Dual PPIMV squeal

[chrisrbr]

Hi All,
I solved the noise problem with my JTM45--see "snap crakle and pop JTM" thread.
But made a mod that leads to a squeal at high volume.
The amp now has a dual PPIMV, LAR MAR style.  Switching is acheived using a relay.  It works great from low to pretty darn loud voulme--much louder than I will ever play.   But when the PPIMV is around 7 out of 10 or so I get a very bad squeal.

Any ideas? anyone see this before?  could the relay be capacitively coupling internally causing this feedback? 
there was no issue before the dual PPIMV,  the single worked fine.
thanks
Chris

[Willabe]

Hi chrisrbr,

It might help the guys help you if you would re-post the pics of the amp here, especially the gut shot.


          Brad       

[tubenit]

I think the LarMar PPIMV disconnects the NFB. Perhaps that is a factor in this?

With respect, Tubenit

[sluckey]

The amp now has a dual PPIMV, LAR MAR style.  Switching is acheived using a relay.  It works great from low to pretty darn loud voulme--much louder than I will ever play.   But when the PPIMV is around 7 out of 10 or so I get a very bad squeal.

and from another thread...

So back to my need to add a second PPIMV for a volume boost for solos.  to do this I got rid of the presence knob--never used anyways.
Then I added a DPDT relay to switch between the two PPIMV's.
Works great, but when I turn the PPIMV up to say 6-7 out of 10 I get a high pitched squeal, was not there with one PPIMV.  Granted I will never play the amp that loud, but would like to have it behave as before.

What exactly did you do to the presence control circuit? It's in the NFB loop. Also, when you added the second MV, did you possibly reverse the two signal leads between the PI plates and the PA grids? If so, the NFB loop will now be the wrong phase and cause possible squealing. Can you post a schematic showing your circuit?

[chrisrbr]

What I did with the presence is just wire as if it was turned all the way down to 0.  So I just hooked a 5k to gnd from the NFB point that was going to the 5k pot previously.

The two PPIMV were pretty straight forward-see pic  The signal from the coupling caps is sent to both dual pots. Then each dual pot's wipers are sent to the relay. 

hoping I forgot something stupid
thanks
Chris

[chrisrbr]

will check for "possibly reverse the two signal leads between the PI plates and the PA grids? If so, the NFB loop will now be the wrong phase and cause possible squealing. "

would such a reversal only squeal at low to mid volume?
would love it to be that simple

[sluckey]

would such a reversal only squeal at low to mid volume?

I understood you to say "It works great from low to pretty darn loud voulme--much louder than I will ever play.   But when the PPIMV is around 7 out of 10 or so I get a very bad squeal." I would think it would tend to squeal more at higher volumes.

An easy way to tell if this is a factor is to simply disconnect the feedback wire from the speaker jack (or wherever you're picking off.

[chrisrbr]

thank you, thank you. problem solved. yes, I had reversed the PI wires when I added the extra PPIMV.
I got so caught up in thinking it was something more esoteric like an interaction within the relay---that I didn't see the forest through the trees etc.

amp is dead quiet now.
thanks again
Chris

[Willabe]

All Right!

Chris, the fellas got you up and runnin, and with out the gut pic! sluckey's one of the best. So's tubenit.  

Don't sweat it, any one here will tell you it happens to the best of them.


       Brad