Hi Rob,

Quote Originally Posted by Filterlab View Post
What he said. The 5691’s are the holy grail of 6SL7 replacements. I have the Melz 6H9Cs in at the moment, which are very very good indeed, but these are the best of the lot apparently.

The buffer is on around 14 hours a day (on when I wake up, off when I go to bed - the system plays pretty much all day). That’s about 5,000 hours a year so these should do me four years, or more hopefully.
Nice one. How much did you pay for the quad? IME, NOS/NIB ones are normally around £70 a piece (or the equivalent in dollars).

I rate the 5691s highly, especially in terms of construction and reliability. I'm also a fan of Melz 6H9Cs. I've used both in my Croft. However, there are other 6SL7s that could also be considered as 'the holy grail'. If you get an opportunity, try some 1940s vintage [*must* be that, not later versions] black glass Tung Sol 6SU7GTY or Mullard ECC35 ['tall glass' with short brown base, such as shown here: https://www.ebay.co.uk/p/1008033773?iid=383269849766 ) both of which IMO would give your 5691s a run for their money!

However, as I've said to Andy elsewhere, much of this comes down to the circuit that the valves are being used in and your personal sonic preferences. Enjoy!

Marco.