SLO100 build lost B+

[hiegdk]

So my SLO100 clone, which I've been using happily for a almost 2 years blew a mains fuse the other night. I just got it on the bench and I'm only able to measure the HT on one side of the choke. In reference to the attached schematic with the power tubes out of the amp I'm measuring 530V at E and about -0.5V at D. Does that imply that my choke died?

[glass54]

Quite possibly!
...and there may be a culprit 
(Amp unplugged!) Check C31, 32 and 33 with your multimeter on ohms. May be very unwell?

Regards
Mirek

[sluckey]

Check resistance between nodes E and D. Should be relatively low. Check resistance from node D to ground. Should be relatively high.

[tubeswell]

I'm measuring 530V at E and about -0.5V at D. Does that imply that my choke died?

or the filter cap at that end of the choke

[Latole]

Choke ;
1- read coil ohm
2- Disconnect choke at D and read voltage.
    A good choke will read same voltage you read at E

[hiegdk]

Check resistance between nodes E and D. Should be relatively low. Check resistance from node D to ground. Should be relatively high.

Resistance between E & D is infinite (OL on my meter). Resistance from D to ground is about 2M. Seems bad

[hiegdk]

Choke ;
1- read coil ohm
2- Disconnect choke at D and read voltage.
    A good choke will read same voltage you read at E

520V at E and about 3V at D...

[sluckey]

Resistance between E & D is infinite (OL on my meter). Resistance from D to ground is about 2M. Seems bad

You need a new choke but the filter caps are likely good. A shorted cap did not kill the choke. Maybe a bad output tube caused the choke to fail.

[hiegdk]

Thanks - ordered a new one.

I've also noticed some discolouration of the 100R balance resistors in the faux center tap on the heaters (photo attached). Should I replace them with a higher wattage ?

[sluckey]

I've also noticed some discolouration of the 100R balance resistors in the faux center tap on the heaters (photo attached). Should I replace them with a higher wattage ?

Replace them with 1/2 watt resistors. Those burned resistors are a sign of output tube failure, usually plate to heater short, or screen to heater short. Since your choke was also burned up, the screen was most likely shorted to heater. You will likely burn up another choke if a shorted tube is not replaced.

[hiegdk]

I've also noticed some discolouration of the 100R balance resistors in the faux center tap on the heaters (photo attached). Should I replace them with a higher wattage ?

Replace them with 1/2 watt resistors. Those burned resistors are a sign of output tube failure, usually plate to heater short, or screen to heater short. Since your choke was also burned up, the screen was most likely shorted to heater. You will likely burn up another choke if a shorted tube is not replaced.

Ok, perfect, thanks - I have a new set of tubes on order already. Maybe I'll double check the wiring at the tube sockets too to make sure I don't have any shorts there.

[Latole]

Choke ;
1- read coil ohm
2- Disconnect choke at D and read voltage.
    A good choke will read same voltage you read at E

520V at E and about 3V at D...

I meant to measure the voltage on the choke, side D and not on D itself where, of course, there would be nothing.
The aim was to check whether the choke is conducting or not.

And I've read that it doesn't.
You'll have to replace it after fixing the problem that caused its destruction.

[sluckey]

Maybe I'll double check the wiring at the tube sockets too to make sure I don't have any shorts there.

Good idea. Look for any signs of arcing or burns on the sockets. Also check for any resistance between all output tube socket pins 3 and 2 and also between pins 4 and 2. Do this with and without the tubes plugged in.

[hiegdk]

Maybe I'll double check the wiring at the tube sockets too to make sure I don't have any shorts there.

Good idea. Look for any signs of arcing or burns on the sockets. Also check for any resistance between all output tube socket pins 3 and 2 and also between pins 4 and 2. Do this with and without the tubes plugged in.

So I'm reading around 1M resistance between pin 2 and D with no tubes in, the choke disconnected, and the lead from D to the screen grid resistors disconnected.

[hiegdk]

Over time that resistance has drifted up to over 10M

[hiegdk]

Disregard - probe clips were loose. Reading open loop between pin 2 &3 and 2&4 of each tube socket.

[sluckey]

Reading open loop between pin 2 &3 and 2&4 of each tube socket.

Are those resistance readings still open loop with tubes in the sockets?

[hiegdk]

Reading open loop between pin 2 &3 and 2&4 of each tube socket.

Are those resistance readings still open loop with tubes in the sockets?

Yes

[sluckey]

Proceed with caution when you replace the choke. I highly recommend using a light bulb limiter at first startup.

[hiegdk]

Will do. I have new tubes as well. If the issue wasn't a bad tube what's the next most likely culprit? Filter or reservoir cap(s)?

[sluckey]

Shorted C31 and C32 or shorted tube(s) are about the only logical culprits. It takes a lot to burn open a 200mA choke. Of course, the choke winding could have developed a short to frame.

What was the circumstance when the amp failed? Be specific. There may be some clues.

[hiegdk]

As best as I can remember I had just moved it and the cab across the room during a recording session. After plugging it all in there was a fair bit of hum after taking it off standby, which I had attributed at the time to a ground loop issue with the DI/splitter I was using in front of the guitar player's pedalboard. During the course of troubleshooting that issue the amp blew it's mains fuse when I popped it out of standby. Took it home and put it on the bench and that's when I diagnosed the missing B+ that started this thread.

[Latole]

IMO could be a power tubes failure and a too powerful fuse ( I see 5A on your schematic) who couldn't protect the amp's parts