[djoliverm]

Hey guys! So I bought this guy on Amazon and I immediately wanted to replace those stock Chinese "6J9" tubes. What I could find online was that the 12AX7 tubes would fit and work for this particular hybrid tube headphone amp, so I went ahead and ordered a set of Tung-Sol Gold 12AX7's, and when I got them and put theme in, immediately you get a high pitched squeal, which has nothing to do with the volume that he amp is set at (i.e., at "zero" it's just as loud as at any other volume"). I used an FFT on my phone and the it seems that the peak of this sound was around 1800Hz, and also tapered off at other higher frequencies. Here is a screenshot of that.
 
Thinking that they were defective, and not finding anything online about this particular problem, I returned them for a refund, and ordered a pair of JJ 12AX7's, which are the number one seller on Amazon. Tried these just now and the same exact thing happened. I've tried swapping their positions, and putting them in individually on each of the two sockets, same deal. The stock Chinese tubes however work fine. 
 
Now, I know the 12AX7 are the highest gain of their family, so is there something to them just not really working for headphone amps, and should only really be used for guitar amps? I've read reviews for all of these tubes where people use them in other headphone amps, so it appeared to me that at least in some headphone applications they do indeed work. 
 
I'm just wondering if anybody else has had success with swapping out better tubes on this particular amp. I would greatly appreciate any help, thank you!

 

[SteveHiFi]

It is likely that the tubes on the J69 are not in the audio path, they are there to look good while the audio just gets amplified using solid state components. If you have a spare half an hour, it gets a close inspection here -
 
https://www.youtube.com/watch?v=coSt5HWRvv4
 
Edit - it seems that the tubes may be adding harmonics but I can't watch it all since I'm working!

 

[djoliverm]

So I figured out last night that this particular amp DOES NOT use 12AX7's; I just took it as fact as someone on the Amazon listing had responded to a question saying that it's compatible. The 12AX7's fit, but this amp uses pentode tubes and the 12AX7's are triodes I believe. What sucks is that there are plenty of 12AX7 is very popular, but I haven't found many new manufacturers for the sizes that are compatible with this amp.
 
What I found on an ebay listing of the same amp is that these are compatible: EF861 / 6688 / E180F. I have found that the EF86 is being manufactured new as an EF806S by JJ and Tung-Sol, although I couldn't 100% confirm whether an EF861 is the same as an EF86/EF806. I might order a pair of the Tung-Sols and just try them out, but I believe at the very least they're both pentodes and very similar specs from what I could gather. 

 

[drich1982]

I have the same amp and was curious if you when ahead with any tube replacement and if there was a notable difference.

 

[DJ The Rocket]

It is likely that the tubes on the J69 are not in the audio path, they are there to look good while the audio just gets amplified using solid state components. If you have a spare half an hour, it gets a close inspection here -

https://www.youtube.com/watch?v=coSt5HWRvv4

Edit - it seems that the tubes may be adding harmonics but I can't watch it all since I'm working!

Are you serious? Can you name ONE single amp out there that does this, using tubes for decoration and literally nothing else? Seriously, I'd like to know. I'm having difficulty believing that ANY manufacturer does that, but if they do, I'd like to know now so I can avoid their products.

For this particular amp, it's now well-known that while the amplification is done with solid-state components, the tubes are used in a buffer stage to add coloration, achieving the "tube sound" everybody loves (or hates). That's a far cry from the decorative, nonfunctional tubes you suggested.