Siemens EL 34

@pehare well, not to rain on the parade, yet perhaps I can share this if it helps, and what I learned recently. You and I have both owned the SLI-80, and have heard what some may refer to as parallel single ended triode switch mode sound in that amp. You also know what UL switch mode sounds like in that amp... did you like the UL setting in that amp? IF so, what you have right now with Will’s amp is similar in UL mode, one might stretch to say - when using the same tubes and caps.

Here is the catch, for the QS Mid Monos, the "50% Triode" caption and unique description is something I asked Mike at QS about directly two weeks go. I don’t think it’s secret or proprietary info. I asked about the circuit function, and what it really means in his words. This was the reply - "It is 50% ultralinear which is half way between triode and pentode operation". A friend has a pair of the QS mid monos and I brought over my Dennis Had Inspire Hot Rod amp to compare. The Mid Monos were classic EL34 sound, a bit more forward stage, midrange was truly present, yet not nearly as recessed or set back type of sound stage as you might hear with one of the little single ended Inspire amps Dennis use to sell or the SLI-80 in triode switch mode. Depends on what type of PSE/Triode sound you might be looking for. As you know, Will also makes a type of parallel single-ended / triode wired version of his amps too, fwiw. I wonder if Will could reconfigure your current 70 amp and get it closer to what you want, instead. I don’t know if that would require a transformer change or anything. He’d be someone to ask. Would be interested in his response back to you.

Thanks decooney for all the valuable info. Will and I did discuss his triode version but given my power requirements decided UL would be the better choice. It does get loud here sometimes:) That and I’d heard another ST-70 (Chris Keller’s version which he builds UL) in my system and liked the sound.  At some point I'll try some KT66's for a different flavor but overall it's a great amp.

@pehare have you tried GL KT77s in that amp by chance? 

I'm considering searching out a good very good designer/builder to commission and build a nice pair of monos for me in single-ended or parallel-single-ended (if I can find the right builder who is capable of the project with a proven circuit).  To start I would likely use really good EL34s or KT77s. More than a few friends have run the newer KT77s at my suggestion and all of them see quite pleased.  Maybe a little more power and retaining a bit of that 34 velvet.  Will be interested in your feedback on KT66s in Will's amp later on if you try it down the road. 👍 

decoony, no I have not tried them.  I'll probably play it as is for 6 mos. or so before I try different outputs.  Yes, I've ready somewhere that the Upscale Audio folks recommend KT77's as an upgrade in EL34 based amps.  

There's a minty pair of AES/Cary six-packs for sale on Ebay and craigslist not too far from my area - have you by chance tried those?  Too many output tubes for me but I've heard they're great sounding.

@decooney I tried Gold Lion KT77's back when I owned a Music Reference RM9 MK2.  I preferred Gold Lion KT88's and Siemens EL34's in this amp, as the GL KT77's lacked emotion and just sounded too clean & lean in my experience.  YMMV.