![]() In the future, I plan on implementing some nested feedback and/or global feedback as an experiment.Ĭlick to expand.My first inclination way back when was to drive the 211 through an IT. Without any feedback, I am getting 25 Watts/8 Ohm 5% THD+N. This two-stage driver circuit has a gain of almost 100, which is plenty to drive the 211 well into A2 territory. I ended up with an SRPP 6SN7GT coupled to a 6BX7GT plate loaded with the Hammond 126B (A Shishido type circuit, although when I designed it, I was not aware that it resembled a Shishido). Base measurements indicated that the 126B had/has good bandwidth and distortion and I did not have to get a second mortgage. Of course nobody (DIYAudio, etc) had anything good to say about the 126 series because they were made by Hammond, but I bought a pair anyway. I always wanted to try an interstage transformer with the big transmitter type tubes, but was shocked at some of the prices until I found the Hammond 126 series. My 211 driver circuit has evolved too over time, but it is two stages plus the output section for a total of three stages. These tubes appear to be much more linear than Triode #1 in the dissimilar envelopes, and the output impedance of this combination should be capable of interfacing to nearly any output triode operating in Class A.Ĭlick to expand.I remember chatting with you briefly at HiFi Haven about our 211 driver circuits. A single power supply with two outputs could power the 6SL7-6SN7. However, in a design with a capacitor-coupled output triode, the cathode follower and its attendant costs could be eliminated. This is the result of wanting a driver capable of pushing the 211 into A2. This circuit isn't as easy to implement as yours, primarily due to the various power supplies required to operate it. These are all actual measurements on the bench using ARTA software. However, it's flat throughout the audio range, with a -1dB point at about 50kHz. Frequency response is not quite as extended as your design, probably due to the use of two gain stages rather than one. At 160V pk-pk, distortion rises to approximately 0.25%. THD at 1kHz with a 20V pk-pk output swing is less than 0.04%. Visible clipping now occurs at about 275V pk-pk. Based on hours spent with SPICE simulations, I also added a nested feedback loop between the 6SL7 and 6SN7.ĭistortion performance and output swing at the output of the 6SN7 have improved immensely with all this work. This circuit has undergone significant changes, including different tube types (the totem pole was initially also a 6BL7), optimized component values and new filament wiring to support independent elevation. I'll repeat that measurement soon, but my focus for the last year has been the pre-driver. I measured the performance of the follower years ago in the prototypes, and it was entirely transparent. The most current (and hopefully final) driver design uses three tubes a 6SL7/6AQ7 at the input, a high voltage 6SN7 totem pole, and a 6BL7/6BX7 cathode follower. Just to elaborate on this a little, I'm nearing the end of a long-term 211 project myself.
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