After building out the EZ260 Amplifiers I was going to build the EZ125 Quad KT88 amp. I had started working on the schematic, but got side tracked. I had the idea of build a mono tube amplifier for some of my ham radio gear. If you have ever listed to a tube receiver they have such a great sound that I figured I might try to build a ‘retro’ fit amplifier to add to my HF station. I started looking at the Dynaco ST-35 circuit but came across a website that had a nice simple amp built from the Silvertone 69A schematic. The guy made a few mods and I figured I should do the same. I decided to try to use a 6SL7 instead of the 7025. The 7025 is a low noise 12AX7 from what I found it. While the 7025 has a bit more gain then the 6SL7 I figured that it would still be fine and I can have an all octal amp. Some of the goal was all tube, and octal just to build an amp with the 6SL7.
After scanning schematics and coming up with a plan I sat down looking for small chassis to see how small I could make it with all required iron and tubes. I came out selecting a Hammond 6″ x 10″ x 2″ aluminum chassis. The bonus for me was Hammond also makes a nice cover for it and I liked the look of it. Reminds me of old Dynaco stuff.
I got the dimensions of transformers from Edcor and cut out some cardboard patters to mock things up. Given the size of the transformers and the open space left I think the board dimensions came out to be a max of 9.75″ x 4″ or something close to that. Transformers and choke were ordered from Edcor and hopefully would arrive quicker then the rest of the needed parts. Here’s what I used –
Power Transformer* – Edcor XPWR-163-120 550V (275-0-275) at 150mA center tapped, 6.3V (3.15-0-3.15) at 5A center tapped and 5V at 3A
Choke – Edcor CXC100-7H-150mA 7H, 150mA
Output – GXPP10-8-8K 8k Primary, 8ohm Secondary
*Note that with this transformer the voltage to the 6V6 might be on the high side depending if you use GZ34 or 5U4GB rectifiers, this will also affect the bias resistor selection.
Now to create the schematic. The few changes were to support the 6SL7 instead of the 7025 and to add a few more options on the build, not many, some could have been done better like the way the feedback is done if you use the cathode bypass capacitor and a few other things that I can’t think of right now. The design was also changed to run in Ultra Linear mode which hopefully is something better then the original. Other then that this is a simple schematic.
The board was laid out and the usual foamcore prototype of the PCB made. It all looked ok. PCB’s were sent out to Advanced Circuits, and these were not too crazy of priced as I made them with standard thickness FR4 material and they were not too large. I will say that the standard thickness (0.062″) should not be used for anything but the smallest tube PCB’s. Even with all the mounting points on the board it still feels flimsy when putting in tubes, I’ll stick with 0.093″ and pay the upcharge. Advanced PCB turn out the boards quickly and I was off and running.
The collection of parts begins, fortunately I had a good amount of parts that could be used so other then transformers and chassis most of the rest was ‘In Stock’. I found some NOS 6SL7’s on Ebay for not too much money and picked up the rest of the tubes from Tube Depot as I recall. I picked up some Tungsol 6V6’s as the outputs as well as some JJ 6V6S’s as they were not too expensive compared to KT88’s! I had a bunch of 5U4GB’s and a couple of GZ34’s and was good in the rectifier department. I also added a volume control to the amp since the original use was for connecting up to something where I may not have access to line level and would have to the flexibility of some adjustment.
After collecting all the parts, I assembled the PCB’s. These are pretty simple boards and went together quickly. On to the chassis. The way that I do them is print out a 1:1 patter of the mechanical layer of the PCB. It has cross hairs for anything that needs to be drilled. It is easy to tape on this sheet to the top of the chassis and center punch all the markers. Then the rest is easy. No fussing with measurements!
The first amp was assembled and quickly tested. The original bias resistor value (250 ohm) was too low. When testing with the GZ34 tube bias was excessive and the value was increased to help reduce the bias. I used a tube socket bias probe to check, this made it easy to see what was going on without much fuss.
I only had one so I swapped it in place of one of the EZ260’s and it sounded good, but lacked the punch that the more powerful amp had. That was expected and while it lacked ‘Big Sound’ it sounded good. Still needed the second amp to really test it out. So off to finish the pair of them off.
The usual chassis drilling, sanding, and painting was done. This time for the finish I found a light textured black paint that reminded me of the Dynaco finish that I liked. My paint scheme went like this – Primer, Black Satin Paint, Black Texture. I think that it’s mandatory to shoot some satin black as the texture coat has a better and more durable layer to stick to. Paint dried for a couple of days to make sure things were hard and durable. After this everything was assembled into the cases. Assembly was a little tricky as the transformers had to be put in first before securing the PCB, but the rear 2 standoffs needed to be installed in the chassis as they are under the transformers! It was tight, but not really that bad.
After both amps were done I fired them up with the iPod as the music source, Dynaco PAS-3 preamp, and the Polk Audio SDA-2’s speakers. They sounded very good. They lacked the punch that their larger brother had, but at normal listening levels they sounded clean and enjoyable. I just kept reaching for the volume control but you hit that point where you needed more power and sound degraded. Then a light bulb went on over my head. These would be perfect for a computer sound system! I had previously used a poorly made Chinese KT88 Singled ended stereo amp for my computer sound system, but it had suffered a large EXPLODING capacitor that lead me to believe I should junk the POS and go back to solid state. Well here were the replacements! In the smaller room with some smaller desktop sized modern Polk Audio monitors these amps came alive. They crank loud enough any more would be too loud, but they sound great in this configuration. They are silent (unlike the Buzzing from the Chinese amp) and fit perfectly on top of each speaker (yeah I know).
I have used these amps for months now and I did run into an early 6V6 tube that failed. One of the Tungsol’s went bad and decided to red plate. Again, this is likely due to the excessive voltage that I’m running as well as having the bias a bit hot. This failure happened about 1 month into running the amp. I has the second set of JJ 6V6S and had read they handle the voltage just fine and are very durable. They now have been in for over 8 months of long on time daily use.
For V2.0 I would make a few changes to the original circuit, one would include a resistor under the cathode bypass capacitor in the first stage of the amp. The second would be to modify the bias scheme to use adjustable bias. I don’t like the cathode bias, but it’s simple and never needs adjustment (really can’t be adjusted). I had toyed with the idea to make this into a stereo amplifier as the next version but not quite sure yet. I like the 6V6 and it sounds good and looks good being all octal as well.
Click the buttons under ‘Project Resources’ to see the gallery, schematic, and related datasheets.