Tube Lab  

Of course, but most of our viewers don't have the ability to tackle such a mod, so we stick to what is possible.

Hey Frank. I’m worried you’re going to think I’m following you around RU-vid. Haha! Seriously though, it’s interesting that we both go to so many of the same channels, mostly based on audio. I am always awestruck by your knowledge of the history of tubes and audio, and the fact that you can remember all those details and how each tube sounded. I could easily see a book being written with all your knowledge being written about your experiences and history with audio and tubes, speakers, etc., etc…. I’m actually thinking of titles for the different chapters as I’m writing this…lol. I’m not sure if I mentioned to you that I do a livestream every Sunday night at 9:00 EST with a friend of mine who’s a recording engineer, producer, mastering engineer, and was the head of academics for almost 30 yrs at a recording university in California. His name is George Borden. You’re probably familiar with some stuff he’s done. We just talk about music, how it was recorded, different albums, audio gear, songwriting, etc., etc…. If you ever happen to be awake on a Sunday night, the channel is just his name. No pressures, just throwing it out there. Janos came and joined us for a show back in October.

HI NIck.................feels good to know to have at least one follower hahaha... Have a nice day NICK and as always a pleasure to meet you ....@@NickP333

Great to hear from you in Germany! We share a lot of things (for free), including full schematics for the Kit Amps, but the re-basing project was a huge investment in time and money, so we really can't share the methodology, it took months to figure how to do it on a professional level.

You could change the socket or make a socket adapter.

Yup it can be confusing! Gm (mutual conductance) testing numbers tell us how much gain we will get out (let's call it volume) for a given amount of input voltage on the grid (again let's call that input volume). The Gm testing number is always referenced to something, normally either the minimum acceptable value or as we do, as a percentage of NOS (new old stock). So for example a Sylvania 6sn7gta with testing numbers of 100/100 (100%/ 100%) is a perfect NOS tube with perfect matched sections. In reality NOS typically ranges from 85 - 105, with as you mentioned Tung's testing on the lower end of expected Gm. How does this affect sound? It doesn't so long as the gain (volume) is enough for your system. BUT the Tung's will play at a lower volume than the Sylvania and you will hear the difference in volume right away. So you turn it up! For sound quality you want the closest to NOS you can find/afford, those tubes always sound the best. And that's why testing numbers are so important (and for matching sections).

It depends, so the 6GU7 and 12AU7 are both general purpose dual triodes with very similar specifications. There are two major differences between the two, the first is that the filament voltage is different. With the 12au7 running off of 12.6 volts at pins 4 and 5 and the 6GU7 running off of 6.3 volts at pins 4 and 5. The other major difference is that pin 9 is used as a heater tap on the 12au7, allowing it to be run at 6.3V and on the 6GU7 it's used to connect an internal shield to ground. So in order to use the 6GU7 in place of a 12au7 you would need to account for these two factors. On our amps we use an external DC filament supply so it's easy to switch the voltage for the tubes. If your amp doesn't allow for this, it might be possible to modify it to allow for an external filament supply. Some amps that run the 12AU7/12AT7/12AX7 series of tubes also run them on 6.3 volts using the tap on pin 9. If that's the case, then you would just need to re-wire the sockets a bit to get them to work. Or get / make an appropriate adapter. Of course if you're building an amp from scratch that was intended to use a 12AU7, it's easy to make the necessary changes to run a 6GU7. It's also easy to set it up with the external DC supply I mentioned earlier so you could run either tube by just changing out the filament supply and leaving pin 9 disconnected. This is exactly what we've done with the prototype testing preamp, and both tubes operate at the same point and settings just fine. Warning though, electronics and tube amplifiers can have very high voltages present, which can be lethal. Exercise extreme caution when working around them and always consult a professional technician when in doubt.

@@charleslambton7836Thank you. And forgive my ignorance--I thought the 6SN7 was an electrical equivalent of the 12AU7.

@@jensastrup1940 No worries! So the 12AU7 is commonly called the direct equivalent of the 6SN7, but that's not technically the case. The actual direct equivalent is the 6CG7. What happened was that in more modern equipment, 12v heaters became more common and so the 12AU7 took on many of the same rolls that the 6SN7 had been performing before it The 12AU7 is actually descended from a slightly different line of tubes. Before the 12AU7 there was the the 12AH7, which was a similar tube in an Octal bottle. The 12AH7 and the 6SN7 and all the other medium MU octal dual triodes evolved from single triodes like the 6J5, 6C5, and 6L5. Which in turn evolved from earlier single triodes like the #76, #56, and #27. There are a few other transitory tubes mixed in here and of course versions of these tubes in different heater voltages and bases. This is all just a lot of unnecessary history, but basically it means that all these tubes perform similarly to each other and were all designed to be "General Purpose". So in many cases they can substitute for each other as long as you have the correct socket, pinout, and heater supply.

Yes in v6&9, the driver/phase stage. This stage brings up the voltage for the power tubes as well as splits the phase for class AB operation. We actually have a Preamp Set for the R8 that uses the 6gu7 in combination with the re-based 7f7 (Sylvania 6SL7gt equivalent) and it sounds amazing, bringing the warmth of the 6SL7 into the preamp stage and the clarity of the 6gu7 for the driver stage. You could also use the 6cg7 in v6&9.

@@tubelab194 ok thanks.