NOSValves Posted December 20, 2003 Share Posted December 20, 2003 I think I have found a SET amp I'm going to build for myself! I may vary this design into a set of mono blocks rather then the dual mono setup. I will be hearing this setup in February at the audio Karma audio fest in <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Detroit on my own speakers. I'm sure the setup at the fest will not be optimal but should give me some solid impressions. I will either use the specified Iron or upgrade to the DS-025 from Magnequest. I probably use some custom made Power transformer to get all the needed taps in one package rather then the filaments on a separate transformer. Not sure about that yet. This is based on the Loftin-White Design from 1929 http://www.wardsweb.org/audio/index_audio.html what do the SET experts around here think? I sure hope this doesn't turn into another militarized zone. Its to close to Xmas for battles.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /> Craig Quote Link to comment Share on other sites More sharing options...
Audio Flynn Posted December 20, 2003 Share Posted December 20, 2003 Craig So what is so interesting about this 2A3 design for you? Quote Link to comment Share on other sites More sharing options...
Tom Mobley Posted December 20, 2003 Share Posted December 20, 2003 Same question. I ran across this while I seaching the web for 2A3 designs and schematics, but it didn't make my top five because I saw it as basically resistor coupled. Looks like the whole thing depends on that resistor between the 6SL7 plate and the 2A3 grid. Is that wrong? I'm interested why you found this one attractive than some of the others? It is a little different. Eliano or somebody could easily crank out a power tranny for this, eliminate all those separate little transformers. Is that a Good Thing? Tom Quote Link to comment Share on other sites More sharing options...
neo33 Posted December 20, 2003 Share Posted December 20, 2003 "I think I have found a SET amp I'm going to build for myself!" Can you build one for me too, Craig . Quote Link to comment Share on other sites More sharing options...
leok Posted December 20, 2003 Share Posted December 20, 2003 The design looks nice. I like the direct coupling and the "Ultrapath" cap. As usual, I would not have a cap on the gain stage cathode .. esp not an electrolytic, but that's always easy to experiment with. The gain will be a bit low, but if you plan to use a preamp, then that's the way it should be .. too much gain just ads noise. Another thing you might consider is replacing the output transformer with a parafeed inductor/transformer set from Magnaquest. Now that's not so easy to experiment with. Even I found riping the transformer out of the Moondogs to be a chore. Parafeed's an elegant aproach. I was impressed with the improvement in the Moondogs. Leo Quote Link to comment Share on other sites More sharing options...
Tubinhard Posted December 20, 2003 Share Posted December 20, 2003 Can you believe this Neo guy above me? Flames you for three days straight, and then wants you to build him some amps. I don't know Craig, are you competent enough to handle soldering a few caps and a handful of resistors? I mean, you can't even handle a freaking Eico. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted December 20, 2003 Author Share Posted December 20, 2003 Tom, To tell you the truth I think most of the things that are tagged as being a huge difference in these designs I truly think are very small and subtle. I'm liking this circuit because it represents itself for what it truly is without romantic tag names thrown in. It will surely to a respectable job for a reasonable cost. I refuse to invest gobs of money in a amp that can not play whatever I throw at it with ease and do it all very well ! not just certain types of music. This is going to be a adventure and by no means one that ends with this first project. Craig Quote Link to comment Share on other sites More sharing options...
neo33 Posted December 20, 2003 Share Posted December 20, 2003 Deano, leave me alone, sheesh! Quote Link to comment Share on other sites More sharing options...
Tubinhard Posted December 20, 2003 Share Posted December 20, 2003 "I refuse to invest gobs of money in a amp that can not play whatever I throw at it with ease and do it all very well! not just certain types of music." Well, someone could come back and say that the monetary investment in the best iron and parts, results in a low power solution that can handle just about everything thrown at it. Did you read this yesterday when I posted it? Good article. Check out the graph. http://www.aikenamps.com/Crowhurst_blocking.pdf Quote Link to comment Share on other sites More sharing options...
NOSValves Posted December 20, 2003 Author Share Posted December 20, 2003 Well Deano people can say what they want but the Iron used in this design is not cheap !! I am almost to the point in this delusional hobby that I listen and believe almost nothing I read or hear its all based on opinion. I am of the opinion that there is only one way to find out and that is by doing not reading and taking anyone elses opinion with to much grain. Its great to get others points of view but there point of view or ears are not your own ! So you can only find the facts by actually experiencing things yourself. Craig Quote Link to comment Share on other sites More sharing options...
neo33 Posted December 20, 2003 Share Posted December 20, 2003 "Its great to get others points of view but there point of view or ears are not your own ! So you can only find the facts by actually experiencing things yourself." Craig, that is a pretty solid statement you made and I am with you all the way on that. Quote Link to comment Share on other sites More sharing options...
Tubinhard Posted December 20, 2003 Share Posted December 20, 2003 I guess your original post was just really saying that if you're going to go with a SET amp (that you feel can't do the full range of things you like to do with your system) -- you're not going to throw almost 2 grand into it. You'd rather throw that kind of money at something that can give 100% -- to 95% of what you listen too. Quote Link to comment Share on other sites More sharing options...
Daddy Dee Posted December 20, 2003 Share Posted December 20, 2003 Craig, Well, since I've been a customer whom you know all too well doesn't have much tech knowledge or skill, my response is definitely non technical. It's a cool looking amp, for sure. A couple of things I'd mention are: Wouldn't there have been some pretty significant improvements over a design from 1929? Of course, I'd do well to assemble a kit, but I'd also be inclined to build from a tried and true circuit design like JFL's Horus. my $.02 Quote Link to comment Share on other sites More sharing options...
Tubinhard Posted December 20, 2003 Share Posted December 20, 2003 Now Dee, you should know all too well -- "there is nothing new under the sun". All of these designs, and their variations -- are older than anyone on this Forum. Well, except maybe Maron. Quote Link to comment Share on other sites More sharing options...
Wardsweb Posted December 20, 2003 Share Posted December 20, 2003 A little background, as the builder of this amp: Direct Coupling: There is no cap or interstage transformer between the driver tube and the output tube. They are "directly coupled". In most amps a capacitor or interstage transformer is used to couple the tubes for AC signal, while allowing for a DC offset of 100 to 200 volts. If we expend the 100 or 200 volts to raise the cathode voltage of the output tube we can eliminate the coupling device, but the plate voltage of the drive tube must be just right to set the output tubes' bias condition. By use of cathode resistor bias on the output tube a self correcting condition is obtained as long as the values are close to begin with. That is to say that if too much current goes through the output tube then the current across the cathode resistor causes a greater voltage drop across the cathode resistor, which raises the cathode voltage of the output tube. Since the grid voltage on the output tube is fixed by the plate voltage of the drive tube, the grid of the output tube is now more negative compared to the elevated cathode voltage of the output tube. As the grid of the output tube becomes more negative with regard to its' cathode it reduces the current through the tube and therefore auto-limits the current through the tube. Viola; auto-bias. Ultra Path: The ultrapath caps do act as filters by reducing AC potential between the plate and cathode of the output tube. They should be seen as filter caps. Notice the large values of inductance employed in the high voltage filtering of this amp. This allows for smaller filter caps of higher quality. As regards the high voltage to ground AC filtering of the final cap stage, it is completed through the ultrapath and cathode bypass capacitors together. The final choke in the high voltage supply blocks audio signal from the output circuit from traversing the prior filter cap and the other channel. This is to compartmentalize the different stages of this circuit and avoid the "smearing" effect which can be a complaint when two caps of different characteristics are in parallel, such as an electrolytic bypassed with a polypropylene. It also improves stereo separation. Isolation: The key to isolation of the simple ultrapath output circuit in this design is the high value chokes which are placed between the main filter cap and the audio stages. Notice that these large chokes are on the same order of magnitude as the primary inductance of the output transformers and are run well under their rated specs for good audio linearity. So each channel is isolated from the power supply and from the other channel by the very high audio frequency impedance of the chokes. The cathode bypass cap on the output tube really bypasses the signal loop which includes the driver tube, the cathode resistor of the output tube, and the grid of the output tube. It is also a high quality polypropylene type, as is the cap from the high voltage to the cathode of the driver tube, a drive stage ultrapath arrangement. Why is the cathode bypass cap on the driver tube an electrolytic? Size is the problem with big values like that. Also, it's from a stash of really good Sprague "Extralytics". The 2uF ultrapath cap from high voltage to the cathode of the driver tube will always have 10 times the impedance of the 20 uF ultrapath cap in the output stage circuit, limiting the contribution of this cap in what is an AC voltage divider between the driver stage and the output stage. This 2uF cap may be what was meant by "the first filter cap". Careful analysis reveals a much larger effect of the caps around the output stage bypassing the 2uF ultrapath cap on the driver tube. Indeed that cap is not necessary, but I hoped it might benefit midrange and high frequency clarity in the driver stage. It can safely be omitted. Note that this is a situation of bypassing like type caps (poly with poly) for the most part. (The Sprague "Extralytic" caps in the cathode bypass of the driver circuit are of really super quality, my pick of probably 30 or more cathode bypass caps I've auditioned, including Black Gates, silver tantalums and many other Sprague Extralytics.) Now this is how I hear it. Your mileage may vary. Flame away Quote Link to comment Share on other sites More sharing options...
Tubinhard Posted December 20, 2003 Share Posted December 20, 2003 Yikes. What is there to "flame"? Dare I ask which method you prefer? Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 20, 2003 Share Posted December 20, 2003 Build yourself a breadboard, Craig. You've got a DC tube power supply, you can breadboard without the aid of a power supply transformer and filament tranny/trannies. You can noodle with any driver and output configuration until your heart is content. Your only limited by the amount of voltage/amperage your bench supply puts out. James makes a pretty nice universal OPT, and they are still cheap, and look nice. (potted.) You got those Scott 7591 OPT's, try a PP 2A3. Hell, you probably have a half a million OPT's to play with. You can homebrew a killer 2A3 amp, and you won't even come close to 2 grand. You could pretty much brew up anything you want. Uh, if you have cobbled up a breadboard for testing, please disregard my reply. Good post from Wardsweb, I'll have to read it 15 times. (Reminds me of the Email's I get from my local audio buddy.) Quote Link to comment Share on other sites More sharing options...
NOSValves Posted December 20, 2003 Author Share Posted December 20, 2003 Luther, Are you following me Dean did I tell you this guy was awesome or what ?? Mark, You will love this guy he is right up are alley ! Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted December 20, 2003 Author Share Posted December 20, 2003 Mike, Your correct except my bench supply doesn't have the 2.5 filament voltage require so I would have to rig something up for that no big deal ! Craig Quote Link to comment Share on other sites More sharing options...
3dzapper Posted December 20, 2003 Share Posted December 20, 2003 Wardsweb, thank you for your excellent explaination of your choice of circuits and welcome to the forum. Except for the voltages, which of course would be higher, would your design work well with 300b valves? Rick Quote Link to comment Share on other sites More sharing options...
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