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In Reply to: RE: "the venerable 6SN7" posted by Triode_Kingdom on July 02, 2021 at 15:54:31
I've never seen the schematic for the 6SN7 version, so I can't give you any information on it. The VTA site says of the 6SN7 board, "Same exact size, parts layout, and hookup connections as the boards above.", with "above" being their 12AU7 version. But parts layout doesn't necessarily mean the same value parts, although a quick visual inspection of the two photos showed many of the color bands on the resistors were the same.
As I was watching the hockey game I started to wonder how different brands of 12AU7s can sound different in that version of the amp, yet the 12AU7 and 6SN7, two obviously different tubes, would sound "no better". Giving him the benefit of the doubt, perhaps he meant different, but no better, but I'm obviously speculating.
Also keep in mind that the AK comment was second-hand information.
Follow Ups:
There are several aspects of the circuit I posted below that seem less than optimum. However, I'm honestly not all that eager to criticize the work of a company that appears to be sincere in creating good value for the money, and which is also producing an American-made product with good quality parts. I think the best approach here is just to advise the OP regarding the better sonics of the 6SN7 and let it go at that. He seems to believe the 12AU7 wouldn't be offered by VTA if it wasn't capable of high-quality sound, and it's probably not worth anyone's time to argue the point further. :)
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Buy Chinese. Bury freedom.
that's not NECESSARILY my view. but I would be curious as to why that particular tube was chosen when it would appear several other choices exist which SEEM to this amateur to have better characteristics in this application?
Nearly any circuit or topology can be bettered. IF you are willing to either spend more or make some other compromise.
Many years ago I worked for an engineer. I ran HIS experiments in semiconductor manufacture. He was experimenting and DID get good results. But his process change required SEVERAL additional steps, including an additional run thru the furnace which was a 4 hour run PER BATCH. so while it was measureabley better, it was a NON-starter to due the original process being 'good enough' and the additional time.....which was NOT compensated for by the slightly higher yield......
this was a semiconducot manufacture process, so I won't bore with details......But I DID get a terrific lunch out of the deal and the engineer DID publish (internally) a report detailing his findings using data I'd accumulated.
Too much is never enough
"why that particular tube was chosen when it would appear several other choices exist"
As has been stated already, the 12AU7 is still in production and made in huge quantities for the guitar market. Everyone has heard of the 12AU7 and 12AX7 because of the guitar market, so it is natural for a vendor to make use of the tubes that people are familiar with and comfortable with.
Of the tubes I mentioned earlier; 37, 76, 6P5G, 2C22, 7119, 5687, 6CG7, 6F8G, and the 6SN7, only the 6CG7 and 6SN7 are still produced as far as I know. It makes no sense for a vendor intent on selling quantity to use a tube like the 6F8G that now go for almost $100 if you can find them. So the use of these older tube types is relegated to those who DIY.
I should also mention that beyond the 56 and 6J5 archetypes, there is an entire range of much newer tubes that provide not just low distortion but serious transconductance and high Mu. Again, with the exception of some super high end vendors that use the Russian tubes, these exceptional tubes are not ever used by commercial vendors. So tubes like the EC86, EC8010, C3g(T), D3a(T), 7788(T), and E180F(T) and several others are only used by the DIY builders.
Despite the 12AU7 based circuit not being "my cup of tea", I greatly admire VTA's helpfulness, and their entrepreneurship. Hopefully nobody thought otherwise.
I read a suggestion that made sense.
Leave the AU7 in the outer positions. The LTP, as I've heard......
Put a 12BH7 in the center position......Available at reasonable cost.
Too much is never enough
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