Hi guys.
Can a brand new preamp tube suddenly go bad?
I am working on an old (and heavy, 31 kgs!) McIntosh MC275. While measurering voltage on different sections, it suddenly started to hum on one channel. I had nothing connected to the inputs. And it hums independent of the volume setting.
After checking all possible ground connections, I began swapping the KT88 power tubes, hum did not change. Then I swapped the preamp tubes. It has one 12AX7 for both channels, and then for each channel a 12AU7, 12BH7 and 12AZ7 (mine uses 12AT7's). When swapping the 12AU7's, the hum changed to the other channel. Then I used 2 working 12AT7's (cause I don't have 12AU7's) and hum disappeared almost completely.
Does that mean that one 12AU7 has become bad?
Thanks in advance.
Can a brand new preamp tube suddenly go bad?
I am working on an old (and heavy, 31 kgs!) McIntosh MC275. While measurering voltage on different sections, it suddenly started to hum on one channel. I had nothing connected to the inputs. And it hums independent of the volume setting.
After checking all possible ground connections, I began swapping the KT88 power tubes, hum did not change. Then I swapped the preamp tubes. It has one 12AX7 for both channels, and then for each channel a 12AU7, 12BH7 and 12AZ7 (mine uses 12AT7's). When swapping the 12AU7's, the hum changed to the other channel. Then I used 2 working 12AT7's (cause I don't have 12AU7's) and hum disappeared almost completely.
Does that mean that one 12AU7 has become bad?
Thanks in advance.
Sure. Tubes are prone to failure especially when new and in addition after transport that has shaken them.
Just get a new ECC82 and enjoy the music.
That 12AU7 is going to be particularly sensitive because the cathode sits over 100 volts above ground, and the Mac's filament supply is grounded. So there's a good chance that some heater-to-cathode leakage could cause some hum.
Without seeing the schematic, I am thinking you probably need to elevate the filament supply.
You will need a series 2-resistor divider from B+ VDC, to get 60VDC to ground, and a bypass capacitor across the bottom resistor to ground.
But you need to check that the rest of the circuitry will be OK with all the filaments at + 60VDC.
Whether you elevate the 6.3V filaments, or you do not elevate the 6.3V filaments, then . . .
If you order new 12AU7 tubes, I recommend the JJ ECC82 from eurotubes.com
They are extensively re-tested at eurotubes after the shipment comes from JJ in Slovakia.
The JJ ECC82 has a Spiral Filament, which many 12AU7 and ECC82 do not.
The original 12AU7 Amperex data sheet said the filament was a spiral filament.
The failure of most parts, including tubes, conforms to the "Bathtub Life Curve":
Most failures Either happen early in the life cycle, Or happen late in the life cycle (fewer failures in the middle of the life cycle).
Just my opinions and experiences
Happy troubleshooting, Happy Listening!
You will need a series 2-resistor divider from B+ VDC, to get 60VDC to ground, and a bypass capacitor across the bottom resistor to ground.
But you need to check that the rest of the circuitry will be OK with all the filaments at + 60VDC.
Whether you elevate the 6.3V filaments, or you do not elevate the 6.3V filaments, then . . .
If you order new 12AU7 tubes, I recommend the JJ ECC82 from eurotubes.com
They are extensively re-tested at eurotubes after the shipment comes from JJ in Slovakia.
The JJ ECC82 has a Spiral Filament, which many 12AU7 and ECC82 do not.
The original 12AU7 Amperex data sheet said the filament was a spiral filament.
The failure of most parts, including tubes, conforms to the "Bathtub Life Curve":
Most failures Either happen early in the life cycle, Or happen late in the life cycle (fewer failures in the middle of the life cycle).
Just my opinions and experiences
Happy troubleshooting, Happy Listening!
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petertub,
12AU7 are all 'supposed' to work well at up to 180V, by specification. But . . .
I once owned a Harmon Kardon kit amplifier that had a 12AU7 used as a concertina.
(the cathode had raised voltage; and because it was a concertina, it could not have a bypass cap from the cathode to ground)
It had hum and 'drill motor' noise.
I am sure that 12AU7 did not have spiral filaments, and did not have really good filament insulation materiel.
That was not OK in my book.
We learn as we experience problems.
The Dyna Stereo 70 abuses the 7199 triode's filament to cathode.
"Sound Practices" issue 10 has an article "Save those 7199s!" by Matt Kamna.
A little advice like that can go a long way.
Your Mileage May Vary
12AU7 are all 'supposed' to work well at up to 180V, by specification. But . . .
I once owned a Harmon Kardon kit amplifier that had a 12AU7 used as a concertina.
(the cathode had raised voltage; and because it was a concertina, it could not have a bypass cap from the cathode to ground)
It had hum and 'drill motor' noise.
I am sure that 12AU7 did not have spiral filaments, and did not have really good filament insulation materiel.
That was not OK in my book.
We learn as we experience problems.
The Dyna Stereo 70 abuses the 7199 triode's filament to cathode.
"Sound Practices" issue 10 has an article "Save those 7199s!" by Matt Kamna.
A little advice like that can go a long way.
Your Mileage May Vary
6A3sUMMER
True that some tubes break, also that pushing against the limits will have even more tubes break.The alternative is to modify a well-known high quality amp and in the process one might
introduce other problems.
In that light i suggest to buy a new ECC82 , should work. If it doesn't, then the serious supplier will
replace it. Most "McIntosh MC275" will work very well in spite of 100Vk/f , just get a decent ECC82.
petertub,
Good points.
I think not all "McIntosh MC275" are exactly the same. Whether they are from the factory, or whether they are Clones.
Many companies have to pound in the quality; partly because of the parts vendors wide variance Within the allowable specifications.
Some product designs are aimed at specific areas of the allowable specification.
Tubes and Semiconductors are often hand selected for a particular circuit or product.
I think that 12AU7 tubes are very often used with a self bias resistor that is bypassed by a multiple uF capacitor.
That kind of service is far easier to make work optimally than: a Concertina or a Cathode follower.
I set aside over 700 Very expensive beryllium oxide insulated stud mounted switcher transistors. They were within the parts specification, but did not perform as well as some other parts that were also within the specification.
When the switcher supply was re-designed to work with the spread of characteristics of that part, I was able to get those expensive 700 otherwise rejectible parts back into the production line. Waste not want not.
Just looking at all the vacuum tube threads will show that many manufacturers had more than one version of tube amplifier with the same model name.
Many of those changes were to solve problems that did not work consistently with all parts.
Just my opinions and experience
Good points.
I think not all "McIntosh MC275" are exactly the same. Whether they are from the factory, or whether they are Clones.
Many companies have to pound in the quality; partly because of the parts vendors wide variance Within the allowable specifications.
Some product designs are aimed at specific areas of the allowable specification.
Tubes and Semiconductors are often hand selected for a particular circuit or product.
I think that 12AU7 tubes are very often used with a self bias resistor that is bypassed by a multiple uF capacitor.
That kind of service is far easier to make work optimally than: a Concertina or a Cathode follower.
I set aside over 700 Very expensive beryllium oxide insulated stud mounted switcher transistors. They were within the parts specification, but did not perform as well as some other parts that were also within the specification.
When the switcher supply was re-designed to work with the spread of characteristics of that part, I was able to get those expensive 700 otherwise rejectible parts back into the production line. Waste not want not.
Just looking at all the vacuum tube threads will show that many manufacturers had more than one version of tube amplifier with the same model name.
Many of those changes were to solve problems that did not work consistently with all parts.
Just my opinions and experience
I wouldn't even think of modifying the amp, just try a different 12AU7 that may not have an issue in that position. It's not uncommon for tubes to exhibit poor specs, especially newly manufactured ones.
If the amplifier follows the schematics above, I wouldn't even think of elevating the heaters, especially if there are 12AT7's instead of 12AZ7's, 'cause the heater to cathode voltage rating ist just 90 V for them. Just buy another ECC82/12AU7, as others yet suggested.
Best regards!
Best regards!