Choke loaded VFET SET amp
Posted: June 14th, 2015, 9:25 pm
Finally, success. David Berning and I have been working for several months now to produce a truly unique and interesting amp. It is a choke loaded Single Ended Transistor amp using two Yamaha 2SK77 VFET transistors I found in my parts bin. I have had them for about 40 years and forgot about them until recently. When I brought them over to Dave's we ran them on his curve tracer and lo and behold they looked similar to a 300B.
I will not go into all the details of the project but the driving consideration to the design was to protect the 2SK77s at all costs as they are no longer available. As a result we had a lot of false starts because on a slew of protection circuits that has a habit of turning the amp off at the wrong times. We will have a meeting in the fall to go into the amp in detail but we are both very busy with our schedules now. We're thinking of having a SET day will all SETs invited at Dave's house as all my speakers are tri-amped.
Just briefly, the amp puts out over 15 watts at 8 ohms, 17 watts at 6 ohms and over 11 watts at 4 ohms. Frequency is flat from 20KHz to almost 20 Hz with just a slight drop at 20 Hz. It is stable into any load from open circuit with a 1uf capacitor on the outputs to any resistive load. We listened to it compared to Dave's 845 SET and they are virtually indistinguishable. Surprising, the lowest frequencies, ie below 20 Hz were better with the 845, not what we expected but Dave said that is probably because the 2SK77s are higher impedance than the 845s through his ZOTL impedance converter used with the 845s and the output chokes like lower impedance.
I'm including three photos, the 2SK77s in comparison to a TO3 and TO220 transistor to give you an indication of the size of the beast, The underside of the amp showing the circuit board, and the completed amp. SInce this is a "retro" project I decided to paint it with a metal flake glitter paint called "cherry bomb" for effect.
Tom
I will not go into all the details of the project but the driving consideration to the design was to protect the 2SK77s at all costs as they are no longer available. As a result we had a lot of false starts because on a slew of protection circuits that has a habit of turning the amp off at the wrong times. We will have a meeting in the fall to go into the amp in detail but we are both very busy with our schedules now. We're thinking of having a SET day will all SETs invited at Dave's house as all my speakers are tri-amped.
Just briefly, the amp puts out over 15 watts at 8 ohms, 17 watts at 6 ohms and over 11 watts at 4 ohms. Frequency is flat from 20KHz to almost 20 Hz with just a slight drop at 20 Hz. It is stable into any load from open circuit with a 1uf capacitor on the outputs to any resistive load. We listened to it compared to Dave's 845 SET and they are virtually indistinguishable. Surprising, the lowest frequencies, ie below 20 Hz were better with the 845, not what we expected but Dave said that is probably because the 2SK77s are higher impedance than the 845s through his ZOTL impedance converter used with the 845s and the output chokes like lower impedance.
I'm including three photos, the 2SK77s in comparison to a TO3 and TO220 transistor to give you an indication of the size of the beast, The underside of the amp showing the circuit board, and the completed amp. SInce this is a "retro" project I decided to paint it with a metal flake glitter paint called "cherry bomb" for effect.
Tom