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)

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.
Ha, sounds very possible to me.. However although they do get a hand slap and a bit of a required explanation to the reasoning as you have stated above, Whats wrong with the sound?

If it performs and sounds as it should than fact is its still a good sounding amp right? Beyond this only other suggestion as it seems your buddy has his hand on the pulse of the circuit design, and knows a thing or two why not just put in a few bucks and develop the first version with dual Rectifiers as he says will be better anyway and eliminate the diodes under the tube already in there?

Just curious because you bought it used so not sure on the pricing, but if its a good basic design take a few steps to improve it I say. I have done this with some chinese designed pieces myself and with a few updates and direct drop ins of higher grade parts they will compete with the best.

I have no experience with Cayin or with your amp just mentioning my thoughts on it.

Good Luck
Good suggestion and already done. The Cayin seems to have excellent output transformers so it is worth fixing up. My amp was apparently mis-manufactured. We saw 5%-10% harmonic crossover distortion at 10W output. The biasing was so crazy we never did figure out if it was supposed to be biased AB1 for 55W of power, or if it was supposed to be Class A1 at 30W. My manual says Class A2 at 30W, but it can't be A2 because according to two engineers I talked to, is is not an A2 circuit.

So my friend biased my amp to be Class A1 (where it seems to deliver 25W) and changed to cathode biasing. He also, just for fun, changed the switch that allows triode or ultralinear mode to toggle between pentode and ultralinear. In its original bias configuration triode mode sounded quite poor. If pentode sounds better than ultralinear (I don't know at this point because my friend is tweaking the amp and I am traveling so that I haven't heard it) then I will leave it like that. Otherwise he will set the pentode back to triode mode.

He also used the rectifier tube in conjunction with two of the diodes to create a kind of hybrid bridge rectifier. It has some of the advantages of tube rectification and actually utilized the rectifier tube.

His report. The amp sound extremely good now. I am looking forward to hearing it when I get back.
If your theories are correct the Cayin is being misrepresented. On the other hand, if you like the sound what's the problem, I have a Cayin phono stage, good for the money. My Art Audio amp has SS rectification and I love the sound. Don't necessarily agree that tube rectification is inherently better than SS rectification. Ultimately, sonics are much more important than design.
If what you say is true about the bogus tube rectification, then it's extremely disturbing.

It's becoming difficult to trust anyone, or any company these days.
I have two cayin products and five VAS products. The same distributor handles both in the USA.

I have the cayin 70 and 100 units. Never any problem.

Please contact the distributor,

There service and support of me has been excellent.
How did you measure the 5-10% crossover distortion? I have never heard of Class A1 and A2 either. Is this something your creative friends invented?

Since you got it 2nd hand, is it possible that the previous owner changed the rectification?
I am the 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.
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.

-- Al
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.
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.
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......
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./
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.

Which goes to my point about who can be trusted.

It's a sad commentary about values in our present society.
I am not asking you to trust the findings of me and captkhz. Check the thread on PinoyDVD for what we hashed out so that it doesn't have to be dealt with again in this thread. We have no agenda other than to alert other to this stuff to that they are not mightly disappointed as I was.

VAS did not do this. They did not design a circuit that has a rectifier tube that does nothing but glow in the dark. Cayin built this circuit.

In the PinoyDVD thread there are pictures from several amps (mine not included) which show the same diode setup as is in my amp. If you don't think I am being forthright here, and you think that the pictures are bogus, try to find a Cayin A70T and have a look for yourself.

I guess my agenda is to try to put a halt to what looks like flim-flammery. And as I stated in the PinoyDVD thread, if someone can come up with a reasonable explanation for the presence and utility of the 5AR4 in this amp, I am more than willing to accept the explanation, apologize for wasting people's time and bandwidth and quietly disappear.

But the guy that sold me the amp was all jazzed about the $125 Mullard 5AR4 that he had used with this amp. As far as I can tell, he wasted good money thinking that the Mullard had some sonic effect. Many argue about the effect of rectifier tubes on the sound of amps, but the argument becomes ridiculous when the tube does nothing but glow. If Cayin is producing products that are disingenuous it only makes audiophiles look more suspect in terms of the objectivity of their hobby than ever.
No agenda on my part, had not heard of Cayin prior to looking at this repair. The truth will out. If you own one of these a simple test. Pull the 5AR4 and see if the amp plays, simple as that. I hope this unit is the only one that was wired this way. Better to know what is going on. Please post results of the test!
But the guy that sold me the amp was all jazzed about the $125 Mullard 5AR4 that he had used with this amp. As far as I can tell, he wasted good money thinking that the Mullard had some sonic effect.

That was my second thought after reading the initial post.
I think what sets Cayin apart here is that they pushed the limits too far and got caught. It is ok to sell stuff that doesn't do anything good or anything at all for that matter, but you've got to leave a possibility of reasonable doubt.
I agree with Jeff. As a tech. , my reasonable boubt left me when it was discovered that the factory power transformer, without a secondary HF center tap, could only be set up with a bridge rectifer,meaning no 5AR4 can be used legit. and no choke can be used legit. and the voltages are only right with a 4 diode bridge set up....Oh, and also the 700 UF filter WILL work with a bridge, not the 5AR4 full wave circuit (not reliably, anyway). Also the Pwer transformer has the yellow 5 v fil. winding for a 5AR$/5U4. Strange! Bridge type power trannies never have a bridge sec. and a 5v tap both. This power trans is indeed set up right for what they did....a bridge and a fake 5AR4 use. I cannot come to any other conclusion....I tried......
"As far as I can tell he wasted good money thinking that the Mullard had some sonic effect."

I disagree regarding whether it was a 'waste of money', at least from his point of view. Perception, often with a bit of gulibility, is the only justification extant for the purchase of many things, especially assessories, but often in the basic components as well. What is the difference between a lot of product puffery, bought hook line and sinker, and a little more graphic deception. Just a different form of Bling perhaps.

Sorry, I couldn't resist.................:-)
Yeah, and maybe you can complete your system using this unidirectional ethernet cable.
Newbee, I hear you, but, we run the risk of becoming too benign and cynical. If we, as Americans' allow ourselves to go along with the B.S., we collectively get what we deserve.
You bought an amp that was manufactured in china(cap omitted intentionally), and you're surprised that the rectifier tube is bogus/counterfeit/fake? Counterfeiting ia their national vocation/pastime/hobby!!
I just took a look at the Cayin website. The A70t in the photo has nine tubes but the description only lists 4 KT-88, 2 12AX7, and 2 12AU7. I don't see any claims made for tube rectification or any rectifier tubes specified in the owners manual. So, it does appear to be a styling gimmick only. Lots of products are sold on appearance versus performance. For the record I have not heard or seen a Cayin product. The appearance may be misleading but the published info on the website is factual.

Who told you it was tube rectified and where did they get their info? Maybe it was just assumption on someones part?

I don't see anything to get excited about. A safer bet would be VAC, Cary, Manley, Quicksilver, etc. You gets what you pay for IMHO.
Without having a stock Cayin in front of me, I have no way of verifying the assertions in this thread. And, while I'd surprised (and disappointed, yet again), after dealing with the Chinese for the past couple of years, I have to say that I'd really not be all that surprised.

As has been pointed out, the Chinese transformers are of pretty high quality. Combine that with using good (or very good) coupling capacitors, and the implementation of classic tube amplifier designs from the 1920s - 1960s resulted in producing good sounding tube amplifiers. They also are capable of putting out some nice chassiswork, provided, in my opinion, it does not cross the line of being cloying.

North American and European customers have bought these products over the past decade because they've previously been inherently cheaper than domestically made products given that level of sound quality. Now that the cost advantage has all but evaporated, and the level of support and customer care (and, actually giving a flying flip) from the Chinese companies themselves has thus far been to put it kindly, indifferent, my opinion is that you are going to see a whole lot less of their stuff imported into our markets as time goes by. In fact, it's already become reality.

I don't want to say too much beyond that now. Suffice it to say that there were reasons for me feeling compelled to give up the line I represented.
I've been doing a bit of research into the marketing of this Cayin model. While I have yet to see any marketing that states tube rectification specifically, some spec. sheets at dealers specifically mention the 5A5R as a tube rectifier. Also, the Cayin AT-88T does not have this extra tube, SS rectification. Seems like at least some deception involved if this is indeed not tube rectified.

Having said that, my Cayin Phono-One is superbly assembled, very disappointed in parts quality, stock capacitors are a joke and stock resistors are garden variety. Replacing the stock caps with V-Caps and Jantzen Z Superior caps has resulted in a way better preamp. I will eventually swap out stock resistors as well.

If this theory of Cayin deception is true I will be very disappointed in Cayin, Steve at VAS seems like a good guy.
could not agree more with Rodman99999
I think I can speak for him when he brought the amp. to me for study, repair, he thought it was tube rectified because of the nice gz-37 tube there on the amp! I agree, Quicksilver a good option and even Dyna as well
Trelja, they don't provide good service and support, so instead by purchasing through you then, that will ensure the good quality of service and support, right?
Toufu, Steve Leung handles Cayin here in North America. The Chinese push off service and support to the people who respect them in their respective markets. From my dealing with him, I know, like, and respect Steve.
Kind of like seeing a car with a big blower sticking out the hood, only to find out it's only attached to the hood not the engine.
LOL re 'blowers', but what I really love is air foils on production cars! Talk about the need for a baby's jock strap! But, given time, we'll probably see that too. :-)
bottom line to all this good technical input is how does the amp SOUND?
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.
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? ;-)