Yeh, is it maybe too laid back ya think? However, I did like Terry's Sherwood after an overhaul and using EH 7868s.
It had lot's of crisp detail that my Fisher using 7591s seems to lack.
If your not happy with the sound of those 1960's production integrated amps that use 7591 (or 7591A's), it's not only these tubes that are giving you what you're hearing.
The stereo amps designed in the 1960's were designed to offer a particular sound. Even if you restore those amps to operate as designed, you still may not like the overall sound.
You won't get changes that are leaps and bounds audibly different by simply retooling to use 7868's or 6GM5's. Initially, that's because they're basically tubes with the same specs. Second, the signal feeding them contains the same info from the same circuit path using the same electronic components as it did before.
If you really want a noticeable audible change in those amplifiers using 7591's, rework the coupling cap stage to try to get the sound you're looking for. Polyester/foil caps will generally leave it with that original vintage feel, and are usually very cost effective (CDE, Mallory). If you're looking for a tiny bit brighter sound, use metallized polyester film caps like Illinois MWR series (a very good cap and
highly cost-effective). If you want it a bit brighter, use polypropylene film and foil such as Sprague 716P, Hovland Musicaps, or MultiCap PPFX-s series. Stepping up a bit more bright, you'd be using metallized polypropylene like Solen PB series, Illinois MPW series, AuriCap, MultiCap PPMFX series, (many, many others).
The Sonicap (Gen I) cap is one of the brightest on the market, but can easily be bypassed with their Gen II series which pops the mids up, and cuts the harshness without losing detail.
If your wallet allows for excellent 'designer' caps, I'd recommend Jensen or AudioNote paper and oil with copper foil. You can also get these from Angela Instruments with their own brand labeled on them; but I've heard they're made on the same production line by Jensen in Denmark. Of course, there's also the Mundorf Silver/Gold metallized polypropylene in oil; as well as a few brands of nice teflon/foil caps out there, too.
YMMV.
Oh, and be sure to connect the inner foil side to the grid side of the tube. This has a direct effect of the frequency response being passed through those coupling caps (it's a reactance/impedance issue). Some may not hear much of a difference, but it sure looks a LOT better on a scope.
. . Falcon
. Do you know the primary impedance? I would guess 6.6k, but if its closer to 7.6k, you could build an amp that uses the new 6973, I've always wanted to do that. Lots of reserve in the transformer then for ba-bass, as trannys for 7591s are usually medium to biggerish...
