The PPIMV (Post-PI-Master-Volume) thread.

novosibir · Post by **novosibir** » Tue Oct 31, 2006 8:36 am

Tonecat wrote:So which method is better sounding? Using the 250k with no bias resistors and the 2.2M on each section of the pot, or a 500k pot with 400-430k bias resistors?

The first one - simply because when the pot is NOT cranked fully open, the output tube's grids 'see' less resistance coming to it.

Keep in mind, that any resistor to the tube's grids is forming a low pass filter together with the tube's input capacity. Therefore when the resistance is lower, then there will be more highs remaining with the pot more or less closed.

Dimed although there's no difference soundwise between both!

Larry

Tonecat · Post by **Tonecat** » Tue Oct 31, 2006 8:47 am

Thanks Larry. How did you come up with the 2.2M value resistor and I am also using shielded cable for my PPIMV. Is it best just to hook up the shielding on one end and is it OK to use negative bias voltage for the the shield hook-up point? Thanks for your help!!

novosibir · Post by **novosibir** » Tue Oct 31, 2006 9:06 am

Tonecat wrote:Is it best just to hook up the shielding on one end...

Yep! Only on ONE end!

Tonecat wrote:... and is it OK to use negative bias voltage for the the shield hook-up point? Thanks for your help!!

Yep again! The negative supply voltage is as good as a shielding as ground, but much easier to hook up directly on the pot's lugs

Larry

rockstah · Post by **rockstah** » Tue Oct 31, 2006 10:34 am

novosibir wrote:
rockstah wrote:Larry talk to me more about the 250k dual pot and the 2.2m resistors - not sure where they go
Ok - step by step:

- The coupling caps are connected to the input lug's of the pot
- The bias supply voltage (which usually feeds the both 220K) is connected to the output lug's of the pot (of course to both)
- The pot's wipers are connected to the power tube's grids (or to their swamp resistors)
- Now simply solder each a 2.2M on each section of the pot from the wiper to the output lug - a small 1/4W resistor will do it

These are only for safety, because if the pot's wiper fails, then the output tube's grid is w/o bias voltage and will immediately jump over the Jordan.

But with the resistor applied and a failing wiper, although this side can't be regulated anymore, but nevertheless there's bias voltage on the output's grid - and the tube(s) will survive

Larry

like this?

edited - scroll down

robert · Post by **robert** » Tue Oct 31, 2006 10:45 am

No! The 2M2 resistors must be installed between the center lugs (to grids) and the left lugs (from bias supply).

Regards

Robert

rockstah · Post by **rockstah** » Tue Oct 31, 2006 10:50 am

like this!

novosibir · Post by **novosibir** » Tue Oct 31, 2006 11:54 am

Exactly like this!

Larry

rockstah · Post by **rockstah** » Tue Oct 31, 2006 11:55 am

thank you Larry!

rockstah · Post by **rockstah** » Tue Nov 07, 2006 6:32 pm

hey Larry can u check my work here on the type 2 master volume?

i want to run it by you before i fire it up but im pretty confident shes correctly done.

from the caps on board red, right lug - front side of pot( closest to chassis),
black right lug back side.
sheild grounded to left lug.

from pot, center lug, red front side ( closest to chassis), to v6 and v7
from pot, center lug, black, backside of pot , to v4 and v5
shield grounded to left lug

used a 2meg resistor and 220k in series to make up the 2.2m resistor
2.2meg center and left lug back side of pot
2.2meg center lug and left lug front side of pot

hope it doesnt take too much work on your part to go through this for me.

appreciate you looking through it for me man!

if u need better pics let me know.

Mark

novosibir · Post by **novosibir** » Tue Nov 07, 2006 7:27 pm

rockstah wrote:hey Larry can u check my work here on the type 2 master volume?
appreciate you looking through it for me man!
if u need better pics let me know.

Don't need better pixs, all's clearly visible!

Perfect, Mark! I'm shure, this PPIMV will work properly w/o oscis!

But don't forget the nuts now to tighten the boards

Larry

rockstah · Post by **rockstah** » Tue Nov 07, 2006 7:40 pm

novosibir wrote:
rockstah wrote:hey Larry can u check my work here on the type 2 master volume?
appreciate you looking through it for me man!
if u need better pics let me know.
Don't need better pixs, all's clearly visible!

Perfect, Mark! I'm shure, this PPIMV will work properly w/o oscis!

But don't forget the nuts now to tighten the boards

Larry

thanks so much Larry - ya know i have never put the nuts on the boards yet on my plexi builds - they have never been in a headcase much less right side up before!

thanks again,

Mark

rockstah · Post by **rockstah** » Tue Nov 07, 2006 7:59 pm

hey Larry,

so these resistors dont effect tone or the sweep of the pot?

tell me again how they function exactly?

Mark

novosibir · Post by **novosibir** » Tue Nov 07, 2006 8:14 pm

rockstah wrote:tell me again how they function exactly?

That I'll do tomorrow, if I don't forget about - because it takes me about 3 times more time to explain anything in English, than in German.

But now I still have to finish my work - already 2:15 am here in Germany

Larry

rockstah · Post by **rockstah** » Tue Nov 07, 2006 8:27 pm

novosibir wrote:
rockstah wrote:tell me again how they function exactly?
That I'll do tomorrow, if I don't forget about - because it takes me about 3 times more time to explain anything in English, than in German.

But now I still have to finish my work - already 2:15 am here in Germany

Larry

ok thanks Larry! i just realized why i havent had a ppimv in my amp - doesnt sound very good.

i dont hear any PO but it has no gain... well barely any - maybe has to do with the last time i tried it i used a 500k dual... or was it 1M dual pot.. anyway

novosibir · Post by **novosibir** » Wed Nov 08, 2006 12:16 am

rockstah wrote:i dont hear any PO but it has no gain... well barely any - maybe has to do with the last time i tried it i used a 500k dual... or was it 1M dual pot.. anyway

That's impossible, when you have choosen the 'right' resistor values!

The resulting resistance with the pot dimed is about the same as with a 500K pot and 470K bias fed resistors - and even more as with the 500K pot and the stock 220K bias fed resistors.

Check your above mentioned 2M resistors! Maybe you took by accident 2K ? That could explain the "has no gain... well barely any"

Larry