Balanced or Un-Balanced Phase Inverter Valve?

[mascis]

Yeah however much I like a good ol' sine wave, I'll use a load box.

A few questions;

What volume should I run the amp at? Half way? The volume I normally like to play at?

Secondly, Matt, if I measure it the way you describe then I'll be measuring the 'AC signal swing'. Is this measuring the voltage difference between the two sides? or will I be measuring the voltage on just one side? Is it the same type of measuring if I measure it Martin's way?

[matt h]

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[mascis]

So I got a balanced GE valve delivered today and took your advice Matt, just played them and compared it to the Mullards I have, and I seem to slightly prefer the sound of the Mullard... When it's being driven the Mullard just seems to be a little smoother and warmer, the GE still sounds really good just a little colder and brighter, more accurate, the difference is subtle though.

I don't know how much this has to do with the valves being different brands or that ones balanced and one (presumably) isn't, but the bottom line is what sounds good to my ears. I can't find any balanced Mullard ECC81's so that's not an option.

I've switched the two Mullards around to see which of the two sounded best in the phase inverter. It's hard to tell any difference to be honest, I'm guessing one may be more balanced than the other but it's hard for me to tell. I might ask my tech to run some tests next time I need to go to him for something, just so I know. To be honest I still don't feel 100% confident trying to do those tests myself on a live amp. Nothing wrong with your guidance on here, just my lack of knowledge.

Thanks for all your help anyway, you guys are great!

[matt h]

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[Structo]

When I have rolled preamp and PI tubes before, I have the amp out of the case to make it easier.
Then I warm up the amp and play for a bit.
Then turn on Standby and pull the tube I want to experiment with.
(Be careful, tubes can get real hot!)

Plug in the sub tube, the heater current is on so it will warm up.
Flip Standby off and play the amp listening for changes.

May not be the best way but it seems to work safely.