I.m assuming that switching the kt88's didn't fix the tble so we can rule out the tube.The bias adj pot is small and delicate and easily damaged. As to the hum,it usually indicates a bad ground connection.It could also be the cathode resistor going open when it warms up. My idea is to shut the amp off and when its cool 1hr or so ,to turn it on measuring your bias voltage. If it changes and eventually goes to zero,I think the cathode resistor is going open.Usually looking at the resistor you can tell if its been overheating. So either the bias pot or the cathode resistor is my bet but thats just a guess. 
Excuse my ignorance, I agree that the potential problem might be either the pot or the serial resistor, but which one is the serial resistor on the circuit diagram ( is it the R17 or R18 equivalent on the L channel? )and what value is that? Is it the 10 Ohm 3W version?? Is it possible that you depict that from the circuit diagram please? Cheers
YES the 10 ohm 3 watt that is connected to the kt88 that you get no readind on. THE bias is actually measuring the voltage drop across the 10 ohm resistor. If the resistor is open then no current flow through the resistor and no voltage drop(bias)across the resistor. Each kt88 has a 10 ohm resistor connected to the cathode. this is where the 4 test points connect.
Hi Guys ! Have built a similar amp to Yours . Would suggest You to raise the 6,3 V filamentvoltage to about 100 V for the tubes having high voltage on their cathodes ! Like the 6SN7 driver and
long tail pair splitter and perhaps also the upper 12AX7 triode (if possible). In a stereoamp the two uppertriodes might be in one 12AX7 . My suggestion is because there´s a limit for how much voltage a tube can stand between cathode and filament !
long tail pair splitter and perhaps also the upper 12AX7 triode (if possible). In a stereoamp the two uppertriodes might be in one 12AX7 . My suggestion is because there´s a limit for how much voltage a tube can stand between cathode and filament !
keithgreenhalgh said:YES the 10 ohm 3 watt that is connected to the kt88 that you get no readind on. THE bias is actually measuring the voltage drop across the 10 ohm resistor. If the resistor is open then no current flow through the resistor and no voltage drop(bias)across the resistor. Each kt88 has a 10 ohm resistor connected to the cathode. this is where the 4 test points connect.
Hi keithgreenhalgh, after your post, I got myself a 10 Ohm 5 Watter and proceeded to take the amp apart - talk about many screws.... I think I undid about 25 screws to have the bottom plate out ! I did not see any burnt/charred/heated components, especially that problem resistor. Nevertheless, I replaced it. Since replacement, I managed to get a voltage adjustment on V2 again and it stayed being able to be adjusted !! Thanks a lot. However, it only solved half my problem. The hum still remains after the tubes are heated. So, I swapped V2 to V3 and vice versa, and funny enough, the hum swapped from L to R channel. I think all in all, the tubes had approx 500hrs of use running 0.55V bias. Does it sound as if its the tube's defect? That hum after the tubes have warmed up, is it a representation of a malfunctioning tube? Your expert advise would be greatly appreciated. Cheers, Spencer
The hum is caused by the heater filament touching the cathode.The heated is run on 6.3 volts a/c and is insuated from the cathode but if the heater touches the cathode the 60 cycle hum will be heard on that channel so the kt88 needs replacing.The heater cathode short does happen from time to time but I suspect your amp was damaged(dropped) on the way to delivery.This can also cause heater to cathode shorts.I don't think that running your amp at .55volts would cause this problem but some people think this is to high(55ma cathode current).This is what thee manufacture tells you to set it for.The other problem also seems like it could be caused by a drop.the cathode resistor(10 ohm) may be broken(carbon resistors are somewhat fragile) and a burnt resistor usually stays bad when burnt.Anyway I am just guessing from here.You could start a new tread called Kt88 shorted heater to cathode and ask other people if they have had similar experiences.
Thank you for such informative explanation. So, if you were to replace the KT88s, what brand would you go for? I'm not an esoteric hi fi nut, so something that's good - better than the factory tubes would be fine. I heard the EHs are good, how about the Genalex Gold Lions? And where would you get them and roughly how much are they? Cheers.
Because each tube has its own bias adjustment(good design)it is only necessary to replace the one bad tube.When you get a new one you will have to set the bias and maybe check it in 6 months time.I would go for a new chinese kt88. The gold lion were british from the 1960.s. they are used and like buying a used car unseen,you dont know what your getting.If you google vacuun tubes you should get lots of people selling kt88,s.They were one of the most popular tubes ever made.Also google kt88.There is an article called kt88 and 6550 tube directory.This article is 18 pages long and gives unbiased facts about the different kt88,s out there and how to tell if a used one is good or not.I was curious since you live in australia,when the amp was delivered to you did it travel over any rough roads.If yes ,this could account for your problems.
Re: Yaqin MC-100B circuit diagram
hi , can u email me the original file to abel_woo@yahoo.com thanks ....
halx00 said:
hi , can u email me the original file to abel_woo@yahoo.com thanks ....
salas said:
Thanks for the pointer - I think I'll go for the Gold Lion. Are there different versions of the Gold Lion? Is there a 2 getter flash and a 3 getter flash version? Whilst we're on the subject, any suggestion to the replacement of the 12AX7s? Which brands to go for and do they have to be a matched pair? Thanks for your help.
Re: Re: Yaqin MC-100B circuit diagram
I was reading the manual I got with my amp in chinese and I noticed it said 220volt + or - 1%
This means that your voltage is much to high This would definately cause problems after a while. Also expermenting last night I found the bias voltage does differ in tr or ul mode.Someone had told me it didn't matter which mode it was in when you set the bias but that is wrong
I don't know anyone here that can translate the manual to english,perhaps you know someone down there. Also be careful buying kt88,s as there are tons of fakes out there(re:the article I mentioned earlier)Most seem to be the same chinese tubes you have now with fake lables and double the price. Its a scarey world out there.
I was reading the manual I got with my amp in chinese and I noticed it said 220volt + or - 1%
This means that your voltage is much to high This would definately cause problems after a while. Also expermenting last night I found the bias voltage does differ in tr or ul mode.Someone had told me it didn't matter which mode it was in when you set the bias but that is wrong I don't know anyone here that can translate the manual to english,perhaps you know someone down there. Also be careful buying kt88,s as there are tons of fakes out there(re:the article I mentioned earlier)Most seem to be the same chinese tubes you have now with fake lables and double the price. Its a scarey world out there.
aristocrat said:
There is one version with a top and 2 side getters. As for 12AX7 get the Tungsol reissue again from New Sensor Russia. Maybe the vendor can match those too upon request. With the above tube compliment you get quality as good as best NOS but brand new and in controlled prices IMHO.
salas said:
Would the 6SN7s make any difference if you were to upgrade them? What would you upgrade them with? Cheers.
By the way, the box that the amp was sent had a 240V version label on it, so unless I take it apart and check the output of the 6.3V filament, I wouldn't know. How about taking just one tube out and checking the socket on the 6.3V element? Can this be done without stuffing the amp up? Do you have to take all the tubes out, or can one just take a KT88 out and check the socket? Cheers
aristocrat said:
Yes they will make a much noticeable difference. I would get Sylvania VT231 or the cheaper and a bit less lush JAN Sylvania WGTA chrome top brown base for very good sound, but these are NOS and I don't know what you are willing to spend or suffer to get. They are probably too much for your amp's cost anyway. Reality dictates 6SN7 EH, and I don't know what the Tungsol reissue sounds like but it has carbonized plates as far as I can see. Anybody knows if its better than it's EH sister?
Just pull out a 12AX7, set your DVM on AC, and measure between pin 4 and 9. Don't take a KT88 out. It will change the AC load enough. And keep something connected always to the output of your amp. A speaker, or better a 8R2 20W resistor.
When I checked the voltage between Pins 4 and 9 on the 12AX7, it reads 6.55V, so I gather that the mains transformer is probably the 230 or 240V version which is good news. If it were the 220V version, the filament output might be 10% higher which would bump the filament voltage to close to 7V. From that I extrapolated that it shouldn't be the voltage problem which led to the shortened tube life of one of the KT88s. Either that or it was just unfortunate that it had a short between the heater and the cathode on the tube. I shall go and get another one or a set. Thanks guys
I bought this Yaqin MC-100B Amplifier. When I received and hooked it up on my system, there is no sound! I checked everything:
- all tubes are in the right position
- the CD player is well connected to the apm's input.
- the optional button switched to "CD" is checked.
- the speakers are also well connected.
And I found one thing, there is a input switch in the front. If I switch to 0.25v, and turn the volume to maximum, then I can hear very tiny sound of the CD that is playing. So that means the apm has received the signal, but why it doesn't work properly?
And also, when I switch on the power, there's electrical noise which is quite loud.
It's marked "110v", but Is it possible that it actually is "220v"? (I live in Canada, it has to be 110v here)
Can anyone give me some advise please? thanks.
- all tubes are in the right position
- the CD player is well connected to the apm's input.
- the optional button switched to "CD" is checked.
- the speakers are also well connected.
And I found one thing, there is a input switch in the front. If I switch to 0.25v, and turn the volume to maximum, then I can hear very tiny sound of the CD that is playing. So that means the apm has received the signal, but why it doesn't work properly?
And also, when I switch on the power, there's electrical noise which is quite loud.
It's marked "110v", but Is it possible that it actually is "220v"? (I live in Canada, it has to be 110v here)
Can anyone give me some advise please? thanks.