Amp crackle and half power weirdness

AcousticBoom

Member
Messages
81
Hey all, I recently bought a JCM 800 4210 (1984 - newer circuit).

A little back story...

First thing I did was replace the electrolytics and give the sockets, jacks and pots a little deoxit. The old tubes would also crackle and pop like cooking bacon when turning the amp on so it was time for new tubes as well. It came with 6550's and decided to put KT66's in it. I bought new production Gold Lion's.

Originally to bias the amp, I had bought a bias scout from tube depot. All my test measurements after assembling the tool came out perfect. However, when I connected it and turned the amp on, it blew the 500mA fuse (slo-blow). From that point I was done with the scout and decided to use the transformer voltage method (70% dissipation if anyone cares to know). Everything was running fine as of this past Saturday. (Edit: I also replaced the tube that was in the scout at the time of the blown fuse with another of the same rating).

Now for the problem I experienced last night...

I haven't had a chance to crank the amp yet and have been playing at slightly-louder-than-bedroom levels (MV cranked and channel volume at 9 o'clock). While in the middle of noodling around, the amp crackles and then loses about half its power. The crackle was similar to a scratchy pot - different than when powering on the old tubes. I played around with the volume and gain knobs, didn't help. I turned off the amp completely, waited a few minutes, and everything came back on OK. It played just fine for the rest of the evening.

Any theories why this might have happened? I would think maybe one of the tubes is bad, but since blowing that fuse, maybe something else got fried in the process? Is it safe to be playing the amp if it does it again? Maybe some suggestions of tests I can do?

Thanks!
 
Messages
826
One possibility is that the output tube sockets have 'arced' at some time in the past (usually caused by playing without a speaker). This can leave a kind of 'burnt track' which can then cause insulation breakdown under normal playing conditions.

Have a very close look at the sockets (above and below the chassis) to see if there is any suspicious discolouration, tracking or burning.

(Just my 2 cents worth, as they say. It could be some other problem.)
 

Blue Strat

Member
Messages
30,197
Intermittent problems are the worst to track down.

Could be anything including guitar, cable and effects (if there were any connected).

See if you can replicate the problem over time.

BTW, 70% is only going to shorten the life of your power tubes. I'd never set new production tubes that high.
 

AcousticBoom

Member
Messages
81
Thanks for the responses.

I'll examine the sockets again when I can get the amp back on the workbench. Nothing stuck out to me as being damaged or burnt when I initially inspected it.

I've been so focused on the amp that I haven't even considered what was plugged into it. Now that I think about it, I have had an issue with a cable in the past where the sound would intermittently drop out completely. That cable had a date with the trash man and never came back! :rimshot

Blue Strat, where do you usually set your NP tubes at? 60? Out of curiosity, what would set them at if they were NOS?
 






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