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Old 30th January 2010, 12:58 AM   #1
ksporry is offline ksporry  China
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Default Meng Yue 6P1

Well, I finally got my Meng Yue.
The first thing I did was check if the transformer was a 240V one... It wasn't They just didn't bother replacing that one as asked.
Now I need to find a solution for that. Can anyone tell me how I can lower the net voltage from 240Vac down to 220Vac? I tried a search online, but without success, so if someone has a solution for me here, then great!

I did install an earth wire. Interestingly the circuit board has one screw that connects the pcb to the chassis. Actually quite good as it works as a star earth. So I connected that directly to the mains earth pin. Without checking the bias, I just gave it a go with my B&W CDM 7NT's and to my surprice it produced sound!, and not even bad sound! I could hear no hum. I turned teh source off and dialed up the volume, but no hum whatsoever. This pleased me. Maybe it's because of the earth I put in.

While I was putting in the earth connection, I noticed that the main caps inside were rated at 220V, which seems quite low actually. SHould I fear thes eburning out if I do use the amp at 240V? I already nticed the input transformer warming up (whilst the output transformers stayed quite cool)

Now I need to find some speakers to go with it (I will continue to use my normal speakers with my main system). I've started to build some big tower speakers, but I started wondering if maybe it will be too big, as I wont have room for 2 pair floorstanders downstairs.
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Old 30th January 2010, 05:22 AM   #2
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Check out what has become a defacto Mengyue thread:
Meng Yue Mini schematic?

Some short answers:
A 56 ohm 20 watt (or greater wattage) resistor placed in the live feed to the on/off switch will drop the voltage to almost exactly the required 220v. It will need to be mounted on a tagstrip or bolted down underside of the chassis depending on the type you get. It gets hot, so you need to make sure it is clear of all wiring. Do this modification as soon as possible, as the amp is really stressed on 240v.
The main caps you see are part of a voltage doubler circuit, so they never see more than 200v. They are not related to the mains voltage.

Interestingly, the sound on mine improved as soon as I dropped the mains voltage as above, so you may be in for better yet.

You will need some high efficiency speakers if you expect high volume levels, as this amp puts out somewhat less than the advertised 10 wpc. However, I am running mine quite happily into some KEFs from the 1970s, which are rated for an amp minimum output of 20W, and they go fairly loud.

Plenty of mods and comments on the little amp on the thread listed.

Gary
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Old 30th January 2010, 06:37 AM   #3
Svein_B is offline Svein_B  Norway
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The main concern with 240V mains and a PT rated for 220, is the heater voltage for tubes. This can be cured with some resistors in series with the heaters.

An increase of 5-10% in the B+ will normally not be a problem.

Svein.
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Old 30th January 2010, 07:44 AM   #4
Trebla is offline Trebla  United Kingdom
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So long as the max plate voltage isn't exceeded there should be no problem.
The heater volts can be adjusted as above.

If you want to try something else, then a bucking transformer works well.
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Old 30th January 2010, 10:26 AM   #5
ksporry is offline ksporry  China
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thx for the advice, just as a pointer, it is a Meng X1, which utilises 8x 6P1 tubes, and in theory should provide 24W (though I doubt that much is achievable from what others tell me). here is a link on ebay: MENG X1 6P1 6AQ5 x8pcs Vaccum Integrated Amplifier NEW on eBay (end time 10-Feb-10 22:29:34 GMT)

Considering that the unit is quite different physically (though not in principle), I am struggling to figure out the schematics, so putting resistors in the heater circuit would be tricky for me to figure out. I could make some pics with the back open though, for you guys to have a look.

The 20+ Watt resistor sounds like a plausible solution to me (that one is easy to trace). I also like the idea of an external transformer, but I don't know where to get a bucking transformer that would do what i require (especially specs wise I wouldn't be sure what to choose)
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Old 30th January 2010, 10:36 AM   #6
Svein_B is offline Svein_B  Norway
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Very often amps specified for 220V work OK at 230-240.

Measure your heater voltages (AC). If they are more than about 6.6V you may shorten tube life. If this is (nearly) OK, and the amp does not run unreasonably hot - do not worry.

Svein.
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Old 30th January 2010, 12:11 PM   #7
ksporry is offline ksporry  China
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I think there's more to it than just the heaters. Normally you have 285Vdc (in case of the Meng Yue Mini) coming from the same transformer going to the rest of the amp. If the input is increased with 10% (220 to 240), then the output will also be increased with 10%, meaning it's no longer 285Vdc but around 315 Vdc, and I don't believe that can be good for the amp...
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Old 30th January 2010, 05:45 PM   #8
Trebla is offline Trebla  United Kingdom
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You need to carefully measure the voltage actually at the plates/heaters when the amp is on and the tubes in place. The voltage from the power supply will drop when under load.
Don't forget to connect some old speakers or some resistors to be sure of not doing any damage.
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Old 31st January 2010, 12:07 AM   #9
ksporry is offline ksporry  China
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I can measure voltages, but I cannot measure current. It would mean putting my multimeter in series after breaking a connection, and that is a little too dangerous without the use of a power lab for me.

I forgot the photos, but you can see them in teh advert on ebay: MENG X1 6P1 6AQ5 x8pcs Vaccum Integrated Amplifier NEW on eBay (end time 10-Feb-10 22:29:34 GMT)

I don't know what the power consumption is for this unit. It's not the mini, so I cannot assume it will use the same amount of power. This unit has 24W, rather than the 10W for the mini. This means that power consumption will be higher. As a result, the 57 Ohm resistor will not be suitable as it is for an amp that uses less power.

I found the following Auto transformer online that MIGHT be suitable: AT0501 : 500VA 0-115-220-230-240vStep up/step down autotransformer(Screw terminals)

This transformer seems to be able to deal with many different voltages. However, this is the ONLY place in the whole world that it exists (I can't find it on the internet anywhere else). Apparently there is no manufacturer for this unit (I wonder if gnomes use some kind of magic to construct it), and I cannot find documentation on it/ I want to know if I can use this AT to pump in 240V and suck out 220V. The VA is most likely sufficient.

I prefer to drop the power BEFORE the main input transformer, because that way the transformer itself will live a little longer as well.

I don't know enough about electrics to start making bucking transformers, and that might result in me blowing up the galaxy, so I won't go that way.

are there any power conditioners for audio equipment that would regulate teh voltage to 220V ?
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Old 31st January 2010, 07:19 AM   #10
Svein_B is offline Svein_B  Norway
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Well, if you want to drop the voltage before the PT, the easy and cheap solution is the previously suggested Bucking Transformer.

The primary V should match your mains. The secondary voltage is your desired voltage drop. Secondary current capacity should match your intended load.

Svein.
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