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Very nice, Anand! I think DIY is the way to go with tube gear. They are not nearly as difficult to make as DIY SS, and there are a lot of schematics available for those who don't want to deal with circuit design. I personally am partial to octaull small signal tubes. They just seem to sound fuller bodied, and present a wide and deep soundstage. I like the idea of using TV damper tubes for rectifier tubes. They are cheap, plentiful, and have a very stout beap, uild quality. The 6AH4 was a vertical deflection tube, so it has outstanding slew rate and voltage handling characteristics. Very stout and well built tubes. The iron is not cheap, but compared to Lundahl, getting the chokes/transformers made from Heyboer can be achieved at very reasonable prices. The total cost to build this was still pretty reasonable.
No major microphonics issues with the 6AH4 tube. I am floored at just how good this sounds. Thomas Meyer is one smart engineer with tube circuits, to be sure. I think I may never go back to commercial tube gear ever again. I can't begin to fathom just how much this would cost if commercially made. The DIY guys were right on about the low mu tube for preamps. Would really like for Roger M. to provide some insight as to why they work so well, and why the low mu approach isn't used more often.
Thanks for the invitation. I do whatever I can do encourage the DIY community. I looked at your schematic which appears fine. The reason you don't have any microphonic problems is you have a gain of about 2x (6 DB). The 3.5 to 1 output transformer helps reduce all forms of noise. I have made a few preamps with output transformers like that. You have already touched on the reason we don't see these commercially. The transformers are expensive and a mystery to many tube designers. It would be interesting to see some measurements of your preamp especially to see the frequency response and distortion as it is a single ended, gapped (I hope) transformer design. Being one of the first to use the 6DJ8 in the RM-1 and RM-4 in 1978 I selected it for its low noise and constant gain (low distortion). By itself it has too much gain for a line stage and many designers don't know what to do about that problem. In the RM-5 I employed some local feedback to let the customer select his gain. I don't use it for everything, but when I make a phono or tape preamp that is the tube I use. The 6SN7 is popular for its low mu but it is hard to find quiet ones. As to why others don't use low mu tubes, I have no idea other than many designers don't do much research into things other than what is already being done. How do you think I chose the 6DJ8 and became an early adopter of that tube?
Roger, thank you very much for weighing in on this. The output transformers were made by Heyboer, based on the the lundhal spec sheet and this circuit diagram. He also made the four chokes. Based on the performance I am getting from the 1625 amps (who made all the iron), I figured that the Heyboer iron would sound as good as the lundhal (at a MUCH better price point). I have limited tools to measure this. I do know it is (subjectively) the quietest tube preamp I've owned to date. I have had several preamps with the 6DJ8. I would agree that the tube has the potential to sound outstanding, but the microphonics issues with them drove me crazy! I do have a NOS DAC (Wavelength Audio COSINE) with a single 6GM8 (similar tube) and output coupling transformers that I still enjoy very much.Thanks Again, Roger! Always value your insight.
Low noise and low microphonic 6DJ8/6922s are not always easy to find. It is why I created the RAM LABS Tube tester in 1981 so I could find the good ones. Testing by hand, as most still do, was just too laborious. Sometimes I send back whole batches when I find the yield is too low to bother with. What you find out there unselected may well be disappointing. However there is nothing lower noise for a low level application. A good one is as quiet as a JFET.....and sometimes better.
6T4 for a low mu preamp as well. Its what AudioTropic used to use in their preamps. Chosen for low noise/microphonic and low mu.
Cool. Thanks for that, Josh. I'll let my buddy know about that tube. He has made a few 12B4 preamps, but so far, the 6AH4 sounds the best to date. I just like the sound from the octal tubes. BTW, TV damper tubes make great rectifiers. No more fooling around with 5AR4's.
