How do I know when to replace Melos SHA - 1 Tubes

Hlt,
"How can I tell if my tubes need to be replaced? "

If you hear excessive hiss, hum or microphony, replace the tubes.

Also, if there is a noticeable channel imbalance, you should consider replacing the tubes.

To 100% sure, swap the tubes between channels. If the noise or imbalance follows the tube, Bingo.

There is another reason for changing tubes: lack of dynamics and less bass punch, a sign of weak tubes. Unfortunately the only way to find out is by swapping in a new pair of tubes (NOS or new production) that are matched and measure very close to 100%.

I agree with the above NOS recommendations and would like to add the king of the hill (IMO): Amperex USN-CEP gold pin 7308, a super-quiet and matched-section 6922 developed for Sonar applications.
The equivalent european tube is Philips Netherlands E188cc, I believe developed for the professional Studer reel-to-reel recorders.
Have fun!

Casouza, thanks for the tip on those Amperex/Philips tubes! Probably impossible to find, right?

You'll never really know for sure, it'll just always be in the back of your head, distracting and gnawing at you. The seeds of a conspiracy planted to induce paranoia, overtake your senses, and the world as you previously knew it.....or thought you knew it?

:-)Just kidding!...............................or am I? :-)

Casouza, I guess they're easier to find than I thought. But do I want to spend $300!
http://www.ebay.com/itm/7308-6922-6DJ8-AMPEREX-MATCHED-PAIR-AUDIO-TUBES-USN-GOLD-PINS-/380378537392?pt=Vintage_Electronics_R2&hash=item589053ddb0
.

Nsgarch, no need to pay U$ 300
Try Upscale Audio Gold Grade tubes.
Best wishes