AirTight ATM-300 - new Coupling Caps

Well, concededly, I haven't read this thread - I just can't stomach R's drivel (be careful...) - but I've owned your amp in a 2nd system for 30 yrs (LS3/5A 15 ohm based) and just can't believe you'd be tempted to chop around on the internals with some non-manuf tech. It turns my stomach.....and I haven't posted here in 30 yrs.

I do have to say, that 15 yrs ago I almost sold my ATM300B because, while dynamic for an 8W SET, I thought the mids were a bit, um, unsophisticated. That was until I put a matched pair of original Genalex GL B749s in there and it's been singing since....so I know the itch to make it better (other tubes: 1997 WE 300Bs, red letter RCA 12BH7s, c. 1942 RCA rectifier, smooth plate, Silltech Double Crown Ruby PC).

Yes, I know, original GLs don't pop up everyday, but there are options out there (1958 Mullard goal post getters sound quite nice...). Try a nice PC on it (PW Maha Samahdi.....?), as I've found it really responds to PCs.

Jupiter OIPs, whatever....It's not the caps, you'll have to trust me on this. 

@asa thank you for your thoughts. Btw. I completely agree with you about Raul’s drivel, I just ignore it :)

To be honest, replacing coupling caps is not dramatic, it’s actually a very simple procedure in this amp, and reversable if I don’t like the result.

Do you happen to know what taps are used on the output transformers of your ATM300, 4 ohm or 8 ohm? I ask because I recently soldered them over to 8 ohm from 4 ohm. I don’t like it so much on the 8 ohm taps even though my speakers are very easy to drive, never dip below 7.2 ohm (Devore O/96). 

With your 15 ohm LS3 you are perhaps using the 16 ohm taps?

Anyway I have rolled alot of 12au7 tubes, I now use 1950’s long plate Mullards. I don’t want to depend on hard to obtain tubes to be able to get good sound from my amp. I did a quick search for the GL 749, and I cannot find any beside the reissue..

Since you say that I have to trust you about the caps, have you tried different caps in your amp? I have replaced coupling caps in other amps, and they always changed the sound quite substantially and to the better.

Yes, changing the caps on anything is usually not a big deal, technically. I just had a tech replace all of the PS caps in 2 Joule Electra pre's I still have around. But change is change....I think I have the 15ohm LS3/5As on the 8 ohm taps, sounds really nice, dreamy. I've had this system evolving for 35 yrs now and I can actually say at this point it is so involving that I just don't want to change anything, and that's a hard place to get to in this hobby/quest/obsession, if you know what I mean....that's probably where my reaction to your cap swap came from....on the GL 749s, you won't find anything out there, too rare these days, particularly as a matched pair, would be $2500+ if you ever saw them I'm sure...you really should, though, think about PC choices. It's not a radical change sound-wise on the amp but definitely worthwhile to look into. I had an Electraglide Fatboy on mine for years; very spacious, but a bit sluggish. A friend passed away and gave me the Siltech and it sounds nice on the amp; smooth, extended, dynamic, no weak points....it's atrociously expensive, though. And I just saw a Harmonix Takumi 1.5M PC sell on another site, my gut tells me that might sound good on the AirTight (I should have bought it myself, but I have too much gear around that I need to sell...)...just a thought. Let us know how the changes sound....

Dear @asa  : Even with your " stomach " :

" whatever....It's not the caps, you'll have to trust me  "

It depends where and wich caps, certainly your recomendation on those OIP goes with your statement or worse than that and " trust me on this. "

Btw, here you have a different " stomach " :

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R.