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I'm here to prove the old saw that a little knowledge is a dangerous thing. I've been mining the OTL online archives (Thanks again so much for putting this up, I'll buy you a beer!) and came across a whole thread re: subbing a 6SL7 for the CCS in M-60s. I've got the 12 tube MA-1s, but the input circuit is the same?. I'm also using 12SX7s, so we're talking 12SL7 for the sub. Found a few 1950's vintage SL7s, took the best two and stuck em in V3. So? You may ask...well dummy me counts uhh 1...2...3 aha! Third tube! That must be V3. So I sticks the SL7s in the socket(s) and listen. Holy Cow! Opens things right up. My word! A little more distortion, but I haven't changed the grid resistors and added the zeners as Ralph recommended in above mentioned thread. Long story short, I changed the wrong tubes, and put the SL7s in the cascode position. I've since put it back right, with no harm apparently done. Amps are working fine. I'll wait til another day to put the SL7s in the proper position. My questions, after all this blather, is why did it sound so darn good with the wrong tubes in, and is there maybe a possible tweak or mod to be explored?
Follow Ups:
it's a good idea to change the resistors and zener BEFORE actually installing the 12SL7 as CCS, if you want to go ahead with that. And yes, all Atma-sphere amps have essentially the same input circuit, except that the MA2 uses higher voltages but with the same topology.
Lew, now I'm confused. I thought the CCS is SS and in the tail of the balanced cascode gain stage.Tre'
Have Fun and Enjoy the Music
"Still working the problem"
Lew, I just read Atma-Sphere's threoy page. Now I know why you said what you said. I think this is wrong, but I will study more. If the top tube is acting as a CCS then there would not be a load resistor on the plate and the signal would be taken from the cathode of the top tube or (better yet, from a distortion point of view) from the plate of the bottom tube. But, as I said, I will do more study.Tre'
P.S. If there is a CCS in the tail, the top tubes can't be CCS's. AFAIK that would not work.
Have Fun and Enjoy the Music
"Still working the problem"
I know nothing about what tubes are what in the standard products, i.e., V1, V2, V3, etc. AFAIK, the input is a dual-differential cascode with a CCS in the tail (on the cathode side). I am assuming that there was a post by Ralph suggesting that one could substitute the SN7 that serves as a CCS in the stock circuit with an SL7, if one also changes the associated resistors and the zeners that set the grid voltage of the CCS. The higher mu of the SL7 could in theory be advantageous in that it would raise the impedance of the CCS, but I wonder whether there is enough voltage to allow the SL7 to operate at the necessary current. I personally never saw a post regarding this substitution, nor do I remember anything about this, but I ain't perfect.I hope this clarifies what I was trying to say and what I think the original poster was trying to do. You know more than I do, but I think you are correct; a circuit with CCSs on both sides in theory would not work. In fact, I doubt you could put CCSs on each half of the anode side of a dual-differential amplifier, because the current has to rise and fall wrt each side.
I just realized what you meant by "I thought the CCS was SS". I figured out that SS probably stands for solid state. Unless there has been a recent major change in the Atma-sphere products, the CCS is derived using an SN7, not a solid state device.
Lew, got it. Thanks for the clarification. A 6sl7 might make a better CCS because it has more gain but I don't think it will stand up to the current. Well, maybe. It's really plate dissipation, not current and with it in the cathode circuit the voltage would be low so...that might be fine.
Thanks again...Tre'
Have Fun and Enjoy the Music
"Still working the problem"
I agree. The plate dissipation of an SL7 is something I no longer can quote without looking it up, but I think it would be a stretch to use it as a CCS in an Atma-sphere. I don't know how much current is now being used in the dual-differential cascode, but lets assume at least 2mA per phase for a total of 4mA seen by the CCS, more likely it's higher than that, and then too one has to be able to bias the SL7 within the -300V available from the B- supply. But if it works, it works.
Here's the link to the original discussion. I see Lew had his doubts back then, also. Thanks for the replies, Tre' and Lew, I do appreciate them.Frank
"Here are some tweaks if you want to use the 6SL7 as the V3 CCS tube:Change the cathode resistor on V3 pin 3 to 32K.
Add a second 47 volt zener diode in series with the first on the grid of V2, raising the grid voltage up to roughly 90 volts. The 499K resistor could be changed too (but its not too critical) to 332K or thereabouts.
Distortion is further reduced and dynamic range is slightly improved.
Finding decent 6SL7s is a problem- most of the current production tend to be microphonic. This is a problem in the circuit as microphonics in the CCS tube are definately audible. "
I would say if you follow the above it should be better than stock. But you need to make the circuit changes not just throw the tube in.
Tre'
Have Fun and Enjoy the Music
"Still working the problem"
Most likely operated a tube outside it's linear operating curve. If you like it, no one can argue, but it's not right.Tre'
Have Fun and Enjoy the Music
"Still working the problem"
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