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

Showing 1 response by pryso

pryso
1,903 posts
 
I own a Cayin A-300P. This is a stereo 300B push pull amp (20 wpc) with input selector and volume control that functions as an integrated amp. The chassis has a tube location marked 5AR4 but it is covered by a metal cap, yet the Operating Manual claims a "SOVTEK 5AR4 is used for rectification". Obviously production was changed without making the appropriate correction to the manual.

I must say I don't see a significant problem with this. I find the sonic performance to be very good, with or in spite of the SS rectification. The overall fit and finish is also very good. Regarding parts quality, I bought this from a friend who has a strong background in electronics and he had upgraded a couple of caps and diodes. I didn't hear it prior to the upgrades but he assured me the basic design and parts selection made it an attractive choice at its price point. It is quiet, smooth, detailed, dynamic, spacious, and has more apparent power than its rating.

So my only question, should I attempt to remove the 5AR4 label from the chassis? ;-)
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