Hi
I am having a rather unusual problem with this amp. When I first got this amp, there was power but no sound. Upon observation, I realised that non of the tubes were lit. I left it aside and a little while later I tried again. This time, all the tubes were lit and there was sound. I kept turning on the amp every now and then and the tubes were all litting up. The next day, when I turn on the amp, again, they were all dead. After sometime, I tried again and the tubes were litting up as usual. I kept doing this for a few days, and it was litting up at times and dead at times. Finally I took out the board and resoldered the whole board. R81(470K) was blown. I replaced it. After putting back everything, I turned on the amp and all the tubes were litting up and the amp works as usual. I having been turning the amp on and off for the past few days and the amp was working OK. I rebiased the amp and finally decided to put back the amp in the cabinet. After that I plugged in a signal and let it run for about 2 hrs and turned it off. After about 3hrs later, when I turned on the amp, there was no sound and all the tubes dead( not litting up). There are 3 wires from the transformer for the filament voltage going to the board. Blue(3.15V AC) going to WP5, Black(0V CT) going to WP7 and Brown(3.15V AC) going to WP8. I pulled out the blue and brown wires and measured app 6.7V ac across it , but when I plugged back the wires to the board, the tubes were not litting up. I realised that the blue wire(3.15V) was very hot and had turned slightly brown at the transformer side. What can be the problem, is it due to a problem on the board or the transformer itself or the tubes. Please assist.
Thanks
I am having a rather unusual problem with this amp. When I first got this amp, there was power but no sound. Upon observation, I realised that non of the tubes were lit. I left it aside and a little while later I tried again. This time, all the tubes were lit and there was sound. I kept turning on the amp every now and then and the tubes were all litting up. The next day, when I turn on the amp, again, they were all dead. After sometime, I tried again and the tubes were litting up as usual. I kept doing this for a few days, and it was litting up at times and dead at times. Finally I took out the board and resoldered the whole board. R81(470K) was blown. I replaced it. After putting back everything, I turned on the amp and all the tubes were litting up and the amp works as usual. I having been turning the amp on and off for the past few days and the amp was working OK. I rebiased the amp and finally decided to put back the amp in the cabinet. After that I plugged in a signal and let it run for about 2 hrs and turned it off. After about 3hrs later, when I turned on the amp, there was no sound and all the tubes dead( not litting up). There are 3 wires from the transformer for the filament voltage going to the board. Blue(3.15V AC) going to WP5, Black(0V CT) going to WP7 and Brown(3.15V AC) going to WP8. I pulled out the blue and brown wires and measured app 6.7V ac across it , but when I plugged back the wires to the board, the tubes were not litting up. I realised that the blue wire(3.15V) was very hot and had turned slightly brown at the transformer side. What can be the problem, is it due to a problem on the board or the transformer itself or the tubes. Please assist.
Thanks
If the filaments connect to the board with push-on connectors they could be oxidized and making poor or no contact and intermittent depending on many things. The fact that they are hot shows that the main resistance is at the connection and not the tube filaments. The wires from the transformer just before the connectors could be corroded and offering resistance.
The fix would be to cut off the connectors off and hopefully you will see clean, no-corroded wire. Solder these directly to the board's push-on terminals.
Check out this YouTube channel. A very experienced guitar amp tech shares his experience.
https://www.youtube.com/user/psionicaudio
Steve
The fix would be to cut off the connectors off and hopefully you will see clean, no-corroded wire. Solder these directly to the board's push-on terminals.
Check out this YouTube channel. A very experienced guitar amp tech shares his experience.
https://www.youtube.com/user/psionicaudio
Steve
I might have misunderstood your description. Post a picture of the wires in question connecting to the board. Picture and a 1,000 words........
Apparently the blue wire makes a short on the board that's why it gets so hot and the tubes don't light up. It looks like some solder on one of the tube filament connections makes a short with the chassis because the problem shows up after putting everything back in place. The fact that every now and then the problem shows up means that it's not a very tight short. This can be checked by unscrewing the board but not completely as long as there is no contact between board and chassis.
It's also possible that the 3.15 pole with the blue wire somehow touches some ground trace on the board although this is less likely as the amp functioned
normally when you had taken it out.
It's also possible that the 3.15 pole with the blue wire somehow touches some ground trace on the board although this is less likely as the amp functioned
normally when you had taken it out.
R81 and R83 form a voltage divider to elevate the heaters (except for the heater of V6). R81 is rated for 0.75 Watt. So for R81 to blow, the voltage over R81 had to be close to 600 V. Maybe this is possible with the tubes not drawing any current, combined with a short to ground in/around the heater winding.
But one short to ground at for instance a tube socket doesn't explain why the heaters don't light up. The only thing one short to ground would do, is to undo the arrangement to elevate the heaters (it's like shorting R83). The two 10 A fuses didn't blow either.
Could there be two shorts to ground, of which one is intermittent? Do you read infinite resistance between the heater circuit on the circuit board and ground with all the tubes removed and the three wires of the heater winding disconnected?
But one short to ground at for instance a tube socket doesn't explain why the heaters don't light up. The only thing one short to ground would do, is to undo the arrangement to elevate the heaters (it's like shorting R83). The two 10 A fuses didn't blow either.
Could there be two shorts to ground, of which one is intermittent? Do you read infinite resistance between the heater circuit on the circuit board and ground with all the tubes removed and the three wires of the heater winding disconnected?
I have replaced all the 3 filament wires from the transformer to the board using pure copper wires. I have been turning the amp on and off for the past few days and the problem did not show up.
Thanks