KT77 SE Stereo Amplifier

     KT77 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 50 years) it had been on my mind for awhile. I ended up using couple of new release Gold Lion KT77’s, Audiocap Theta coupling capacitors, Transcendar output transformers, and a vintage Olson electronics power transformer.

Chart KT77

     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 Vintage 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. Trancendar’s web site listed detailed specs on the transformer which helped a lot.

KT77 stereo schematic

     KT77 amplifier schematic.  The coupling capacitors in this project are Audiocap Thets’s. I usually am happy using Mallory 150P’s, but I thought I’d try the Audiocaps. 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.

KT77 amplifier

     I find myself listening to Triode mode most of the time, though Ultra-linear and Pentode mode sound nice to. The tubes you see here are EL34’s which I used before the KT77’s arrived. 

KT77 chassis
KT77 chassis 2

     The chassis is a standard Bud 12×12 aluminum chassis with a dark metallic paint.  Drilling and punching the chassis is the least fun part for me. The output transformers are Transcendar  TT-336 OT.

KT77 inside chassis
KT77 se2 amp rear

Inside view of the chassis and rear view of the chassis with the usual hardware.