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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
worrioe33
1 posts
 
Hi Ttbolad
I am in process of buying Cayin A70T fix bias version.
I have few questions to your friend who fix your amp.
1) Crossover distortion elimination over biasing, how to do?
2) How to modify it to Class A1?
Thank you.
Looking forward to hear from you.
pryso
1,927 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? ;-)
helomech
1,670 posts
 
Similar to Line Magnetic, there are plenty of gray market Cayin amps out there. Maybe your example is one of them?
trelja
3,223 posts
 
@helomech, "Similar to Line Magnetic, there are plenty of gray market Cayin amps out there. Maybe your example is one of them?"

Honestly, that's a flat out poor (but effective, so it seems) excuse some Chinese manufacturers and their North American representative who don't accept customer service and care as Job One use when facing an actual issue a customer encountered.

Did the factory build the amp or did they not?  If the faceplate says Cayin, Line Magnetic, or John's Lovely Tube Amp, do they build product to different standards with differing levels of appreciation, care, and support depending on where it's headed?  In other words, do they appreciate the business of some customers more than others?

As someone who previously imported the major competing line to Cayin a decade ago, I knew how much I paid for product.  I also knew how much the "grey market" sellers in China did, as I also had those price lists.  Considering the total cost of getting a component over here, a customer would save precious little, if any by taking that route.  By the way, how exactly did they obtain a 120V or whatever other version in their 240V market from the manufacturer, and for what purpose?  As for me, I couldn't care less.  You bought something with our name on it, you're now in the family, and come under my care.  You have a problem?  I am truly sorry to hear that, and now my responsibility is to help you.

As I mentioned Steve Leung being the importer of Cayin products in the North American market, and my holding him in very high esteem.  Since then, like me, Steve has also given up the Cayin line he once handled.  It's fairly easy to read my prior posts here to understand how both Cayin and the line I represented have fallen down in this market since we walked away
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