I bought a YAQIN MC-13S tube amp 6 months ago. That's my first tube amp. It uses 4x 6CA7T, 2x 12AX7B, and 2x 12AU7. I liked how it sounded compared to my solid state amp, so it became my main amp for the last 6 months. I estimate the total play time to be around 1,000 hours.
Today I noticed the left speaker is silent. I swapped speaker wires and source wires around; confirmed that the problem is in the amp.
Next I measured the bias voltages for the output tubes; on the working pair (right channel), the bias is 0.4V, which is at the top of the recommended range; on the left channel, however, the bias measured dead 0 for both tubes.
Judging by naked eyes, all 8 tubes are glowing like normal when powered up. Before I go buy a tester, or order a new set of output tubes, I want to make sure the problem is indeed with the tubes, rather than something else. This brings to my question:
Is it safe to take the 2 good output tubes from the right channel and stick them into the sockets for the left channel?
I know they are the same type of tubes. But I'm worried that maybe something is wrong with the circuit that will blow the good pair as soon as I put them in. But I guess I don't have much to lose, since if I buy new tubes, it'll be a set of 4. Anyway, looking forward for some expert advice before doing the tube swap.
Thanks a lot!
Today I noticed the left speaker is silent. I swapped speaker wires and source wires around; confirmed that the problem is in the amp.
Next I measured the bias voltages for the output tubes; on the working pair (right channel), the bias is 0.4V, which is at the top of the recommended range; on the left channel, however, the bias measured dead 0 for both tubes.
Judging by naked eyes, all 8 tubes are glowing like normal when powered up. Before I go buy a tester, or order a new set of output tubes, I want to make sure the problem is indeed with the tubes, rather than something else. This brings to my question:
Is it safe to take the 2 good output tubes from the right channel and stick them into the sockets for the left channel?
I know they are the same type of tubes. But I'm worried that maybe something is wrong with the circuit that will blow the good pair as soon as I put them in. But I guess I don't have much to lose, since if I buy new tubes, it'll be a set of 4. Anyway, looking forward for some expert advice before doing the tube swap.
Thanks a lot!
Schematic here: China-HiFi-Audio.ru - ????? ????????????? ????????? ????????? YAQIN MC-13S
You should measure the voltages of both EL34B at pins 3,4,5 and 8.
I would not change the output tubes before doing the voltage measurements.
You should measure the voltages of both EL34B at pins 3,4,5 and 8.
I would not change the output tubes before doing the voltage measurements.
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Measuring from tube pins is probably more problematic because one slip of the probe and you can damage the amp or tubes. It's really easier and safer to open the amp up. I don't recommend noobs to open anything up with potential over 500v, you should be fine with this amp.
If you are not confident opening it up then you should bring it to a tech.
Or be safe and learn something and do it yourself, it's not hard or dangerous if you can follow rules.
First rule is when measuring is to keep one hand behind your back or in a pocket, this lowers the chance of current passing from one hand to the other through your heart in case of an accident. Also wear shoes with rubber souls and don't do it in a damp environment like basement or bathtub
Open the amp up and don't touch anything inside with your hand.
Take the black probe from your meter and place it to the chassis ground.
Turn the amp on and measure various things with the red probe (again with one hand in pocket or behind your back).
Unless you have Parkinson's or you are currently kicking a habit you should do fine.
Measure pins indicated in previous post.
If you are not confident opening it up then you should bring it to a tech.
Or be safe and learn something and do it yourself, it's not hard or dangerous if you can follow rules.
First rule is when measuring is to keep one hand behind your back or in a pocket, this lowers the chance of current passing from one hand to the other through your heart in case of an accident. Also wear shoes with rubber souls and don't do it in a damp environment like basement or bathtub
Open the amp up and don't touch anything inside with your hand.
Take the black probe from your meter and place it to the chassis ground.
Turn the amp on and measure various things with the red probe (again with one hand in pocket or behind your back).
Unless you have Parkinson's or you are currently kicking a habit you should do fine.
Measure pins indicated in previous post.
That .4v you measured before was the quiescent current through a 10R cathode resistor, which is 40mA. (well this is an educated guess from schematics on the web)
When you swap the tubes keep your eye on that voltage reading. It will take a few seconds for the tubes to start conduct and then if you see a reading over the max .4v then shut the amp off or the tubes will be damaged. If this is the case then the amp needs to be brought to a tech.
Good luck!
When you swap the tubes keep your eye on that voltage reading. It will take a few seconds for the tubes to start conduct and then if you see a reading over the max .4v then shut the amp off or the tubes will be damaged. If this is the case then the amp needs to be brought to a tech.
Good luck!
Update: did tube swap; turns out all tubes are good. Something else is wrong with the amp. Any recommendation on reputable shop for further diagnose? Los Angeles area would be nice. Thank you!
I'll also try to reach out to the manufacture in China to see if they have any recommendation on diagnose the problem.
I'll also try to reach out to the manufacture in China to see if they have any recommendation on diagnose the problem.
Hi again,
I would not swap the tubes if it was my amp., my advise is to proceed as the following:
Check the voltage on pin 3 (plate) it should be approx 350 volts, if not, check connections through the output transformer to the powersupply. If this is ok then check pin 4, also approx350 volts. If this is ok check bias, pin5, 0.4 volts over cathode resistor should give
A plate current of approx 40 ma (as written before by famousmockingbird ) this indicates a bias voltage on pin 5 of approx -30 - 32 volts, if not check all connections back to the potmeters for bias adjust, if the bias voltage is wrong ( below about 25 volts, the tubes may be dead. Get the amp fixed before replacing tubes.
You probably must open the amp to do the voltage measurements.
I would not swap the tubes if it was my amp., my advise is to proceed as the following:
Check the voltage on pin 3 (plate) it should be approx 350 volts, if not, check connections through the output transformer to the powersupply. If this is ok then check pin 4, also approx350 volts. If this is ok check bias, pin5, 0.4 volts over cathode resistor should give
A plate current of approx 40 ma (as written before by famousmockingbird ) this indicates a bias voltage on pin 5 of approx -30 - 32 volts, if not check all connections back to the potmeters for bias adjust, if the bias voltage is wrong ( below about 25 volts, the tubes may be dead. Get the amp fixed before replacing tubes.
You probably must open the amp to do the voltage measurements.
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If it were me, I would compare various points of both channels, start from the input and work the way to the output Since one is working, you are likely to see some big/obvious different at some point.
Prior to start the above, I will check the output transformer to make sure the broken channel is not open. I will also check the input mechanics to make sure there is no lose wiring/soldering. If the OT is bad, then you can either claim warranty, get a replacement tranny, or write it off.
Prior to start the above, I will check the output transformer to make sure the broken channel is not open. I will also check the input mechanics to make sure there is no lose wiring/soldering. If the OT is bad, then you can either claim warranty, get a replacement tranny, or write it off.
If amp is switched off and tubes are loosed , check resistance between both plates /pin 3 and pin 3/ in the sockets. R must be about 100-200 ohm. And check resistance between pins 4 and 4 in both sockets. R - about 50-150 ohm. This will give You results about output transformer without opening the amp box.
I removed the top cover and output tubes, left input tubes in. Then I powered it up and measured the voltage between the different pins on the tube sockets, and the voltage across various capacitors.
All the capacitors appear to check out; the measured voltage matches the voltage printed on the circuit board (either 250V or 450V).
One big difference between left and right channel is that, on the right (working) channel, the voltage between pin-8 and pin-3 is ~460V. On the left (broken) channel, the voltage will flash ~230V before very quickly drop down to 0. If I re-measure immediately, I get either straight 0 or single digit numbers; if I wait a couple minutes before measuring again, I get ~230V reading for a split second.
I also noticed that there is a pair of pins that measured ~37V on both good and bad channels.
Re azazello: "check resistance between both plates /pin 3 and pin 3/ in the sockets"
Thank you for the suggestion, but I don't quite understand. Do I loosely insert the tube to make sure there is contact, but not completely so I can access the pins? Also what is the "plate /pin 3"? Are you referring to the resistance between pin3 of one socket and the pin3 of the other socket? Sorry I really don't know much about electronics, despite growing up watching my dad fixing hundreds of analog TVs. Should have learned something from the old man
All the capacitors appear to check out; the measured voltage matches the voltage printed on the circuit board (either 250V or 450V).
One big difference between left and right channel is that, on the right (working) channel, the voltage between pin-8 and pin-3 is ~460V. On the left (broken) channel, the voltage will flash ~230V before very quickly drop down to 0. If I re-measure immediately, I get either straight 0 or single digit numbers; if I wait a couple minutes before measuring again, I get ~230V reading for a split second.
I also noticed that there is a pair of pins that measured ~37V on both good and bad channels.
Re azazello: "check resistance between both plates /pin 3 and pin 3/ in the sockets"
Thank you for the suggestion, but I don't quite understand. Do I loosely insert the tube to make sure there is contact, but not completely so I can access the pins? Also what is the "plate /pin 3"? Are you referring to the resistance between pin3 of one socket and the pin3 of the other socket? Sorry I really don't know much about electronics, despite growing up watching my dad fixing hundreds of analog TVs. Should have learned something from the old man
Problem SOLVED! My very own stupid dumbass forgot to check the fuses; turns out the left fuse is blown.
I knew the separate fuses for left and right channel. But, since the tubes all light up I assumed the fuses are fine. Very wrong.
Now I still have another question: on the rear panel, the manufacture have spec'd 0.5A fuse for 110V AC, and 2.5V fuse for 230V AC. This seemed wrong to me. 0.5A at 110V AC is 55 Watt, which is way too low for 40Watt output, especially for a tube amp. I think they got it backward; should have been 2.5A at 110V, and 0.5A at 230V. What do you guys think? (the working right channel does have a 0.5A fuse right now)
Thanks for all the help and sorry for the false information I provided.
I knew the separate fuses for left and right channel. But, since the tubes all light up I assumed the fuses are fine. Very wrong.
Now I still have another question: on the rear panel, the manufacture have spec'd 0.5A fuse for 110V AC, and 2.5V fuse for 230V AC. This seemed wrong to me. 0.5A at 110V AC is 55 Watt, which is way too low for 40Watt output, especially for a tube amp. I think they got it backward; should have been 2.5A at 110V, and 0.5A at 230V. What do you guys think? (the working right channel does have a 0.5A fuse right now)
Thanks for all the help and sorry for the false information I provided.
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