#102 died yesterday :( how,why?

[norburybrook]

got a call from Alan D last night saying his original #102 had died he said the power light was off and no OD, he;d swapped the OD tube and sound came back but it was noisy, he also said he'd smelt burning.


he was prepping it for Eric Clapton to play on Sunday.

went over today and I opened it up and as he'd mentioned the power LED wasn't working I looked at the 6v wiring first. I could see the two 120R center tap resistors were bunt out.

I took the valves out and fired it up and checked the heater voltage...it was reading 175v AC!!! I'm surprised the amps actually powered up and made sound!

Anyway I presume the PT is knackered then?

why did this happen and what could cause this?


It's a Hammond 290EEX bassman power transformer by the way.


M

p.s. I've lent Eric my own #102

[sluckey]

I could see the two 120R center tap resistors were bunt out.

A very common cause for burning these resistors is a short between pins 2 and 3 of one or more of the power tubes, either internal to the tube or an arc on the socket. A common cause for this arc is playing the amp loud with an open speaker load. Be sure to look for signs of arcing on the sockets.

[norburybrook]

I could see the two 120R center tap resistors were bunt out.

A very common cause for burning these resistors is a short between pins 2 and 3 of one or more of the power tubes, either internal to the tube or an arc on the socket. A common cause for this arc is playing the amp loud with an open speaker load. Be sure to look for signs of arcing on the sockets.

thanks Steve on closer inspection I can see an arc on one of the OT tubes between 2 and 3, you're a clever guy . Is the PT definitely f***ed then?



M

[Charlie Wilson]

Not nessasarily. That’s why there is a fuse. Did you use Belton sockets?
CW

[norburybrook]

Not nessasarily. That’s why there is a fuse. Did you use Belton sockets?
CW

yes I did Charlie, the fuse didn't blow, ( I Only have a mains 2 amp fuse as its a 50w amp not an 500ma HT one as well) the amp was making sound albeit a noisy one. Once I'd measured 175vAC on the heaters I didn't wan't to go any further as i presumed the transformer had shorted internally somewhere hence the large voltage.

Alan said he'd been doing a lot of speaker swapping with that amp and NOT turning it off in between.....so short burst of open speaker from a cranked amp....so now I know how and why




M

[sluckey]

Replace the sockets that show signs of arcing. May be a good idea to just replace both.

[norburybrook]

Replace the sockets that show signs of arcing. May be a good idea to just replace both.

the arcing looks like its only from the metal pins at the top where the heater cable is wired in the top holes, the brown base is completely clear.

what about the 175v AC heater readings?

M

[xtian]

The PTs heater coil is not creating 175vAC...I can only guess, if you measured heater voltage while a bad power tube was shorting HT to the heater coil, that's what you saw. The bad noise you heard was from the toasted heater balancing resistors. So, replace heater balancing resistors, check grid and screen resistors, power up without power tubes and check voltages...

[norburybrook]

The PTs heater coil is not creating 175vAC...I can only guess, if you measured heater voltage while a bad power tube was shorting HT to the heater coil, that's what you saw. The bad noise you heard was from the toasted heater balancing resistors. So, replace heater balancing resistors, check grid and screen resistors, power up without power tubes and check voltages...

indeed it was a surprise. I tested without any tubes though. I'll replace the resistors and have another go.



M

[xtian]

While you're in there, you may want to disconnect the PT from everything and measure it's secondaries unloaded...for peace of mind if nothing else.

[norburybrook]

the main secondary is measuring 320-0-320 just fine.

here's the socket ,I've removed the 100R resistor. It was on the blue wire. you can see the black charing on the cable ends.

#102 socket.jpg

M

[Colossal]

Sorry to hear about that, Marcus. I have nothing to offer, but seeing your thread title, I've now got A New Day Yesterday by Jethro Tull stuck in my head, but singing "my 102 died yesterday, and it's long gone nooowwww."

[sluckey]

Check resistance between pins 2 and 3 without tubes. Then repeat with tubes. Should be very high, or open, or OL.

[norburybrook]

Check resistance between pins 2 and 3 without tubes. Then repeat with tubes. Should be very high, or open, or OL.

is that with the amp off?

if so then they're 3M without tubes and 11M with tubes


M

[pompeiisneaks]

It's probably not a bad idea to check the resistance betweens pins 2 and 3 on the power tubes out of circuit as well...

I don't know for sure on power on/off myself...

~Phil