Jim Posted November 6, 2015 Share Posted November 6, 2015 (edited) So all this amp stuff going back and forth has me wondering if I should upgrade my little Chinese amp with new capacitors and resistors or not. Will the upgraded caps and resistors really make a difference in sound? Or is it more based on the transformers? The amp runs on 6N9P,EL-34B,5Z3P tubes and I switched them out to the new Shuguang 50 Year Anniversary 6CA7,GE 5U4G NOS,and Tung Sol 6SL7GT NOS Looking for opinions, and anyone who may want to take this on if it would make sense. I am attaching pictures of the underside to show what's there. I know Maynard had looked at pictures when I was contemplating getting it. The white glue like stuff is rubber like. Edited November 6, 2015 by Jim Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 6, 2015 Share Posted November 6, 2015 G'day Jim, Caps and resistors look fine to me, Wima MKP for coupling are good. I have seen so many Chinese amps that the designers can't do basic math so a schematic would be helpful. If you don't have a schematic it shouldn't be too hard to figure out what's going on. OR a compete rebuild of the circuit using the tubes you have is easy enough. IF you want to spend money do it on output transformers hands down. How much power and distortion does it claim? How much power do you need/want? What's your budget? 1 Quote Link to comment Share on other sites More sharing options...
Jim Posted November 6, 2015 Author Share Posted November 6, 2015 (edited) G'day Jim, Caps and resistors look fine to me, Wima MKP for coupling are good. I have seen so many Chinese amps that the designers can't do basic math so a schematic would be helpful. If you don't have a schematic it shouldn't be too hard to figure out what's going on. OR a compete rebuild of the circuit using the tubes you have is easy enough. IF you want to spend money do it on output transformers hands down. How much power and distortion does it claim? How much power do you need/want? What's your budget? Here is the link to the newer version, but it's the same one I have. http://www.ebay.com/itm/6N9P-EL34b-Single-ended-Class-A-Tube-Amplifier-Power-Audio-DIY-KIT-13w-2-Latest-/251525310960?hash=item3a9013a9f0:g:V60AAOSw7ThUkC4D Power wise, I think I'm OK,just looking to see if it would be worth the upgrading and how much SQ would be better with the upgrades. I have a Sansui AU-70 integrated I'm getting refinished by Craig, and wanted to use this for either a smaller room, or if it sounded better than the Sansui, use it for my external hard drive stuff. It's rated at 8 watts in triode, and 13 watts in pentode. It starts distorting once it's above 11 o'clock on the volume control. So if I read correctly, only thing you'd suggest at this point would be transformer upgrade? Edited November 6, 2015 by Jim Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 6, 2015 Share Posted November 6, 2015 So if I read correctly, only thing you'd suggest at this point would be transformer upgrade? Well yes and no. Usually these circuits need to be looked over so just changing the circuit could yield much better results. I know lots of people that buy these kits just to revamp the circuit and yes sometimes upgrade the transformers. Did you put the kit together or did somone do it for you? Do you still have the schematic or can you get it? If not I will have to spend a bunch of time looking at pictures to figure the circuit out.......and I don't want to have to do that If you are worried about posting the schematic just PM it to me or I can give you my email. Quote Link to comment Share on other sites More sharing options...
Jim Posted November 6, 2015 Author Share Posted November 6, 2015 PM sent Quote Link to comment Share on other sites More sharing options...
mike stehr Posted November 6, 2015 Share Posted November 6, 2015 The white glue like stuff is rubber like. Can a guy pick that stuff off prior to unsoldering? I wouldn't want to breathe whatever that stuff is touching it with a soldering iron. Not to mention the havoc it would wreak on a soldering tip. Quote Link to comment Share on other sites More sharing options...
Jim Posted November 6, 2015 Author Share Posted November 6, 2015 It looks like it can be taken off for the most part. Quote Link to comment Share on other sites More sharing options...
Jim Posted November 7, 2015 Author Share Posted November 7, 2015 That is one of the reasons they can sell this amplifier so cheap and it really is a deal for the price. I paid $169.00 for this new plus of course the shipping, so I don't think I'd have a problem doing some upgrades to it. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 7, 2015 Share Posted November 7, 2015 Let's have some fun with this one!!!!!! I am drawing a schematic of the amp as we speak for you tube heads to look over. For starters the preamp stage triodes look to be wired in parallel. The power stage.......well that has me confused. The yellow wire (ultra linear connections?) from the OT is hooked up to the screen yet there is a 100 ohm resistor from plate to screen?! Oh the Chinese make me scratch my head Plenty to work with here so I will first get a schematic going and take it from there. It sure would be helpful to know the load impedance of the output transformer. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted November 7, 2015 Share Posted November 7, 2015 It sure would be helpful to know the load impedance of the output transformer. Jim, if you have an AC output wall wart of any voltage (as opposed to the more typical DC types), a multimeter, and would be comfortable doing a little work under the chassis, we can tell you how to measure the output xfmr impedance. It's extremely simple to do. If you have sent JP pics of the underside of the chassis, he can tell you how to verify that the power supply caps are discharged so you can take the measurement in perfect safety. Maynard Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 7, 2015 Share Posted November 7, 2015 (edited) Hey Maynard! The seller told Jim that if he wants the higher power mode that he would have to connect something, there is no UL/Triode switch on this amp. From the pictures in Jim's first post you can see three wires from the output transformer, red, blue, and yellow. The red wire goes to the plate and the yellow goes to screen, I can't see the blue wire but it must go over to B+ somewhere near that big 450V cap. BUT there is also the common 100R connecting screen to plate for triode mode. I would almost be so inclined to tell Jim to just snip those yellow wires at the socket on Pin 4 of the power tubes and tape them off, it would probably sound a lot better that way. What do you think? On another note I don't even think the heater wires are twisted. EDIT: I am now thinking it might be prudent to double check the output transformer because for the UL tap connection to be the correct % it would have to be hooked up correctly, whenever I have wired UL transformers the red wire goes to B+ and the blue wire goes to the plate. I spoke with Jim and he only wants triode mode anyway so I guess if they are backwards it wouldn't matter. That yellow wire has got to go. Edited November 7, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 7, 2015 Share Posted November 7, 2015 Hey Jim, when you get a second can you post a pic of inside the amp where the speaker connections are. Thanks! Quote Link to comment Share on other sites More sharing options...
Jim Posted November 7, 2015 Author Share Posted November 7, 2015 It sure would be helpful to know the load impedance of the output transformer. Jim, if you have an AC output wall wart of any voltage (as opposed to the more typical DC types), a multimeter, and would be comfortable doing a little work under the chassis, we can tell you how to measure the output xfmr impedance. It's extremely simple to do. If you have sent JP pics of the underside of the chassis, he can tell you how to verify that the power supply caps are discharged so you can take the measurement in perfect safety. Maynard Maynard, I don't have any of those tools available to me,sorry. JP, I will upload more pictures in a little while. I'm actually listening to it right now so I can burn in tubes a little. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted November 8, 2015 Share Posted November 8, 2015 Hey Maynard! The seller told Jim that if he wants the higher power mode that he would have to connect something, there is no UL/Triode switch on this amp. From the pictures in Jim's first post you can see three wires from the output transformer, red, blue, and yellow. The red wire goes to the plate and the yellow goes to screen, I can't see the blue wire but it must go over to B+ somewhere near that big 450V cap. BUT there is also the common 100R connecting screen to plate for triode mode. I would almost be so inclined to tell Jim to just snip those yellow wires at the socket on Pin 4 of the power tubes and tape them off, it would probably sound a lot better that way. What do you think? On another note I don't even think the heater wires are twisted. EDIT: I am now thinking it might be prudent to double check the output transformer because for the UL tap connection to be the correct % it would have to be hooked up correctly, whenever I have wired UL transformers the red wire goes to B+ and the blue wire goes to the plate. I spoke with Jim and he only wants triode mode anyway so I guess if they are backwards it wouldn't matter. That yellow wire has got to go. I think JP is correct based on looking at the pics. It seems that the amp is wired for triode operation but has an ultralinear tap connected as well. I've never encountered such an arrangement before. JP, to me it looks like this wiring simply creates a parallel RLC (with the latter being the capacitance between the screen and plate) filter which will have a varying impedance depending on the frequency. Does that make sense? In any event Jim, I wouldn't touch anything right now if you are satisfied with the sound of the amp in general. If not, you can decide if you want to try snipping a couple of wires which will create a more typical circuit arrangement. Maynard Quote Link to comment Share on other sites More sharing options...
Jim Posted November 8, 2015 Author Share Posted November 8, 2015 Maynard, I'm looking to see if it makes sense to rewire it,and potentially upgrade it to get better sound out of it, if it's possible and not to demanding on the financial end. If I can upgrade the SQ I may do it,as the platform is already there. The 3 dimensional sound is there at times, but I have to "concentrate" when listening.Don't know if I can get better out of it or not though. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted November 8, 2015 Share Posted November 8, 2015 Maynard, I'm looking to see if it makes sense to rewire it,and potentially upgrade it to get better sound out of it, if it's possible and not to demanding on the financial end. If I can upgrade the SQ I may do it,as the platform is already there. The 3 dimensional sound is there at times, but I have to "concentrate" when listening.Don't know if I can get better out of it or not though. Well, the output xfmrs are usually the weak link as JP said. If you decide to try an upgrade, keep in mind that replacements are likely to be a different size from the factory units and will require re-drilling of the chassis. The amount of available space will dictate which replacements can be considered. There's also the question of whether they will make as much difference as you are looking for. It's a gamble. The soundstage issue is more likely to be due to blending of the 2 channels as a result of using a shared high voltage supply (I seriously doubt that the supply rails are split- hard to tell from the pics). Replacing the xfmrs won't fix that. And, as JP also said, sometimes you have to reconfigure the whole amp. Is it worth the monetary investment? I don't know, as there is no way to predict the final outcome. So, it may be worth trying JP's suggestion to simply cut the yellow leads. Don't do that before discussing things here first! By the way, that gooey stuff on the connections is probably some kind of silicone rubber, or siliconized latex (why that was used is beyond me). That would have to be picked off totally before any resoldering could be done. Possibly a time consuming task! Maynard Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 8, 2015 Share Posted November 8, 2015 (edited) Hello Gang! I have been working on the schematic from the pictures. Here was my method. I drew a rough schematic from pictures. The power supply is tough to follower so I did some fudge factoring. I knew the bias resistor for the power tubes so I just adjusted B+ to get the dissipation just under 25w. For the preamp section I can see the last cap is 250v so it must be lower than that. Open loop we get 22db of gain total. I couldn't see the feedback resistor, it's probably out near the speaker terminals. The company states 8w from a 2V input signal. I made an assumption here. If you do the math out 8 watts would be 8Vrms out into 8 ohm load. I think they measured 8V and didn't convert it to RMS to calculate power correctly. 8V peak is 5.65Vrms into 8 ohms is 4 watts, which sounds about right to me for this valve. Okay so 2V in to 8V out is a gain of 4, or 12db. From this data I can estimate the feedback resistor around 2k value. They claim 1% distortion but I get around 1.5% at full output power of 4 watts. Open loop mode the distortion rises to 5% at full output. I did poke around the web and you can find a lot of different schematics for this amp that are similiar, they all showed either 3.5k or 2.5k primary load impedance, I went with the middle of the road of 3k for these calculations and simulations. I know how scientific of me with all these major fudge factors I don't really like the input stage. I think I could make the amp much more linear overall with much less feedback, or none at all. The other thing about feedback is I would like to see if they used any compensation, the cap should be found empirically injecting sqare waves and adjusting until the ringing is minimized. If I made any gross mistakes please let me know. I might mess around with the front end and see what I can come up with, if anyone else has any ideas please share. I am thinking since Jim wants to stay with triode mode, I might try a half mu follower, the output Z will be lower to drive the triode, also the active load has better PSRR and linearity. Edited November 8, 2015 by xxJPMxx Quote Link to comment Share on other sites More sharing options...
Jim Posted November 9, 2015 Author Share Posted November 9, 2015 OK JP, Here are photos of everything under the amp as well as the top Quote Link to comment Share on other sites More sharing options...
Jim Posted November 9, 2015 Author Share Posted November 9, 2015 Sorry for the crappy photos, let me know if better are needed. Quote Link to comment Share on other sites More sharing options...
xxJPMxx Posted November 9, 2015 Share Posted November 9, 2015 It's tough to see but I think the resistors I am looking for are in heatshrink tubing. I also don't see a cap for compensation. Thanks for taking some more gut shots Jim. Quote Link to comment Share on other sites More sharing options...
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