On another related Cayin subject, I recently bought a used Cayin A70T(it hasnt arrived yet), but have found that there is more than one version of this intergrated. So, can I tell more from the serial number, or other markings as to whether it is 30watt class 'A' output, or the newer 50watt 'PP' output. Even the manuals online don't seeem to agree. And nobody ever mentions the biasing process needed for different output tubes. I'll know more when the amp arrives. thx in the meantime for your input....greg 
First of all, they are both PP. One is class A the other is class AB1 (the more powerful one). I recently got a used one and it is a bogus mess as far as I can tell. I found that out after I took it to a friend who has 35 years experience building everything from SET amps to AM radio transmitters (tube) and the corresponding power supplies. He wanted to see how it bench tested, even before we listened to it.
Here is what we found. The frequency response is excellent 10Hz to 60kHz, and the build quality is very good. But that is the end of it. My manual says that mine is Class A2 at 30W. But there was no grid current source, so it could not have operated in class A2. More likely the 30W version operates in Class A1. The bias circuit was so bad that there was massive (5% - 10%) crossover distortion at 10W output. The values of the "auto bias" circuit resistors were way off. Looks like someone grabbed the wrong resistors out of the parts bin.
The power supply was incredibly strange. It looks like the rectifier tube was just standing in the socket for show. At the base of the socket were 4 solid state diodes that function as a bridge rectifier. These were doing the rectification. The choke input filter was completely bypassed. That is, the power supply only looks like it is tube rectified. The tube seems to be doing nothing.
So we were speculating that the 30W version was originally class A and probably tube rectified. But the 5AR4 is marginal in this situation, according to my friend. Most kt88 amps uses two of these tubes for the rectification. So they may have had problems with the power supply and hence went to SS rectification. But, since the amps were in production, they couldn't very well remove the tube socket and close the hole. So they just left the tube in the amp, but went to SS rectification. Or more likely, when they decided to make the 55W version, the decided to go to SS rectification. So it appears that either the power supply was originally tube rectified and that either reliability or the evolution to the AB1 version necessitated going to SS because the single 5AR4 couldn't handle it. In either case, the situation sucks.
I have no explanation for the bad biasing circuit in my sample.
So here is what we are thinking about doing...
Bias the amp Class A using a cathode bias as opposed to the so-called auto-bias. Revert the power supply to tube rectification using either a GZ37 or a 5V3 which can provide 350 mA into a choke, as opposed to the 250mA that the single 5AR4 is capable of (provided that the power supply trannies have a suitable tap).
If you don't believe this situation, you can find a photo of the circuit somewhere on the web (don't have the link at the moment) which shows the diodes strapped across the base of the rectifier tube. The photo is pretty high res so you can zoom in and see what is going on.
All of that being said, I think that it probably sounds great in either version assuming that there is no mistake in the bias circuit (which I must assume is what happened to mine). I would be interested to know if the 30W versions actually used the rectifier tube. If so, I can sort of forgive the presence of the tube in the 55W version since there was a pragmatic evolution. If not, then shame on the company for the deception.
Bear in mind that I am no electronics expert; so take these comment with a grain of salt. However my friend who is highly skilled, was very puzzled with these finding, to say the least. In any case, he is going to straighten out the amp. Given that the output trannies are very good, and the circuit is basically simple, it has the potential to sound very good. Hence the effort to get it right. My amp is a little over two years old. Maybe it was built during the transition to the 55W version and mine got a little screwed up - neither fish nor fowl so to speak.
If any one else has an explanation for the 4 diode rectifiers strapped at the base of the rectifier tube to function as a bridge rectifier, I would sure be interested in hearing it. And if I have mis-stated anything technically, please correct me. I do not want spread bad information, or to induce audiophile angst in anyone, because that is rampant enough. But, this is quite a mystery to us. And it all started with noticing the huge crossover distortion.
Interestingly enough, the guy who sold it to me had it for two years and enjoyed it. And I though it sounded pretty good as well - albeit kind of hard a moderate volumes. He also swore to me that a Mullard 5AR4 was the only way to go. I even though I heard diference in the rectifier tubes that I tried. We audiophiles are sure capable of fair amount of self-delusion.
If anyone can shed some light on this, it would be interesting. And I don't mind being told that me and my friend are full of s***, provided there is some kind of accompanying explanation. You dealers - got anything to say about that power supply issue? And why is it not possible to get Cayin circuit diagrams? I also have a Yaqin MC100B, and the diagrams are readily available and ACCURATE. (The Yaqin is dirt cheap by comparison - output trannies don't seem to be as good though). Why won't Cayin release those diagrams. There is NOTHING proprietary going on and it would be a great help in trying to decide what to do with the amp.
