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Old 14th March 2012, 05:49 PM   #1
Edriz is offline Edriz  United States
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Default Bias help Music Angel L$, EL84

Hello DIY. This is my first post and also my first tube amp.

I hate joining a new forum and starting right out looking for assistance, but I don't know where else to go. I recently purchased a Music Angel Mini, L4 using EL84 tubes. So far, Im satified with the performance and sound of this amp but my concern is Bias.

The amp came with no Owner manual and I havent been able to find any information on how to bias the amp or what settings. I emailed the seller and was told it was biased at factory with the factory installed tubes. However, I have a set of EL84M Sovteks I'd like to try but don't dare until I know more about how to bias this amp.

Can someone maybe point me in the right direction, maybe where to obtain the information I need? Im not really a DIYer but bias adjustment I think, anyone that used tube amps is something that needs to be known.

I'd surely appreciate any help or assistance,
Thanks
Ed

Last edited by Edriz; 14th March 2012 at 06:41 PM.
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Old 14th March 2012, 09:42 PM   #2
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Most push/pull EL84 amps are cathode biased, so there is no adjustment. Of course there are EL84 amps with fixed bias, and some with Dave's EFB. If nooone has a manual to verify, you might have to open it up. Make sure it's unplugged first.
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Old 14th March 2012, 09:59 PM   #3
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Hi.

A schematic would help...

It is probably cathode biased with a common resistor shared by two EL84, bypassed by an electrolytic cap, or, better, with an individual resistor + cap for each cathode.. In this case you don't need to worry about it, just buy matched EL84's.

You can measure the voltage across this cathode resistor and divide it by the resistor value to get the current (which should be somewhere between 35 and 45 mA for one EL84 - double that for two tubes if there's a single shared resistor, and the two/four currents should be very close).

Last edited by Vincent77; 14th March 2012 at 10:07 PM.
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Old 14th March 2012, 10:10 PM   #4
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Appears to be a variation on the Meng Yue Mini. Same board layout and parts:

Meng Yue Mini schematic?

jeff
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Old 14th March 2012, 10:23 PM   #5
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If that's the schematic, then the good news is that each EL84 has its own cathode resistor, which prevents the pentode from self-destructing in case of current drift.

The bad news is that the voltage amplifier/phase splitter stage is cr*ppy, you'll need balanced 6N2 tubes and/or the 50k pot shoud be adjusted for minimum distortion when you change the input tubes. BTW, ditch the chinese 6N2 and get some russian 6N2P-EV.

Last edited by Vincent77; 14th March 2012 at 10:27 PM.
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Old 14th March 2012, 11:34 PM   #6
Edriz is offline Edriz  United States
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Amazing. Thank you all for all this information. I will be ordering the Russian 6N2P-EV very soon. It really does sound alot better then I expected. Once I learn what to do, maybe a few mods will make the amp even more satisfying. I followed the link in Vinylkids post and alot of good reading there. I'll be back to read more tomorrow. Never got into tubes in my 40+years in the audio hobby but I think Im gonna like them
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Old 17th March 2012, 09:48 PM   #7
Edriz is offline Edriz  United States
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I opened up the bottom of my amp to do some investigatin, Seems there is a ground wire hooked from the chassis to the ground on the IEC so that takes care of one concern.

Looking at the solder joints, it looks as if 1/2 way through, someone got in a hurry for some reason. 1/2 look real good and the rest looked hurried. Seem to apprea solid just not nice and smooth like you'd expect. I think next rainy day, I'll clean up the solder that needs it.

Other then that, I'm pretty impressed with the over all performance. I think it's a decent amp for this price. Much more then i expected, thats for sure. I've actually given it a permanent spot on the rack so it's a keeper.

Thanks everyone for all the help. I shurely appreciate it
Ed
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Old 17th March 2012, 10:32 PM   #8
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Quote:
Originally Posted by Edriz View Post
I opened up the bottom of my amp to do some investigatin, Seems there is a ground wire hooked from the chassis to the ground on the IEC so that takes care of one concern.
Good to know it's got a proper ground.

jeff
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Old 23rd March 2012, 12:33 PM   #9
Edriz is offline Edriz  United States
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It looks like the original driver tubes are 6N3. I bought a pait of JAN-5670W to swap in and when i did, I got a wicked loud hummming. What would cause that?
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Old 28th March 2012, 05:07 PM   #10
Edriz is offline Edriz  United States
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I wanted to ask, would going with a different brand 6N3 change the sound, performace and if so, what should I expect?

I cant tell what brand the original 6N3's are. Even with using a magnifying glass, I cant seem to find a name.

I started looking for some 6N3's but still not sure which brands to try and which, if any to avoid. Any recommendations?
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