yaqin 100b kt88-98 bulb blown and smoking coming from the amp - diyAudio
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Old 28th March 2013, 12:13 PM   #1
youngie is offline youngie  United Kingdom
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Default yaqin 100b kt88-98 bulb blown and smoking coming from the amp

yaqin 100b kt88-98 bulb blown and smoking coming from the amp. removed the valve and the amp is still working. will i be ok to play the amp with 3 kt88 valve or will i bust the amp. what valves are cheap and worth replacing the kt88. kind regards mr young
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Old 28th March 2013, 08:11 PM   #2
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Default In short, no...

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Originally Posted by youngie View Post
yaqin 100b kt88-98 bulb blown and smoking coming from the amp. removed the valve and the amp is still working. will i be ok to play the amp with 3 kt88 valve or will i bust the amp. what valves are cheap and worth replacing the kt88. kind regards mr young
Smoke coming from any item of electronic gear is a BIG red flag! IMHO, you shouldn't even plug it in to the mains until you have determined what smoked and why.

As for replacing the tube, your amp is a push-pull design. As such, it needs both KT-88's (or whatever) to operate properly. It might put out sound with only one - but likely nothing you will want to listen to for very long.

Also, if you just buy a replacement tube and plug it in the amplifier will likely still not sound right. There are internal adjustments on that design that are used to balance the circuit - any time you change a tube (or pair of tubes) those adjustments need to be tweaked. All tubes are slightly different, even from the same tube manufacturer, which is why the amplifier manufacturer provides those adjustments.


You will first need to troubleshoot and fix the original problem (smoke), then replace the tube (you might need to buy a matched pair to get it working properly), and finally adjust the bias (inside the amplifier) to get the unit back to the performance standards you paid for.

If you do not feel comfortable working inside a unit that has exposed 400+ volts (which is quite dangerous and can be deadly if you touch it), I would suggest you take the amplifier to a qualified repair shop and get it fixed properly.

If you decide to try it yourself, you will also need some basic equipment such as a voltmeter, amplifier schematic diagram and service manual, parts to replace whatever smoked, soldering iron, etc. to do any work inside the unit, and you will need to know how to use them.

Good luck - and please be safe!

~ Sam
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Last edited by rfengineer2013; 28th March 2013 at 08:13 PM.
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Old 28th March 2013, 10:13 PM   #3
youngie is offline youngie  United Kingdom
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thank you so much sam . i found out it was just a broken valve. kt88. so i will need to buy a new one. what are the option i can buy.. i have also brought a xiang sheng preamplifier model 728a.. i am using ma gr20 but when i turned up the amps today the base was sounding really muffled. i have the preamp fitting into the yaqin 100b at the back not the front of it does it matter. i have my marantz 6003 going into the preamp. have i set this up the best way or not. any help would be great. kind regards mr la young
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Old 28th March 2013, 10:21 PM   #4
Hi_Q is offline Hi_Q  England
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Bulb (actually a LED) is run from the heater supply via a 330 Ohm 3W resistor.
The heaters are fed with a grounded center tapped transformer heater winding.
The LED failure could be a red herring as they are prone to failure - why? - too much heat applied when soldered, excessive strain due to leg bending or no reverse protection diode when fed with AC? Who knows but the smoke is another thing. A good visual inspection is required around the suspected tube socket. It could have been a screen grid failure which has taken out its limiting resistor (180 Ohms at 3W) or perhaps the cathode resistor has roasted (10 Ohms at 3W). Perhaps both? Good visual check should find it and some deduction will no doubt lead to the reason for the circuits demise. Don't put that suspect tube back in the amp until it has been thoroughly checked out. All of what Sam says about the Dangers and setting up is valid and the assistance of a qualified tube doctor/engineer/technician is highly recommended.

LOL we both posted at the same time - glad you found the reason but check for badly burnt resistors.
Also understand now that Bulb was in relation to the tubes glass envelope :-)

Last edited by Hi_Q; 28th March 2013 at 10:27 PM.
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