[Redwingnine]

Wanted to get some feedback/opinions on the Stax Headphone amps, specifically the tubed units.  I've seen some posts that suggest that the tubed units do not sound as good as the solid state offerings.  
 
Well, my experience is that the Stax Tube head amps sound outstanding.  I have a SRM 006t II, and a SRM 007 tA, and they both sound great.  What I did notice was that the units did sound a little closed in when I first picked them up.  However, after swapping out the Sovtek stock tubes for NOS RCA tubes, the character of the sound improved markedly.  The closed in sound was replaced with increased clarity, more life like.  I was a bit surprised that the NOS tubes would cause that much of an improvement, but they did.
 
The SRM 007tA is an absolute joy to listen to.  Currently using SR 404 Signature phones, but will be getting a pair of SR 507's very shortly.  Really looking forward to using the 507's with the SRM 007tA. 
 
Who else here has changed out the stock tubes, and what was your results/impressions?  

 

[spritzer]

Rolling different tube manufacturers will give you a change in flavor but for any real improvement you need to change the tube type.  This requires internal changes though.  Moving from 6CG7's to 6S4A's in the 007t is a gigantic change. 

 

[MDR30]

Rolling different tube manufacturers will give you a change in flavor but for any real improvement you need to change the tube type.  This requires internal changes though.  Moving from 6CG7's to 6S4A's in the 007t is a gigantic change. 

Is that possible to do with the 006t as well, and are instructions available?

 

[Redwingnine]

  Rolling different tube manufacturers will give you a change in flavor but for any real improvement you need to change the tube type.  This requires internal changes though.  Moving from 6CG7's to 6S4A's in the 007t is a gigantic change. 

 
I think it’s more pronounced of a difference that you indicate.  The small signal current production tubes are just not all that good sounding.  The NOS RCA tubes is more than a different flavor, it’s a big step in the right direction sonically. 
 
I’m not so sure about the tube you mentioned.  It’s only a single triode as opposed to a dual triode.  I would think something along the lines of a 7044 or 5687 tube would be more in line with your stated objective.
 
Stax has to sell the amp with Sovtek tubes for production and marketing purposes.  The end user has no such restrictions, so a change to NOS tubes can be easily achieved, and it seems to provide a marked improvement in sonic performance. 

 

[spritzer]

None of those tubes would work.  The dual triodes are driven as one triode but the real problem is Stax running a 330V tube at 700V.  The 6S4A is the perfect tube for the job.  Not easy to rewire the board though. 
 
Quote:

 

Spoiler: Warning: Spoiler!

 

Is that possible to do with the 006t as well, and are instructions available?

 
Not with the 6S4A's.  The only dual triode that is higher voltage and has two identical triodes inside is the ECC99.  Higher filament current though.  Plenty of dissimilar triodes though but they won't help us. 

 

[Redwingnine]

   
Not with the 6S4A's.  The only dual triode that is higher voltage and has two identical triodes inside is the ECC99.  Higher filament current though.  Plenty of dissimilar triodes though but they won't help us. 

 
According to the tube data sheet, running the 6S4A as a A1 amp lists the plate voltage as 250V, same as the 6CG7.  The max plate voltage for the 6S4A is 550V, but that is as a vertical deflection amp, not for audio.  
 
I maintain that the Stax amps with good NOS tubes sound excellent, quite a bit better than the stock tubes.  I don't think it's worth the hassle to modify the Stax amps when NOS RCA's provide a marked improvement.  
 
There was a version of the Stax amp that ran ECC99 tubes, but it was discontinued.  Not sure why. 

 

[spritzer]

The output stage of the 007t (or the 006t as they are identical) is just a grounded cathode amp except the cathode is tied to the - rail.  The ratings for TV use are more appropriate here as we want maximum voltage swing. 

 

[Redwingnine]

No doubt the 6S4A is a stout tube.  Not sure where one gets schematics of the amp drivers. 

 

[spritzer]

The SRM-T1 schematic can be found online but not the 007t. Very similar though, the driver stage for the tubes has been beefed up on the 007t and different part values due to the dual tubes per phase. Next step for mine is to add a CCS for the output tubes which should be interesting.

 

[Redwingnine]

It looks like the SRM 006tII may already employ CCS. I see the Red LED's, which looks very much like the DIY Torpedo Head Amp.

 

[spritzer]

There are CCS's in there but not for the output stage, just 66K plate resistance. 

 

[Anders]

The SRM-T1 schematic can be found online but not the 007t. Very similar though, the driver stage for the tubes has been beefed up on the 007t and different part values due to the dual tubes per phase. Next step for mine is to add a CCS for the output tubes which should be interesting.

Yes, that is interesting and I am waiting for the result.

 

[spritzer]

It's on my list but who knows when I'll get to it.  It was initially postponed as I needed normal bias sockets (it will be used to drive my old Stax so it needs NB) but I got a bunch of them on Friday.  
 
I did completely refurbish a SRM-T1 I've had for years yesterday and given the state of the plate resistors it made sense to change it to ECC99's.  Very nice indeed now...