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

[Mjh36]

"I got an idea, without desoldering anything, can I alligator a whole separate presence circuit for testing? Take alligator from the NFB resistor and have it go to a pot/cap arrangement and test what that does?"

You should also add a new cap to that new pot for testing in that manner.
That would give two possible outcomes, I think:
1 no difference which means the current pot is shorted to ground
2 big difference which would mean the current pot is open (given that currently nothing happens)
3 You can't hear any difference lol

I couldn't hear any difference on this either! Turning both knobs did nothing.

Here's how I had it set up, using a .68uf cap. Just kind of did a mirror image of the existing one, utilizing the same 4k7 resistor that's in there and a new cap and pot. I used a 100k pot with 5k6 parallel to make it close to 5k.

 

[dtier]

I couldn't hear any difference on this either! Turning both knobs did nothing.

Here's how I had it set up, using a .68uf cap. Just kind of did a mirror image of the existing one, utilizing the same 4k7 resistor that's in there and a new cap and pot. I used a 100k pot with 5k6 parallel to make it close to 5k.

View attachment 131513

Looking at your pics, I can't tell if your output transformer secondary common, (orange wire) is grounded. Its on your output jack sleeve with a black wire that should go to the chassis to complete the feedback circuit. Good Luck!

Dave

 

[Mjh36]

Looking at your pics, I can't tell if your output transformer secondary common, (orange wire) is grounded. Its on your output jack sleeve with a black wire that should go to the chassis to complete the feedback circuit. Good Luck!

Dave

Thanks I actually had to look very closely, I never noticed that wire. Connects to ground though so that checks out.

 

[Chris-in-LA]

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

 

[Gene Ballzz]

Most lower priced attenuators totally destroy the frequencies associated with a Presence circuit!
Just Sayin'
Gene

 

[Jon Snell]

Two hopes come instantly to my mind;
No Hope and Bob Hope.
Only joking.
10v is nominal dependant on the condition of the valve and various other things.

10volts proves the 4k7 is in circuit and working.
Varying the pot/hanging a capacitor across it/other listed tests, proves only one thing to me ... there is no feed resistor. Either 47k or 100k depending upon the model number and builder.
Check you have signal across the 4k7 by playing music into the amplifier and connect a 220n capacitor in series with the cathode end of the 4k7 and ground to another amplifier. If you have no signal to the other amplifier, there maybe no feed through the 47k/100k resistor.

Good point with the MV addition,if it uses a tandem pot (which is wrong way to do it), that will reduce not only the output as a volume control but the NF loop as well and as mentioned above, could severly impact upon the actions of the presence control by reducing not only the signal to the output valves but the NFB as well. In turn reducingthe actions of the presence control.
If you require an MV use a single pot and shunt the two drives for the output valves together with it. That will maintain the NFB and fix your issue.

 

[Mjh36]

Most lower priced attenuators totally destroy the frequencies associated with a Presence circuit!
Just Sayin'
Gene

Yessir I agree, that's why I use my trusty JohnH 100w M3.

 

[Mjh36]

Two hopes come instantly to my mind;
No Hope and Bob Hope.
Only joking.
10v is nominal dependant on the condition of the valve and various other things.

10volts proves the 4k7 is in circuit and working.
Varying the pot/hanging a capacitor across it/other listed tests, proves only one thing to me ... there is no feed resistor. Either 47k or 100k depending upon the model number and builder.
Check you have signal across the 4k7 by playing music into the amplifier and connect a 220n capacitor in series with the cathode end of the 4k7 and ground to another amplifier. If you have no signal to the other amplifier, there maybe no feed through the 47k/100k resistor.

Good point with the MV addition,if it uses a tandem pot (which is wrong way to do it), that will reduce not only the output as a volume control but the NF loop as well and as mentioned above, could severly impact upon the actions of the presence control by reducing not only the signal to the output valves but the NFB as well. In turn reducingthe actions of the presence control.
If you require an MV use a single pot and shunt the two drives for the output valves together with it. That will maintain the NFB and fix your issue.

Ok excellent I will get on that thank you very much. Though somewhat a bit over my head with the MV. I'm just getting into this sort of thing, but everything I've come across seems to use the dual pots for the PPIMV. And I've heard these do reduce presence control and NFB but at maximum setting it should be effectively out of circuit, atleast enough to hear some presence adjustment.

I will try this test out right now.

I did measure voltage across the NFB resistor and it's -10 Volts. That may be obvious, I don't know much, just giving as much info as I can.

 

[Jon Snell]

That is correct and proves the far end of the NFB resistor is connected.
One side of the NFB feed resistor is the winding of the output transformer, so effectively 0v and ground, the other end will be the voltage on the 4k7 resistor.

 

[Mjh36]

Two hopes come instantly to my mind;
No Hope and Bob Hope.
Only joking.
10v is nominal dependant on the condition of the valve and various other things.

10volts proves the 4k7 is in circuit and working.
Varying the pot/hanging a capacitor across it/other listed tests, proves only one thing to me ... there is no feed resistor. Either 47k or 100k depending upon the model number and builder.
Check you have signal across the 4k7 by playing music into the amplifier and connect a 220n capacitor in series with the cathode end of the 4k7 and ground to another amplifier. If you have no signal to the other amplifier, there maybe no feed through the 47k/100k resistor.

Good point with the MV addition,if it uses a tandem pot (which is wrong way to do it), that will reduce not only the output as a volume control but the NF loop as well and as mentioned above, could severly impact upon the actions of the presence control by reducing not only the signal to the output valves but the NFB as well. In turn reducingthe actions of the presence control.
If you require an MV use a single pot and shunt the two drives for the output valves together with it. That will maintain the NFB and fix your issue.

I get signal through to another amplifier from that point you highlighted on the 4k7. So NFB resistor is ok?

edit: well I did a couple times, I'm having trouble keeping audio it's gone now. But it did clearly work at first...

 

[Jon Snell]

I would blame the MV addition and remove it.
Seems you have done all the usual checks, oscilloscope time is next if you want to leave the MV in there. You will see what is happening then.

 

[XTRXTR]

4) I like what @Jon Snell is saying over what I said

 

[Mjh36]

I would blame the MV addition and remove it.
Seems you have done all the usual checks, oscilloscope time is next if you want to leave the MV in there. You will see what is happening then.

Thank you for your help. I could keep audio I think it's just my Mustang Micro Headphone amp needs to charge. But audio was playing.

So by all accounts the circuit seems intact and as far as we can tell not faulty. I'll try reverting it back to non-master and try it out. Maybe also try desoldering the NFB wire leading to the current presence pot arrangement, and alligator clip that to another pot/cap, so the stock one is completely out of circuit for that test.

 

[Jon Snell]

You have proved the Presence pot is intact and connected, I wouldn't touch it.
No sense in possibly adding potential dry joints etc to what appers to be working.

If you remove the MV and wire the grid drives wrong, the amplifier will howl and be uncontrollable. In that case, wire the drives the other way around.
This is due to being out of phase and becomes Possitive Feedback and we all know about microphones too close to loudspeakers don't we 🙃 !
If it makes no difference, the NFB is not connected.
Have fun.

 

[XTRXTR]

At one point I tried a Trainwreck 3 PPIMV I think on my 2204. One Pot mixing the PI plate outputs. The Presence circuit still worked. I later changed it back as the PPIMV had insane Odd order harmonics and sounded harsh. Later still I found I an oscillation at around 147kHz which took weeks to find out it was a cold solder joint on the Cold Clipper plate feeding the next stage. I should test the PPIMV out again with the MV in place as well. Then I could control the distortion levels on either side of the PI. It might be a cool thing...or not.

 

[FleshOnGear]

You’ll figure it out, @Mjh36! Sorry you gotta go through all this trouble, but it will be worth it. I had a few problems with my ‘72 when I first bought it, and it was worth every moment of troubleshooting to get it just right.

 

[mickeydg5]

I get signal through to another amplifier from that point you highlighted on the 4k7. So NFB resistor is ok?

edit: well I did a couple times, I'm having trouble keeping audio it's gone now. But it did clearly work at first...

There is going to be signal there whether the 100k negative feedback resistor is connected or not. There is always signal in the cathode circuit above the 4.7k (5k) at bottom of the tail.

Try this easy test.
With the amplifier OFF, place your multimeter across the 100k negative feedback resistor on the board and measure resistance.
What does it read?

 

[dtier]

There is going to be signal there whether the 100k negative feedback resistor is connected or not. There is always signal in the cathode circuit above the 4.7k (5k) at bottom of the tail.

Try this easy test.
With the amplifier OFF, place your multimeter across the 100k negative feedback resistor on the board and measure resistance.
What does it read?

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

 

[Mjh36]

There is going to be signal there whether the 100k negative feedback resistor is connected or not. There is always signal in the cathode circuit above the 4.7k (5k) at bottom of the tail.

Try this easy test.
With the amplifier OFF, place your multimeter across the 100k negative feedback resistor on the board and measure resistance.
What does it read?

Amplifier off, measuring across 100k NFB resister I get = 4.3k

Should it still be 100k?

 

[dtier]

Amplifier off, measuring across 100k NFB resister I get = 4.3k

Should it still be 100k?

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.