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Boyuu EL34 A9 Tube Amp
Boyuu EL34 A9 Tube Amp
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Old 31st January 2018, 05:37 PM   #331
dmnstewart is offline dmnstewart
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Join Date: Jan 2018
Default A9

Works very well on my NAD cd player. Very little hum.
Have to use a preamp(NAD) for my Technics record player (magnetic cartridge))
and my REGA 78 rpm turntable (magnetic pick up) But results are very good despite some mis-match.
Have put in bypass capacitors on input valve/tube, with increase in output with some very low hum. Nothing that affects listening quality.
Will now enjoy music before any more modifications.
Current speakers which are Mission/Denon 88 Db soon to be replaced by B & W speakers.
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Old 1st February 2018, 04:07 PM   #332
PrebenDK is offline PrebenDK  Denmark
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Default A9 schematic

The schematic in my post # 316 is not good to see or read, so i made a redrawing.
Attached Files
File Type: pdf Boyuu NS-11D A9 original schematic.pdf (48.9 KB, 142 views)

Last edited by PrebenDK; 1st February 2018 at 04:25 PM.
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Old 4th February 2018, 08:21 AM   #333
easygoing is offline easygoing
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Default power output low

Some what puzzled .Ive just built one of these , it works - but with only a little power, 2 - 3 watts.
Have fitted the by pass 470uf , grid resistors , and 100r ground resisters on the 6.3v heater .
Currently using the supplied tubes. Both channels are the same .
All seems good , except that i only get a max of 6v peak on the 8r output loaded with a 8r2 10w resistor , above this the positive clips and the negative goes to about 8v before clip. If i move the resistor to the 4r output i can get 9v peak before clip , the 8r terminal shows 11v on scope. If increase the load to 4r on the 4r terminal i get a max of 6v before clip.
The signal on the 6n9pj is a clean sine wave,even when the output is virtually a square wave. ( i use 600 - 800 hz for testing.)
Ive checked and rechecked the connections and measured all the values , all looks good. Transformer wires go to the correct places.
Ive measured the voltages and compared to those on page 32 and there are differences , i seems to me that there is too little voltage on the output transformer. At the 39r resistor 342v , B1 - 330v , EL34 pin3 324v , pin4 329v , pin1/8 23v. right channel is similar.
that puts 6vdc across the output transformer , compared to the measurements on page 32 that one has 10v . As the dc is a little higher i would have expected arround 11v.
Does any one have any ideas whats wrong here? Also, Was expecting the transformers to get a bit warm , but they dont.
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Old 4th February 2018, 03:49 PM   #334
PrebenDK is offline PrebenDK  Denmark
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Hi easygoing
It seems to me, that the screen grid is to high (329v on EL34 pin 4). Check the wirering according to what 6A3Summer wrote
Quote:
Originally Posted by 6A3sUMMER View Post
Be sure that the output tube screen is wired to the screen tap, and that the output tube plate is wired to the plate tap, otherwise you will get low gain, and low power.
The A9 has normally a screen tap at about 50%, so the voltage shut be about 325v.

If the wirering is correct, check the output transformers according to what ygg-it wrote in his post #28.
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Old 6th February 2018, 01:58 PM   #335
easygoing is offline easygoing
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Thanks for reply, had more time today, let it warm up properly.
Secondary resistance seems good 0.9R on 8R and 0.6R on 4R. On primary i have - red to blue 99R , blue to brown 114R , red to brown 212R.
with 100Vac @ 600hz across primary i get 3.41v on 4R and 4.52V on 8R outputs. which seems about right.
Measurement and connections on EL34 - 1+8 25.2v 2x1k and 220uF all to ground, 2+7 6.2Vac , 3 325v brown wire , 4 330v 100R to blue wire, 5 32mV 330k to ground and coupling cap, 6 not used.
Tried swapping red and brown wires , made no difference.
Output still the same as it was, ie 6v at cippling on 8R.
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Old 6th February 2018, 04:58 PM   #336
Alllensoncanon is offline Alllensoncanon
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Boyuu EL34 A9 Tube Amp
The El34 cathode (pin 8) is at 25V. The current over the 500 ohm cathode resistor looks to be =25/500R or 50mA. This match the EL34 "Triode" mode curve and not the expected UL mode. Your output power also seems to match that of a SE EL34 in triode mode. I wonder if your EL34 tube screen is some how blown open? Have you try switch tubes (ie. between left and right channels)?
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Old 7th February 2018, 01:00 PM   #337
easygoing is offline easygoing
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Yes i did that , Also I've tried the KT77's i got to use once its all ok. Still the same.
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Old 7th February 2018, 01:52 PM   #338
easygoing is offline easygoing
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I am wondering if boyuu have fitted EL84 trandformers in error , how would i test for this?
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Old 7th February 2018, 07:11 PM   #339
Alllensoncanon is offline Alllensoncanon
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Boyuu EL34 A9 Tube Amp
Quote:
Originally Posted by easygoing View Post
I am wondering if boyuu have fitted EL84 trandformers in error , how would i test for this?
Put a scope probe on both pin 4 (screen) and pin 3 (Anode) and compare the AC signals. What percents of pin 4 signal is compared to pin 3? Are the the signal in phase? EL84 transformer are higher impedance. Your measurements (plate and speaker) indicates around 4K. Not untypical for EL34 use.

Last edited by Alllensoncanon; 7th February 2018 at 07:14 PM.
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Old 8th February 2018, 02:36 PM   #340
audiowize is online now audiowize  United States
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Quote:
Originally Posted by easygoing View Post
Some what puzzled .Ive just built one of these , it works - but with only a little power, 2 - 3 watts.
That may be "working condition" for this amp. I see a manufacturer's rating of 12W at 1% THD. This amp is ultra-linear with no feedback. These are mutually exclusive conditions, you are being lied to! I would wonder if you could get more than 1W at 1% THD. This is very standard practice for Chinese amps; no surprises here.



Quote:
Originally Posted by easygoing View Post
The signal on the 6n9pj is a clean sine wave,even when the output is virtually a square wave. ( i use 600 - 800 hz for testing.)
The driver stage has a ton of headroom.

Quote:
Originally Posted by easygoing View Post
At the 39r resistor 342v , B1 - 330v , EL34 pin3 324v , pin4 329v , pin1/8 23v. right channel is similar.
Those are dead nuts on in terms of what the schematic shows, other than slightly high screen voltage.

Quote:
Originally Posted by easygoing View Post
that puts 6vdc across the output transformer , compared to the measurements on page 32 that one has 10v . As the dc is a little higher i would have expected arround 11v.
What's important is the DC current across the cathode resistor, which you have measured to be correct.

Quote:
Originally Posted by easygoing View Post
Does any one have any ideas whats wrong here? Also, Was expecting the transformers to get a bit warm , but they dont.
The power transformer looks to be adequately sized for what it's doing. It should warm up after a few hours of use. As you've said before, you have about 10V across the OPTs at about 45mA, which is less than 1W of power dissipated. They aren't going to get warm.

You bought a $200 tube amp from China, YGWYPF. Try triode strapping it.
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