Crate v18 212 amp wont turn on anymore???

[HIVibesSoundsystem]

Hey all, first post, never had a problem before, this amp is great, however i had it plugged into my audio interface from the line out of the amplifier, with a 1/4" instrument cable, and i unplugged the speaker out. it played well for an hour or two, one day. i went to play it again the next day and a few minutes later the amp just turned off and won't turn on again it just makes this low pitched poppopop.... i was messing with the plug on my guitar while this was happening btw. what could have blown on the amp and how can i tell what blew? took a look at the tubes and they look ok, except for one has a small black spot on the top of one of the larger tubes, but the circuit board looks fine.
thanks,
Jordan

[Enzo]

Oh NO NO NO!!!

One cardinal rule of tube amps is to NEVER operate one without a speaker or load on the output. You cannot just unplug the speaker and continue to play the amp. There are ways to get around this, but your amp I think lacks an FX return or a Power AMp In jack. One way to use the preamp would be to remove the two power tubes, and then the preamp can be used alone.

Your amp also does not seem to have protection diodes. It could be something simpler, but the usual death from speakerless running is the output transformer arcing over inside. You might haqve damaged the power supply instead/also, and in this amp that is not a good thing either. Nothing will look different. This will not be easy for a novice to troubleshoot. I am assuming you are not a technical type? Have a meter and know how to use it?

[Chuck H]

Oh Enzo... Why do you always assume it's an output transformer

Just kidding (inside joke). I'm going to back Enzo on this one (with contrary hopes for the best). But that amp needs to be seen by someone familiar with electronics. And better if they're also familiar with guitar amps. The possible failure list, as compared to the problem description, has too many possible causes to diagnose without some bench tests.

[Enzo]

Well, it COULD have been the power transformer. I always hear the tubes should be balanced. One of the big rubber feet is missing, and the amp is not well balanced now. Do you think that might be part of the problem?

[HIVibesSoundsystem]

OH! well you learn something everyday.... maybe i shoulda used the output for the spring reverb... Well i have some technical skills and a meter. is the output transformer that big one next to the tubes? and is the power supply the medium or two small transformer looking things where the power cable connects.

thanks for the quick replies

[Enzo]

The OT is indeed the large transformer. You can go to Geofex.com and under Tech Tips find the transformer short detector. Very simple and easy to make and use.

Your power supply is a square circuit board, should say "V series SMPS" om it. Careful, that SMPS is VERY dangerous when powered.

But let us not assume anything yet.

Did you check the high voltage fuse on the main board?