Amp Died After Power Surge

[Dmessick]

Well, I'm a dumbass and now I'm trying to recover my own stupidity of not using a surge protector where I plugged in my amp.

I am working on a Rocket that I recycled from someone else's project, which I stripped down to just a bare bones rocket with a bypass cap switch. Chassis layout is similar to standard rocket. I had been testing and playing through it at home for about a week and it sounded great as expected... However; at one point, I saw a spark come from my IEC socket area and the amp went dead. The socket in the wall was dead too and I found the house circuit break had tripped. After switching back on the circuit breaker, I checked the amp fuse and it was fine, the amp powered on but no audio. I could hear a transformer hum, not coming through the speaker. Heaters light up, B+ voltages are all there (although a little hot), so the power transformer seems to be working.

Since I was getting power everywhere, I thought maybe I blew a tube(s). Replaced the PI and got sound for a second before amp went dead again. I don't want to keep killing tubes and I am at a loss as to what to check next. When I chopstick the PI cathode, I get a loud pop through the speaker so the OT seems to be working.

Is anyone up for some troubleshooting help?

[dorrisant]

Can you post your voltages? Maybe post some pics?

[xtian]

Start troubleshooting as if this was a new build--pull all the tubes and measure AC voltages. Then recto goes in, and measure DC voltages, etc.

[Dmessick]

Thank you all for being willing to jump in on this with me. Hopefully I can get some pictures up in the next 24 hours. Below are the voltages I measured today. This is my third amp build and with each one I'm trying to dive in a little deeper to fully understand how these work. I think this one is going to force me to gain a better understanding of the power section really functions.

V1a - 12ax7
anode (plate) - 127V
cathode - 1.28V

V1b - 12ax7
anode (plate) - 83V
cathode - 0.88V

V2 Cathode follower - 12ax7
anode (plate) - 218
grid - 127
cathode - 127

V3a PI - 12ax7
Pin 1 - anode (plate) - 174
Pin 2 - grid - 15
Pin 3 - cathode - 37
Pin 6 - anode (plate) - 250
Pin 7 - grid - 33
Pin 8 - cathode - 37

V4, V5 - EL84
Cathode - 9.4V
Anode - 323V
Screen - 321V

V5,V6 - EL84
Cathode - 9.4V
Anode - 321V
Screen - 320V

Secondaries: 510V
B+1: 331V
B+2: 320
B+3: 250
B+4: 220
B+5: 213

SS rectifier

[Dmessick]

Here are some pictures of the build so far. This amp came to me a little unexpected and because the chassis and board weren't in great shape, this amp is meant be my platform for experimentation. It was quiet and sounding great before the power surge, and now it is extra quiet I am a little embarrassed to pose these pictures because I know the lead dress leaves a little to be desired, but I was making due with what I had, and I knew that due to the state of the chassis, this was never going to be a showroom amp anyways. The Woody sticker was a special request from my nephew...

[Colossal]

Maybe this is from an older photo, but there is no second connection to the power switch

Power Switch.jpg

[Dmessick]

Great observation Colossal and thank you for taking the time to scrutinize the photos. The angle on the pictures isn't very good so it does not show that he switch is DPST toggle switch and the second connection is on the bottom. The pilot light, which is running off the heaters, comes on and B+ seems to be getting everywhere. My next thought is to start testing caps and resistors to see if maybe something got fried.

[Mark]

With a power surge, I'd expect the diodes and maybe the filter caps on the first stage of filtering to have a hard time. However, this isn't the case with your amp as the DC voltages are all present. Do you have another amp that you can use to test the valves?

Valves are pretty sturdy, I'm surprised the P.I. valve popped so easily. Also what could possibly induce a high current situation in the P.I.?

There are only caps on the input and output. The rest of the circuit is resistors. Resistors never go short circuit, they go open circuit, you would expect to see discolouration/burn on the resistors themselves.
The exception being the cap from pin 2 to ground.

I would want to know my output stage is working and work back. The reason for that is I can insert a signal and get an output.

https://ampgarage.com/forum/download/file.php?id=8003

https://ampgarage.com/forum/download/file.php?id=7458

[Colossal]

Great observation Colossal and thank you for taking the time to scrutinize the photos. The angle on the pictures isn't very good so it does not show that he switch is DPST toggle switch and the second connection is on the bottom. The pilot light, which is running off the heaters, comes on and B+ seems to be getting everywhere. My next thought is to start testing caps and resistors to see if maybe something got fried.

Ah! I was duped! I forgot about those Carling DPSTs having that lug on the bottom. Ok, well then, yes, I would start at the very beginning and meter your parts and look for other abnormalities.

[Dmessick]

Thank you all for taking the time to provide your feedback. Sometimes we have to be reminded to step back and take a systematic approach to troubleshooting our circuits. I wanted to circle back and provide my train of thought so that hopefully someone else can learn from my experience, like I have learned from many others here.

I have a KT66 Rocket that I am finishing up (quarantine has really helped me clear up some projects...) so I decided to test the preamp valves in that amp. Plugged in the three valves, fired it up, and got audio. Now I know the tubes are working properly. I put the tubes back in the Rocket and still no sound.

Now I know the valves are good, voltages are reasonable, possibly an issue in the audio path? With the amp on and speaker connected, I broke out the chopstick and started tracing the schematic. I found I was still getting a pop on the PI cathode wire. I had previously re-soldered all the joints on this wire so this time I just replaced the wire instead. I am now getting audio through the amp and so far, everything seems to be back to normal. I am using this amp as my main practice amp this week so I will continue to monitor it to make sure it holds up. At this point, I'm not certain if the power surge was the cause of the issue, or possibly exacerbated the issue with the cathode wire.