Chinese amp: 2A3 6SN7 6SJ7 5Z5P push pull

[Alexontherocks]

HI everyone.. I just wanted to submit for your evaluation an amplifier I found on the internet.

It is manufactured in China, soldered board with Shuguang valves.

The circuit is the follwing:



and finished product:



After extensive research it would seem that the chinese tubes have improved to acceptable levels and can sometimes surpass other manufacturers in terms of realiability and average quality.

I wanted to have an opinion on the circuit and if there is any way to improve output and quality in the signal path.

Thank you!
Alex

[hidnplayr]

I Dont like the phase splitter.
I would make R3 and R7 have the same value, and replace R6 by a CCS

[Alexontherocks]

Interesting point... I have some supertex DN2540 regulators I can use in place of R6A and R6B or IXYS if I can find them..

[andyjevans]

Is this a 5Z3 rectifier? Can't find 5Z5.

What I'd do with this is replace R6 with a CCS as said and eliminate the first stage with its feedback loop. Take the input direct to one side of the 6SN7 with something like 270K from grid to ground, and ground the second grid. The first stage and the feedback can only make the sound worse. If you need more gain, do it in the preamp, otherwise if you have enough gain you could replace the 270k with a 100K volume pot.

Andy

[hidnplayr]

Also, notice that there are no grid-stoppers.
So check for grid-current and/or oscillations.

[Alexontherocks]

Yep sorry for that... 5Z3P

Checking the costs...by eliminating the first stage it maybe actually cheaper to redesign it on a PCB board.

Are there proven applications for th 2A3 I can look at for reference?

By the way andyjevans, thankyou for your input I'll try to redesign it taking out the first stage.

[Alexontherocks]

hidnplayr... thanks for the observation.

[andyjevans]

What would be nice is to use plate chokes on the 6SN7. The Hammond 156C are excellent for the price, and even better when two are used in series. This would raise the quality and give a little more gain than resistor loads.

Also think of better coupling caps - the Russian teflon ones are really excellent. FT-3. Need some space in the chassis.

But seriously, since you have to put all this in a chassis anyway and get some transformers, you may be better off just building a 300b SET from scratch. I'm assuming that what you get for your money here is the PCB plus the tubes.

Andy

[Alexontherocks]

yes Andy....the project seems counterproductive since the main goal of somwhat saving on costs is frustrated by the necessary modifications...

I probably would end up spending the same amount of money I would spend bulding 2x Pete millet's push pull monoblocks with KT88 in ultralinear operation. The results would most certainly be better and the cost just about the same (considering what I have at home). The interesting thing about Millet's design is that I can actually use 2A3 (but with slight voltage modifications).

BTW the link to the chinese product is: http://stores.ebay.it/diyaudiokit/Po...=p4634.c0.m322

I just did not want to post it to avoid "marketing and spam issues"

Alex

[hidnplayr]

Originally Posted by andyjevans

What would be nice is to use plate chokes on the 6SN7. The Hammond 156C are excellent for the price, and even better when two are used in series. This would raise the quality and give a little more gain than resistor loads.

I dont see how that can work in a long-tailed pair configuration??