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Boyuu EL34 A9 Tube Amp
Boyuu EL34 A9 Tube Amp
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Old 2nd February 2015, 11:37 AM   #11
316a is offline 316a  England
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Location: West London
Chinese amps with their 220V or 230V primary mains transformers are notorious for over running HT and LT supplies on 240V UK mains . You may find close to or above 7V on the heaters where it should be 6.3V and the same for the 5V rectifier winding . Watch out for capacitor ratings , I once had a Chinese amp made by Dared for repair which was poor engineering at it's utmost worst . This used hot melt glue to fix components to the chassis (which ran very hot in operation when the fuse didn't blow) also over run the reservoir capacitor as well as the heaters . This had a 220V primary mains transformer and the fix was to add series resistance to the HT rail and a common mode choke to the heater supply with an NTC thermistor in series with the mains transformer

316a
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Old 2nd February 2015, 05:53 PM   #12
sundaycyclist is offline sundaycyclist  United Kingdom
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Hi all - thanks for the useful replies. I only got one 10k resistor so I assume I just need to cable the jump from the valve to the board.

What I can't work out is the connections from the input board - the output board and the volume control board. If anyone can supply me with a photo (Celcius?) I'd really appreciate it.
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Old 2nd February 2015, 07:02 PM   #13
sundaycyclist is offline sundaycyclist  United Kingdom
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Thanks for the info on hot running stuff - a bit over my head at this point so I'm just plugging on with what I have.

Can someone help me with the transformer wires? I'm not completely sure if the 315v supply comes out of the top in this picture (i.e. on the same side as the mains input) or on the bottom side closest to the board.
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Old 2nd February 2015, 07:09 PM   #14
ygg-it is offline ygg-it  Italy
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Connect the 10k resistor from the center tap of the volume 100K potentiometer to the grids of the parallel triodes. Connect the left tap to the ground and the right tap to the input.
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Old 2nd February 2015, 07:54 PM   #15
ygg-it is offline ygg-it  Italy
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Quote:
Originally Posted by sundaycyclist View Post
Thanks for the info on hot running stuff - a bit over my head at this point so I'm just plugging on with what I have.

Can someone help me with the transformer wires? I'm not completely sure if the 315v supply comes out of the top in this picture (i.e. on the same side as the mains input) or on the bottom side closest to the board.
From the photo i see 315V "should" come from the top since it is twisted with the ground black. But since I don't own the amplifier, just check the voltages (easy 5V or 315V...)

Check the photo here:
http://www.china-hifi-audio.com/imag...tube-amp-3.jpg
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Last edited by ygg-it; 2nd February 2015 at 07:57 PM.
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Old 2nd February 2015, 09:05 PM   #16
sundaycyclist is offline sundaycyclist  United Kingdom
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Grazie tanto
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Old 2nd February 2015, 09:11 PM   #17
mdamp is offline mdamp  Australia
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Quote:
Originally Posted by celsius235 View Post
Click the image to open in full size.
This is my rebuild after I was disappointed with the sound these kits give out
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Old 2nd February 2015, 10:46 PM   #18
sundaycyclist is offline sundaycyclist  United Kingdom
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Spot on YGG-IT. I checked using a multimeter and the top delivers 315v.
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Old 3rd February 2015, 06:25 AM   #19
ygg-it is offline ygg-it  Italy
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Quote:
Originally Posted by mdamp View Post
This is my rebuild after I was disappointed with the sound these kits give out
I disagree, these Chinese kits sound very well self alone and use very good components, but, as I did too, we can adjust the design for our personal taste.

For example you and me got the same Chinese amp (more or less) but we have choosen two completely different modifications:

1) you put negative global feedback, for which I dislike the sound (I have tested) and leave it without.

2) I put cathode bypass capacitor in the driver stage: again I skip the intrinsic local feedback, while you leave un-bypassed.

3) I put a switch to move to triode mode, and, again, you stay with the screen feedback of the ultra linear operation.

So for me Zero-feedback works the best, for you it seems that you like to have feedback: just personal taste, but not bad Chinese construction.

I maintained the triodes in parallel to lower the noise, since this is the main issue of these amplifiers

We both increased the bias of the final tubes, but you increased too hot I think. Are you sure that the stock output transformer doesn't saturate with so much DC? Also 24W is too much against the max dissipation of 25W: do you know that the loudspeaker load is an ellipse and not a straight line?

If you move from EL34 to KT66 you will get an additional improve.

Check my modifications:
http://www.diyaudio.com/forums/tubes...iew-mod-2.html
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Old 3rd February 2015, 08:44 PM   #20
mdamp is offline mdamp  Australia
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All I'm saying is once I replaced the 6N9P and readjusted the schematic to take 12ax7 I would never go back to the Chinese 6N9P but the EL34B are very good the OPT are not so good but the main power Transformer is good, the Tube sockets are cheap and not so good the chassis are dangerous and no room to get in there, components are low grade which I replaced with better ones. But for what you pay for these kits it is worth rebuilding them to a higher standards. But at the end of the day it comes down to choice and sound that you are comfortable and what sounds right to your ears, for me I like the punch in the preamp of the 12ax7 gain has.
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