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Old 1st November 2007, 12:04 AM   #21
gasman is offline gasman  England
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Hi Les, newer schematic differs to yours cant seem to attach it, can send you it email. cheers.
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Old 1st November 2007, 12:57 AM   #22
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OK Gasman, I tried to send you some info myself but can't access your e-mail address. If you can send the schematic I will reply with what data I have. Cheers, Les
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Old 4th November 2007, 06:28 PM   #23
gasman is offline gasman  England
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Thanks Les, Looks like I have recieved the wrong schematic and yours is definately the correct one,did you get the photos I sent to you? the board layouts appear nearly identical as far as i can see from the 2 sets of photos, will run the amp for a week or two now and then decide on tweaking, the mains transformer seems to be running a lot cooler now with very little hum, and since biasing sounds excellent, Thanks again, Brian.
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Old 4th November 2007, 07:55 PM   #24
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OK Brian, yes I received the photo's alright and they sure are very similar. The original schematic was very useful as a starting point for re-draw, mine are still not 100% but almost. I fitted the 220nF 630V Audio grade coupling caps today and they certainly improved the bass and lower mid range, not a lot but noticeable. I also fitted the voltage equalisers across the main smoothing caps while I had the pre-amp board out. I intend to order the parts from Maplin soon to fit the mains spike suppressor in place. Maplin's do all the components :-
1 OFF, VDR MAPLIN HW13P
2 OFF, X CLASS 0.047 CAPS MAPLIN N68CN
Not too pricey either so worth doing I think.
Yes it is a nice amp when everything is set up right, it has it's design of manufacture flaws in places but for the money, who can really complain. Enjoy! Les

BTW I did visit that site with pics of the burnt MC10K, a different amplifier but does appear to have a problem with reliability. I cannot put any of my views on this until I get one that I can investigate as I am sure there has to be another reason like for example incorrectly set standing bias, but John did a great job of repair.
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Old 9th November 2007, 02:17 PM   #25
gasman is offline gasman  England
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Thanks Les, Will be visiting John as he only lives a couple of miles from me, What thoughts have you on the 680 uf caps on my amp, 680 uf, as opposed to the 470 uf fitted to your MC10L, cheers Brian.
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Old 9th November 2007, 07:56 PM   #26
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Quote:
Originally posted by gasman
Thanks Les, Will be visiting John as he only lives a couple of miles from me, What thoughts have you on the 680 uf caps on my amp, 680 uf, as opposed to the 470 uf fitted to your MC10L, cheers Brian.
That's very interesting Brian and something I failed to notice. I did notice however that the original value of the capacitors was 330uF, so Yaqin went from that to 470uF and then to 680uF. I do not have any issues with the present 470uF on my amp and I would have thought that even the lower values of 330uF would be fine for a valve amplifier.
Your amplifier also sports proper test points for the bias adjustment, it's a pity Yaqin did not silk screen the factory bias settings onto the PCB. I wonder what other changes we can find. When I fit the new 6CA7's I am going to try to set up the output stages slightly different.
First set the standing current, probably 30mA, then slightly adjust one of the pots for minimum 100Hz on the scope. This wil give equal but opposite currents in the output transformer. I also want to be able to adjust the phase splitter for minimum overshoot on a square wave input (minimum harmonic distortion) but yet to decide which is the best resistor to adjust to achieve this, the 51k looks a possibility.
Les
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Old 9th November 2007, 08:39 PM   #27
gasman is offline gasman  England
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It will be interesting to find how the 6CA7,s sound when you have set them up Les, I,ve heard good things about them as replacements for the EL34,s, incidently slightly off topic I know, I am thinking of changing the Burr Brown 2604 op amps in my MHZS CD66 tube output cd player, have you got any ideas on what might be a worthwile upgrade, cheers,Brian.
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Old 12th November 2007, 09:46 PM   #28
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Can't help you with the DACs Brian, not enough experience here but a good surf around may bring up something. I am pleased with the 6CA7's and my many visitors think they sound great. I keep looking at the power supply of the Yaqin and can't help thinking something should be changed here. I am posting up the HT part on the Forum and hoping someone can add advice or support some of the concerns I have, maybe I am worrying too much. well I have added some voltage equalizers across the caps for starters and just wondering if it is worth adding some 6V 200mA wire ended pea bulbs to protect the transformers if any of the output valves went down. I would like to hear from anyone who may have tried these. Yep, I could use a fuse, I know, but a small pea bulb would be easier to fit in the amp and take up minimal space. The switch-on surge taken by the bridge rectifier must be a bit harsh, I just hope it can take it. It must be even greater in your version with 680uF! Also, should I be concerned about the 800V piv rating of this rectifier? It's just that my experience keeps telling me the margins are a bit too close for comfort, just hope the rectifier manufacturers are providing a good safety margin.
Still it's nearly 18 months old now and no smoke yet from the HT circuits! Cheers, Les
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Old 12th November 2007, 11:21 PM   #29
gasman is offline gasman  England
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Glad to hear its sounding nice with the new valves Les, Yes the power supply does concern me sometimes, I havent had any problems with the initial surge on switch on since I changed to the 2.5 amp time delay fuse, it would be interesting to see if anyone else has come up with a solution to address this, have run the amp for 12 hrs the other day, admittedly the mains transformer got a bit on the warm side, I think a bit of extra ventilation may help there as it doesnt seem to get quite as warm when the lid is off, My next door neighbour works in a sheet metal fabrication shop, might pick his brain to see if something can be done with the green top hat cover. Also I rechecked the bias when I had the lid off the other day, V4 had crept up from 300mv to 326mv, must be a little bit of drift at the pot, will keep an eye on it, I usually leave it an hour after adjustment before I take my final reading, will check it once a month to be on the safe side,I have bought a set Svetlana 6n1p,s to try, will have them Weds the specs look the same as the chinese 6n1,s so I,ll assume they are a drop in replacement, I,ll let you know how they sound, thanks for the drawing,Brian.
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Old 12th November 2007, 11:42 PM   #30
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I think you will notice quite a difference with the svetlana's Brian.
I often wonder if it is possible to chrome plate the pots, perhaps you could inquire. Might be possible to drill holes or slots on the rear part so that they are not visible from the front, just a small amount of ventilation would be of great benefit I am sure. The down side to fitting 6CA7's is the fact you have to unplug all 4 before you can remove the top plate so I scratched marked them I II III IIII on the hidden side so I get them back in the right sockets LOL!
Les
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