KT77 Single Ended Stereo Amplifier
I have finished the KT77 / EL34 single ended stereo project. I have to say, it sounds far better than I expected for a low power amp. I would have never believed 4 watts of tube power could sound so nice without hearing it myself. No flaws in this single ended amp. I am impressed with the amount of clean bass and clarity of the high frequencies. Being that I have never heard a single ended stereo amplifier ( well that I can remember in the last 45 years) it has been on my mind for a while. Since I like using PC boards it was hard finding a board with a circuit that interested me. I know what you are thinking, why not build one using point to point wiring? It would not be an issue to do it but usually my preference are PC boards. I've used Dirty PC Boards for smaller projects with great results, but this one is made with a Chinese board and I think the next project will be point to point wiring. A word about output tubes. It confuses me hearing how everyone's description of the sound of a tube is colorful and different! Well here you go,, my two cents. So far I have tried JJ-Kt77's, TAD EL34B-STR Premium's (which are relabeled Shuguang tubes), and Sovtek 6L6WXT+'s. The TAD's sound the best. The JJ-KT77's and the Sovtek 6L6WXT+'s seem to have a hole missing in the strength of stringed instruments such as violin, fiddle, steel guitar, banjo and such. I have ordered a couple of new release Gold Lion KT77's. We will see what they sound like when they get here and I'll add a screen shot below of power just before clipping for the KT77's.
Is there a noticeable difference between 5AR4 rectifiers in the amp? Not really, at least between the JJ's and the TAD select Chinese version. Ive read that the 275B sines in a single ended amp but it needs a maximum of 8uf for the first cap, so I may try it in the next project.
3/16/16 - Received the KT-77 Genalex Gold Lion re-issues from New Sensor today. They are some nice sounding tubes. They have a nicer sounding top end then the EL34's. in this amp. They are well rounded tubes, but this amp is not broke in yet so I expect things to sound even better as time goes on. Don't be afraid to give them a try in your EL34 Stereo as they are drop in replacements and are capable of 70w continuous in push pull configuration.
3/29/16 - Coupling capacitors, what a controversy, from low cost to expensive boutique parts. So far I used Mallory 150P's in my amps, with the exception of the Audyn caps that I used in the Tone Control. The fun of all this is experimenting with different things. I had ordered a set of Audiocap Theta's for this amp and replaced the Mallory 150's with them. All I can say is that there is a noticeable difference (or it is my mind) that the mid to high frequencies are more clear with the theta's.
When figuring out some of the final values for a SE amplifier it helps to use simulation software to get in the ballpark. Using Glassware Audio's SE Amp Cad software I was able to see the results of using KT77's in this amp. It is a handy piece of software. You can enter in values including transformer specs to see what the scenario results would be. Trancender's web site listed detailed specs on the transformer which helped a lot.
KT77 amplifier schematic. The coupling capacitors in this project are my favorite, Mallory 150P's. I always get great results with them. As I usually do, the pre-amp tubes have regulated DC on the filaments. The amp is dead quiet, nice. With only a 6.3 volt tap on the power transformer I would usually have used a voltage doubler circuit with a regulator and the pre-amp tubes in series. But, the power transformer I used was a new old stock made for Olson Electronics, remember them? The secondary voltages were a little higher I'm guessing because it was made for 110 - 115 volts. So there was enough headroom to uses a full wave bridge and a regulator to push out a solid 6.2 volts DC for the pre-amp tubes. The output tube filaments are running at 6.6 volts which is not as high as the 6.9v limit on the power tubes.
Transformers painted and mounted and vent holes with brass inserts. Usually I would use Edcor transformers but I decided to spend a little more on the output transformers since I already had the power transformer. The output transformers are Transcendar TT-336 OT.