Tube amps like this must never be operated without a load or the OT becomes a flyback high voltage generator which will arc the tube or socket unless you have clamping diodes on the plates. Arcing happens between the positive screen voltage and negative voltage peeks on an unloaded plate, because the screen and plate pins are next to each other. Clamping diodes prevent the plate voltage from going negative. Some amps are unstable without a load and oscillate at a high frequency. In any case, momentarily disconnecting the speaker may be the root cause of the failure. You could add plate clamping diodes, but they need to be very high voltage and fast. The lack of feedback may be involved but that gets complicated and unpredictable.
Followup — finally had time to replace the resistors. They are at the top of the circuit board.
I set the bias and right now I'm playing music through it. It sounds great.
Some notes:
1. There were some comments about the tubes being mounted sideways instead of vertically. As you can see, this is a commercial product, and there's not much I can do about that without making a new case, in which case I might as well just try to build a new amp from scratch.
2. The resistors I used are the same specs as the schematic. They are 10 ohm 1W wirewound resistors, made by Ohmite. The spec sheet on the Mouser website says that they are rated for 275ºC. I know that they are close to the EL34s, but I don't think the temperature there will get that high. I know that they are small, and that there were comments about how I should get the physically largest resistor I could find for this purpose, but no other resistor had a stated temperature rating that high.
Just from appearances, it looks like the old resistors might be made by Yageo, which are rated to 200ºC according to their spec sheet. I don't know if the 75ºC difference in temperature rating is really of any help, but it makes me feel better.
Thanks for all the help! We'll see how I did 5 years from now.
I set the bias and right now I'm playing music through it. It sounds great.
Some notes:
1. There were some comments about the tubes being mounted sideways instead of vertically. As you can see, this is a commercial product, and there's not much I can do about that without making a new case, in which case I might as well just try to build a new amp from scratch.
2. The resistors I used are the same specs as the schematic. They are 10 ohm 1W wirewound resistors, made by Ohmite. The spec sheet on the Mouser website says that they are rated for 275ºC. I know that they are close to the EL34s, but I don't think the temperature there will get that high. I know that they are small, and that there were comments about how I should get the physically largest resistor I could find for this purpose, but no other resistor had a stated temperature rating that high.
Just from appearances, it looks like the old resistors might be made by Yageo, which are rated to 200ºC according to their spec sheet. I don't know if the 75ºC difference in temperature rating is really of any help, but it makes me feel better.
Thanks for all the help! We'll see how I did 5 years from now.
i would rather have those resistors fail than poop my output transformers when the tube fails...
