I also was after the same thing that the author of this thread was going for, using the EL34 (6CA7) tube in this amplifier. The manual shows that it can use 6L6, 6CA7,
KT-66, 6550C, KT-90, KT-99 or EL-34 tubes. But after a call to Cary, there is an adjustment needed for using EL34 (6CA7) tubes. I could only get the bias to 90.
Gerald in support at Cary will e mail you a two page pdf file that shows how to do it. Here are the steps: Take out the tubes. But first turn the bias of the tubes all the way down. Move the bias screw counterclockwise. The bias needs to be as low as possible before proceeding. Then take the tubes out. Flip the amp over and remove all of those screws to remove the bottom plate. After the plate is removed, loosen the Large UCC power cap on the left and just lay it (still connected) outside the amp. Then you will see two 1002F resistors. Jump these resistors. The pdf does not state but I called Gerald and he said the jumpers should be at least 18 gauge. The photos in the PDF are really good and make it easy to see what to do.
I had already modified the bottom of the amp by drilling ventilation holes in. So I soldered four seperate wires to the four different sides of the resisitors. Actually, to make it a point solder and not a surface solder, I put the wires through a small hole in the connecting devices. Then I put these four pigtails through different ventilation holes in the bottom. Then after screwing the bottom cover of the amp back on, I used 2 wire nuts to connect the wires for each jumped resistor.
If I would have just soldered a jumper as described in the PDF then I would have had to undo the whole thing just to go back to the KT-88's.
Performance: since there is no resistor the bias does not have to be turned up much. The sound is fantastic.
Here is my brief summary. KT-88's sound like a muscular solid state amp but with tube beauty.
The El-34's with the bias at 90 (before the bias mod) sounded "vintage". Interesting. Like turning on a radio found in an old attic.
After the jumping the bias resistors the meter easily gets to 140. The El-34's make it a powerful real unmistakable tube amp. This is the sound that I was searching for but I can easily see how lots of people will not like it.
If I want to go back to any of the other tubes then I lift up the amp and undo the wire nuts and cover each of the four wires with a wire nut.
Mod I am considering. Getting a surface mounted rocker switch to put on the back or bottom of the amp to connect or disconnect the jumper.
In hindsight, I might have used tiny alligator clips to attach the wires in the amp. There are a lot of soldered joints in there and my iron was not that steady and I was very worried about putting a major hurt on it.
Supplies: For the Cary way. Screwdriver, soldering iron, solder and two jumpers.
For my way: Get 18 gauge wire, four small wire nuts (two to use now and two to use if you want to disconnect it), Drill and file to smooth out the holes. Consider tiny alligator clips and then cover with electrical tape as needed. (Insert laughs here for people who really know what they are doing with a soldering iron.) Get a pack of 6-32x3/8 machine screws. Some of those removed to get the bottom off will not be able to be reused. Get Fast Acting (Fast-Blo) 0.5 Amp 250 Volt fuses 1 1/4 inches x1/4 inches. I went through two of these immediately because the bias was way too high after the resistors were jumped.
Equipment: stock eh KT88's, cary 6SN7, RCA blackplate 6SN7's, Tungsol VT-231, RCA Smokeglass VT-231. Best 6SN7 combo for me was two blackplate RCA 6SN7 and two Tungsol VT-231's. The EL34's are SED Winged C (great deal used). All but non-stock purchased here at Audiogon.
But first, call Cary and get the PDF called "CAD 120S Bias Circuit Mod for EL34 use" e mailed to you.
