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-   -   Yaqin mc10L (http://www.diyaudio.com/forums/tubes-valves/110483-yaqin-mc10l.html)

gasman 21st October 2007 08:45 PM

Yaqin mc10L
 
Hi,All, Bought an mc10L, lovely amp but its blown a few of fuses on switch on, the fuses are fast blow 2 amp, was thinking of trying a 2.5 amp fast blow to cure this nuisance fuse blowing but could this possibly damage the circuits, this has only happened on switch on otherwise apart from the power transformer getting a bit on the warm side possibly due due it being 220 volts, I measured our U.K voltage at 242 volts, Any body got any ideas or has come across the same problem would appreciate any help, Thanks Gasman. :smash:

hnb2907 22nd October 2007 05:55 PM

Hi,

Maybe you could try putting a NTC thermistor in series with the fuse. I'm currently playing with some of these, rated at 2A 120ohms. That'll reduce the inrush current. It will also drop about 5VAC off your 240V, which may be useful if your amp is intended for 220V. Maybe using a 2A slow blow would be a better option?

However, the NTC runs at about 110C, so I've put mine in a ceramic terminal block and heatshrinked about 2inches of the cables going to it.

Have a search on the forum for "CL-90" or "NTC", there's some more info from the folks here, who are definitely more knowledgable than me! :)

It might be worthwhile having a look around the insides to make sure that there's no obvious faults. Check the sticky safety thread first though...


Cheers,
Chris.

gasman 22nd October 2007 08:44 PM

Hi Chris, thanks for that, I will search and investigate the ntc thermistor route, slowing or reducing the incoming current could possibly be the answer, Had the lid off this afternoon and checked the circuits for any obvious faults couldnt find anything, inserted a 2.5 amp fast blow and checked the voltage at the iec socket which was reading between 241v and 245v on the meter, have switched on and off 6 times with no problem had the amp on for approx 2 hours no problem ,Hooked it up and and it sounds fine, it is a lovely sounding amp for not much money , looking forward to tweaking it but naturally i want to make sure its safe first, may also see if i can borrow a variac and regulate the voltage down on that and try the 2 amp fuse again. Thanks again.

Hi_Q 22nd October 2007 09:39 PM

Hi Gasman, I have had no problems with fuse blowing on my MC10L but I am using an anti-surge fuse, these tend to be used on the mains input circuit of valve amps with solid state rectifiers. There is quite a surge when power is applied, especially if you happen to apply power when the mains waveform is at a peak. I was worried about the UK mains voltage as well and used an auto transformer to provide a 220V tap but last week, after doing my routine bias check, I measured the heater volts and found them at 5.9V. So I took out the auto transformer and ran direct from the mains and the heaters are 6.32V so I reckon it is safe to run direct from the mains.
BTW don't quickly turn the amp off and then back on, the negative bias for the valves takes a bit longer than the HT to come up with the result that the EL34's have a hard time and draw a lot of current for a brief instant in time. I first noticed this on my valve current monitor box I made which incidentally I find much better to use than measuring the cathode voltages.
Les

gasman 23rd October 2007 12:09 AM

Hi Les,Thanks, You are doing exactly what John chambers of Champ electronics recommended to me, i,ll pop and pick some up tomorrow, which amp rating are you using is it 2amp or 3 amp? Also what do you think of the sound of the mc10L, Sound quality on mine is absolutely 1st class to my ears for the money. thanks again. Brian.

Hi_Q 23rd October 2007 11:10 PM

Hi Brian, well I had been using my faithful Yamaha for 30 years, I had forgotten the sound of a decent valve amp. Then I visited by brother who had just built his own WAD amp and I was very impressed. So I tried the Yaqin MC10L and within 2 minutes the Yamaha was shifted to the bedroom to collect dust. I found the Yaqin very energetic in the highs but it acquired a nicer feel when I replaced the valves with better quality. Currently using Russian military grade 6N1P's.
You should be ok with 2A anti-surge, the original fuses fitted were 2.5A quick blow.
It did concern me that maybe you are living in an area with a higher mains voltage, it does vary somewhat and if a little high it is quite possible that it could slightly increase the switch on surge.

There is supposed to be a 10nF/400V capacitor across the mains transformer primary. I have not dismantled the Yaqin this far to see it :D but is a possible candidate. All I know is that it is 400V but AC? or DC?, one would assume AC. Capacitors normally go short once and stay that way or blow to smithereens. But I did come across a case of one that blew a fuse, seemed to repair itself and repeat the performance a number of times before it finally blew apart!:D
Les

gasman 24th October 2007 02:05 AM

Hi, Les, Thanks for your reply, i fitted a 2.5 amp time delay fuse today and have switched on and off several times no problem so it appears to be solved, On sound quality this amplifier is excellent i have been experimenting with single and biwiring on the different taps, biwiring on the 4ohm tap onto Jamo d830/concert 8 speakers just in front at the moment, using an MZHS valve output cd player, sounds just slightly lively at the top end at the moment but its early days yet, will let it burn in a few weeks before i start tweaking, probably just change the 6n1,s to start with, I also have a Classe cp35 pre with a Samuel Johnson PPA 100 power amp, have done a brief comparison with these and the MC10L and it is not that far behind, only slighty lacking in a bit of low end grunt but the mc10l sounded clearer, when you consider that the cp35 and s/j ppa100 retailed at 1200 and 2600 new the mc10L is a total bargain, could I be selling soon?, I have changed the stock 12 ax 7,s for siemens ecc83,s in the MHZS CD66, vast imrovement, and i will be changing the op amps next see if i can glean any improvement, Cheers for now, Brian.

rk-one 27th October 2007 12:26 PM

Hallo, i've the mc-10l too, and i like to know from les what kind of 6n1p has used ( could u post name of the shop or ebay seller ?) and if is it possible to swap it whit the old one without any regulation (must be matched or doesn't care?).What's changed with the new ones?

I'd like to know also if someone is able to make an "how to" (with some pics if possible ) set bias of el34 on mc-10l


Thank You

Gerry

Bob01605 27th October 2007 11:55 PM

Try Ebay for the Russian 6N1P tubes
 
Try Ebay for the Russian 6N1P tubes. There is a guy in the Ukraine called "Mycomponent" that sells them cheap - I bought a lot of 16 6N1P tubes from him at $20 US dollars + shipping. Even with shipping it came out to about $2.12 US dollars per tube. Nice tubes - sound great ... Check out the link below for his Ebay web store.

http://search.ebay.com/_W0QQsassZmycomponent

Bob

Hi_Q 28th October 2007 12:40 AM

2 Attachment(s)
Hi Gerry, I use the svetlana 6N1P available from Watford Valves.

http://www.watfordvalves.com/product_detail.asp?id=1479

For biasing the output stages you need a good multimeter to connect across each of the 10 Ohm resistors associated with each valve.
The original voltages I measured as being around 330mV (0.33V) but I set my outputs to 300mV and seems to be ok. Running a bit lower will extend the life of the valves. I recently checked my amp after it had been in use for 12 months and found each valve had dropped to 250mV so I adjusted each control to bring them all back to 300mV.
I think that when the day comes that you cannot get 300mV any more, then its time to change the valves. But before you put the new ones in make sure you back down the controls for minimum voltage with the old ones in circuit. Then let the new ones heat up for a good 15 minutes before setting each as required.
Les


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