How do you have the EQ set during these tests?

Hey thanks I hear ya. I checked and checked on the wiring. Since we are dealing with this focused area of the circuit I feel confident I got it wired right. It sounds like how my presence sounded before I even started messing with everything.

At one point a couple days ago I actually did remove the MV and put in 220k resistors to go stock. I didn't hear a difference so I put the master back in.
EQ-wise I've been trying all settings, set low to max, or with just maxing one thing like bass, with others low. Tried that every way I could.

I wish I could make a sound example but I don't think it would matter since nothing is happening that I can hear anyway.

 

[mickeydg5]

Is it possible that the 100k and 4 ohm tap are causing the problem? Just the values chosen. I'm still getting not much response from any presence combinations.

I mentioned it in an earlier post. Those two combinations make the use of a .68uF/680nF capacitor less appealing.

 

[mickeydg5]

Not likely. Seems weird that none of the presence arrangements yield any results. Are you sure you have it wired properly? Can you record an audio clip of you playing while turning the presence control? Remove all your alligator clips and play the amp as it is so we can hear it.

All setup plays a part.

 

[FourT6and2]

I mentioned it in an earlier post. Those two combinations make the use of a .68uF/680nF capacitor less appealing.

The presence cap value has nothing to do with his presence control literally not working.

 

[FourT6and2]

It was in the original description and provided picture.

What are you talking about?

 

[mickeydg5]

What are you talking about?

you posted "without being able to see exactly how you've wired everything up"
Read the fuucking thread.

 

[Mjh36]

It's hard to offer any advice without being able to see exactly how you've wired everything up. But the presence control should have a drastic effect, in any of the configurations, if you've wired it properly. Here are my best guesses:

1. You could have made a mistake in your wiring. I would literally remove the stock presence control and replace it with a dedicated pot, cap, and resistor/no resistor instead of using alligator clips and switches. Keep it simple to eliminate unknowns. It shouldn't take more than 5 minutes to do it.

I made a quick video kinda going over how it's set up, I'll have to do a sound test tomorrow for you guys to hear as I'm turning the dials:

 

[mickeydg5]

Hey, please for no reason should anyone think I am right unless you can understand, or overstand.

 

[Pete Farrington]

When using the 25k / no shunt, there's loud squeal and oscillation when turning it down towards 0 on the knob.

I triple checked all wiring. Just really nothing, besides the initial "boost" of connecting the NFB in general

The above comments indicate something basic is wrong.
With negative feedback, closed loop gain is lower than open loop.
For the opposite to be happening then the feedback must be positive.
It’s easy to accidentally flip the power amp signal polarity when installing types 1 and 2 master volumes. Doing so would reverse the feedback from being negative to positive.

Tried all combinations. Low master volume, High master volume.

Your master volume is a complete distraction to this process. I suggest you set it to max and leave it alone, or remove it, until the presence issue is sorted.

Apologies the ampgarage doesn’t allow files or attachments to be viewed unless signed in. It’s unfortunate but there’s not much I can do about it. I think that thread makes it worth the faff.

 

[Mjh36]

Your master volume is a complete distraction to this process. I suggest you set it to max and leave it alone, or remove it, until the presence issue is sorted.

Gotcha, I generally do have it set max or almost to for these tests so far. I'm trying all variables, but I'm running it high most the time since that's close to stock. I even put the 220k resistors in for an hour or so and reverted it to stock, and didn't hear anything different presence-wise.

The above comments indicate something basic is wrong.
With negative feedback, closed loop gain is lower than open loop.
For the opposite to be happening then the feedback must be positive.
It’s easy to accidentally flip the power amp signal polarity when installing types 1 and 2 master volumes. Doing so would reverse the feedback from being negative to positive.

Wait, so if I disconnect the purple wire in the negative feedback circuit, it should get louder? Because you're right I'm getting the sort of the opposite.

With master at max, if I disconnect the negative feedback wire from the pot, it oscillates and gets crazy.
With master at say half, the guitar signal stays intact. But it's quiter disconnected, then louder when I connect it again.

 

[Pete Farrington]

1) with presence at zero the filter resonates at 47Hz passing only low frequency due to the 5k resistance.
2) with presence at halfway the filter resonates at 94Hz passing only low frequency due to the 2.5k resistance.
3) with presence almost max the filter resonates at 11,708Hz passing lower frequency due to 20R (ohms) resistance.

But note that at just 100R (ohms) the frequency cutoff shelf is 2,341Hz and at 500R (ohms) it is 468Hz.

In comparison, using a .1uF capacitor with the pot maxed getting 20R (ohms) the frequency cutoff would be 79,618Hz

Please point out what construes my upside down thinking and then expound upon it.

The frequencies being calculated above seem to be irrelevant to the operation of the circuit.
Whatever, basic RC filters don’t resonate.
I’m sorry it’s a hassle to view them but as the topic is of interest to you, you may find roberto’s simulations enlightening.

 

[Pete Farrington]

so if I disconnect the purple wire in the negative feedback circuit, it should get louder?

Yes.
For the moment I suggest to remove the master volume and reassess.

 

[Mjh36]

The above comments indicate something basic is wrong.
With negative feedback, closed loop gain is lower than open loop.
For the opposite to be happening then the feedback must be positive.
It’s easy to accidentally flip the power amp signal polarity when installing types 1 and 2 master volumes. Doing so would reverse the feedback from being negative to positive.


Your master volume is a complete distraction to this process. I suggest you set it to max and leave it alone, or remove it, until the presence issue is sorted.

Damn you crazy man you did it Pete!!! Well... you all did. I couldn't have done this with out everyone. But what you just said Pete made me rethink something.

My master volume wasn't a distraction, it needed more attention!

After you said that, I looked at it. And I remember earlier in the thread someone mentioning of reversing the wires but I never did think much of it. Until I looked again closely at the LarMar PPIMV schematic just now. I didn't see that you actually need to connect them to the right tube. I had my wires switched going to V4 and V5!

You see, when I got the amp it had a MV mod, but with a 100k dual pot, no safety resistors, and wired backwards. I "fixed" it by putting in the 250k, safety resistors, and wired normally 0 to 10. But I unintentionally switched which wires go to which power tube. It never even occured to me that it would make a difference so I never thought about it. I just figured it's symmetrical, two 220k resistors and two power tubes.

I'm trying just a quick alligator test, the stock 5k/.68uF but no shunt, so dc thru the pot and there's that glorious presence once again. I assume all other presence wirings will work to some degree now.

 

[Mjh36]

What I was saying is that the amplifier is fickle. Yes you can swap the control grid wires on that PPIMV to see if one way or the other gives better results.

The other thing is when an output amplifier is driven hard or overdriven there is a need for enough negative feedback to control/tame the output.
That is why I asked about volume settings. Higher settings in an amplifier such as this will cause the output amplifier to be unstable therefor a certain amount of negative feedback is required. So when you disconnect the feedback circuit at higher levels howl can be expected.

I should have swapped those wires that was the problem!

 

[Pete Farrington]

I should’ve clicked earlier when you mentioned that hiss increased as the presence was turned down

 

[XTRXTR]

I should’ve clicked earlier when you mentioned that hiss increased as the presence was turned down

It's Pete's fault - lol