After completed the construction of Kit One, I measure the voltage according to manual, without any tubes plug-in, it seems all in normal rating. But when I plug-in all tubes and test it again, I found the terminal 3 of front 300B (cathode pin) was arround 7v (indicated 70v on manual), and the back 300B was 65v firstly and drop down then. The front 300B was so hot then the back one which just a little bit warm. (touch the glass)
I tried to swap the 300B tubes, cathode pin capacitor but still in vain.
BTW, all the high voltage seems drop down step by step as times goes by. say the HT on 300B terminal 1 from 0v to 320v after powered on, after 5 secs, it begin to drop down from 320v...315v, 310v, 300v and so on. I turn it off immediately so I don't know how low it will drop down. But the front 300B's cathode terminal (pin 3) was stayed at 7v from power on. So I think the problem is on it but I cannot identify it.
Anyone can help me!!!
I tried to swap the 300B tubes, cathode pin capacitor but still in vain.
BTW, all the high voltage seems drop down step by step as times goes by. say the HT on 300B terminal 1 from 0v to 320v after powered on, after 5 secs, it begin to drop down from 320v...315v, 310v, 300v and so on. I turn it off immediately so I don't know how low it will drop down. But the front 300B's cathode terminal (pin 3) was stayed at 7v from power on. So I think the problem is on it but I cannot identify it.
Anyone can help me!!!
Very possibly you have a simple wiring error. I don't have the AN circuit in front of me but it probably has a paralleled resistor and cap in the cathode. If the cap is reversely connected you may experience the symptoms you describe. As a first step measure the resistance between cathode and ground when the amp is not powered and if you get a value lower than Rk try to find out why.
Alternatively the resistor between grid and ground may be missing.
Alternatively the resistor between grid and ground may be missing.
Thx,
I check it once and once again and confirmed the cathode capacitor solder in correct position. The capacitor positive terminal solder to cathode pin and the negative solder to the ground.
Also the cathode resistor was worked fine and measured in 900R.
No resistor between grid and ground directly according to the schmatics.
I check it once and once again and confirmed the cathode capacitor solder in correct position. The capacitor positive terminal solder to cathode pin and the negative solder to the ground.
Also the cathode resistor was worked fine and measured in 900R.
No resistor between grid and ground directly according to the schmatics.
I am talking of the 330k resistor. When measuring from 300B grid to ground do you measure 333k?
If this doesn't help disconnect one side of the .22uF cap and see if the fault persists.
If this doesn't help disconnect one side of the .22uF cap and see if the fault persists.
thanks~~
I found it. It causing by 7805 5V filament reglator for the 300B tube. the GND (case of 7805) was touched chassis, after I re-sit the 7805 and heatsink, the symptom disappeared and my kit one sounded great~~
It is careless mistake....
anyway, thanks for your help, analog_sa!
nice to talk to you!!
I found it. It causing by 7805 5V filament reglator for the 300B tube. the GND (case of 7805) was touched chassis, after I re-sit the 7805 and heatsink, the symptom disappeared and my kit one sounded great~~
It is careless mistake....
anyway, thanks for your help, analog_sa!
nice to talk to you!!
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