EL34 burn out on Yaqin MC10T - diyAudio
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Old 25th February 2012, 03:04 PM   #1
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Default EL34 burn out on Yaqin MC10T

I received my MC10T from factory one month ago, and V3 tube (EL34 type) is yet burn out, after just 50 hours use. I have noticed a BIAS deviation, getting lower than nominal 370mV, with this tube before it burns out. Tubes are original EL34-B from Shuguang.

Does anyone have encountered that problem?

I plan to change EL34 quad, but before doing that, I would like to find why V3 has burn out so quickly. V1 is also burn out, as a consequence of V3 burn out I suppose.

Thank you for your help.
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Old 25th February 2012, 08:37 PM   #2
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First off... Don't believe the Chineese made toobes that came with the amp are actually any good. They are lousy and the number of hours you got is about right. Switch to higher quality tubes and you won't have many, if any, problems ay all. It also helps to delay the turn on of the high voltage to the output stage. You will get lots more tube life if you do. I have A Chinese Audioromy FU-29 amp that sounds absolutely wonderful but like you I got very little time out of the included Chineese made tubes. I use RCA 829 JAN NOS tubes in the output and Russian small signal tubes. Now at well over 150 hours on those and zero trouble!!

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Old 25th February 2012, 09:14 PM   #3
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One of the problems is that there is an SRPP in there, but the filament voltage is at 0V.

So the tube's maximum filament/cathode voltage is exceeded which may fry the odd tube, which in turn may make an output tube glow.

It's really the only problem with these amps (which is impressive for a chinese amp), but you will need to elevate the heater voltage.
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Old 25th February 2012, 09:59 PM   #4
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If it's only the tube in the V3 position, you may have a bigger problem. But first, get rid of those Chinese EL34's!!!! Unreliable and thin. The JJ KT77's sound great...that is if you can get a good set. For more bass, get the EH 6CA7. In other words, replace all the output tubes, it will sound 100X better!
"No man really becomes a fool until he stops asking questions" - Charles Proteus Steinmetz

Last edited by TubeHead Johnny; 25th February 2012 at 10:03 PM.
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Old 26th February 2012, 11:50 PM   #5
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Thank you for your advices. I definitively think I have to change my EL34B tubes with more reliable ones.

I hesitate between EH 6CA7, JJ KT77 and KT66. I have found a lot of positive comments for EH 6CA7 and JJ KT77. Have you experienced KT66 with Yaqin MC10 amplifier? Any brand to recommend for KT66? It seems it should give a really great sound.

About delaying the high voltage, does it have a wide impact on tubes life time?
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Old 27th February 2012, 05:40 AM   #6
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Talking delay HT to output tubes

I have some amps that have Flying Cs in them now for 9 years. The amps have a solid state PS for HT, 645v applied on turn on. No problems with "Cathode Stripping". I think the problem is overblown. My two cents.
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Old 27th February 2012, 06:01 AM   #7
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I never had problems with "cathode stripping" either. That is for preamp tubes, i don't have much experiences with final tubes like EL34 and similar. A friend does and uses ss rectifiers; when he has a failure it's always for other reasons than "cathode stripping".
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Old 28th February 2012, 11:36 PM   #8
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Thank you for your responses. I feel that the cathode (circular top part inside the tube?) of V3 is not in the right axe, that should explain problem with it.

For now, I have ordered a new set of tube, I will see... crossing fingers! This time I have taken KT66 tubes, hoping getting a excellent sound at the same time.
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Old 29th February 2012, 12:46 PM   #9
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You might have trouble fitting the KT66's base into the holes in the top of the MC-10L. If it's anything like the MS-20L the holes are just big enough to get the El34's in...
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Old 29th February 2012, 06:52 PM   #10
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You're right, that's why I plan to use tube saver sockets to plug the KT66.
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