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
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.

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.

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.