Question about the headphone amplifier in tubed preamps

I think the general advice on that was to fill those slots with cheap tubes -  like Electro Harmonix, though used Sylvania 6SN7GTB could fit the bill too (and should last a LONG time). I'd hate to leave the slot empty or covered in tape.

A pair of 6SN7 in some kind of follower configuration, probably output coupled via electrolytic capacitors, does not make a super headphone amp by today's standards. But it still might sound nice with high impedance headphones like Sennheiser HD600.

Tube designs which use transformer coupled outputs can use basically the same circuity for preamp and headphone amp outputs. The problem for OTL (transformerless) amps is providing enough capacity on the output caps to not roll off bass when driving headphones, while also not compromising the cap choice when used as a preamp that doesn't need very much capacity. That's hard (if not impossible) to do well. So Cary made the choice to use a separate output stage just for headphones. The problem is it's probably not up to par with the preamp stage.

Another interesting option for you -- Schiit has a solid state 6SN7 tube available to replace vacuum tubes. You could get two of those for the headphone section and then you'd never have to worry running down the tube life while they are sitting there doing nothing. And, if you decide to use headphones, that circuit is immediately available.

If you go to Schiit's web site, and look in "Accessories" under their product lineup, you'll see them listed as LISST tubes. I bought one for my Lyr 3 headphone/preamp and was quite impressed with how good it sounds. I very slightly prefer the glass 6SN7 but was surprised as to how close the LISST was to the glass tube.

Finally, they do have a disclaimer about using these in amps made by other companies, so be forewarned. However, I'd suspect that is mainly CYA wording from their lawyers. But, still something to consider.

@mulveling  and @mlsstl  thanks for the info & advice.  I will look into those Schitt SS 6SN7s.  Currently I have the EHs that it came with in those slots.   Thanks again.  Stuff like this makes this site great. 

@mulveling you are without doubt one of the most knowledgeable contributors to this Board. Always learn something from you. 

@fsonicsmith1  Cheers :)

Happy when I can provide useful input for others - I way way too into this hobby lol