tube fanatic Posted December 12, 2016 Share Posted December 12, 2016 This project was inspired by William who constructed an amp using the same tube from a schematic posted on another site. Unfortunately, the amp did not meet his expectations so he had to modify the circuit to get it to work as it should. I'm sure he'll post comments about that. For those who are unfamiliar with what spud amps are, the term seems to have originated by someone calling an amp which uses only 1 tube/channel a "one tuber." Since potatoes, under their various names like spuds are tubers, the amps began to be called spuds. I couldn't resist using a similar term for naming these. The 6CY7 was originally registered by GE in April 1957. It's a dual section triode with one section designed for use as the vertical oscillator, and the other for a vertical amplifier, in televisions. As is the case with similar tubes, it is very useful for audio purposes as the oscillator section makes an excellent voltage amplifier (driver), and the amplifier section usually has a high enough plate dissipation to make it useful as an audio amplifier. Linearity of the latter section is often good as well making a low distortion operating point possible. As is the case with most high-mu triodes (oscillator section of the tube), grid-leak bias works extremely well. That method not only eliminates a cathode resistor and bypass cap, it also allows the tube to self-adjust over its life span. In combination with the cathode biased output section, this is a total plug and play amplifier which never needs any adjustment. Simply turn it on and enjoy! My goal here was ultra low distortion at the expense of some power output (the designer of William's amp seems to have preferred the reverse). The chosen operating point allows an output of 600 milliwatts, with roughly 1% 2nd harmonic distortion, into a matched load (i.e. 8 ohm load on the 8 ohm opt tap). With the usual drop to around 4 ohms somewhere in the bass region, output power increases while the distortion rises to only about 3%. This is extremely good for SET operation. Considering how little power most Klipsch speakers need, the distortion when listening is obviously much lower. Many are now laughing at the minuscule amount of power available. I can only say that with most Heritage and Reference Series models the available power is more than enough to create very loud, clean, levels in small to medium size rooms. The semi recent discussion in this section about the ridiculously low power needs of the La Scala drives home this point very well. The real surprise here is the sound of the amps. I didn't expect them to sound so good. They are totally dead quiet with an ear against the speaker grilles- not a trace of hum or hiss is noticeable. Sonically, they do absolutely nothing wrong, and everything right: enormous sound stage (typical of good SETs), midrange to die for, silky smooth top end without any trace of hardness, and lots of clean bass. I'm not sure why some say that SET bass is mushy and smeared. It isn't! Another plus is that the amps run almost cold. After 5 hours of continuous use, the power transformers barely get warm. These are amps which you can use all day without any worries about heating up the room. As noted on the schematic, I've included the locally famous "ear bleed filter." Although it can be eliminated I urge its inclusion, especially if you find your speakers to be on the bright side. It allows a good bit of high frequency attenuation which can significantly reduce top end brightness resulting in a very smooth presentation. Installation can be on the front panel if you want the ability to adjust it as needed, or it can be internal as a "set it and forget it" adjustment. If you are handy enough to build a pair for yourself, and have never heard what good SETs can do with Klipsch speakers, you must not deny yourself the experience! Mine are constructed on very small chassis (only 6 X 8). It's not necessary to go that small. And, if you are careful with the lead dress and component arrangement, the amp should work flawlessly at the outset. Attached are ample pictures which can be used as a construction guide. Note the heat vents which I installed over the power resistors. Although they do not get overly hot, keeping the underside of the chassis as cool as possible ensures extreme longevity (besides, Carl likes heat vents, so I even included an extra one..........). Post any questions you have, and don't hesitate to start a build thread if you decide to construct a pair. There are enough guys on here with experience to help you with any situation which arises. The schematic is attached as a PDF. I don't think there are any mistakes, but if you see anything which looks weird, please advise immediately! Due to size limits, I have to attach the pics in separate threads. MOST IMPORTANTLY, DO NOT FORGET THAT TUBE AMPS OPERATE ON VOLTAGES WHICH CAN KILL YOU! DO NOT BUILD THESE AMPS IF YOU ARE NOT WELL EXPERIENCED IN WORKING WITH SUCH VOLTAGES IN A SAFE MANNER. Maynard little tater.pdf 4 Quote Link to comment Share on other sites More sharing options...
davesans Posted December 12, 2016 Share Posted December 12, 2016 Very nice work Maynard that is a very cool little amp. Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted December 13, 2016 Share Posted December 13, 2016 That should have been a fun project. That's a nice spud! Quote Link to comment Share on other sites More sharing options...
Schu Posted December 13, 2016 Share Posted December 13, 2016 Any pictures under the hood? 1 Quote Link to comment Share on other sites More sharing options...
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