NOS Tung Sol 6550 VS Tung Sol 6550 New Production

I think that the 6550 the OP is referring to is used in the power supply of his preamp.  I don't think it would have the same impact that a set of NOS power tubes in an amp would.  If I'm wrong, let me know.

U

I also have the ARC Ref 6 and thought I would try the NOS Tung-Sol.  I like a rich, sweet midrange, which is what the stock Sovtek has.  I did not like the Tung-Sol.  It has more extended highs, a deeper, tighter bass and is more dynamic.  However, the midrange is sacrificed for these qualities--all in all I did not find it musical.  I much preferred to original Sovtek; it has a very sweet, rich midrange.  I thought there might be an issue with break-in time, so I did put 50 hours on the Tung-Sol with only a slight improvement.

tattoo

some people who have answered you here seem to have only read your title line, then didn’t read what you actually are asking -- which is these titled tubes used in an ARC Ref 6 linestage...

the 6550 position in the Ref 6 is in the rectifier section, it is NOT used as a power tube in the signal chain at all, so its impact on the sound of the unit will be somewhere between minimal and none...

so I would not worry so much about using these ungodly expensive NOS USA 6550’s from the 1950's and 1960's and stick to what ARC suggests to use in terms of modern day produced 6550’s for the specific purpose in your unit

you may want to direct this query to greg christensen at arc (technical / service manager) -- ask him whether they have done usage and listening tests using old stock american 6550s in the Ref 6 rectifier circuit and what they have found

this is one of the hazards of posting a question like this on a forum like this... you give people license to wax lyrical on stuff they have feelings about, but they are not answering you on your specific application -- details matter!

good luck

you may want to consider the Shuguang KT88-T nature sound series. among the best sounding 6550/KT88 I have heard, in a power amp, but do not know what it would sound like as a rectifier.