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Old 25th February 2008, 08:48 AM   #1
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Default mengyue mini valve amplifier

hi i have one of these little amps it is the EL34 version i must say the sound is very good i paid 160 for it. the guy who had it before me had changed the valves but he said he never ajusted the bias. i am hopeing that it is a fixed bais, as i have not a clue were to start with byassing has an y one got one of these amps
all help would make me very greatful
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Old 25th February 2008, 11:20 AM   #2
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Actually I think you want self biased in this case as "fixed" bias is more likely to need adjusting. I tried to find a schematic for you but google came up dry. I suspect it is probably PC board based making it hard to trace the circuit but why not try to locate the cathode resistors on the output tubes and report back the color code on them?

Be careful of leathal voltages and stay way from those power supply capacitors!

mike
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Old 25th February 2008, 04:50 PM   #3
kevinkr is offline kevinkr  United States
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Simple question, are there any pots other than a volume control (if fitted) that are accessible either directly or through small holes in the chassis? If you see no means for adjustment it is most likely that the amplifier is cathode biased and no adjustments are required. (Or possible without mods.)

If you are unfamiliar with tube/valve amplifiers, and it sounds like you are -- you should read the safety and newbies thread here before taking the bottom cover off of this amplifier.
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Old 25th February 2008, 04:59 PM   #4
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hi there is 4 pots near the front here is a pic
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Old 25th February 2008, 05:13 PM   #5
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here is another pic maybe you can see better from here
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Old 26th February 2008, 12:19 AM   #6
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i don't think they're for bias - something to do with the PP aspect of the amp I think (I had and modded a similar amp a while ago but it had different tubes.) Can't remember right now what they do though......


Fran
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Old 17th December 2008, 04:16 PM   #7
Merlinb is offline Merlinb  United Kingdom
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Two of the pots are for hum-balancing the heater supply (one for each channel). The other two pots adjust potential dividers used in the paraphase inverters, to get proper balance (one per channel again). The cathode resistors are big grey ones in mine, though I can't remember the value off hand.
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Old 21st December 2008, 09:02 AM   #8
Ian444 is offline Ian444  Australia
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I've been running a Mengyue Mini for about a year and spent the last couple of days drawing up a schem and checking it out. It's a sweet & nice amp just the way it is, but the urge to modify is strong...here is the schem of mine: (as it came, unmodified)

http://i198.photobucket.com/albums/a...6P1Mengyue.gif

As you can see mine is the 6P1 model with 6N2 phase splitter tubes. Build quality is a bit shabby. My amp is specified for 220V and that is what I run it at in Australia with an APC Line-R 600 (www.apc.com). I find I have 6.3V on the heaters on one channel and 5.6V on the other channel. If I run it at 230V I get 7.0V and 6.3V. I should be able to sort that out with a low value resistor in the higher voltage heater cct. I also found a DC voltage on the control grid of one of the output tubes, around 4V to 6V. Could not isolate it, seems to be coming from the control grid itself, swapping capacitors and 6N2's and 6P1's did not isolate the "problem". Grid voltage rises and cathode voltage falls, not what I expected. Amp still sounds good so maybe not a problem, but a question I need to find an answer too. Just learning here...

I would like to find a thread somewhere for people with cheap Chinese amps in general, so we could share schems and mods or rebuilds with another circuit entirely etc. I have 3 different Chinese amps to help my learning curve I find PCB's a pain to modify and rework, so a point to point with tag strips is not far away, in fact, quite close But as I said at the start it sounds good as it is.
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Old 21st December 2008, 09:44 AM   #9
Ian444 is offline Ian444  Australia
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"I find PCB's a pain to modify and rework"

that is - when you have to pull out 6 valves and a selector switch and volume pot, otherwise no problem....
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Old 21st December 2008, 10:17 AM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by Ian444
I also found a DC voltage on the control grid of one of the output tubes, around 4V to 6V. Could not isolate it, seems to be coming from the control grid itself, swapping capacitors and 6N2's and 6P1's did not isolate the "problem". Grid voltage rises and cathode voltage falls, not what I expected. Amp still sounds good so maybe not a problem, but a question I need to find an answer too. Just learning here...
I live in the UK, and my mains voltage is 245V, but the design is the same as yours, so my HT was up over 300V. It was remarkably powerful, but the output valves and PT were on the verge of melting! I completely rebuilt mine (made a turret board) for less (safer!) power, and better performance.
One problem I encountered was that one of the heater supplies was connected (or shorted?) to ground on one side, inside the transformer, which obviously caused problems with my hum-balance pot. I had to remove the pot and just leave it with one heater leg grounded, without any problems. I wonder if your odd heater voltages, or grid voltage, could be due to something similar?
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