Yaqin MC10l

Neskahi

This is my first journey into tubes. Purchased off Audiogon from a highly rated seller. But when I hooked it up this morning it blew the 5 amp fuse after
about 20 seconds. It started a faint popping in the speakers that gradually
became louder then one last pop blowing the fuse. I checked all my wiring
and all good. Then I notice that in my haste I had tubes V4 and V1 [EL34's] reversed according to their labeling so I fixed that. I then replaced the fuse and it blew again. Watching the tubes as it warms up, the popping seems to coincide with a flashing or arcing in tube V4 at the last moment before
blowing again. The amp came with a spare set of tubes so I have them to try out when I get more fuses.
The speakers are 1.2TL's and were hooked up both times the fuse blew.
Most of the popping seemed to come from the right speaker.
Since this is my first tube amp, any recommendations would sure help.
Thanks.

michaeljhsda2

What brand of tube are you using? Is the V4 tube seated all the way into the socket or just partially seated? Some of the Russian tubes have a shorter base? (not sure if that's the right term) so they don't fit all the way into the socket. In my situation with my MC10-L, the glass portion of the Russian tube was hitting the amp case and didn't allow it to seat all the way into the socket..that bothered me so I never powered the amp on because I was afraid it would arc. Instead, I reinstalled the Shuguang tubes which have a taller (longer?) base and they seated all the way into the socket. Not sure if this info helps.

Neskahi

The tubes say Electron Tube, EL34B, made in China. The other set I have is labeled
JJ E34L. Pins look the same length and appear to seat below the case. The arcing that I
saw was inside the tube. When tubes are warming up, is it a smooth transition to a glow, or should I see any arcing? I am beginning to suspect a bad tube.
By the way. This is the Yaqin you linked to here on the forum. It was a indeed a good deal. My wife did not know until it arrived and asked how much it cost. I told her
it cost 4...... big ones. She sat there for a minute and saw how pretty it was and
thought, 4 thousand? Had her goin for a good laugh.

Neskahi

One more thing. The Chinese tubes have 8 pins and the JJ tubes have 7.

polkfarmboy

Check the bias with a multi meter and report your readings

Make sure the powers down and the speakers are hooked up when checking with the multi meter. You should be reading between 350 and 400 mv's

Try using a circuit that has nothing else hooked up to it and make sure your speaker wires aint touching as these amp dont have an auto shutdown feature like most mordern day avr's etc...

How much money did you pay for this because you can buy them brand new from ebay shipped for like $500

michaeljhsda2

Never should arc. Was it the chinese tube that blew? If not, I would install the chinese tubes and then check the bias. Make sure it's between 350 400 mV.

polkfarmboy

I am using the electron made in china tubes

michaeljhsda2

Sounds like the Shuguang EL-34B's that came stock with the amp. They work great but you're right, could just be a bad tube or the bias was set too high (perhaps it was biased with the JJ's in it but you reinstalled the Chinese tubes instead...just thinking out loud).

Neskahi

I'll have the Fluke meter hooked up the next time I power up and watch the bias.
Speaker wires are ok. I use bananas.
It was 4 hundred big ones, with an extra set of tubes, interconnects, no freight, and no
paypal fees. Cheapest way I have found to try out a tube amp. Just a little bad luck
at this time.

Neskahi

It was in fact the Chinese tube that seemed to arc or flash.

michaeljhsda2

Since the amp is manual bias, I'm wondering if having the 2 tubes installed in the wrong sockets made V4 arc and then blow.

heiney9

Sounds like a poor tube.............I hate Chinese tubes in general for this reason.

H9

Neskahi

Swapped tubes today and it works fine. Bias all at around 340mv.
It's a little early to say but my first impression is that the 1.2TL's
may be a little too much for this Yaqin. Going to try the 2.3's that
Ben did my xover's next. Thanks.

michaeljhsda2

Neskahi wrote: »

Swapped tubes today and it works fine. Bias all at around 340mv.
It's a little early to say but my first impression is that the 1.2TL's
may be a little too much for this Yaqin. Going to try the 2.3's that
Ben did my xover's next. Thanks.

How does it sound on the 2.3's? Was it not able to drive the 1.2TL's loud enough? It drives my 1.2 TL's to about 116 db in my 13 x 18 ft room with great sound quality.