Matched P.I. Tubes?

[paddy]

Hello to all.
First let me say what a great site this is,and give
my thanks to all the contributors who's posts have
allowed me to build my first ODS with relative ease.

My question relates to the use of a 12ax7 with both sides matched
in the P.I.position.
I have read on some sites that this is critical if you want your amp
to "sing"and not have dead spots.I have also read the opposite and
that an unbalanced P.I. makes for a great sound.
I have a good selection of 12ax7's,a tube tester and a scope.
My build is the '97 ODS with no P.I. trimmer and fixed 91k/100k
plate resistors.
The amp sounds great but like most people here I want it to be as good
as it can be.Should i use a matched tube,install the trimpot and balance
everything? Will this make my amp sing? or should I leave it alone?

Many thanks given in advance for any help

Cheers
Paddy

[MarkB]

I doubt you'd hear a difference unless there's a gross imbalance.










Now someone will tell you the exact opposite.

[paddy]

Thanks Mark
you're probably right
does anyone else have any opinions or even better--experiences?

[llemtt]

Yes you probably hardly hear differences if you run the power stage clean and only little differences if you overdrive it.

Consider these also

- matched tubes become unmatched with use
- if you builded the usual schema you have only one bias trim so power tubes are biased with different operating points (best matched power pairs are still within 5-10%)
- main reason to AC balance power amp is to maximize clean output

install a PI trimmer, a 25k to have enough range, then whenever you need rebalance you amp

teo

[paddy]

Thanks Hemtt,
i'll probably just leave things as they are unless somebody
else can persuade me otherwise.

cheers