Been busy building again. :)

[Dru Edwards]

Originally Posted by clintj

Found some leaky caps in the bias circuit, so I'm replacing them as soon as I have spares in hand. They're passing enough DC current to keep bias voltage from being strong enough, so I think the power tubes are going into overload and popping the fuse. Hopefully that's the only problem. I'm also making it a rule to never start a build or have an amp and not have a spare set of tubes in hand. Heck, that'll be a rule for all repair work, too.

I've heard of the term leaky caps before ... is it that the signal leaks or does the cap actually leak something else?

[clintj]

Caps are supposed to pass virtually no DC current - AC only. Leaky ones let DC pass, which pulls down the DC voltage I need for the power tubes to bias correctly.

[Dru Edwards]

Originally Posted by clintj

Caps are supposed to pass virtually no DC current - AC only. Leaky ones let DC pass, which pulls down the DC voltage I need for the power tubes to bias correctly.

Thanks Clint, my amp building knowledge builds a little each week

[moon]

I guess the oscilloscope is earning its keep.

[clintj]

It definitely is. That was money very well spent. Now I just need a big resistive load bank so I don't have to listen to a constant 1kHz tone.

[clintj]

Recapped the bias circuit, and replaced a preamp tube that died. Sounds good, but shows a little crossover distortion. Quick bias adjustment once the tubes cool off should fix that quickly, then she'll be ready to head home. Very fun amp with a lot of blues attitude. With a little hotter bias point for easier breakup, it'll sound a lot like Clapton with my Les Paul plugged in.

[Dru Edwards]

A Clapton Les Paul tone - that's gotta be sweet.

Your friend is going to be getting a great amp. I hope he appreciates the effort you put in.

[clintj]

Oh, he will. This particular model is "the one that got away" for him. He had an original JTM years ago, and sold it for way cheap because it was too much amp for most gigs. Now he has a detached outbuilding with a loft and no close neighbors so he can crank it up to his heart's desire. He just got the heads up to expect delivery tomorrow. Now to pull out my spare tube and pack it up for transport.