The 6N1P draws more heater current than the tubes in the 6DJ8/6922/ECC88 family, so anything in that group should be interchangable in a component designed for the 6N1P. There are a plethora of NOS, vintage tubes available in both that and the 12AX7 family to choose from. Generally speaking, British tubes(ie: Brimar, Mullard) will have a warmer presentation and those from the USA and Germanic nations(ie: Sylvania, RCA, Siemens, Telefunken), cleaner, more defined. A lot of the Amperex tubes fall somewhere between. Just my observations, based on various components/circuit designs. Happy tube-tasting!
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I think that Rodman's assessment of the various tube's sound is pretty much right on.
However, I have found that it really depends on the circuit that the tube is used in and how it's used. I'm a Mullard man, (love that midrange), but in some devices they suck.
Only way to know for sure, is to just try a few different ones. Sorry, this can get quite expensive, but really is the only way.
Tube rolling can be fun, rewarding, frustrating, and expensive.
Before rolling a lot of tubes and money, I would suggest two things. Put at least 200 hours on the amp. Buy a Synergistic Research Black fuse for your Vincent amp. Unlike tubes, these fuses come with a 30 day return policy. I think you will be happy in the sonic improvement.
I would start with the 12AX7. Usually, rolling the input driver tube has little if any affect on the sound. You can try the reissue Mullard or Tung-Sol gold pins, which cost about $30 from Upscale Audio for the platinum grade (I use that Mullard in my CAT and they sound great). If they have no effect, then chances are the more expensive NOS types may not have one either, and you would have saved a lot of money. If there is an improvement, then the sky's the limit.
The 6922/6DJ8 family is technically not a direct replacement for the 6N1P -- they have a higher mutual conductance. Without the schematic I can't tell if that would be a factor but those tubes can safely be substituted for the 6N1P.
I just sold one of your amps. Really fantastic amp you have there. I liked the Mullard 12AX7 10M as it stood out as a musical never harsh tube that still let all the detail through, with tons of dynamic prowess. I have a big batch of these if you are interested. All the 6N1P's sound about the same to me, so I would stick with the stock tube here.
You might try a 6DJ8 variant which was specified for the high end of its day: Tektronix, HP, German Post Office. By far the best small signal tubes I've ever heard were Telefunken 6922's, specced for HP test equipment, the stuff that put men on the moon (1962 vintage, I believe). When I was collecting, very few people understood what an HP designation really meant, and they commanded no premium at all !!!!
Absolutely, upgrade your fuses(per davidpritchard’s recommendation) and the power cord, as well. Make your changes one at a time, keeping notes on your observations and(please) let us know of the results. Have fun! BTW: Parts Connexion has Hi-Fi Tuning's Golds and Silvers on sale. ie: (http://www.partsconnexion.com/fuse_hifituning_gold_main.html)