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In Reply to: Re: Sound Labs posted by Lew on July 02, 2002 at 06:55:49:
Thanks for that Lew, I see more clearly now. I did enquire a while ago at Southern Electronics regarding Polystyrene Caps. They had a good stock of film to produce 0.8uF max.
I find the AS amps to be more harmonically right in the Mids-Highs. The AR amps have a more solid bass. Your much larger AS amps are bound to have a better bottom end than mine though.
How large are you custom amps. I keep reading snippets about them and I know that you have implemented most of the tweaks but I have not actually seen a full rundown of their specification.
I was fortunate enough to get a couple of dozen of 2uF polystyrenes from Southern Electronics off their "last run" of them. I will use same in the M1s. My amps are essentially MA2s with six 6C33C output tubes per chassis instead of twenty (?) 6AS7s, as per the MA2 spec. Thus I have a little less power than an MA2 but a little more power than a standard MA1. The current configuration includes type 6900 tubes as input voltage amplifier (in the standard Atma dual-differential cascode topology) running 11 to 12 mA of current. The CCS for this circuit uses an ECC99, stacking the two sections in cascode mode. This input circuit feeds three discrete CFs per chassis, such that each CF tube drives only one pair of 6C33Cs, one section per output tube. (Coupling caps are Rel teflon.) This enables me to set bias current separately for each output tube. The CFs are made from 12SX7s. Then each 12SX7 CF section has its own CCS, made from a single section of an ECC99. I have now acquired just about enough 7241s to replace all the 6C33Cs in both chassis', if I should require a big boost in power to supra MA2 levels. I am loathe to convert the amps, however, because the 6C33Cs have been so reliable. The power supplies have the usual hexfreds, lots of capacitance, pi filters, separate LC isolation networks for the plus and minus supplies to the driver stage and for the bias supply. I am only sorry that I have no room left to install the constant voltage sources that would constitute the upper half of the full SLCF config.
Lew, you have some very special amps, I didn’t realise that they were that good. I have only just finished on the input stage rebuild of my MA1,s and I am already thinking forward to my next project after I have installed some Sound Lab speakers. You don’t have to guess for too long what it will be – fabulous looking bigger amps. I hope you don’t mind if I talk to you at some time in the future about the finer points of your circuit design when I put my chassis design hat on.Coming back to the polystyrenes. At least Dr. West has reduced the capacitor value by half. Even if I get the 0.8uF there won’t be too many of them.
Thanks for the nice compliment, but the proof of the pudding is in the eating. I don't assume that the tweaks make the amps special, but I did have fun and learn a lot by going through the thought processes necessary to make the changes from stock. When I first bought the MA240s in disassembled (aka "kit") form from Ralph, I really had no clue about audio design, how tubes work, etc. I had to hire a local guy to help me with initial construction of the basic design, upon which I have since improvised. Also, don't forget that I have not deviated from the Atma-sphere gospel. I just used novel tube types and many of the tweaks that have been discussed on this site. Ralph himself helped me initially to build the amps with the special bias feature, by providing a schematic. (This would not be practical with production amps that use so many 6AS7s per chassis, nor am I certain that the advantages of my set-up outweigh the disadvantages related to the added complexity of the circuit, increased parts count, etc. For one example, I have six coupling caps per chassis!) I AM infatuated with the final results, however.
Your too modest Lew. I thought that the only kit offered was the M60 and that these are now discontinued. I never realised that kits for larger amps were ever offered. When did you start to build yours.
As stated, my amps started life as basic MA240s. That was a model that Ralph offered up until about 1996-7. Along with the Novacron, these are/were the only two Atma amps to utilize the 6C33C output tube. The Novacron was offered as a kit, I believe. OTOH, I am not sure that the MA240 was ever offered as a kit. I think what happened was that I approached Ralph at that time, and he had exactly one pair of MA240 chassis' remaining, as he had already made the decision to discontinue it. He offered to sell me the last pair of chassis' and all parts necessary to build the amps, just to clear the deck. So you could say it was a "kit", but the MA240 may never have been commercially offered as such. You know the thing about Ralph's design is that it is so basically safe and sound that one can tweak it extensively and hear any and every small difference in sonics that results, without much risk of provoking dangerous instability. (Warning: knowledge and understanding of tube electronics is required, however. I did nothing on my own until I had studied the subject for a year or two. Also, always keep one hand in your pocket if the circuit is "live", or if there is charge in the big caps.)
Thank you for the history on the MA240.
Don’t worry, I won’t frazzle myself. The first 15-20 years of my employment was as a toolmaker but then I started up on my own I have spent the last 20 years building my own special purpose machinery. This has involved excessive levels of electronics. I’m just new to audio electronics. It’s a great hobby.
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