Well even though I have not yet finished the last tube amplifier I felt compelled to finish a small project that was on the drawing board for a while. The EZ215. This is a stereo amplifier with the expected output of 2 channels at 15 watts each. The amplifier is based a bit on the EZ10 with a bit of difference in the coupling circuit. Also the output tubes were upgraded to 5881 (similar to 6L6’s but smaller).
The goal was a compact Stereo amp with a bit more juice then the EZ10’s. The 5881 is a smaller brother to the 6L6 tube and hopefully does not present too much load on the light weight driver, the 6SL7. This amp uses cathode bias and has a balance adjustment for the phase inverter (‘Borrowed’ the idea from another project). Target B+ is 450 volts or less so the use of single 450 wvdc caps can be used and likely a GZ34/5AR4 to keep the starting voltage from exceeding the capacitors voltage. The recommend voltage for cathode bias is 430 volts on the plate. This accounts for the somewhere near 30 volts of difference due to the cathode bias scheme.
Edcor is the supplier for the transformers, they have a few that look good that were off the shelf. Circuit boards and the Edcor transformers have been ordered. I have most of the other parts except the 5881’s which I also ordered from the Tubedepot which had the best price and I could use my repeat customer 5% discount coupon I had from the last order.
The amplifier name is back on track as 2 channels at 15 watts, hence 215. I am very conservative with the expected power output so will see if it has to be changed to the EZ210 or hopefully the EZ220…
I got all the parts and really only needed a few things ordered due to all the other parts I have amassed from the other projects. I had the Hammond chassis and bottom cover from a spare I had from the EZ260 project, and most of the other passive parts, connectors, etc.
After I posted the schematic on DIY Audio an eagle eye caught a slight mistake I had made in the circuit. Too late boards were already in hand. It was a minor fix, cut 2 traces, add a jumper and it was fixed (on each channel). Board went together very easy. I started with the 5881 bias resistors at a total of 300 ohms as one area that I figured I would have to change depending on rectifier type and how the transformer worked.
I test built a unit with the typical construction you see in the other amps, with the one exception that I had collected some 3 terminal binding posts. And for a change I would bring out the amp 4 ohm and 8 ohm speaker taps.
Transformers and choke mounted and I wired up the board. One thing that was really nice is when I was finished I got that feeling, dang, I have to build another one… Then I remembered this was a stereo amp, all done!
I connected things up to the variac, scope and signal generator and lastly the dummy load, hit the switch and turned up the variac. No smoke or fire so that is always a good sign. I noticed that the signal was a bit odd looking and figured I had the phase of the output transformers backwards and they of course were. Once fixed I check the bias at idle and it was bit high, with about 20 watts of dissipation sitting on the plates that are rated for 23 watts. I changed out the bias resistor from a total of 300 ohms to 360 ohm (2 x 180 ohm 5w) and that brought the dissipation down to 17 watts. This seemed OK now.
The sine wave looked clean and not skewed at any point in the sweep. No low frequency instability was noticed and the 10 kHz square wave looked pretty good with only a small bump in the leading edge, but otherwise very clean and square. With the GZ34 rectifier tube B+ was right at 404 volts and output was about 21 watts per channel both channels driven. I popped in a solid state rectifier and it output went up to 25 watts with a B+ of 434 vdc. The only potential gotcha was that it was about 480v until some of the tube started drawing current. This was still under the surge voltage limits that the capacitors had specified. Bias with the solid state rectifier was a bit hot so I decided to keep it all glassy. For the tubes I used a quad of Tung-sol re-issue 5881’s and some old school GE 6SL7’s and finally a JJ GZ34 rectifier.
I hooked things up to the usual set up, an old iPod, Dynaco Pas-3 (dead stock and old), and the old Polk Audio SDA-2’s and picked my usual test songs… I hit play and immediately looked at the tone controls on the Pas-3 (they suck as they are super sensitive) but they were dead center. I sat down and cranked the music and could not believe how good the EZ215 sounded! I played Rockabilly, Enya, Led Zepplin, Blues Travelers, Bill Cosby and it all sounded fantastic with very clean mids and highs. The base was unbelievable for this tiny amp as well. I spent about 2 hours just enjoying the music. About the only thing that was annoying was the stupid bright power lamp that I have put on all amps (This is a simple fix).
About the only thing that I could say might be a problem was the capacity of the power transformer. After a few hours of elevated listening it got pretty warm. I may be over worried about it but I think I would like to use a larger core the next time. Other then that I have just been listening to it and enjoying the tube sound from this little amp.
I am going to build another of these amps and try it with 6L6’s and see how it works out, likely use one of the spare power transformers from the EZ260 and a 5U4GB rectifier to keep voltages hopefully below the limit of the 450v caps that are in the circuit.