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If you can affort and find, try NOS Tung Sol 6SU7, which is 100% replacement of 6SL7. This tube sounds fantastic.
For 6SN7 I highly recommend NOS Mullard ECC33, brown base, short bottle, but thats only if your amp can take that tube, ask Almarro guys directly. Other than that, try Italian made NOS Fivre 6SN7, brown base, they will match Tung Sol perfectly.
Thanks both of you for the responses. I will look into the Tung-Sols. The Mullard seems to be harder to come by unless I buy a new reissue but those have mixed reviews. I think this will be an interesting tube match. I might even get two different 6sl7 to mix and match with the other 6sn7. This is fairly new to me, but it is interesting. I guess it is not too cost prohibitive because you only get one choice of power tubes with the Almarro and they are cheap.
There are so many varieties of 6SL7 and 6SN7 tubes available that it is hard to generalize. But one thing I would say is certain: for a hobbyist, as opposed to a manufacturer, use only vintage Old Stock. Why bother with modern reissues when superior sounding vintage tubes are available at such reasonable prices? A mfr needs a large supply of identical tubes. A hobbyist doesn't need hundreds of the same tube; he just needs a couple of whatever tube sounds best.
For the 6SL7 slot my very favorite is a Sylvania VT-229 (military designation for a 6SL7) black plate from the 1950s or 1960s. I have several purchased on EBay which sound superb. My suggestion is to try several varieties of vintage tubes by buying inexpensive "used but still test good" on EBay. You will quickly form your own opinions about what you like and what you don't.
For those 6sl7/6sn7 users,don't be afraid to swap 6sl7/ 6sn7 or vise versa,or even 6sl7 on one channel and 12ax7/ 6dj8 12au7 on the other channel.it won't hurt your preamp.I've done this a hundred times ,I even use 12ax7,12au7.12at7,12bz7 & 6dj8/ecc88 etc. with an adapter with no problem in my socket modified Dyna Pass II.
You have now been given a year's worth of the best tube options for your amplifier. I have been there and done that. Before buying even more tubes, I suggest upgrading the fuse in the Almarro amplifier to a Synergistic Black fuse or if on a limited budget the Synergistic Research SR-20 fuse.
I think the sonic improvement with a fuse upgrade will be greater than any additional tubes you currently own. Hopefully there is a Synergistic Research dealer in your country that provides the Synergistic Research 30 day trial policy.
Congratulations on owning a nice sounding amplifier.
Hi,You can plug in any 6sn7 gt,gtb,gta wgt,w,& A / 5692/VT 231 or 6sL7 / 6SU7. without any problem with your amp or preamp,I been doing these any time and I even have an adapter for 12AX7/12AU7/12AT7/12BZ7 to the 8BD BASE socket and they work fine if you are only looking for sound..This statement is false. While it is true that you won't damage anything, the fact of the matter is that the two tube types have different characteristics and will not operate correctly at all if exchanged. For example, in many cases a 6SL7 can saturate if placed in a circuit designed for a 6SN7. The tube may well fail! If it does work, its guaranteed to have higher distortion- which is to say it will be a lot higher.
Miniature tubes can be made to work in some cases as suggested, but miniature tubes have a much lower cathode/filament arc-over spec and may instantly fail if placed in some circuits in which a 6SL7 or 6SN7 is perfectly comfortable. And again the miniature tubes have different operating points and so can't be swapped with the expectation that the circuit is going to sound as intended (read: more distortion).
FALSE?,Maybe you haven't tried but I've been doing this for almost 10 years and not a single tube fail,a living proof.BTW,my preamp has stock 12ax7.I'm sure you've gotten away with it. I've done the same. But I can tell you for a fact that all those different tube types have different characteristics, each requiring a certain set of operating conditions for best results. I've already described what those results are about.