EL34 in Cary CAD120S bias info needed

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.

I have the CAD 120's. What preamp are you running? I had EL34's in a V12R, I love the sound with the stock KT88's. What are you trying to do soundwise? You have a fantastic amp. Good luck.

I have a Parasound Halo P3 Preamp. The connections to the amp are by balanced lines. Inexpensive ones but I do not recall where I got them.

In my initial setup there were three different amp hums. I have learned to ignore the really low level transformer hum. Cable placement changed the second hum. The third hum required getting a HumX by Ebtech to isolate the ground.

Like you, I was very pleased with the sound of the KT-88's and still think that it is wonderful.

The EL34's I imagine have the biggest difference in sound of any of the available tubes and that is why I went to these just for fun. The bias will not get to 140. It is just a guess but the 6L6 requires 0.9 amps for the heater and the EL34 requires 1.5 amps. Maybe that is why the resistor needs to be bypassed. I would like to hear from someone who knows this stuff to comment on why the resistor needs to be bypassed.

To answer your question about what I am trying to do soundwise: I am still figuring that out. Why does my wife get new dishes when the old ones look perfect?

The sound is intoxicating. There are so many ways in that it falls short of the KT-88's but I am not rushing to go back. The good thing is that having the jumper connections outside the amp make it easy to switch back and forth. Paradoxically for me that makes me less likely to switch back. The disease is AudioMasochism.

Hi,

I've been new to the audiophile hobby but I guess I've picked it up fast because I'm a very experienced RF engineer with good knowledge about transistor amplifier theories.

Anyway enough about me so you won't get bored. I happened to own a few months old CAD 120S MkII with Clarity caps upgrade and a good used SLP-98P. I've upgraded the Clarity caps to Vcap CuFT reference ones and Mundorf S-G-O for the SLP98. They've been broken in properly close to 300 hours for the Vcaps and 70 hours for the S-G-O ones. They really sound wonderfully crystal clear and wide soundstage with deep accurate image for the Maggies 3.7i speakers.

I also own many NOS and NIB tubes ranging from Psvane TII, Svetlana/SED Winged-C in Petersburg, Tungsol, EH gold pin for EL34, KT88/120 and 6550 for the 120S MkII. I biased all except the EL34 at 135mA. The EL34 is maxed out at 110mA or a few less when fully warmed up for two matched quads. The other two quads were maxed out at 100mA. However I could not tell the sonic differences between them. And all EL34s sounded just as wonderful as the KTs or 6550, even more and tighter bass from the EL34s with better midrange as they are known for. Another plus is they draw less current therefore the amp runs cooler noticeably by at least 300-400mA lower. That said I don't think you have to bias to 140mA for better sound as they are required so. Remember with 10% recommended limit and a typical 120-130mA for these low power tubes 110mA sound just about right.

I've not opened up the boxes of Psvane TII 6CA7/EL34

The 6SN7s as driver are EH GP and Psvane TII as front end. They are Sylvania JAN WGTA or CHS V231 which have not been rolled in yet but soon for the SLP-98, which also got the Psvane as driver. And for the phono input they are Telefunken or Amphoronics? or something by England.

If you got some extra cash around try the MEL 6SN7, they sound excellent! Probably the best.