pre-amp tubes: matching and care

[jadzuke]

New to the tube scene. I have a pre-amp with 6 Sovtek 12AX7LPS tubes that need replacing. Compatable replacements are $4 to $100 ea. A guy on CL has some nice JAN Philips 12AX7WA for $30 ea, unmatched and ungraded. Is it critical to match tubes in a pre-amp? Are cryo treated 7025 worth the money? Is there a break in procedure. Is it true to not touch these with your fingers?
thanks

[TheGimp]

Originally Posted by jadzuke

New to the tube scene. I have a pre-amp with 6 Sovtek 12AX7LPS tubes that need replacing. Compatable replacements are $4 to $100 ea. A guy on CL has some nice JAN Philips 12AX7WA for $30 ea, unmatched and ungraded. Is it critical to match tubes in a pre-amp?

It unfortunatly depends on your circuit topology. For instance
a cathode bypassed common cathode amp with cathode self bias will be sensitive to Mu variations, so the gain from tube to tube will vary if not matched.

There are other (Triode in this case) topologies that are less sensetive to Mu.

If you could post a schematic it would be most helpful.

Are cryo treated 7025 worth the money?

Not in my book, but neither are cryo treated rifle barrels, or bananas.

Is there a break in procedure.

They may change sound profile as they break in, however there is no set procedure that I'm aware of.

Is it true to not touch these with your fingers?
thanks

Probably best not to get finger oil on them as they get hot. However it should not cause a change in performance.

Cheers.

[SY]

Originally Posted by TheGimp

It unfortunatly depends on your circuit topology. For instance
a cathode bypassed common cathode amp with cathode self bias will be sensitive to Mu variations, so the gain from tube to tube will vary if not matched.

There are other (Triode in this case) topologies that are less sensetive to Mu.

If you could post a schematic it would be most helpful.

Not in my book, but neither are cryo treated rifle barrels, or bananas.



They may change sound profile as they break in, however there is no set procedure that I'm aware of.



Probably best not to get finger oil on them as they get hot. However it should not cause a change in performance.

Cheers.

+1.

I'd stay away from designer stuff and collectors' items. So far, the JJ's are the best I've tested and they're under $10.

[Eli Duttman]

The Sovtek 12AX7LPS is a very good tube and very likely better than Philips JAN.

Drop Jim McShane a note stating the equipment to be retubed. Jim will tell you whether or not you need culled tubes and why. A straighter shooter you'll not find.

[jadzuke]

Thanks Guys. I', still stoopid. Whats JJ'S tubes? I'll look for a schematic

[zigzagflux]

Originally Posted by jadzuke

Thanks Guys. I', still stoopid. Whats JJ'S tubes? I'll look for a schematic

Actually not a what but a 'who':

JJ-Electronic - Vacuum tubes, Capacitors, Amplifiers - Home

[Eli Duttman]

Originally Posted by jadzuke

Thanks Guys. I', still stoopid. Whats JJ'S tubes? I'll look for a schematic

JJ is a tube manufacturer whose plant is in Jamona, Slovakia.

Sovtek and ElectroHarmonix are brands used by New Sensor on the products of their Saratov, Russia, plant.