Can't remember exactly how long I've had this speaker APPJ for, (PA0901A), but it must be not far short of 10 years, and the other amp I have, the Little Dot MKV+ for several years longer than that. Both are modded with the LD having a far greater number of upgrades than the APPJ which was relatively simple to do, it only requires upgraded coupling and cathode caps. The caps should be of best quality possible to affect sound beneficially, so the Miflex's that member Wushuliu mentioned could be a good bet for coupling, and the Kaisei's for cathodes are fairly cheap but good.
As I've had the amp for quite a while now I'm in a good position to to evaluate it properly, and I've got used to using different tubes from what I started out with so I thought it might be interesting to do a summary of my impressions of the amp now for anyone interested... if there's still anyone out there that is! I know there are a couple of members with that version APPJ.
Build: To answer a question from a member a while back I can say that the amp is reliable.... touch wood.... the upgrades have made no difference to this. This is good because the internals, being surface mount are not really made for modding but if they're left alone should last really well. But if the upgrades are done with care that shouldn't affect this, however I managed to mess it up, along with several other members! So build quality is not bad.
Pairing: It should pair well with anything from simple no crossover to multiple crossover amps, mine is single crossover, from sensitivities from about 85db upwards, mine is 87db and people have said it shouldn't work with such a low sensitivity, 90-95 would be much better. My answer to this is it works fantastically well, I use it in my average sized dining room, I like it quite loud and I never feel the need to go past 12 oclock so plenty in reserve. It doesn't clip or distort at all and is dead quiet. It would pair with warm or more neutral speakers, mine are fairly warm sounding.
Sound: The sound is beautifully balanced, liquid with good frequency response pre mods. Post mods it just pushes everything to the max, soundstage, resolve, textural detail, dynamics, frequency extension are all top level. The sound is transparent but with a nice touch of tubes, but it is not over warm or syrupy.
Comparisons: From reviews and my own experience I would put this at top level post mods. Other amps may be more neutral, or more tubey sounding according to taste. To see it compare favourably to the much more expensive and highly modded Little Dot is pretty impressive and difficult to believe.
Tubes: I think I'm in a good position to know what my preferences are now I've lived with them for several years. I won't go into past tubes, people can look back in the thread for that. So starting with my previous favourite the Tungsol 5751 has a glorious liquid, yet detailed sound. The TS14AF7 has great resolve, detail, treble, transparency, I thought it was slightly thin occasionally and one dimensional soundstage, but still excellent, Philips 12AX7 has great punch richness, resolve, textural detail, frequency extension, 3d soundstage. The Mazda 12AX7A silver anode is lighter and taughter sounding with light extended treble, great resolve, really open soundstage, there is a really exciting percussive edge to the upper register like when a real guitar is being played near by. Can be a tad thin on thin sounding music but on most music sounds quite brilliant IMO with great textural detail. That and the Philips are my fav tubes now... with the Philips in top position. This tube is the best I have heard, and it is unlikely to ever again come out of it's socket on the amp. It has texture in abundance, the frequency extension is impressive with powerful bass, mids and treble all in perfect balance. The tube has much of the percussive attack of the Mazda but is better for all types of music due to its richer tonality whereas the Mazda can be on the light side. The dynamic response of the tube and all that textural detail all go to making the soundstage impressive and many layered although it is not so open sounding as the Mazda. I think the Philips is perfect for this amp because the amp naturally has a tight attack and quickness of response, therefore the fuller sound of the tube makes for a better combination on the amp. This is the best ever sound I have heard coming out of this little amp. I think I have finally overcome my prejudice against the 12Ax7 tube which I now regard as a superb tube taking into account all the variants we have gone into here, and it is known to be a very linear tube.
I've gone into this at length because having lived with the amp and tubes for some time now I feel in a good position to comment finally on the setup we have here.
That's where I'm at atm and I doubt anything will change as I'm very happy with the amp. I would be interested to hear what others with the headphoe APPJ, the subject of this thread thought in comparison.
I believe this amp displays all the best qualities of SET tube amps: speed, openness, transparency, and the SQ indicates to me that the transformer is good quality too, which is fortunate on output.
Edit: I added a concluding section to the paragraph on tubes...
