Hi everyone. I recently purchased this amp on eBay, yet low and behold.. another dud... Of course, the guy I purchased it from claims "it was fine when I used it"... Anyway, it seems to have some sort of a balance issue. The stereo balance is never spot on like with my solid state gear. Everything being played through this thing is either on the left or fades to the right. Both channels work mind you, but one is always much weaker than the other. I purchased a complete set of brand new tubes. That didn't help. My father, who works on old shortwave, A.M., F.M. tube radios looked at it but couldn't find anything physically wrong. He said if he had a schematic for it, he might be able to isolate the problem.. Anyone hear have any suggestions as to what might be wrong? Does anyone happen to have a schematic of this amp? Any help would be greatly appreciated. 
attenuator tracking issues ?
maybe this could be used http://www.audioxpress.com/magsdirx/ax/addenda/media/blencowe2996.pdf
maybe this could be used http://www.audioxpress.com/magsdirx/ax/addenda/media/blencowe2996.pdf
A lot of times, accurate schematics are not available. The best schematic is the one you draw based on your amp circuit. It does not take too long and well worth the effort when trying to troubleshoot or modify an amp.
If the left / right signal level is that different in level, it maybe the input pot / volume control. You could disable it (the volume pot) temporarily by putting two 100k input resistors (matched of course) in place for a test.
If the left / right signal level is that different in level, it maybe the input pot / volume control. You could disable it (the volume pot) temporarily by putting two 100k input resistors (matched of course) in place for a test.
What doesn't make sense is the fact that the OP says that the sound will be louder in one channel, but then FADES over to the other channel.
Sorry, I didn't mean that it would fade back and forth while listening. Just when it started, it would play much louder in the right channel. I was running my laptop through it and didn't realize I had the rca's connected to the amp backwards.. I switched them, so then the left side was obviously louder. Sorry for the confusion...
Does it do this after the amp warms up or does it just randomly fade over?
When I first received the amp, the channels would be centered when it was cold or just turned on. After about ten minutes or so after it had warmed up, is when everything would shift. After a couple of days that this had started, it remains this way as soon as you turn it on. No warm up anymore..
When I first received the amp, the channels would be centered when it was cold or just turned on. After about ten minutes or so after it had warmed up, is when everything would shift. After a couple of days that this had started, it remains this way as soon as you turn it on. No warm up anymore..
That sounds like a problem with the RCA leads coming from your laptop, or the soundcard settings, or the mini-jack going into your laptop. Have you tried running the amp from a CD player or some other source? While your father has the amp, ask him to check that the mains earth is connected to the chassis, as most Meng Yue's do not come with that fitted standard, and you need it to prevent possible electric shock.
I found out shortly after my last post here that the OP is my brother!! I knew he got a new amp but didn't know the name or anything.
So anyway, he's got it over at my place and we have it connected to my NHT 2.9's. Just for giggles before I took off to work, I put my NOS RCA 5751's in his amp. When he fired it up, the left channel was loud, the right channel quiet. He let it play that way for several hours. He shut it down after about 3 hours, swapped out the 5751's for his new Mullard 12AX7's. When he turned it back on, it was playing normal in both channels, but the sound was a little distorted like something was being overdriven, but nothing other than those two tubes were changed. He let it play that was for another couple hours before shutting it down once more and swapping back in my 5751's. This time around, it did a flip-flop and now the right channel was loud and the left channel quiet. It was this way until I came home.
Since he was using my preamp and old iPhone running Pandora, SomaFM, etc, etc as the source, he set the volume on the amp at around the 3 o'clock position. Later through the day, I asked him to turn the volume all the way up (on the amp) as I was thinking that maybe the cheap volume pot may be unbalanced unless wide open, however, nothing changed.
I'm wondering if there's something wrong with the PC board that the tube sockets are mounted to as when he swapped the driver tubes all three times, the channel balances changed... ALL THREE TIMES!
Do you guys have any ideas?
So anyway, he's got it over at my place and we have it connected to my NHT 2.9's. Just for giggles before I took off to work, I put my NOS RCA 5751's in his amp. When he fired it up, the left channel was loud, the right channel quiet. He let it play that way for several hours. He shut it down after about 3 hours, swapped out the 5751's for his new Mullard 12AX7's. When he turned it back on, it was playing normal in both channels, but the sound was a little distorted like something was being overdriven, but nothing other than those two tubes were changed. He let it play that was for another couple hours before shutting it down once more and swapping back in my 5751's. This time around, it did a flip-flop and now the right channel was loud and the left channel quiet. It was this way until I came home.
Since he was using my preamp and old iPhone running Pandora, SomaFM, etc, etc as the source, he set the volume on the amp at around the 3 o'clock position. Later through the day, I asked him to turn the volume all the way up (on the amp) as I was thinking that maybe the cheap volume pot may be unbalanced unless wide open, however, nothing changed.
I'm wondering if there's something wrong with the PC board that the tube sockets are mounted to as when he swapped the driver tubes all three times, the channel balances changed... ALL THREE TIMES!
Do you guys have any ideas?
Sorry, I didn't mean that it would fade back and forth while listening. Just when it started, it would play much louder in the right channel. I was running my laptop through it and didn't realize I had the rca's connected to the amp backwards.. I switched them, so then the left side was obviously louder. Sorry for the confusion...
This means your source/input is the problem. The right and left issues follow the way you inserted your RCA cables. It seems your right channel is working correctly because IT IS LOUDER, with the RCA cable connections swapped, but the left is louder once the cables are switched over.
Either your cables or your sound card/laptop setup are the problem here. Try an input from a dedicated CD player and new cables and let us know the results.
Sorry. I think my post came in almost simultaneously with yours.
If that is the case, could it be possibly unmatched tubes? Does the system use separate tubes to drive each channel, ie one 5751 for each channel? If it does, then the 5751 seem suspect as your 12AX7 seem to work ok. The overdriving might be happening due to the higher mu of the 12AX7 compared to the 5751 leading to "clipping" of the driver stage.
I think you are going to have to get a look at the output of both channels on an oscilloscope. What you describe could be a high-frequency instability which is swamping the one channel (and instabilities can be random like this) and making it seem to go low or distorted. The PCB layout and build quality that I saw on their 6P1 amps leaves a lot to be desired, and with tubes like EL34s it could be a recipe for instability.
Nope. There's definitely nothing wrong with the NOS 5751's as they are matched and I have been using them in my amp without a single issue. My amp is perfectly balanced with these tubes.
With his Mullard 12AX7's installed, I don't really think they are getting overdriven. If that was the case, he would have been getting distorted sound all along with this amp. It only got that distorted sound today after swapping my 5751's out for the 12AX7's.
I'm a firm believer in point-to-point wiring, especially when it comes to output tubes. I would never buy or build an amp with PCB mounted output tubes.
What exactly could be causing that "high frequency instability" you are speaking of? Is it a flaw in the design, something wired wrong, or some components used with wrong values or something?
In order to view this amp's output on an oscilloscope, he's going to have to take the amp to our father so he can check it out on one of his oscilloscopes. I have no clue when it comes to using one of those.
Even p-p wiring can be susceptible if it is not done correctly. As for the cause of this one (if that's what it is) it could be any of the failings you mention. Without having a schematic, we won't have a chance at determining whether the design is at fault, or whether wrong values had been used.
The alternative comes down to reflowing the solder on the PCB and hope
An oscilloscope with a knowledgable operator is really your friend here.
Gary
The amp is not standard if it accepts 5751 or 12AX7, so the previous owner has been in there doing "stuff". What's the chance of a close-up pic underneath? (bottom cover removed) The PCB traces for heater power will at least have been hacked/modified. You brothers must be a "chip off the old block" (as we say in Australia) with tubes in the blood.
The standard (unmodified) Mengyue Mini EL34 uses a 6n2 for the front end tube, with no DC blocking cap on input. Any DC coming from the source computer (leaky output electrolithics perhaps) can upset the DC bias of the front end tube and can cause weird results.
My sugegstion would be to fit two good quality 33nF capacitors in series with the two leads coming from the pot wiper (where they are soldered to the main PCB), see if this makes any difference.
The 6n2 is a 12AX7 type tube, only the heater is 6.3V to p4-p5, so to use a real 12AX7 or similar the socket needs to be rewired.
/Tibor
My sugegstion would be to fit two good quality 33nF capacitors in series with the two leads coming from the pot wiper (where they are soldered to the main PCB), see if this makes any difference.
The 6n2 is a 12AX7 type tube, only the heater is 6.3V to p4-p5, so to use a real 12AX7 or similar the socket needs to be rewired.
/Tibor
Even p-p wiring can be susceptible if it is not done correctly. As for the cause of this one (if that's what it is) it could be any of the failings you mention. Without having a schematic, we won't have a chance at determining whether the design is at fault, or whether wrong values had been used.
The alternative comes down to reflowing the solder on the PCB and hope
An oscilloscope with a knowledgable operator is really your friend here.
Gary
My brother has tried contacting the company for a schematic, but no luck. Our father is very knowledgeable with the oscilloscope so that shouldn't be a worry. It's just a matter of him getting the time to look at it.
The amp is not standard if it accepts 5751 or 12AX7, so the previous owner has been in there doing "stuff". What's the chance of a close-up pic underneath? (bottom cover removed) The PCB traces for heater power will at least have been hacked/modified. You brothers must be a "chip off the old block" (as we say in Australia) with tubes in the blood.
A couple things here don't make sense to me. One, the amp has "12AX7" printed right on the chassis under the 9-pin sockets, so apparently it is wired up to use 12AX7's. The second thing is that a 12AX7 and 5751 are identical minus the lower gain of the 5751. Same goes for an ECC83.
And yes, all of us have always loved and have had tube amps at some point in our lives except for one other brother of ours. I don't think he ever got into them. Then again, his systems have always been sub-par so to speak, so it doesn't surprise me that bit. LOL
Again, it has "12AX7" printed right on the chassis under the sockets, so I'm pretty sure it's wired up for them. Also, for the entire weekend he has been using my KingRex Pre-amp and iPhone 3G (been reduced to an iPod touch) as the source. The KingRex does not pass any DC at all due to the nature of its design. In fact, that was one of the key points of this preamp.
Pot wiper I am assuming you mean the volume pot?
Not to be too pedantic, but that is a common misconception. It is true that all the 12**7s share the same pinout but their operating characteristics are different: that's why they are given different nomenclature.
In fact, the 12AX7 and 5751 operating parameters are different, just as the amplification factor, plate resistance, and transconductance are different. For instance at Vp = 100 and Vg = -1 the 5751 will draw 35% more current than the 12AX7. That's clearly not related to your problem, but it is also clear that they differ in more than just amplification factor.
There appears to be two versions of this amp in circulation. Lot's of images on the net of the 12AX7 version.
Gorgomat's version:
http://www.diyaudio.com/forums/tubes-valves/157134-meng-yue-mini-schematic-27.html
jeff
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