This project was really my first build it all tube amplifier. After finishing the Tubes4HiFi ST-70 I started looking for a KT88 monoblock that had a lot of glass. I came across many interesting amplifiers but found an odd ball one from Grommes Precision. This was their 260A and it had some interesting things, for one it was not an ultra-linear amplifier, and it featured a screen regulator (if you could call it that). It also had a OB2 gas discharge regulator on the bias. I always liked that tube as it glowed nicely. One other thing that caught my eye was the dual regulator tubes. It seemed like a robust amplifier and odd enough to build.
Transformers were custom made from EDCOR Electronics, and IMO very reasonable. As a matter of fact the power transformer made it into their online catalog. The Output transformer was one of their off regular Ultra Linear version even though I didn’t use that functionality.
Lastly the idea that this was going to be a PCB based amplifier and one that could easily be built I came of with the ‘EZ’ idea. That idea was to have NO spaghetti of wires coming out from all parts of the PCB, but rather all nicely terminated at the back side of the board.
The board can be built with a few options, including the option to run Ultra-Linear or Screen Grid Power Supply was simply adding a jumper or cutting traces. Don’t want tube rectifiers, not a problem provisions for stuffing with semiconductor diodes were also provided. Lastly if you can’t scrounge up an OB2 tube provisions for messing with the voltage divider chain on the bias was also provided.
While I’m not super technical on the topology of this amp (Much repeated here is what was told to me), the input stage is self biased to get some extra gain, and the phase inverter is a nice long tailed pair. Screen power is by a 6L6GC driven by a resistor divider (option for Zener diodes also provided).
My PCB CAD system is by Altium and I had to make all the Tube and related odd component footprints. Took a while to make them, but figured they would be reused on other projects.
Parts ordered, transformers and chokes ordered from EDCOR (4 weeks lead time) and I began to lay out the board and cram things in. The idea that components could be mounted on either side (well except for tubes and a few things) was interesting, and I ended up stuffing a lot onto a small board, yet while keeping the heat away from capacitors and other sensitive parts. The planned chassis was a Hammond 2″ thick 10″x12″ chassis. So not a lot of room for tall capacitors, given the use of 1.2″ standoffs.
The initial board was printed out and spray glued to a piece of foamcore. This allowed me to test fitting the components and see how it looks. This was a good idea as a few parts needed to be moved around a bit to fit nicely given the larger sized parts that I had later received. All good, time to send out to the PCB fab house.
The Gerbers were sent to Advanced Circuits for a run of 10 boards. These were 2oz copper and 0.094″ this to support the plugging in of tubes. I also made sure their were a LOT of places for supporting the boards. I hate cheep amps that the PCB or chassis flexes when putting in tubes. And yes, they were expensive.
In the mean time parts were flowing in, the iron came in quicker then expected and the EDCOR transformers are beautiful. Tube sockets, caps, resistors, fuse sockets, cords, etc all arrived and source from Tube Depot, Antique Electronic Supply, Digikey and Mouser.
After collecting all the parts and the finished circuit boards I assembled one of the boards. I then made up one of the chassis so I could mount things up and not have things rolling around the bench. In retrospect it was not necessary to build the chassis prior to testing, but had to do it anyway. I connected up a make shift 8 ohm dummy load and popped some fuses into the amp. With some trepidation I flipped the switch. Nothing exploded, no blown fuses and the tubes were warming up nicely. The cool thing was the OB2 had a nice purple glow. Bias was previously set to maximum negative and other pots were centered. After a bit of warm up Bias was adjusted and scope connections were put on the input and output of the amp. I cranked up a 1kHz tone and the signal looked clean. After doing some testing it seemed the amp was flat topping in the 45 watt range. This seemed a bit low for the amp, but it is what it is. After doing some sweeps with the signal generator I noticed an odd wave form at 100hz that looked like fuzz on the lower portion of the sine wave. It seemed that their was some odd oscillation going on at low frequencies. Not having much experience in debugging amps I hit up the DIYAudio board for some help, some very smart folks that are willing to help out a newcomer. While I had heard many theories about what could be wrong I found out that while probing the first stage with a volt meter the oscillations on the waveform were gone and the signal was clean! It seems that something was unstable in the design of the circuit and my guess was that the use of paper capacitors provided some form of loading that is not happening with the poly-xxx style that are being used. I added a 10 megohm resistor to ground from the first stage to the second (called the Cheezistor as it was a cheezy fix). Signal was clean, and otherwise good to go. I hooked it up to a test speaker and it made sound!
I built up 2 new PCB’s and kept the original as a spare since it was getting pretty hacked up from trying to fix the issue. I got both new boards up quickly and finished the second
case. Then painted them up with the brown texture coating, then a coat of clear to make it a bit less texture-y. The amps assemble very quickly since most all of the wiring is out the back of the PCB, the exceptions are the input and the bias connections. The amplifiers fit nicely into the small chassis and look nice. It’s a bit tight to the bottom of the chassis with the choke and the filter caps, but it works out just about right.
Now the real test. How do they sound? My test set up is a Dynaco PAS-3 with the source being a iPod. The PAS-3 is tired and has never been touched. Speakers are Polk Audio SDA-2’s and music is everything from the Blasters to Enya. I sat down with the iPod on a few tunes that I really enjoy and was blown away on how good the system sounded. I sat and listened for hours, and most everything I played sounded nothing short of spectacular. So I played them louder! And to my surprise they sounded good and to the point of the saying that tube amps are not fatiguing, they weren’t. The only other tube amp I had was the Tubes4HiFi ST-70 (with upgraded 60 Watts/Channel) and that’s my mark. That amp was very good sounding and enjoyable as well, but definitely different sounding. The Tubes4HiFi amp sounded faster and a bit sharper then the EZ260 amps. Not a good or bad thing, but I like the sound of the EZ260’s better as they seemed to have a better voice and yet be easy to listen to. I can’t say anything bad about the ST-70, just like the EZ260’s better, they look better as they have a lot more glowing glass as a double bonus, I can add the sticker on the bottom that says – Sandy Ganz Built!
As a later test I had built up a pair of Dynaco MkIII’s and found that I did not enjoy the sound as much as the EZ260’s. They had Triode Electronics driver board and otherwise stock build. Again they sounded good, but not the same as the ST-70 or EZ260. These are currently my main stereo amplifiers, and only the EZ10’s get more time on them since they are connected up on my computer for sound.
That’s the project, schematic, plenty of photos are all just a click away, look at the buttons near the top of this page and you can see it all.