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
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......
captkhz
7 posts
 
I think they (designers) ran into problems...took the easy way out...5ar4 failures, we still want large cap. filter values...what to do...throw in the diodes...what about the tube people want...5AR4...keep it on there! Still does not explain the power transformer with a HV secondary without the center tap for 5AR4/5U4 type full wave circuit. THIS TRANSFORMER MUST USE A BRIDGE RECT!
Anyway, this amp. in question does sound good after fixing the bias and trying the "pentode mode" (G2 tied to B+). driving K-horns. The pentode mode is a mod./
newbee
4,767 posts
 
With all due respect to Ttbolad and Captkhz, before we all jump on this band wagon, consider that this consumer alert came to A-Gon via the first post of an unknown person. We've made a lot of charitible assumtions about new posters in the past that proved to actually be from folks who had a hidden agenda

I say that knowing first hand that VAS and at least one of his vendors have no problem stretching the truth about the amplifier output of at least one of thier amps and maintaining the lie in their ad's after they were notified (by me, in writing). As far as I know this was not the doing of Cayin. Some dealers got it right in Europe and a least one in NA.

FWIW.
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