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Boyuu EL34 A9 Tube Amp
Boyuu EL34 A9 Tube Amp
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Old 30th June 2019, 05:24 AM   #1011
daniellu is offline daniellu  Australia
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Thanks Chris! good thoughts. I probably will try one day. have a good weekend.
Daniel
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Old 30th June 2019, 09:15 AM   #1012
huggygood is offline huggygood  France
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@ hanskarel
"If you are interested i can put some test results of my Boyuu amp here, with a 1 KHz squarewave at the input"
Yes,i'm interested
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Old 1st July 2019, 06:45 PM   #1013
nitrate is offline nitrate  United Kingdom
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Picked up one of these pre built off a well known auction site. Got it cheap as somebody built the kit and decided it was not for them, so I thought I would take the opportunity to see what these cheap Chinese amp kits can do

I fired it up with 8r dummy loads and did some tests with the scope and sig gen. First, the signals were really jumpy and unstable, so I disconnected and checked all the components and wiring against the stock schematic. All the connections were present and components totally stock, but the soldering quality was dire. Some wires were actually rattling about on their crimps and valve sockets. I'm not having a go at the original builder, they did a good job of getting all the connection correct, but if you have ne or soldered before then it's no surprise the joints were pitifull.

After resoldering EVERY joint I repowered the amp and made some basic checks. All was well, voltages and currents as expected. The intermittent signals now were gone.

INITAL IMPRESSIONS:
The quality of the chassis is excellent, the tubes are ok, the iron is incredible considering the money that is charged. All in all excellent value for the money.

Now the bad points, performance is in MY opinion, rubbish. As stock there is a nightmare buzzing, one cannot get more than a couple of watts without horrendous distortion at any frequency, frequency response falls off after 10khz and forget about anything below 100hz. Full power was measured to be around 5W rms. Most of these problems relate to the output transformers and primitive design, but it is still excellent value for money, just needs a little reconfiguring

I decided to see how much I could improve this little amp without changing any major components or spending loads of money. Read my next post to see how I modified one channel to see how far I could improve the performance of this little amp. I left the other channel stock as a comparison to avoid any placebo effect creeping in

Have fun all!
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Old 1st July 2019, 07:35 PM   #1014
VictoriaGuy is offline VictoriaGuy  Canada
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Boyuu EL34 A9 Tube Amp
Quote:
Originally Posted by nitrate View Post
one cannot get more than a couple of watts without horrendous distortion at any frequency, frequency response falls off after 10khz and forget about anything below 100hz.
Do you have measurements (numbers, and test setup) for those problems?
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Old 1st July 2019, 08:02 PM   #1015
nitrate is offline nitrate  United Kingdom
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This is what I did to one channel of my amp to try and increase performance.

First, the buzzing. We all know this one, floating heater filaments are to blame. Two 100r resistors centered tapped to ground on the preamp tubes stopped the buzz.

Next I tried wiring the el34 in triode mode to see if this helped. It did not do anything. I tried extra bias, this achived nothing as well. Seems the output transformers are the real limiting factor here.

To get around this problem without replacing the opt I decided to apply a generous dab of feedback. I know you are all throwing rocks at me as I type, but I just cannot let the little amp go to waste as I cannot bear to use it as it is.

So here goes, first I properly set up the biasing on the 6n9p as it was not very good. I did this by changing its bias resistor (r104) from 2k to 1k (my amp is the one with 75k at the anodes of the 6n9p triodes. Next i closed the feedback loop by connecting a 2k resistor and 10uf capacitor in series from the 8r tap on the opt to the cathode of the 6n9p preamp tube. Voila! After many measurements i was convinced this was the right way forward. To get maximum power at 50hz i also increased the quiescent current in the el34 by adding a third 1k resistor. This cleaned up the top half of the waveform below 100hz. After more measurments i added a 470pf cap accross the feedback resistor to clean up a little ringing on the edges of squarewaves. This still needs attention but i can live with it for now.

So, after the above mods the scope looks good. Clean waveforms right up to full power clipping with no visible distortion or flattening of the top of the waveform. Max power is now 22vPkPk or 7.5 Wrms. Bandwidth now 50hz to 60khz with no visible distortion, extending down to 20hz at full power with distortion, then down to below 10hz but horrible distortion. 20hz at 2w is fine, but more power causes distortion.

So 20hz to 60khz, clean squares and no distortion, whats the catch?

Well the catch is it now takes 8v PkPk to fully drive the amp. This is unacceptable, hmmm....

I decided to get around the input sensitivity problem by utilising the 'Spare' section of the 6n9p.
So i commenced by cutting the tube in half, using one half as el34 driver with the feedback loop still attached, the other half as an input preamp to reclaim my gain. So simply removing the connections tieing the two triodes together did the trick. After seperation i did not alter any component values as according to the data sheet they should still be appropriate. I powered up to check that the channel still worked as previous whilst running off one triode instead of two in parallel. It worked just fine and the tube was still biased at 1.5mA with anode biased around 170v.

Next i employed the now redundent triode inside the 6n9p as a preamp to boost the input. I used the 2k cathode resistor i removed previously and connected a 100k anode load resistor. I coupled the stages together with a 100nf capacitor and fed the audio input to the grid, using a 470k resistor as grid leak to ground.

After first power up all was not well, the channel was motorboating. Adding a 220uf capacitor to c303 solved this.

Success! Still had the previous performance of 20hz to 60khz flat, low distortion and 7.5Wrms. Input sensitivity was now 500mV.

The only downside to all this is one has to remember to connect the speakers back to front as the signal is now inverted LOL

IN MY OPIONION this little amp is now to my liking and working much better than it was when it was stock. Let me know your thoughts and please down burn me for using feedback!!

Have fun all and carry on building
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Last edited by nitrate; 1st July 2019 at 08:17 PM. Reason: Changed pentode to triode mode top of text
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Old 1st July 2019, 08:14 PM   #1016
nitrate is offline nitrate  United Kingdom
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One last thing, cut out the capacitor that is accross the mains switch. It should not be there. If this was a DC mains supply then yes, the capacitor will reduce arcing and extend the switch life but mains is AC and this cap will pass a small amount of AC when the switch is off. Worst case is the cap will heat up and possibly cause a fire, apart from this its a waste of power. Best to remove it if you have not already.

Thats all for now!

@VictoriaGuy, what measurments are you looking for?

Have fun all, and stay safe
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Old 1st July 2019, 08:41 PM   #1017
huggygood is offline huggygood  France
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thanks Nitrate
for the moment, I have applied only the "mods" of the wire and already it is better.
I tried 6l6gc, 6p3S-e and the best result for me is kt66 with 6h9c.
I will try your mods.
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Old 1st July 2019, 11:38 PM   #1018
VictoriaGuy is offline VictoriaGuy  Canada
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Quote:
Originally Posted by nitrate View Post
@VictoriaGuy, what measurments are you looking for?
You mentioned the distortion, so I was wondering if you had measured THD, or used a spectrum analyzer, or some other method.

Certainly the numbers you quote after your mods are outstanding.

I'm also curious about your comment that pentode connection vs triode had no effect - usually doesn't this increase the power output?

I still have a lot to learn, so your mods are interesting to me.
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Old 2nd July 2019, 09:07 AM   #1019
nitrate is offline nitrate  United Kingdom
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Hi VictoriaGuy,

The THD has not been measured by the use of a hardware / software analyser. It is simply what i can observe on the oscilioscope. After many years of construction i get used to seeing distortions vs clean waves on the scope. When i can see the distortion it usally means its quite bad. When i can't see any distortion on the scope when overlapping the input and output it can still have distortion, but its usally under 2% when measured with an analyser. I don't have a computer in my workshop at the mo, so maybe sombody could try modding a channel on thier amp if they have access to distortion measuring equipment then post the results on here.

I do know that some people will be dissapointed if they reproduce my mods as it makes the amp somewhat linear and removes most of the harmonics that make the sound 'Tubey or Rich'.

Personally i like my amps linear and transparent, but these amps are by their nature very harmonically rich and that MAY be what people who use them like about them.

My comment about running in triode mode with no apparent change in the waveforms surprised me too. That was indeed what i witnessed. Normally the drastic reduction in output impedance in the tube normally improves wave distortions as there is more current available to drive the loads, but in this case it seems the lack of inductance in the OPT stops this from occuring. Maybe sombody with more knowledge about these things could chip in here and help us out.

This was just a weekend experiment just to pass the time, so nothing scientific here, Just having fun
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Old 2nd July 2019, 12:28 PM   #1020
daniellu is offline daniellu  Australia
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Congrats Nitrate!
I love my A9 so much and I listen a few hours everyday. I hardly use my newly bought Yamaha R-N602 these days. The final sound of A9 is an addiction.
Daniel

This is my final mod I drew the changes on it , It suits my taste and I decided to close the bottom cover and never open it again.. (in the near future)
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