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Old 4th February 2007, 11:35 AM   #1
Cobra2 is offline Cobra2  Norway
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Default Chinese EL34 amp to modify

I have one of the cheap china-amplifiers, a copy of this:
http://www.sophiaelectric.com/pages/amp/ampel34.htm

with 2 x 6N1(P) / 4 x EL-34

I have tried to make a schematic of it, and the input-section appears to be like this: (sorry, no expert in making schematics...)

Any suggestions to what can be eaily corrected/modified? or maybe revired for another input-tube?
Any big drawing-errors?
http://www.tubes.ru/techinfo/HiFiAudio/6n1p.html


Arne K
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Old 4th February 2007, 01:36 PM   #2
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Hi,

I think you might have made an error in your schematic. Its a push pull amp right? The 6n1p has to be connected to two el34s maybe at pins 6 & 8? (I guess its one 6N1P per channel?)

Regarding tube substitution; the Russian types are supposed to be rather good to start with, so the benefits might be a bit slight, BUT

If you havent already, have a look at the Duncan amps datasheet locator,
http://tdsl.duncanamps.com/tubesearch.php
Although substitutons seem to be a bit thin the pinout is quite similar to the ECC8* types and they might provide an alternative there, depends what you want to achieve.

Good luck!

Andy
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Old 4th February 2007, 03:06 PM   #3
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Default Re: Chinese EL34 amp to modify

Quote:
Originally posted by Cobra2
I have one of the cheap china-amplifiers, a copy of this:
http://www.sophiaelectric.com/pages/amp/ampel34.htm
Is it me, or is this all weird. Sophia Electric owns a trademark in the phrase "S.E.T." (the serial number is 78163221 which can be looked up at the USPTO, or just search for Sophia Electric) for use with audio equipment which means a) only they can sell audio equipment labeled "S.E.T." and b) it does not mean Single Ended Triode. Instead, it means nothing. So, this amp, called "S.E.T. Music EL-34" is actually a push pull amp and is so called because "S.E.T." does not mean anything in this context.

Sorry to thread jack. This just struck me as odd.
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Old 5th February 2007, 05:01 AM   #4
Svein_B is offline Svein_B  Norway
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Yes the S.E.T. trademark is a bit weird.
In any case I do not really think the cheap China amp is a copy of the Sophia Electric, it is probably rather the Sophia which is a re-branding/OEM of a Chinese amp.

The circuit drawn is obviously not yet correct, but tracing schematics from the live amp is a good exercise.
The 100nf cap is probably connected to the other end of the 300K resistor.
The phase splitter might be a basic paraphase, with the grid of the second triode half being driven by the 330K/20K divider.
The anode of the second triode half (pin 6) will probably go to the other EL34 grid via a 100nF cap.
And I guess there will be some resistor from pin 8 to gnd.

SveinB
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Old 5th February 2007, 07:35 AM   #5
Cobra2 is offline Cobra2  Norway
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correction
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Old 5th February 2007, 07:38 AM   #6
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output

the resistance reading for the trafo seems odd?

Arne K
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Old 5th February 2007, 08:24 AM   #7
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Old 5th February 2007, 08:25 AM   #8
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Arne K
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Old 5th February 2007, 08:44 AM   #9
Svein_B is offline Svein_B  Norway
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Quote:
Originally posted by Cobra2
output

the resistance reading for the trafo seems odd?

Arne K
Well, maybe not so bad. These are DC resistance readings.
I would expect the secondary to be a little higher, probably more like .5 ohm. Since you have it apart, you should measure also the AC ratio, connect an AC signal to one side and measure on the other. The square of this will be the impedance ratio.


Your diagram is looking better now. The 100nf still need to be connected to the other end of the bottom 330K. Please also check the feedback connection, it is a chance it shouild be connected to the 900/90 ohm cathode resistors.


SveinB
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Old 5th February 2007, 08:52 AM   #10
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Question from newbee:
B+ to g2/pin4? Is that correct. Pin 3 is the anode, and in triode mode g2 would be tied to anode. In pentode mode g2 is connected to OTP center/B+ via a resistor.
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