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

Showing 1 response by almarg

I'm an antique radio collector, and I can tell you that during the golden age of radio, in the 1930's, far more than a few companies put extra tubes in their designs that did absolutely nothing (other than to allow the marketing people to claim a higher tube count than the competition). :)

On the other hand, giving them the benefit of the doubt, there are other possible explanations. Perhaps they wanted to use a solid state rectifier circuit, but wanted to utilize a common chassis with some other model, perhaps a predecessor model, which had tube rectification. And they didn't think it would look right to have either a hole or an empty tube socket.

Or perhaps they just made a dumb design mistake, and thought a single rectifier tube would be adequate, and didn't realize the mistake until a large quantity of chassis had been fabricated. Or they wanted to keep its configuration consistent with what was described in manuals that had already been printed.

There are undoubtedly other possible explanations as well, that would be less egregious than intentional deception. We can't really say at this point.

Regards,
-- Al