Higher end Tubes for Ah Njoe Tjoeb 4000

Please don't try them. 6ES8 -- not stable and not interchangeable. 7DJ8 -- different heater voltage. Not interchangeable.
You just need to pay up for the NOS tubes

The Amperex 7308's are very good (esp the PQ versions). The Seimens 7308 is supposedly the ne plus ultra 6922 variant, but its expensive.

Also very good is Mullard

Au contraire, the 6ES8's work fine in the Ah, and they sound
great too. After much tube-rolling, I settled on the 6ES8's
and yes, I wanted to use the much more expensive Siemens
PCC88's or the Tungsrams and others-a very surprising find for
me. YMMV! Have fun.

Hi Douglas

I very recently bought a used 4000. Amazingly good for the money, but when I first turned it on (Stax SRM-T1S/Lambda Sig's only set-up), I was dismayed at how mediocre it sounded. I quickly replaced the Sovtek 6922's with the Phillips 7308's that also came with the machine. Better, but still not so good, IMO. I then started tube rolling using my stash of NOS 6dj8 types that I have collected over the years, since the pre (Melos) in my regular system uses this tube. Here are my findings, and please note that I use the 4000 in an extremely revealing electrostatic headphone only set-up, so soundstaging issues don't really apply the same way; it may be different in your system:

Telefunken 6dj8: Excellent tube. VERY smooth, total lack of electronic glare. Full sounding, maybe a little TOO smooth up top, not quite as dynamic as some.

Siemens (W. Germany) 6922: Great tube. VERY clean and refined sounding. Excellent dynamics. Ultimately, a little too lean for the Stax 'phones. I would imagine that with a different interconnect between the 4000 and the SRM-T1 amp, this would be the one. I use Kimber.

Phillips (Holland) SQ "Miniwatt" 6922: This was the surprise of the three. Not quite as refined as the Siemens, but clean, fuller sounding, and VERY dynamic. And a lot cheaper. That's what stayed in the machine.

All three were FAR superior to the stock Phillips 7308 in every respect,and why anyone would use the Sovteks is beyond me.

As far as the 6ES8's and 7DJ8's are concerned, issues of interchangability aside, don't assume that simply because it is a 6ES8 or 7DJ8 it will sound better. In my experience it's the manufacturer that matters. I have tried (in other equipment) both of those types and I was not impressed; even some "Bugleboy" 6ES8's that I thought sounded awfull in my Melos pre.

Good luck,and I hope this info is of some use.

Frogman,
The 6ES8 is a variable mu tube, not suitable for pre-amp use,
no wonder it sounded awful. The AH! uses the tubes only as
buffers, not as voltage gain devices, giving more leeway in
your choices. As long as pinout is compatible, anything goes
but how it sounds is your call. My 6ES8's are American
Amperex, and they were really cheap in comparison to the
others I auditioned. As always, YMMV. Clyde.