2. amps-preamps
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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
captkhz
7 posts
 
I am the friend...tech. who serviced this amplifier, need to enter this discourse. I am creative, however, not creative enough to invent "class A1"! Anyway, I repaired the xover distortion problem by change in bias current. I would like to install 2 5AR4s, not so fast....not so easy! No centertap on the power transformer for normal full wave rectifier application. I could set up as a bridge if we had a second 5 VAC winding for the second 5AR4 fil. Needs separate 5 v. taps. Another problem, one OR two 5Ar4s likely fail after the first "hot switching" event. Hot switching is when power lapse for few seconds, power returns, 5AR4 cathode is still hot....current surge...charging the large 700 UF filter caps in the Cayin.
SS diode power supply can sound good..true....the deception in the marketing of this product is what is disturbing. Not sure we want to change the power transformer...... Filter values...can fast become complete power supply re-design.
almarg's avatar
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almarg
9,670 posts
 
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
ttbolad OP
42 posts
 
Al, you are thinking the same thoughts we had. We had been giving them the benefit of the doubt as to having made a design mistake and then tried to cover it up. But you would have to be pretty stupid to inadequately design the rectification circuit. And that flies in the face of an otherwise well built product.

As for the idea of a shared chassis that they are trying to reuse, I haven't seen another model like this. And, really, how hard would it have been to plug the hole with some fancy machined aluminum with a Cayin logo.
ttbolad OP
42 posts
 
No, Cayin is mis-representing their products if (and at this point I don't think there is much doubt) our findings are correct. How cynical do you have to be to "fake" a feature? I don't care if the amp sounds like the highest seraphim cherubim, this kind of thing is still going to piss me off.
captkhz
7 posts
 
I agree. for instance, 5AR4 failures as mentioned in my last post may have prompted addn of diodes. The bridge configuration is getting far removed from the orig. topology, however, and gives me pause. I am willing to consider these reasons. Many times "design by inventory" takes over 5Ar4 problems and we have all these chassis' in the warehouse!? A for instance......
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