LarMar PPIMV presence dosen't work when PPIMV Full

[psychepool]

I have a DIY amp with a LarMar PPIMV.

I know that any type of PPIMV affects the NFB, so if I lower it, the presence won't work as expected.

However, the amp I am using is not working even if I raise the PPIMV to full. (Depth is also installed, but it doesn't work either)

If PPIMV is installed, is it correct that Presence does not work even if PPIMV is set to full?

The reason why Presence does not work may simply be due to my wiring error, but I would like to check the cause before removing the chassis and checking it. (I made the mistake of making the cabinet too tight for the chassis size, so taking the chassis out and putting it back in is a huge challenge.)

[maxkracht]

Shouldn't effect NFB unless you are doing something weird... Lowering the signal going to the speaker will mean less signal feeding back. This is the case with any kind of volume control.

[psychepool]

Shouldn't effect NFB unless you are doing something weird... Lowering the signal going to the speaker will mean less signal feeding back. This is the case with any kind of volume control.

Thanks for the reply.

In fact, I only use the attenuator and I don't use the master volume, so I thought of removing the master volume anyway while checking the wiring this time.
However, if the wiring is also repaired while removing the master volume, I would like to ask a question because I would not be able to figure out what caused the presence to not work.

I'll check the wiring carefully. thank you

[pdf64]

My guess is that careless wiring to the master volume has flipped the balanced signal polarity, so the loop is now positive.
Don’t ask how I know to look out for that

[martin manning]

Shouldn't effect NFB unless you are doing something weird... Lowering the signal going to the speaker will mean less signal feeding back. This is the case with any kind of volume control.

As psychepool says above, any PPIMV will affect the feedback loop since it reduces power amp loop gain, and therefore the output, while the input signal remains constant.

The problem here could be reversed leads in the PPIMV, or possibly the power amp is going into oscillation at the higher volume settings.

[psychepool]

I mustered up the courage to rip the chassis out of the cabinet and rewired it. Half of the total working time was spent taking out the chassis. It was a tough fight.

The goal was to remove the PPIMV pot and redo all the related wiring, so I can't remember exactly which part I wired incorrectly before wiring.

This amp is largely a combination of a Bogner Ecstasy preamp (with many features shortened) and a Friedman BE-50 deluxe power amp.

The feedback circuit of the Friedman BE-50 Deluxe looks like this,

nfb wiring 01.jpg

The only incorrect wiring I remember is this point.
(Wired state as shown in the image on the right)

nfb wiring 02.jpg

As I said, my memory isn't clear, but I thought there was nothing wrong with it other than this part.
Depth Presence Response (NFB resistor value) were all in a state of not working.

Now I have taken off the PPIMV and rewired it properly and everything works fine.

Can all NFB-related functions not work at all due to the miswiring shown in the image above? Or could it be the effect of PPIMV?
As I said, even if I set PPIMV to full, it didn't work at all.

Anyway, since the NFB works normally, the sound has become much more luxurious. So far the sound has been overly hot and the highs have been very rough, but in a good way, it feels very well-organized. Even with the presence set to maximum, the amp feels calmer than before. The range of sound options has also expanded. It feels so good.

[martin manning]

Ok, so slight misunderstanding, you were saying that the presence wasn't working at any master volume setting. The error you found explains that, since virtually all of the FB signal is shunted to ground at any presence settings. Is the PPIMV reinstalled now and working correctly?

[maxkracht]

My guess is that careless wiring to the master volume has flipped the balanced signal polarity, so the loop is now positive.
Don’t ask how I know to look out for that

Could you elaborate? I have never paid attention to which wire goes to which power tube control grid and have never had a problem. Have also installed many Lamar style master volumes without issue. Am I missing something fundamental while being very lucky? I have screwed up the output transformer primary or secondary many times, or added or removed a gain stage without thinking and caused a squeal.

[sluckey]

Reversing the grid signal wires will have the exact same effect as reversing the OT primary plate leads.

[maxkracht]

Reversing the grid signal wires will have the exact same effect as reversing the OT primary plate leads.

Well, I'm an idiot. I guess I rarely if ever have a reason to swap those and never thought about it. Every time I get a squeal on a new build I fix it other ways...

[psychepool]

Ok, so slight misunderstanding, you were saying that the presence wasn't working at any master volume setting. The error you found explains that, since virtually all of the FB signal is shunted to ground at any presence settings. Is the PPIMV reinstalled now and working correctly?

The reason I installed PPIMV was to use Ecstasy's Plexi mode, but I didn't feel that PPIMV suited me well, and since I'm using an attenuator anyway, I rarely use PPIMV, so the goal was to remove it this time.

I was thinking of removing it anyway, so I asked the question because I don't think I'll have a chance to check the operation of the NFB pots myself with the PPIMV installed.

Anyway, in conclusion, it works very well and I am satisfied.