Ceriatone Forum

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)

http://www.youtube.com/watch?v=8lNuu7TzzNU (http://www.youtube.com/watch?v=8lNuu7TzzNU)

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  ;D
BTW obviously I would need a speaker plugged in or a dummy load connected yes?

My choice would be a dummy load or your ears will bleed  ;D... 1kHz freq is not a lot of fun to listen to
 ;)

+1 And it's easy to make.

I made mine switchable 4-8-16 just in case...

Niiiiiiiice!

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.

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.

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. :-\

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.
 ;)

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.

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?

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.  ;)

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.

I read the thread on ampgarage and spent a while trying to set mine, but without a scope found it frustrating. I did post some info here
http://ceriatoneforum.com/index.php?topic=3428.msg16597#msg16597 (http://ceriatoneforum.com/index.php?topic=3428.msg16597#msg16597)
but I'm not sure I know what I'm doing to be honest

I know I need to achieve AC 0v on the PI, but so many of my tubes were way out even the one I bought as 'balanced'. Couple with that and the fact the trimmer made very little difference to the AC balance makes me think my meter is just too crap!.

SO my first question is, if I order another 'balanced' Sovtek LPS tube what do I need to ask for? Balanced gain? which is what I think I got or balanced transconductance? (If I knew what it was I'd be better informed when I go to the shop!)

Secondly how accurate does my meter have to be to achieve this 0v AC? +- 1mV? or better? Not sure if mine is good enough but I'd like to try it again.

Cheers guys.

Search E-Bay for "Tektronix Oscilloscope Probe". Tons available BIN. I have a Tek 466 and got a pair of 1X-10X probes for less that $20. Matter of fact, here you go:
http://cgi.ebay.com/Two-New-100MHz-Oscilloscope-Probes-Probe-Tektronix-HP-/160549907389?pt=LH_DefaultDomain_0&hash=item2561852bbd

Hi Hywell,

If you have a 50w OTS and a DMM you also can use the 2 red bias points to measure.
Insert 1kHz signal in the Fxloop and set your DMM for dc voltage. This method is more accurate because it takes the imbalance of the power tubes in account. Your actually measuring cathode balance of the power tubes in this method.

For you 100w guys; the red testing probes have to be tied together for tube v4 and v5. V6 and v7 also have to be tied together. And in this way you also have 2 measuring points were you can measure ac balance.

One other thing: just like when biasing; leave the amp 15 min on before measuring, sometimes tube heat changes the response of tubes.

Thanks!!!

Got it to work. I dug out my old test probes from when I was in school, which I tried before when I first got my scope and they didn't seem to work and I just put them to the side and used the probe that came with the scope. So, I decided to find out why they weren't working. Here it seems that the removable tips were broken and just barely holding together. I probably loaded them to somebody back in school and they broke them. >:( So I put new tips in and presto.

It turns out I had it pretty much set correctly already, but it's nice to know your at the right starting point.

For those using a DMM...are you using a dual probe setup? (two probes beside the black ground probe) I'm thinking this is one thing getting overlooked by some.

So if you inject a 1khz signal into the return jack of the loop and adjust the level of the signal so it measures about 30VAC on pin 3 (plate) of your power tubes. Then connect the dual probes (with black to ground) on pin 1 & 6 of the PI tube. You then want to tweak the trimmer to read 0VAC.

Then mark this spot of the trimmer. Like the others said it's the slight imbalance that's the object. From I can tell it's going to be very slight changes from there.

I know this isn't any different than what others have said. I guess repitition is the key to perfection. Or something like that. :P