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Author Topic: How to set the PI balance using scope  (Read 14452 times)
mcinku
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« on: March 21, 2011, 08:09:27 AM »
This might be helpfull for all DIY guys which are lucky enough to own a scope...

  • Feed the amp with 1kHz (power amp input - loop)
  • Connect your probes on PI plates and set the scope to measure AC
  • Set the math function to sum the two channels
  • Adjust trimmer until you get the straight line (or close to)
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« Reply #1 on: March 21, 2011, 09:43:44 AM »
Thanks for that mcinku
I don't own a scope but I have access to one at work, I will be taking my amp to work for a tuneup  Grin
BTW obviously I would need a speaker plugged in or a dummy load connected yes?
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« Reply #2 on: March 21, 2011, 09:48:29 AM »
My choice would be a dummy load or your ears will bleed  Grin... 1kHz freq is not a lot of fun to listen to
 Wink
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« Reply #3 on: March 21, 2011, 10:35:12 AM »
Quote from: mcinku on March 21, 2011, 09:48:29 AM
My choice would be a dummy load or your ears will bleed  Grin... 1kHz freq is not a lot of fun to listen to
 Wink
+1 And it's easy to make.

* dm1.jpg (120.42 KB, 1000x750 - viewed 681 times.)

* dm2.jpg (141.03 KB, 1000x750 - viewed 658 times.)
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« Reply #4 on: March 21, 2011, 10:58:39 AM »
I made mine switchable 4-8-16 just in case...

* Selectable AMP LOAD.jpg (52.43 KB, 620x702 - viewed 642 times.)
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« Reply #5 on: March 21, 2011, 01:10:16 PM »
Niiiiiiiice!
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sduck
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« Reply #6 on: March 21, 2011, 05:39:12 PM »
This is a great thread - now I'm starting to feel like I know how to do this adjustment.

Only problem - my scope doesn't seem to have a math function - it's an older Heath 4226. Can I use the X-Y mode? Or is there some other way to set the thing? Actually it might have some kind of math mode, just that I don't have a manual and can't find anything obvious on the panel.
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« Reply #7 on: March 21, 2011, 05:54:10 PM »
Hello all
back to the infamous PI pot this thread leaves me wondering what I am missing.
Mcinku for those of us that dont have access to an O-scope, what difference in tone are you noticing once you have it set properly?
I am just wondering if this will be a necessary adjustment for my amp since right now it sounds great. Although I want to get the most out of the amp too.
Would you be able to tell the differennce between an OTS that has PI setup and 1 that has not by ear?
Thanks
Todd
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« Reply #8 on: March 21, 2011, 06:09:02 PM »
Quote from: T Wilcox on March 21, 2011, 05:54:10 PM
Hello all
back to the infamous PI pot this thread leaves me wondering what I am missing.
Mcinku for those of us that dont have access to an O-scope, what difference in tone are you noticing once you have it set properly?
I am just wondering if this will be a necessary adjustment for my amp since right now it sounds great. Although I want to get the most out of the amp too.
Would you be able to tell the differennce between an OTS that has PI setup and 1 that has not by ear?
Thanks
Todd

Perfect balance is not the aim; when it's set slightly out of balance, harmonics and bloom will be more present. You need a balanced tube to get the best out of the amp.
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« Reply #9 on: March 21, 2011, 06:55:30 PM »
Hey Todd, are you using a balanced tube in the PI position? This would be the first step as erwin stated.

I need a second probe to do it properly with my scope, which I'm about to order. But I was messing around with it, and one thing I did (just for proving purpose) was set the trimmer all the way left or right, wait a bit to settle in and then turn it al the way the opposite and then start strumming some chords with some distortion. Since it takes some time for it to settle in, It seemed I could hear when it passed through the sweet spot. (atleast it sure seemed like to me)

Now, this doesn't make it much easier to go back and find the perfect spot, but it did help get some idea of what we're after. Since I'm using a balance tube I find if I set it just a bit off center CCW it gets thing creamier with harmonic content sounding more "swirly" for a lack of a better word.

Any of the experts have any comments on what I'm saying here? or does it sound like my imagination. Undecided
« Last Edit: March 21, 2011, 06:57:52 PM by SoundPerf » Logged
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« Reply #10 on: March 21, 2011, 06:55:44 PM »
Quote from: sduck on March 21, 2011, 05:39:12 PM
This is a great thread - now I'm starting to feel like I know how to do this adjustment.

Only problem - my scope doesn't seem to have a math function - it's an older Heath 4226. Can I use the X-Y mode? Or is there some other way to set the thing? Actually it might have some kind of math mode, just that I don't have a manual and can't find anything obvious on the panel.

Do you have a measurement function? Both waveforms should have the same pp voltage... when Vpp is the same on both plates, sum of both is zero.
 Wink
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« Reply #11 on: March 21, 2011, 07:01:35 PM »
Quote from: T Wilcox on March 21, 2011, 05:54:10 PM
Hello all
back to the infamous PI pot this thread leaves me wondering what I am missing.
Mcinku for those of us that dont have access to an O-scope, what difference in tone are you noticing once you have it set properly?
I am just wondering if this will be a necessary adjustment for my amp since right now it sounds great. Although I want to get the most out of the amp too.
Would you be able to tell the differennce between an OTS that has PI setup and 1 that has not by ear?
Thanks
Todd

Like Erwin stated the goal is to get best harmonics and bloom... I try to achieve it with setting my PI to be perfectly balanced like shown on the video and than I get my guitar and play some chords... the sweet spot is usually very very close to that.
« Last Edit: March 22, 2011, 11:17:36 AM by mcinku » Logged
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« Reply #12 on: March 21, 2011, 07:18:07 PM »
Thanks for the fast responses
I have been avoiding tinkering with this pot so far, I really just set it at 12:00 and forgot about it, mainly because I dont have an o-scope. I had read that the o-scope was necessary because human ears would not detect sweet spot, that made me think well hmm if I cant hear the difference than no one else will either so why bother.
Thanks to yall, now I will be opening up amp today and tinkering. I want the amp to sound as good as it can but honestly my biggest tonal obstacle are my fingers.
From what Soundperf said the difference can be heard while turning pot but you will have to wait for settings to take effect. How long did this take between each change?
Also I bought JJ tubes to start up amp and burn in with, and plan on upgrading later once everything else is working properly. I understand that any changes to the PI will depend on the tube that is used. The 6l6's were matched but I dont know if the Ecc83's were balanced or not?
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« Reply #13 on: March 21, 2011, 07:33:16 PM »
Todd, I'll answer some of your questions, but wait for the rest to chime in too.

you only need a balanced tube in the PI positon. If you didn't specify when ordering then it wasn't checked.

It doesn't take long for it to "settle in". It's more like it just "eases" in when turning.

Oh, and yeah, my fingers are my biggest obstacle too.  Wink
« Last Edit: March 21, 2011, 07:40:58 PM by SoundPerf » Logged
Chris

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« Reply #14 on: March 21, 2011, 07:38:26 PM »
Not to throw the topic off, but does anyone have any suggestions on where to buy and brand of inexpensive scope test probes? I have a Tek 2246 100Mhz scope. It has a 1Mohm 20pf input.
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Chris

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