2. amps-preamps
  3. 1233757559

Cayin A70T has bogus tube rectification?

I hate starting this thread. I am also a little hesitant because

a.) I bought the amp and hate the thought of its resale value after I post this

b.) I am not an electronics expert, so I have to rely on other experts for the facts.

Here is the basic story. I bought the amp from an audiogon member about a month ago. A friend of mine bench tested it and found a very large amount of crossover distortion on the scope. We unbuttoned the amp to look at the circuit and try and figure out what was wrong - but that is a side issue. The real issue is that there were four diodes strapped to the base of the 5AR4 which formed a solid state bridge rectifier. The bridge appears to completely bypasses the tube. The tube seems to just stands in the socket doing nothing but glowing. My friend had questioned this before I bought the amp because he says that a single 5AR4 is marginal for four KT88s. Usually you will see two.

I found a site (which I did not realize was in the Phillipines) where I blogged about this. There was some very good discussion there between very knowledgeable electronics guys and they came to a seemingly inescapable conclusion that the tube was there for show - as a marketing add-on to capitalize on the idea that tube rectification is better than solid state.

Here is the link to the thread on that forum (go to page 18)
http://pinoydvd.com/board/index.php?topic=29032.540

We are waiting for Cayin to respond to this issue, but they are in the middle of Chinese New Year (which last 15 days) and are not available for comment. If the evidence weren't so compelling, I would wait for their response before posting this.

There is a slight chance that we missed something in our analysis - but the fact that one of the contributors to the discussion actually pulled the rectifier tube with the amp turned on, and it kept running (now that I think about it, he didn't say how long he left it on, so he may have been running on capacitance) and the fact that a pretty critical analysis of the circuit does not reveal any function for the tube, I decided to post this on Audiogon in order to bring this issue to light in the USA and Europe.
ttbolad
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ttbolad OP
42 posts
 
I am not asking you to trust the findings of me and captkhz. Check the thread on PinoyDVD for what we hashed out so that it doesn't have to be dealt with again in this thread. We have no agenda other than to alert other to this stuff to that they are not mightly disappointed as I was.

VAS did not do this. They did not design a circuit that has a rectifier tube that does nothing but glow in the dark. Cayin built this circuit.

In the PinoyDVD thread there are pictures from several amps (mine not included) which show the same diode setup as is in my amp. If you don't think I am being forthright here, and you think that the pictures are bogus, try to find a Cayin A70T and have a look for yourself.

I guess my agenda is to try to put a halt to what looks like flim-flammery. And as I stated in the PinoyDVD thread, if someone can come up with a reasonable explanation for the presence and utility of the 5AR4 in this amp, I am more than willing to accept the explanation, apologize for wasting people's time and bandwidth and quietly disappear.

But the guy that sold me the amp was all jazzed about the $125 Mullard 5AR4 that he had used with this amp. As far as I can tell, he wasted good money thinking that the Mullard had some sonic effect. Many argue about the effect of rectifier tubes on the sound of amps, but the argument becomes ridiculous when the tube does nothing but glow. If Cayin is producing products that are disingenuous it only makes audiophiles look more suspect in terms of the objectivity of their hobby than ever.
captkhz
7 posts
 
No agenda on my part, had not heard of Cayin prior to looking at this repair. The truth will out. If you own one of these amps...do a simple test. Pull the 5AR4 and see if the amp plays, simple as that. I hope this unit is the only one that was wired this way. Better to know what is going on. Please post results of the test!
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jeff_jones
418 posts
 
I think what sets Cayin apart here is that they pushed the limits too far and got caught. It is ok to sell stuff that doesn't do anything good or anything at all for that matter, but you've got to leave a possibility of reasonable doubt.
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