xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 (edited) My simulations seem to confirm my suspicions. Running the sim with and without the 100k Schade feedback resistor shows gain/reduction in 2H. Edited November 16, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Hey Matt, feel free to look over my schematic maybe I made some errors. I will have to dig out the article and look at the math for inverse voltage feedback but I think it only cancels 2H so if the circuit has higher order harmonics they will now dominate. Now plug in a 51v zener, bypass with a 0.01uF cap in place of the 2.2K G2 resistor, replace the cathode resistor with an LM317 set with a 27R resistor (@40mA) and leave the 100uF bypass cap in there. Has plot changed? Thanks. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 I can try it but I don't think that is the answer. The issue is the driver linearity especially at higher signal levels, I believe you would want a device with more or less a linear transconductance, a triode has more or less linear mu. A pentode as a driver is really what you want to drive a schade feedback. I will check the FFT with your changes for you still. Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 I agree, however, since we were starting with a dual-triode, I selected the one that I found to perform best as a driver within this context in a noval envelope. There are a few triodes that offer excellent linearity and transconductance but aside from the almost "un-obtainable" 5842/417a, they all have some sort of weird filament voltage that would make it difficult to implement considering Corey already has his tranny's and chassis. AND, for whatever reason, the "hearing" test does not concur with your sim findings even running the source direct without a line-stage adding anything to the signal. Obviously, I prefer the pentodes I listed as more optimal drivers in this config., but, wiring them as "pentodes" requires an additional load resistor and coupling cap tied to ground to power G2 at the correct voltage, in this config., G3 is tied to ground or alternatively, the cathode. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 I am not talking about a triode with a high transconductance, if you compare a triode's plate curves to that of a pentode you will see they look quite different. I see you are familiar with the EF86, that's a noval pentode maybe try that up front. I have to do a few things and then I will be back on later, I will look into it more then. I did try the sim how you suggest, I even went as far as to give the 6V6 screen it's own regulated supply and the results were all the same. Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 16, 2015 Author Share Posted November 16, 2015 All, thanks so much for the help. So: My plan, I think is to keep the PS as is with Matt's suggested changes in terms of grounding. I'm going to build up and see what B+ is after building the PS section. I will order a 100uf cap for C8 and some alternate values for R14 to help put B+ around 305 or so. A note about 6v6 plate and screen voltages: So the north American distributor for JJ electronics (eurotubes.com) is about 15 min from my house so its super easy to swing by and talk to those guys and pick up tubes. This means I end up using a lot of their stuff. I bought the JJ 6v6 S to use in this amp, which is a beefed up 6v6 and per the JJ spec sheet, if I am reading it right, can handle plate and screen voltages of 450V, so I'm not too worried about tube life. JPM, I am planning on using the specified Edcor OPT and PT. I have the iron already so am pretty committed to making it work. I don't have an oscilloscope nor the expertise to use one so I'll have to do the best I can with what I have. Appreciate you guys chewing on this for me, and Matt, thanks for the schematic work. -Corey Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 16, 2015 Author Share Posted November 16, 2015 Didn't see the other replies regarding the audio circuit until I had already posted last thing above. You guys are way more advanced than me, very impressive. I guess I just need to build it one way or the other and see. Matt, sounds like you are pretty confident that things will sound good with your revision, so if it doesn't end up being a bunch more to order the new/different parts, maybe I'll just build it that way and see... I do appreciate both of your input though, shows how much I have to learn. Corey Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 16, 2015 Author Share Posted November 16, 2015 Matt- Also, one more grounding question for you. Since the chassis is now anodized inside and out (NO longer electrically conductive), does this change your grounding advice at all? Just curious. Thanks! -Corey 1 Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Matt- Also, one more grounding question for you. Since the chassis is now anodized inside and out (NO longer electrically conductive), does this change your grounding advice at all? Just curious. Thanks! -Corey Corey, At the three grounding points on the inside of the chassis it probably wouldn't hurt to sand or scrape away a bit of the anodizing to get to the raw aluminum underneath. Aluminum is a pretty good conductor so your ground plane should be sufficient. Matt. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 (edited) I wasn't trying to say Matt's schematic would sound bad I just wanted to show the distortion spectrum. Matt's schematic at low listening levels will be mainly second order harmonic distortion but with increased signal the third becomes dominant, so at higher output levels it will probably sound more like a push pull amp than a single ended amp. I have some time after dinner to mess around with different drivers. Edited November 16, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 Here is another option, instead of dual triodes where one triode is going to be wasted use a noval pentode. Sensitivity is greatly improved so there is no need for a preamp and THD is down to 1.8% at full output. Spectrum looks a little better too. It shouldn't be too hard to build it one way and listen for a while and then change it later on. Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 Here is another option, instead of dual triodes where one triode is going to be wasted use a noval pentode. Sensitivity is greatly improved so there is no need for a preamp and THD is down to 1.8% at full output. Spectrum looks a little better too. It shouldn't be too hard to build it one way and listen for a while and then change it later on. You know what is funny about this? It is NEARLY the Genesis6 Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 16, 2015 Share Posted November 16, 2015 You know what is funny about this? It is NEARLY the Genesis6 Well you know Matt I guess great minds do think alike 1 Quote Link to comment Share on other sites More sharing options...
ToolShedAmps Posted November 16, 2015 Share Posted November 16, 2015 You know what is funny about this? It is NEARLY the Genesis6 Well you know Matt I guess great minds do think alike They sure do Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 17, 2015 Author Share Posted November 17, 2015 JP and Matt- Thank you guys so much. I've got lots to think about. I'll report back once I have decided and built the amp. Appreciate your input and design help! -Corey Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 17, 2015 Author Share Posted November 17, 2015 Hi all- Thanks again JPM and Matt for spitballing this circuit design. I did some checking and it will be about $100 for the extra materials to build the revision as suggested in post#32. Most of this cost will be for a decent pair of 6267 tubes and some decent 0.22uf coupling caps. The rest for new electrolytics and resistors. While this is not a huge issue, I do already have everything on hand for the original circuit. So in the spirit of educating myself, I decided to start with the original circuit and make the following adjustments. As suggested earlier, I will byass the feedback resistor with a 0.01uf 450vdc film cap. Also, as matt suggested, I will replace the resistor on the ultralinear tap with a 21v/3watt Zener "bypassed" with a 0.01uf/350v film cap. Please don't take this as my not trusting ya'lls advice. I don't believe for a second that Matt and JPM's circuit revisions would be anything other than amazing. But I thought I would start original and once I've listened to that for a while, I can order the new parts, make the changes, and then take another listen. After all, that's part of the fun of this hobby. I hope you guys aren't offended or you feel as if I wasted anyones time. I am very very appreciative. I'll report back. Thanks! -Corey Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 17, 2015 Share Posted November 17, 2015 I like the original design and I am sure it will sound great. As suggested earlier, I will byass the feedback resistor with a 0.01uf 450vdc film cap. That is a bad idea unless you want a serious rolloff above 10kHz. You can't find the optimum value of this cap without an oscilloscope. If you have to add this cap use a small value e.g. 330pF I will replace the resistor on the ultralinear tap with a 21v/3watt Zener "bypassed" with a 0.01uf/350v film cap. I also wouldn't do this, the resistor is there for a reason which is to stop oscillations. Quote Link to comment Share on other sites More sharing options...
cfishbowl Posted November 17, 2015 Author Share Posted November 17, 2015 I like the original design and I am sure it will sound great. As suggested earlier, I will byass the feedback resistor with a 0.01uf 450vdc film cap. That is a bad idea unless you want a serious rolloff above 10kHz. You can't find the optimum value of this cap without an oscilloscope. If you have to add this cap use a small value e.g. 330pF I will replace the resistor on the ultralinear tap with a 21v/3watt Zener "bypassed" with a 0.01uf/350v film cap. I also wouldn't do this, the resistor is there for a reason which is to stop oscillations. JPM- Got it. Perhaps once I get it done, I can con my buddy (an electrical engineer) to bring his oscilloscope down for some testing once the original design is done. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 17, 2015 Share Posted November 17, 2015 (edited) For a three stage amp as long as you keep the feedback below 18db I think you shouldn't have any oscillation issues. The original circuit your are building only has like 10-12db of feedback so the cap is not really needed. A two stage amp can never reach 180 degrees reversal. Here is something I think people will like, it's from an old book of mine authored by Norman Crowhurst. Edited November 17, 2015 by xxJPMxx 1 Quote Link to comment Share on other sites More sharing options...
Mallette Posted April 3, 2020 Share Posted April 3, 2020 It will be approved, but without the link. Assuming the link meets the TOS (review them before posting), you may repost it once this one is up. The reason is that mods do not have the time to check every link. Post at will... Dave Quote Link to comment Share on other sites More sharing options...
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