In answer to Mechans 1's apposite question '
has anyone found another single triode we might try' I have two on the go in Ember and Horizon3:
Zenith and Ken-Rad 6P5's, and Hytron E1148/VT232's. My impressions are similar in both amps.
The E1148 looks like a 7193 but has a longer bottle, so our helpful Chinese seller made a new adapter with a longer wire so they wouldn't stretch too much.
http://www.ebay.co.uk/itm/1pc-Dual-E1148-VT232-CV1135-TO-6CG7-6FQ7-ECC88-6DJ8-tube-converter-adapter-/191763536185?hash=item2ca5ff5939:g:XgYAAOSw3KFWdlyy
I could still use the 7193 Franken Ember adapter but the wires were quite tight and I was concerned the soldered connections might eventually break.
The 6P5's, which I believe predate the 6J5's, are Zenith, which is a coke bottle shape, and the Ken-Rad is a straight bottle.
The Zenith is fairly fast, dynamic and very sweet sounding-a very good tube- but unfortunately it has quite a loud, intrusive hum which I can't at present remove by the usual earthing methods. I have another coming to see if it's just these two tubes. Hence I hesitated posting about them as I'm unsure if the hum is universal to this tube type, but if it's not then it's a fine sounding tube to me.
The Ken-Rad 6P5's don't hum and are actually very quiet. They are warm and glowing, a fraction down in dynamics, with a prominent bass. The mids and treble don't suffer though and the whole sound is refined and well ordered in a holographic sound stage. Some might find it too polite but I like them for solo piano and late night listening to relax and unwind. They won't get your adrenaline going though, and hence I don't think they will suit large scale orchestral music or a reverberant church acoustic. Nor are they likely to suit warm/bass heavy cans. They do go well with my K701's and DT 1770's though.
The E1148 was apparently used extensively in British Army tank radios in WW2- the 'E' designating 'Export' . Many were manufactured by Hytron, and there is a similar tube labeled VT232. I can't differentiate the two by their sound. This is a great tube to my ears and has many of the characteristics of the Visseaux. Without going into excessive hyperbolic jargon I would describe them as perfectly balanced with excellent instrument detail, separation, texturing and imaging. Timbre is very good too. They are quiet tubes with the usual big, holographic sound stage we have come to associate with these dual single triode set ups.
If you can't source a Visseaux I would say these, together with the 6L5's, would be worthy alternatives to try.
Here's some pics:
Zenith 6P5 in Horizon
Hytron E1148 in Horizon (note the horizontal plates, each of which glows from both ends, as below)
Ken Rad 6P5 in Ember.