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Originally Posted by moredes /img/forum/go_quote.gif
I'm new to this and not real knowledgeable, but from my perspective, I have a coupla questions. What are you using for a driver tube, and what were your "bad experiences" with the 6BL7's and BX's?
I like the BLGTA's for their 'retained' warmth and openness (I know--kind of a contradiction in terms) but I'm using a Mullard ECC32 to drive those, and that tube may account for my favortism. (by "retained warmth", I mean that in comparison between 6SN7's and 5687's, I found the 6BL7GTA's to be almost as warm as the SN's and 5687's--certainly, the BLGTA's are not as 'fully' warm, but they're not thinned so much that I'd describe the BLGTA's as 'less warm'... I hope that makes sense)
I've tried GE, Sylvania and RCA BLGTA's but prefer the National Union (black[?] is that the right name for it?--it's black around the lower half of the glass and the base is black) for their warmer highs above the others. And the BL's are 'way noticeably warmer than the BX's. BX's screech.
From my experience, either of the 6BLGTA (or ...B) is more dynamic and powerful than any 6SN7 in the output ports. Frankly, I don't hear any difference at all between 6SN7GTA's and B's there.... on the other hand, I don't know what problems you encountered when you used the BL and BX's you bought.
As an aside, I found the 6BL7s to be too shrill for input work.
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First to Frihed 89, the 6sn7gta/b can take up to 450 volts on the plate but the tube is not seeing 450 volts in your MPX3; none of the output tubes are. The high voltage setting in the MPX3 tops out around 400 volts. Most of the later SLAM SE models were set at 375 volts.
By the way, if I had to use 6sn7gtbs I would use the TS tall bottle 6sn7gtb or the Sylvania 6sn7gtb. I like the airiness and neutrality of the TS but the tube can sound harsh at times. The Sylvania sounds warm, rich and smooth but to closed in and dark for my tastes. These are good tubes but, to me, as output tubes they just dont compare to the 6bl7gt.
Secondly, to moredes, a 6sn7gta, say from Sylvania, is no different sounding than a Sylvania 6sn7gtb .... the only difference is the 6sn7tb has controlled warmup. The 6sn7gta was the redesign of the 6sn7gt and the 6sn7gtb added the controlled warmup to the 6sn7gta to make the tubes live longer in TV sets. That is why you dont hear any difference with the 6sn7gta vs the 6sn7gtb.
Thirdly, I agree with moredes, as I stated above, I would take a 6bl7gta over ANY 6sn7gt as an output tube. The 6bl7gtas just have so much more power and a noticeably increased soundstage size. Plus, the 6bl7gta still has VERY good tone as mordes noted .... and less overall coloration to my ears. Two of my three favorite amps use 6bl7gtas as the output tubes. Note: there is no 6bl7gtb. There is the original 6bl7gt and the later redesign 6bl7gta.
Concerning the 6bx7gt, the 6bx7gt definitely has a lighter, brighter tonality and an airier sound than the 6bl7gta. I found them somewhat harsh with my original gain tube choices. But, after some experimentation I have come to enjoy the tube. Both the 6bl7gta and 6bx7gt are exceptionally tough tubes. I use the 6bx7gt in my high voltage Supra. The tube works well in the Supra because the output tubes are being driven almost 50% over spec, so consequently, the 6bx7gt warms up. The 6bl7gta warms up too .... to much so with Senns. So, for an allrounder I get the best results with the 6bx7gt for Senn and Grados in this particualr amp.
Finally, you can use a 6bl7gta for gain. But, I dont think the tube works well for gain at all. The 6bl7gta was typically used as a driver tube to drive big output tubes like 2A3/300Bs etc... in integrated amps .... which is why the tube makes such a good headphone output tube. Remember the high voltage SP amps are only high voltage on the output tubes. The gain tube is still running at the default 300 volts with a much lower current bias. The reason I had the original high voltage amp built was because the default 300 volt/ 10 ma bias didnt bring the 6bl7gta tube alive. The 6bx7gt is an even worse choice as a gain tube as the tubes mu/ gain is only 10.
Here is a one off high voltage PPX3 SLAM that uses 6bl7gtas with 9 pin to octal adapters @ 375 volts. This amp is exceptional with woodied MS1's.
Here is the original high voltage SP amp; an updated old chassis Supra. This one runs at 392 volts but the bias current is a very high 56ma. This amp is a pure powerhouse ....