Quote:
Originally posted by markl
I have a question about the tubes for anyone who can answer. Ray states you can use 12AU7, 12AT7, or 12AX7 for the input tube. This is good in that it opens up lots of possibilities for tube rolling. This is bad because it opens up lots of possibilities for tube-rolling.  Can anyone tell me which of these tube types has more power, more gain, and is likely to last longer in this circuit? My guess is the tube that would work best is the one operating most easily within its abilities that can take it in stride, rather than one that has to struggle to pump out the juice you need. Or, is that faulty thinking, if so why?
Same question regarding the 6SN7s. From my ZOTL experience, I know there are 6SN7, 6SN7GT, 6SN7WGT, 6SN7GTA, 6SN7GTB, each a little different than the last (and even some other weirdos that aren't called "6SN7-anything" but have totally different name scheme that are sometimes substitutable for 6SN7s). Berning recos against using regular 6SN7s in his circuit, he preferes 6SN7GTBs for their slightly better power handling capabilities, IIRC.
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No idea which tube variant to use at the input. The 12AU7 has the lowest gain, but may wind up being capable of maximal detail. I'd probably start with whatever Ray is using, as he's likely to have optimized the amp for that level of gain. In addition to the 12a*7's, you'll be able to use 6201's, 5693's, 5751's, 7025's and several others (bozebuttons mentioned the 7316, and I hope I haven't transposed any numbers anywhere
). It'll be a fun experience...there are a LOT of possibilities.
The problem with the GTB variants, as I understand it, is that in giving the tube higher voltage capabilities, the gap between the cathode and the plate was increased, resulting in higher capacitance. This increased capacitance can obscure fine detail, so in theory these tubes should not be able to handle the level of detail that a top GT variant can produce. There are still some fine sounding GTB's. The Tung Sol "tall boys" are one of my favorites, and were great with the ZOTL and CD3000 combo in particular.
IMO the Sylvania 6SN7GT (aka VT-231) is the tube to beat as an output buffer in the Singlepower amps (Metal base 6SN7W's are also excellent). However, if Ray's circuit is significantly different (and it appears to be), there's no way to know besides experimentation. The Tung Sol round plates or Ken-Rad VT-231's are my tubes of choice at the Singlepower input, but I have no idea how they would sound in Ray's amp.
The cryo thread does need updating. It's inconclusive mainly because I haven't reached any conclusions. I think there was a beneficial effect on Sylvania's and Sovteks. I couldn't hear any difference in Ken-Rads. Since it takes about a week to get a set of tubes up and stable, IME, it's a slow process, and I took a break to just go back to a set of reference tubes and listen.
If there's no change in the sound of the amp as you change tubes, that's almost a sure sign of a feedback loop or servo control in the circuit. The upside is that the designer has much greater control over the final sound of the amp, and it's easier to keep the circuit stable with a smaller power supply using feedback. The downside is that it makes it more difficult to hear the differences between tubes, which are still going to be there, just harder to hear. The actual contribution of the tube to the sound tends to be minimized...which can be good or bad depending on personal preference more than anything else.
