Lar/Mar PPIVM 68 Plexi 100w

[Ampzone]

Larry while I am waiting for the 100k dual pot I went ahead and tried it stock with some EL34's. I am getting trash on the notes and unwanted loud crackling buzzes. It gets more pronounced as the 250k PPIMV pot is turned up. Any ideas? I removed the redundant control grid wires from under the board to avoid an extra RF etc.

Thanks...

[novosibir]

I am getting trash on the notes and unwanted loud crackling buzzes. It gets more pronounced as the 250k PPIMV pot is turned up. Any ideas?

You definitely have parasitic oscillation, what's mostly caused by suboptimal lead dress.
In this case a photo says more than 1,000 words, 3 fotos say more than 10,000 words.
And please don't tell me, that you've placed the PPIMV pot in one of the input holes!

Larry

[Ampzone]

Larry,

The PPIVM is in a speaker jack on back. I'm going to do the chopstick method to see if something quiets down moving the wires. I wonder if the leads going to the control grids need to be shielded, right up to the grid lug? The the shield only goes to were the two wires split to go to their respective lugs. That leaves about 2" of unshielded lead.

The Parasitic OC did not exist prior to PPIMV which I have to consider.

Going to test some stuff now

[novosibir]

If you don't use shielded cables, you have to twist both cables to the pot, but especially the two from the pot to the tubes.
Don't come too close to the OT's cables and NEVER bundle it to them - also keep a respectul distance to the violet NFB cable AND to the PI's cables!

Larry

[Ampzone]

Larry,

I moved the cables around and was able to mitigate the problem a little more. I will say the sound of the PPIMV works well and is more transparent than another MV's I tried. Nice work on your part.

But I am still getting trash on the notes, basically a crackling sound that comes and goes. There is a hum that get's louder as the PPIVM is turned up. I think I read you will get a little more hum with this style MV. It's not bad. I just wish I could find the source of this damn crackling crap.

Nothing is bundled as far as the PPIMV network. I used microphone wire that has a foil shield covering a single drain wire and 2 conductors. The single drain wires goes to the grounds on the PPIMV.

I going to move more stuff around and check the NFB area. I can say the crackling is at it's worst when the presence is dimed.

[martin manning]

I think you need to ground the shield, which surrounds the conductors. The drain wire won't serve the same purpose.

[Chuck H]

If it's the cable I think it is, the "drain" wire is bare and in contact with the inner foil shield and that's how it's supposed to be grounded.

What you need to make sure of is keeping the PPMV leads away from preamp components and leads. The NFB lead should be treated like a red headed stepchild and kept away from everything. In the process be sure not to move any preamp leads close to output leads. Be sure not to ground the PPIMV in the same place as any preamp circuitry, yada, yada... Sometimes a circuit will only misbehave outside of the cabinet. The extra shield (usually glued or stapled) in the cabinet seems to stabilize things.

[Ampzone]

Martin the foil shield is in contact with the raw copper wire that is grounded to the 250 dual pots wiper, so it must be making contact with the shield. But I appreciate your thought on this.

Here's the latest. I had left the (orange/green) bias wires under the board, but both ends of the wires had been clipped, at the PPIMV contact (on the PTP board) and at the V5-V6 control grid inputs (pin 5). So there was not contact on either end. However after removing them and then playing amp upside down and not in the headbox the trashy crackling is now gone. could it have been just the old 100% disconnected wires still acting as an antenna?

Now I am going to put the amp rightside up in the headbox and see how it goes. If there is still no trashy crackling then were home free I hope.

This just leaves the 60 cycle-ish hum which get's louder as the PPIMV is turned up. Once again I am assuming this is just a by product of the PPIMV mod. It goes away when you play.

[novosibir]

the foil shield is in contact with the raw copper wire that is grounded to the 250 dual pots wiper

Hopefully this is a typo!
The wiper is the middle lug on a pot, where the shield NOT has to be connected to! The shield must be connected to the output lugs of the pots, there where the negative bias supply is present! The negative bias voltage is AS GOOD AS ground as a shielding potential. I recommend to connect the shields there, because the Mouser pot housings don't like solder too much and it happens again and again, that a solder bulb simply goes off of the housing. PEC pot housings don't take solder at all (except you're using some special chemics before on the surface), because they're stainless steel.

I had left the (orange/green) bias wires under the board, but both ends of the wires had been clipped, at the PPIMV contact (on the PTP board) and at the V5-V6 control grid inputs (pin 5). So there was not contact on either end. However after removing them and then playing amp upside down and not in the headbox the trashy crackling is now gone. could it have been just the old 100% disconnected wires still acting as an antenna?

Shure, possible! Remove anything from inside the amp, what's conducting and doesn't need to be there anymore! Who's lazy first, later has to do his work again & again

This just leaves the 60 cycle-ish hum which get's louder as the PPIMV is turned up. Once again I am assuming this is just a by product of the PPIMV mod.

An amp with a properly installed PPIMV doesn't hum a yota more than before. This is a bad rumour, because people haven't done their job correctly in the past - and then have explained to the customer, that a PPIMV in an amp... yada yada

If after installing the LarMar PPIMV in an amp you can't get rid of any enhanced & persisting hum, then the bias electrolytics are bad and allow ripple to the bias voltage, what's then fed into the output tube's control grids. If so, swap in two new ones! To go bigger than 10µF is not required.

Larry

[Ampzone]

Larry,

I meant the outside lug on the pot...typo

After looking around I noticed the stock swampers (1k5) were not on each control grid. Only two of them existed in between v4-V5 & V6-V7. So the PPIMV contacts on V5&V6 were going straight to the control grid without going through a swamper. So I changed the value and added 4 new 5.6k swampers on each grid to see what this higher value would do for oscillation.

Now it's even quieter...I cannot get the trash or crackling noise to happen so far. Perhaps the higher value and the additional two swampers on each grid did the trick. The hum was reduced a tad as well.

I'll check the bias caps now. Fingers crossed that I'm golden

[Chuck H]

The hum is suspect to me. Have you tracked the bias at different PPMV adjustment levels? I would check that.

Also, the grid resistors (swampers) are usually best placed right on the grid pin with almost no lead between the resistor and pin. It's ugly wiring but this is where they work best. Am I understanding that before you had NO grid resistors?

[Ampzone]

The hum is suspect to me. Have you tracked the bias at different PPMV adjustment levels? I would check that.

Also, the grid resistors (swampers) are usually best placed right on the grid pin with almost no lead between the resistor and pin. It's ugly wiring but this is where they work best. Am I understanding that before you had NO grid resistors?

Chuck, the hum is not overbearing but perhaps not right? The bias caps (BC 10uf) are newish. As far as the swampers there were only 2 1k5 swampers on it before the PPIMV (between v6-V7 and V4-V5, leaving V5&V6 grids directly connected to the 220k splitters without a swamper), now I attached 4 5.6k swampers, and made them all with no extra lead on either end like you said, attached right at each tubes control grid. Looks like poop, but hey if it works it works. Like I said, so far there is no sign on of the trashy crackling since the 5.6 swampers were added.

I am going to run it through the paces Monday and see where were at. Ive read about the hum with other PPIMV users, but if Larry says no hum then we'll keep checking the bias network.