Good PI tubes...where to get them

[greiswig]

I'm still searching for the magic fairy dust setting of the PI trimmer, and I wonder if my issue is the tubes that I'm trying.

Where do you guys find 12AX7s that are properly balanced for PI use?

Thanks,

[David Root]

NOS or current production?

For NOS specify matched triodes (DC voltage) from your seller. Note this does not guarantee AC voltage or current balance, but it should be close too.
For current production JJ go to Eurotubes, other makes buy half a dozen and test them.

NOS GE gray longplates are good Dumble PI tubes, and not pricey like Amperex, Telefunken, Mullard etc. I bought three and two are pretty close triode to triode.

I also think that the '50s long black plate US mfrs. can be pretty good in that respect. I have a used '53 RCA blackplate 12AX7 that is dead on perfect DC balance. In general I have found '50s and early to mid '60s 12AX7/7025 to be better in that regard than most later.

I would also use a 10K trimmer, not a 5K. That helps if you can't find good DCV balance at a reasonable price.

[greiswig]

Thanks, David,

I'm using a 120k/110k pair with a 10k trimmer already.

[dobbhill]

This is how I set AC balance:
https://tubeamparchive.com/viewtopic.ph ... ht=balance

D

[dogears]

Yeah, but what you want is balance of the system. Try this, adjust your way. Then crank and play. Then swap the output tubes from side to side. Very likely, you will hear and feel a slight difference. One way being preferable. This proves that the system balance matters!

This is how I set AC balance:
https://tubeamparchive.com/viewtopic.ph ... ht=balance

D

[dobbhill]

I agree. The ideal would be to include the output tubes and transformer in the "balance."