Lar/Mar PPIMV

[WilliamDouglas]

Hi Guys,

I want to install the Lar/Mar PPIMV into my new express build and I wanted to confirm the wiring. I can only seem to find a sketch of the connections and not a schematic with the circuit installed. I drew up a schematic of how I interpret the integrated circuit and I wanted to bounce off you guys to to make sure I got it right before I wire it in. The part that confused me is the grid resistors, The drawing I was looking at says to connect the pot wipers to pin 5 of the power tubes. there is no mention of the 1.5k resistors that are currently in the circuit.

Thanks,

William

[oh7hhi]

Hi!

I recommend you to connect pot wipers to grid stop resistors.
Other than that, schematic seems to be correct.

[rock_mumbles]

This is for a different amp so the values are different and it's drawn as two separate circuits ... you need the (grid) resistors between your 2.2M resistor and the output tube grid.

[img:600:430]http://www.rh-tech.org/public/18-Watt/20_Watt_PPIMV.png[/img]

[WilliamDouglas]

Thanks for the response..

Regards,

William

[Structo]

Maybe this will help.

[sepulchre]

Where can I get that two conductor plus shield cable?

[rp]

Saved this from the web:

[chief mushroom cloud]

Where can I get that two conductor plus shield cable?

http://www.apexjr.com/

[xtian]

It has been recommended here that you use two separate shielded cables for runs, rather than two wires together inside a single shield.

[sepulchre]

So if I were to use two separate runs (nice because I already have the wire) do the shields get tied together?

[M Fowler]

So if I were to use two separate runs (nice because I already have the wire) do the shields get tied together?

Yes the shields all go to same ground point.

I used single shielded wire on my PCB Express.

Mark

[jjman]

So if I were to use two separate runs (nice because I already have the wire) do the shields get tied together?

Electrically yes but not necessarily twisted together.

[sepulchre]

Cool, Thanks!

[sepulchre]

Is there some reason that the shielding is connected to the bias voltage? Would it be as effective or even more so if it were attached to ground?

I ask because after installing it on an express build I'm getting what is obviously feedback or low frequency (~300Hz) parasitic oscillation when the Master is turned up around half way. At first I thought maybe it was a microphonic tube but a little tapping and swapping proved otherwise. I tried chopsticking the wires in the area to no avail - they're rather short so there's not a lot of room to move them.

However, the master pots are right next to the presence pot. Could that be the source of the problem? Seems like I read something like that recently.

But my question is still, would grounding the coax shields be more effective at blocking parasitic oscillation than using the bias voltage? Is there some reason the bias V is used or is it merely convenient?

[rp]

I'll let the more knowledgeable here explain why use the B- rather than the B+ I'm curious too though I think it's all the same, but IMO don't troubleshoot a problem you don't have yet. Connect to ground as proper and see what happens, it's also likely you might not even need shielded cable at all, always do what's minimal first and work up from there. I tried grounding the grid leads on V1 in an amp and couldn't sleep knowing I had 250V on a ground - I can't remember what I did 20 minutes ago much less when I go poke around in 5 years, or someone else goes in there not expecting it. Here it seems like a good way to accidentally blow your bias with the amp upside down and not catch your tubes running away. So, I'm just saying I personally wouldn't go putting the shield above ground as SOP but as a last hope kluge after I've exhausted everything else, or as a quick test to see if the problem is actually on those leads. BTW tapping on a tube isn't enough, try an other tube, though with the PI it's probably ok.