Last week I took delivery of one of the chinese "Music Angel" EL34 Push Pull Amps.
I fired it up on the weekend and initial impressions were that it was Ok without being anything special. Stereo imaging was'nt particularly good and it was a bit "covered" indicating too much 2nd harmonic distortion.
So dig out the schematic to look at modification possibilities (see attached):
Hang on - IIRC the larger anode resistor for a cathode coupled phase splitter should be on the cathode coupled side, NOT the "straight through" side as shown in the schematic.
Off with its lid too have a look:
The 83K are in fact 4 x 332K in parallel, the 73K6 is in fact 75K consisting of 4 x 301K in parallel and the 55K "Tail" resistor is 75K (4 x 301K) as well.
Checking the first channel I find that the 83K is in fact on the cathode coupled side, that is the amp is correct and the schematic is wrong.
Checking the other channel and HEY the 83K and 75K are opposite to the first channel. A manufacturing fault.
I corrected this problem and Wow! a totally different amp. The stereo image is now quite acceptable and the "covered" sound has cleaned up significantly.
So last thing, a bit of output tube rolling
- tried Winged C Svetlana EL34s, Much better
- on a whim, I tried some "Fat Bottle" chinese EL34s I bought 5 years ago and had left sitting on the shelf as I had decided they were probably not muich good - WRONG these tubes were simply stunning in this amp, significantly better than the Winged C Svets.
So with 4 or 5 hours effort I have a lovely, cheap EL34 amp.
A few other comments:
- I run it in triode mode all the time
- The input tube is not a 12AU7 but a 6N1 instead. The extra gain affored by the 6N1 is compensated for by simply leaving out the 220uF/6V3 cathode bypass cap.
- full power output is 20W per channel
- frequency response at 10W out is 7Hz to 60kHz
- Absolutely no indication of any transformer saturation at 10Hz 10Watts out telling me that the output transformer is better than average quality
- Zout is 2.3 Ohms
- Gain (to 4Ohm output) as shown is 33.54
- Gain (to 4 Ohm output) with feedback disconnected is 42.85
That means there is ONLY about 2dB of global feedback
Cheers,
Ian
I fired it up on the weekend and initial impressions were that it was Ok without being anything special. Stereo imaging was'nt particularly good and it was a bit "covered" indicating too much 2nd harmonic distortion.
So dig out the schematic to look at modification possibilities (see attached):
Hang on - IIRC the larger anode resistor for a cathode coupled phase splitter should be on the cathode coupled side, NOT the "straight through" side as shown in the schematic.
Off with its lid too have a look:
The 83K are in fact 4 x 332K in parallel, the 73K6 is in fact 75K consisting of 4 x 301K in parallel and the 55K "Tail" resistor is 75K (4 x 301K) as well.
Checking the first channel I find that the 83K is in fact on the cathode coupled side, that is the amp is correct and the schematic is wrong.
Checking the other channel and HEY the 83K and 75K are opposite to the first channel. A manufacturing fault.
I corrected this problem and Wow! a totally different amp. The stereo image is now quite acceptable and the "covered" sound has cleaned up significantly.
So last thing, a bit of output tube rolling
- tried Winged C Svetlana EL34s, Much better
- on a whim, I tried some "Fat Bottle" chinese EL34s I bought 5 years ago and had left sitting on the shelf as I had decided they were probably not muich good - WRONG these tubes were simply stunning in this amp, significantly better than the Winged C Svets.
So with 4 or 5 hours effort I have a lovely, cheap EL34 amp.
A few other comments:
- I run it in triode mode all the time
- The input tube is not a 12AU7 but a 6N1 instead. The extra gain affored by the 6N1 is compensated for by simply leaving out the 220uF/6V3 cathode bypass cap.
- full power output is 20W per channel
- frequency response at 10W out is 7Hz to 60kHz
- Absolutely no indication of any transformer saturation at 10Hz 10Watts out telling me that the output transformer is better than average quality
- Zout is 2.3 Ohms
- Gain (to 4Ohm output) as shown is 33.54
- Gain (to 4 Ohm output) with feedback disconnected is 42.85
That means there is ONLY about 2dB of global feedback
Cheers,
Ian