36w build popping fuses…

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36w build popping fuses…

Post by Billy_Goat »


I built a 36w amp a couple of years ago using the 36w information found here. After help sussing FX loop issues from the good folks here, the amp has been running without issue since.

Recently, I powered up, hit the standby switch… the pilot lamp flickered a few times and the amp went dead. Checked the fuse - open.

So, I popped the chassis out and first checked for any obvious problems - burned components or any other tell tale signs of damage. Nothing.

So, I pulled all the tubes, fired the unit up using a variac and current limiting bulb.

All came up as expected. Measurements all as expected.

Powered down, installed the rectifier tube. Again, all tested fine.

Powered down again, added the preamp tubes. Same as above. No issues.

Powered down and installed the power tubes. All normal.

Amp sounded and worked normally. So, I cleaned the tube sockets and put the amp back together. One of the power tubes was not seated fully when I first inspected so I thought maybe it was loose and that was the cause for the issue.

Next power up, all normal. Played for the better part of an hour.

Next power up, blown fuse again. Same pilot light flicker, pop!

I pulled the power tubes, and tested the amp off/on. On for a time, then standby on. Did this for over an hour. No problems.

Assuming maybe a failing power tube, I swapped in two known working tubes. Powered on, waited for a minute, then turned on the standby. Pop!

My question here is what would be the best path to troubleshoot this as whatever is amiss is intermittent. Am I right to assume this issue is localized to something amiss in the output section? Loose or broken connections or output transformer issue? If output transformer, how would one best test and confirm?

Thanks for responses in advance!

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Re: 36w build popping fuses…

Post by JMPGuitars »

Step 1:
With the amp unplugged, and the tubes pulled, inspect the amp thoroughly with a magnifying glass. And/or take some really good, clear, high-resolution photos of the build from many angles. Look for any possible issues including frayed wires, bad solder joints, loosened bolts, etc. Also make sure all bolts, especially grounds, are nice and tight.
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