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
I've been doing a bit of research into the marketing of this Cayin model. While I have yet to see any marketing that states tube rectification specifically, some spec. sheets at dealers specifically mention the 5A5R as a tube rectifier. Also, the Cayin AT-88T does not have this extra tube, SS rectification. Seems like at least some deception involved if this is indeed not tube rectified.

Having said that, my Cayin Phono-One is superbly assembled, very disappointed in parts quality, stock capacitors are a joke and stock resistors are garden variety. Replacing the stock caps with V-Caps and Jantzen Z Superior caps has resulted in a way better preamp. I will eventually swap out stock resistors as well.

If this theory of Cayin deception is true I will be very disappointed in Cayin, Steve at VAS seems like a good guy.
I think I can speak for him when he brought the amp. to me for study, repair, he thought it was tube rectified because of the nice gz-37 tube there on the amp! I agree, Quicksilver a good option and even Dyna as well
Trelja, they don't provide good service and support, so instead by purchasing through you then, that will ensure the good quality of service and support, right?
Toufu, Steve Leung handles Cayin here in North America. The Chinese push off service and support to the people who respect them in their respective markets. From my dealing with him, I know, like, and respect Steve.