5 Amp Slow Blow Fuse popping!

My ST120 blew the 5 amp fuse yesterday as I was letting it warm up. I came back in the room and all the tubes were out. I pulled the fuse and it was blown. After a quick trip to the Shack, I placed another in the holder and turned on the amp. Tubes powered up for about 15 seconds then pop.

I checked all the tubes and they looked OK. I placed another fuse in the holder and tried it again. Same thing, but I was watching the rectifier. I saw arcing in the rectifier.

Pulled the rectifier and replaced it with a Webber Copper Cap SS rectifier. Replaced the fuse, turned it on, everything powered up and played fine. I let it warm up a bit and re-biased. All is OK now.

I called Bob to ask him about it and he said that is somewhat typical of a bad rectifier. Bob pointed me to Ruby 5AR4 rectifier, so I have ordered a couple of those.

Bob, I just wanted to thank you for being there for us! I really appreciate the fact that I have you to help me with this amp that I have fallen in love with, it was made for my Klipsch Cornwall's!

Hi- Were you still using the 5U4GA that you mentioned in other posts as your rectifier before it started to arc? If so, I would suggest returning to 5AR4 as a permanent rectifier choice. Although the Dynaco manual allows the use of 5U4 type tubes as backups, they employ a direct heated cathode design rather than the indirect cathode sleeve design of the 5AR4. In higher voltage amplifiers designed to use 5U4 tubes, they are generally employed in pairs, cutting the voltage to the heater and allowing longer life. Although I am not an expert, the evidence suggests that long term use of a 5U4 in this amplifier is not the best choice.

kevco wrote:Hi- Were you still using the 5U4GA that you mentioned in other posts as your rectifier before it started to arc? If so, I would suggest returning to 5AR4 as a permanent rectifier choice. Although the Dynaco manual allows the use of 5U4 type tubes as backups, they employ a direct heated cathode design rather than the indirect cathode sleeve design of the 5AR4. In higher voltage amplifiers designed to use 5U4 tubes, they are generally employed in pairs, cutting the voltage to the heater and allowing longer life. Although I am not an expert, the evidence suggests that long term use of a 5U4 in this amplifier is not the best choice.

No it was the 5AR4 that arced, the 5U4GA sounds great. I have ordered a Ruby 5AR4 to replace it.

Jhoman wrote:

kevco wrote:Hi- Were you still using the 5U4GA that you mentioned in other posts as your rectifier before it started to arc? If so, I would suggest returning to 5AR4 as a permanent rectifier choice. Although the Dynaco manual allows the use of 5U4 type tubes as backups, they employ a direct heated cathode design rather than the indirect cathode sleeve design of the 5AR4. In higher voltage amplifiers designed to use 5U4 tubes, they are generally employed in pairs, cutting the voltage to the heater and allowing longer life. Although I am not an expert, the evidence suggests that long term use of a 5U4 in this amplifier is not the best choice.

No it was the 5AR4 that arced, the 5U4GA sounds great. I have ordered a Ruby 5AR4 to replace it.

I went through a spate of rectifiers tubes before I learned that they do not like to be turned off and on quickly . Once they go bad the arcing seems to be the primary symptom.

"I went through a spate of rectifiers tubes before I learned that they do not like to be turned off and on quickly . Once they go bad the arcing seems to be the primary symptom."

Bugs,

Dynaco tube amps do not have any sophisticated "delay ON" circuits. If you turn the amp OFF and then ON quickly (within a few seconds) you can cause a power surge that can take out the rectifier tube. A delay ON circuit could be added but it would also add to the cost of the amp. My advice is > If the amp has been ON for while and you turn it OFF for any reason leave it OFF for about 30 seconds to a minute - then turn it ON again.

Also keep in mind that the rectifier tubes made today are not as "robust" as the USA and UK made rectifier tubes of the 1950's and 1960's. In short, the newer tubes can't take much abuse without failing.

Bob

Hi Bob, thanks for the info, the eye-rolling in my comment had more to do with the fact that sometimes I have to learn things through the School of Hard Knocks rather than eye-rolling aimed at the design of the system.

Got the Ruby in, sounds great, a bit tighter than the 5U4GA, had to crank down the bias, but over all its great!