Hello,
After a particularly long listening session of about 10 hours
my Yanqin MC10L (version 2 ) generated some rather loud pops & crackles via the speakers & blew its onboard fuse yesterday
I replaced the fuse with the spare and then checked all 4 bias were where I left them -at 0.35V, which they were
There are now still a couple of pop & crackles when the amp is powering up and some more on shut down.
I've altered the bias to 0.32V on all 4
Now it doesn't seem to be playing up as much as when the fuse blew, but I've only had 4 test runs of 5 minutes since the incident yesterday
- I've had this amp for about 2 weeks. It's been working just brilliantly(switched on for about 3 hours every day) untill this little hiccup
I'd really be most gratefull for any assistance !
James
After a particularly long listening session of about 10 hours
my Yanqin MC10L (version 2 ) generated some rather loud pops & crackles via the speakers & blew its onboard fuse yesterday
I replaced the fuse with the spare and then checked all 4 bias were where I left them -at 0.35V, which they were
There are now still a couple of pop & crackles when the amp is powering up and some more on shut down.
I've altered the bias to 0.32V on all 4
Now it doesn't seem to be playing up as much as when the fuse blew, but I've only had 4 test runs of 5 minutes since the incident yesterday
- I've had this amp for about 2 weeks. It's been working just brilliantly(switched on for about 3 hours every day) untill this little hiccup
I'd really be most gratefull for any assistance !
James
Seems like there are a few reliability problems with the Yaqins, but you can deal with most of them.
Some questioned the use of F2.5A (Flink, Fast-acting fuse) in the mains circuit, and I would too. Try a T2A like the DIYers here
http://www.diyaudio.com/forums/showthread.php?s=&threadid=110483&perpage=50&pagenumber=2
No doubt the EL34s fitted at the factory are not the best quality types. And if they are running output valves as hard as they do the 300B model, it won't be long before they'll need renewing. A ten hour session could have fatigued the screens in such a setup - the crackling could be screen - cathode shorts... does the amp have an HT fuse? I would fit one quickly if not!
I like to use these inline fuseholders, if I'm in a tight spot
http://www.rapidonline.com/Electric...0mm-5A-In-line-fuseholder-250V-AC-white/80051
and these, if I have time and space
http://www.rapidonline.com/Electric...olders/20mm-Chassis-mounting-fuseholder/71463
If it blows HT fuses (try T500mA), consider stocking up on some EL34s before anything more serious gets blasted when the EL34s really blow up.
The EH300B is good & reliable, and if you want to ride first class, get some Svetlanas.
Some questioned the use of F2.5A (Flink, Fast-acting fuse) in the mains circuit, and I would too. Try a T2A like the DIYers here
http://www.diyaudio.com/forums/showthread.php?s=&threadid=110483&perpage=50&pagenumber=2
No doubt the EL34s fitted at the factory are not the best quality types. And if they are running output valves as hard as they do the 300B model, it won't be long before they'll need renewing. A ten hour session could have fatigued the screens in such a setup - the crackling could be screen - cathode shorts... does the amp have an HT fuse? I would fit one quickly if not!
I like to use these inline fuseholders, if I'm in a tight spot
http://www.rapidonline.com/Electric...0mm-5A-In-line-fuseholder-250V-AC-white/80051
and these, if I have time and space
http://www.rapidonline.com/Electric...olders/20mm-Chassis-mounting-fuseholder/71463
If it blows HT fuses (try T500mA), consider stocking up on some EL34s before anything more serious gets blasted when the EL34s really blow up.
The EH300B is good & reliable, and if you want to ride first class, get some Svetlanas.
Many thanks - yes thinking about it, I ran the amp for more like 14hours.
I don't thing it does have a HT Fuse/s. I came across this schematic that Les posted ....
http://www.diyaudio.com/forums/attachment.php?s=&postid=1349665&stamp=1194907581
I note that he's running 6CA7's, which after a couple hours "researching" / lost in an internet valve vortex, I was begining to consider too.
Unfortunately, I thought that these were exactly what I was after : http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=160266967398&ssPageName=STRK:MEWA:IT&ih=006
-the hunt begins again.. !
I don't thing it does have a HT Fuse/s. I came across this schematic that Les posted ....
http://www.diyaudio.com/forums/attachment.php?s=&postid=1349665&stamp=1194907581
I note that he's running 6CA7's, which after a couple hours "researching" / lost in an internet valve vortex, I was begining to consider too.
Unfortunately, I thought that these were exactly what I was after : http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=160266967398&ssPageName=STRK:MEWA:IT&ih=006
-the hunt begins again.. !
The Power supply is common to both channels, and since power supply noise rejection in SE circuits is effectively zero, you will hear breakdown events from both speakers.
Schematic Useful, and highlights two immediate design errors:
- The 440V supply is uses pairs of series connected 250V electrolytics, with their voltage balancing resistors not provided. The design thus expects big-cap leakage currents to balance within 30% under all conditions, which will not happen for long, even if it held for the amp when new. leakage gets really out of hand after the amp has not been used for a few weeks, so I expect MC10 owners returning from a good holiday may be in for some cracking moments!
Fix this easily with 2W balancing resistors properly accounting for leakage. 150K by default of a calculation. Check resistors are 500V rated.
This job is only for High Voltage trained persons! This amp shows no signs of any HT bleed resistors either, so the dangerous voltage will remain on the amp after switch-off! Danger!
This fault will also be corrected by balance resistors.
The next fault is even stupider, inasmuch as even fewer extra parts would have been needed to do it right. It's the screen resistors - none fitted! at 440/470V found in this amp you need 1K 5W screen resistors on an EL34 - even in UL circuits, as here.
Fixing this also involves handling the dangerous bits of the amp - so be trained or get a trained tech/DIYer to do the fix!
Leaving these resistors out really puts the screens at risk on any EL34, and the more so on cheap valves. sudden failures, or sudden loss of quality will result when the screens get overstressed one day.
There is usually a guitar amp tech nearby, wherever you are, so if you are not HV trained, track 'em down.
Both these fixes are standard fit in Marshalls of various types, so a guitar amp tech is ideal.
Lots of scope to improve the sound on these, while you are at it. Probably worth figuring out a mod pack for these, since everyone wants an amp at their prices!
Schematic Useful, and highlights two immediate design errors:
- The 440V supply is uses pairs of series connected 250V electrolytics, with their voltage balancing resistors not provided. The design thus expects big-cap leakage currents to balance within 30% under all conditions, which will not happen for long, even if it held for the amp when new. leakage gets really out of hand after the amp has not been used for a few weeks, so I expect MC10 owners returning from a good holiday may be in for some cracking moments!
Fix this easily with 2W balancing resistors properly accounting for leakage. 150K by default of a calculation. Check resistors are 500V rated.
This job is only for High Voltage trained persons! This amp shows no signs of any HT bleed resistors either, so the dangerous voltage will remain on the amp after switch-off! Danger! This fault will also be corrected by balance resistors.
The next fault is even stupider, inasmuch as even fewer extra parts would have been needed to do it right. It's the screen resistors - none fitted! at 440/470V found in this amp you need 1K 5W screen resistors on an EL34 - even in UL circuits, as here.
Fixing this also involves handling the dangerous bits of the amp - so be trained or get a trained tech/DIYer to do the fix!Leaving these resistors out really puts the screens at risk on any EL34, and the more so on cheap valves. sudden failures, or sudden loss of quality will result when the screens get overstressed one day.
There is usually a guitar amp tech nearby, wherever you are, so if you are not HV
trained, track 'em down.Both these fixes are standard fit in Marshalls of various types, so a guitar amp tech is ideal.
Lots of scope to improve the sound on these, while you are at it. Probably worth figuring out a mod pack for these, since everyone wants an amp at their prices!
The noises you hear may not be incipient output valve failures, or capacitor breakdown.. you may just have cheap 6N1s that have become noisy.
So if you are looking for something to try without voltage hazards, try one of the Russian ebay vendors of 6N1 that other DIYers here have recommended.
I'd still have the design errors fixed soon though, and would avoid leaving it powered & unattended.
So if you are looking for something to try without voltage hazards, try one of the Russian ebay vendors of 6N1 that other DIYers here have recommended.
I'd still have the design errors fixed soon though, and would avoid leaving it powered & unattended.
Hi, in relation to:
The next fault is even stupider, inasmuch as even fewer extra parts would have been needed to do it right. It's the screen resistors - none fitted! at 440/470V found in this amp you need 1K 5W screen resistors on an EL34 - even in UL circuits, as here.
Fixing this also involves handling the dangerous bits of the amp - so be trained or get a trained tech/DIYer to do the fix!
I don't understand were I have to put the resistor. I have mc10L with 6n1.
Many thanks
The next fault is even stupider, inasmuch as even fewer extra parts would have been needed to do it right. It's the screen resistors - none fitted! at 440/470V found in this amp you need 1K 5W screen resistors on an EL34 - even in UL circuits, as here.
Fixing this also involves handling the dangerous bits of the amp - so be trained or get a trained tech/DIYer to do the fix!
I don't understand were I have to put the resistor. I have mc10L with 6n1.
Many thanks
Hi, in relation to:
The next fault is even stupider, inasmuch as even fewer extra parts would have been needed to do it right. It's the screen resistors - none fitted! at 440/470V found in this amp you need 1K 5W screen resistors on an EL34 - even in UL circuits, as here.
Fixing this also involves handling the dangerous bits of the amp - so be trained or get a trained tech/DIYer to do the fix!
I don't understand were I have to put the resistor. I have mc10L with 6n1.
Many thanks
The screen resistor is in series with the G2 or supressor grid. I see most amps using a 1/2 watt resistor there so you need to ask Rod why he thinks a 5 watt is necessary.
Also, 1K may be too much for screen stopper resistor for Ultra Linear (UL) operation. Putting in too large of a resistor there will have significant effect on the UL feedback path. Personally I will put in less then 200R, if I decided to put in one. For example, the Dynaco ST70 did not have one. If you do put in a screen resistor, put it in series with the screen pin which is pin 4 in an EL34. You will need 4 screen stopper resistors, one per EL34.
Let say even if you have a 20mA of screen current (more than typical). P = I2R meaning even at 1K ohms, A 1W resistor there is enough.
Let say even if you have a 20mA of screen current (more than typical). P = I2R meaning even at 1K ohms, A 1W resistor there is enough.
Some builders even use a 1/4 watt resistor so that it acts like a fuse. If there is an arc the resistor will fail with a loud snap which will alert you to the problem and also "shut down" the tube.
Sorry guys, I have one last question! I would like to replace R112, R111 and R104 with others of the same value and power (1K-1W) .... but of better quality. I suppose I use Helco or Kiwame. It's a good idea? In the circuit there are other resistances that would be better to replace with others of greater quality? Many thanks!
I don't think there will be any benefit to using an "audiophile" type of resistor for the grid stopper. the important point is to get one with the proper voltage rating. A carbon film resistor is all that is required. Some builders like carbon comps for the grid stopper due to their non inductive nature.
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