Yaqin - MC13s

[txtune]

I have had my Yaqin MC-13s for one year. I made sure the bias level was correct from the first time I powered it up, and I continue to check it every couple of weeks. One side does 'drift' out of bias and has to be adjusted.

A couple of months after I got it I heard a 'pop' then one of the tubes lit up and burned out. I thought it must have been a bad tube. Last week it happened again. Any idea what would cause this? Where to start looking?

Thanks

[shooter]

does 'drift' out of bias

leave the tubes out, monitor the bias volts @ the tube socket, pin side, watch it for 10-15min.
stable?
If so, probably to hot for tubes, maybe 


a schematic helps
1 ohm resistors on the cathodes are great if fixed bias
If Self biased the cathode vdc and cathode R come in handy

[txtune]

leave the tubes out, monitor the bias volts @ the tube socket, pin side, watch it for 10-15min.
stable?
If so, probably to hot for tubes, maybe 

It has places to monitor the bias on top, see pictures, how/where do I compare those to the socket pin?

a schematic helps
1 ohm resistors on the cathodes are great if fixed bias
If Self biased the cathode vdc and cathode R come in handy

Schematic attached.

It is self biased. What is cathode vdc and cathode R? Sorry 

[PRR]

Sounds like cheep factory tubes. Get some good EL34s.

hint

[txtune]

Sounds like cheep factory tubes. Get some good EL34s.

hint

I replaced the cheap factory tubes early on. I replaced the 12AX7 and 12AU7 with some vintage tubes. Initially I replaced the EL34 tubes with some of the new Russian Mullards. One of those blew. I then bought some Genalex Gold Lion KT77 tubes, and now one of those has blown. This time when the tube lit up red and the speakers had a horrible noise, I tapped the cover for the center transformer. Just a gentle tap made the noise crackle, then stop, then start back up. I turned it off after that, not sure if that would indicate something being loose or shorting ... or if that is just normal when a tube is burning up like that.

I still have the original Chinese tubes and also some JJ Electronics EL34s from the Slovak Republic that I have never used. Not sure if the problem here is caused by China or Russia.

[txtune]

Get some good EL34s.
hint

I take it you mean the EH EL34 Power Tube. Where are those made? If the problem is with my amp I hate to burn another tube up without fixing the problem first.

I have seen online where people have modified these Yaquin amps and made them superior to how they come from the factory... but there was not much in the way of instructions for a newcomer, such as myself, to accomplish the mods. I will keep googling.

[shooter]

are they blowing in the same socket?
are they blowing on the side that "drifts"?


look at the schematic, the bias voltage that should be on the grid, goes through a connector, (XS102 Pin 1 & 3).  I would look REAL close for burn marks, sloppy fits, poor solder connections.


IF you loose that connection, the tubes go into meltdown.


you can monitor the grid volts with no power tubes in, put + lead in the socket for G1, grid, put - lead on chassis. (should see ~ 45 negative vdc) now thump, bang, bump the wires, connectors, with a wooden stick (chopstick) and see if the meter goes bonkers. (POWER IS ON SO BE CAREFUL)

[txtune]

are they blowing in the same socket?
are they blowing on the side that "drifts"?

Unfortunately I did not take a note of the socket location when the first one blew. Rookie mistake. This last time it blew in V3. V3 and V4 are the side that drift.

look at the schematic, the bias voltage that should be on the grid, goes through a connector, (XS102 Pin 1 & 3).  I would look REAL close for burn marks, sloppy fits, poor solder connections.


IF you loose that connection, the tubes go into meltdown.


you can monitor the grid volts with no power tubes in, put + lead in the socket for G1, grid, put - lead on chassis. (should see ~ 45 negative vdc) now thump, bang, bump the wires, connectors, with a wooden stick (chopstick) and see if the meter goes bonkers. (POWER IS ON SO BE CAREFUL)

Will do. After removing the power and opening the hood, how do I safely discharge any capacitors that could bite me? If I unplug it and let it sit for an hour will they discharge on their own?

Also, should I remove ALL the tubes, or just the power tubes? Or does it matter so long as I remove all the power tubes?

[shooter]

you can have all tubes out for bias voltage check, but POWER needs to be ON.


for just a visual inspection no power needs to be on.


as for discharging caps, there are bleeders, but ALWAYS measure to be sure (NOTE: your meter WILL start to charge caps when measuring with power off.  you want ideally 0vdc but <10vdc and one hand in your pocket should be safe-ish


meter where I drew the "short", neg to chassis, pos to top (+) of caps,


IF you do short out caps, there WILL be a pop/arc/snap so go pee before you do this 

[txtune]

Thanks shooter, your words of warning always resonate!

[2deaf]

Turn the amplifier off.
Unplug the amplifier from the wall.
Wait for B+ to drain down.
Remove fuse F603.
Plug amplifier into wall.
Turn the amplifier on.
Test the bias circuit.

[shooter]

Remove fuse F603.

nice catch, thanks

[txtune]

Wait for B+ to drain down.

B+ is what shooter marked up on the schematic, right? 351V

[2deaf]

B+ is what shooter marked up on the schematic, right? 351V

Yes

[ac427v]

Each power tube has its' own adjustable fixed bias and a 10 ohm measuring resistor. This allows great accuracy but is trickier to bias than an amp with 1 ohm measuring resistors. What voltage reading is your goal when you bias?

[shooter]

built in notes

[txtune]

Initially I biased them at .375vdc ... after the first tube blew I tried to keep them at 3.5vdc

I am going to take it to my shop where I can work on it. Will update this post with findings and, almost certainly, more questions. Everyone here is very helpful and understanding to 'stupids' like me.    I had a flight instructor that referred to all student pilots as 'stupid' pilots... we knew what he meant.

[txtune]

Remove fuse F603.

nice catch, thanks

I have found 3 fuses total, all on the back of the housing. To me it looks like there should be a total of five.

[shooter]

the rest should be inside, guessing black shrinkwraped with leads protruding.  do a good gut shot of the amp so we can go Easter egg hunting

[txtune]

the rest should be inside, guessing black shrinkwraped with leads protruding.  do a good gut shot of the amp so we can go Easter egg hunting

I have taken it apart as much as possible without cutting anything. The first image is for reference. The rest are photos of the interior of the back plate of the chassis where the PEM, speaker wires, fuse holders, etc are coming in. Let me know what else you want to see.

[shooter]

I'm only finding 4 on the schematic, but i'm not trying hard 
3 you have, with those 3 on back out and let stand for 1/2 hour, there shouldn't be anything left to bite hard.


that said, ALWAYS understand and use safety precautions.

[txtune]

I'm only finding 4 on the schematic, but i'm not trying hard 
3 you have, with those 3 on back out and let stand for 1/2 hour, there shouldn't be anything left to bite hard.


that said, ALWAYS understand and use safety precautions.

Ok, so take those three fuses out and let it stand for 1/2 hour. Then put the +lead in a power tube socket (pin 1) and -lead to chassis?

[ac427v]

Just doing an ocd check here. What is your desired bias reading? Hopefully not 3.75 vdc across a 10 ohm cathode sensing resister. More knowledged folks please react. My internet-educated brain says that would give 375 ma of bias current! It seems the goal be a reading of .35-.40 volts?

[shooter]

catching up;
the fuse you want out is F603, 1.6A,  that kills B+ to the tubes, which should be out.
THERE WILL BE VOLTAGE THAT BITES, SO 1 HAND IN POCKET.


gator-clip the black probe to chassis ground
power up NO SHORTING jumper, just awareness
use red probe to measure VDC at each of the 4 G1 pins (tube side) PIN 5
record the 4 readings and post

[txtune]

Will do on Monday, I was planning to leave all three fuses on the back of the device out... however one of the fuses is on the PEM. I should put that one back in right?

[shooter]

yep, you need the bias portion LIVE to measure it

[txtune]

Just doing an ocd check here. What is your desired bias reading? Hopefully not 3.75 vdc across a 10 ohm cathode sensing resister. More knowledged folks please react. My internet-educated brain says that would give 375 ma of bias current! It seems the goal be a reading of .35-.40 volts?

My bad, I typo'd in the post which I believe you are referring. I had started out biasing at .375, after the first meltdown I started aiming for .350 ... not 3.5 ot 3.75. 

[ac427v]

0.350--Great! That's me being Mr. Literal. Funny clip by the way.

[shooter]

 
one of my favorite movies, along with Joe Dirt, Blazing saddles........
I'm the "best of" the lead guy(cleaning fish on the TPS reports :) and the stapler guy


don't "dial" any of the bias pots yet.  what i'm trying to eliminate is loose/bad connections from the bias pots to pin 5

[txtune]

Got it. It will be Monday, at best, before I get to it.

[txtune]

catching up;
the fuse you want out is F603, 1.6A,  that kills B+ to the tubes, which should be out.
THERE WILL BE VOLTAGE THAT BITES, SO 1 HAND IN POCKET.
With the pots for bias all dialed full left I got ~ -51.75vdc on each of the PIN5 sockets.

gator-clip the black probe to chassis ground
power up NO SHORTING jumper, just awareness
use red probe to measure VDC at each of the 4 G1 pins (tube side) PIN 5
record the 4 readings and post

[txtune]

I checked resistance on the bias test points, with no power to the unit. The left side was fairly consistent but the right side, which is the side that drifts' was less consistent.

[shooter]

was less consistent.

like changes a few tenths of ohms or drops out comes back?
what you want while ohming, wiggle jiggle tap n bang on the boards cables etc.. to see if the meter drops out, comes back.....push down on the boards some also.


if it seems good
load the tubes back in, monitor the currents, if one side drifts, swap PA tubes and see if it follows the tube or stays with the socket, take notes, label tubes

[txtune]

I just meant the readings, which can be seen in the pictures. Not sure how close to 10ohms they should be.

What bias level would you aim for, if the given range is .35 - .4vdc? If a tube is run lower than .35 is it equally as hard on it as running higher than .4?

[shooter]

homework time;
those #'s are the tubes current when you /10(ohm) (I=E/R)
the plate voltage * the tube current minus a smig ~~ the tubes power dissipation(P=E*I) (Not audio power)
now that you have the basics;
read this til your head explodes, or the light bulb turns on
IF explosion, glue the pieces back, repeat til lightbulb comes on 


http://www.aikenamps.com/index.php/what-is-biasing

[txtune]

Will do, thanks for the reading material.