'74 Bassman 10 Repair, with photos

pbagley · #1 03-29-2011, 05:48 PM

A report on how I fixed my Bassman 10 (Baseman 50 watt tube head) at home with photos.

A little background - I've had the amp for 5 or 6 years, not sure. Bought it with only 2 speakers in it, and promptly installed 4 new 32 ohm Jensen Mod 10's. It quickly became my favorite amp, and I think I've used it on 30 to 40 small bar gigs (so far). Last July I had it to a two night gig. It was hot and humid. No problems on Friday, but Saturday night it was cracking and popping like crazy. At first I was concerned about vandalism since we had left the stage set up. Speakers looked OK, so I guessed it was a broken component inside the amp.

Life is busy, I have another amp, so the Bassman 10 has sat in the garage until last night.

If you are a tube amp tech, you may be guessing that the problem is a broken grid resistor on one of the power tubes. At least that is what I was guessing, based on finding that problem on a previous Bassman 50 I used to own. I put a Torres Bulletproof kit into that amp to fix the issue.

I was wrong.

Here is the Bassman 10 head, removed from the combo and propped up on top of a 1-15 cab.

I poked around inside the running amp using the all plastic sharpie to keep from touching anything live, trying to find a microphonic component that would produce the cracks and pops. Found that this point was noisey.

A cracked solder joint. A super easy fix. Not sure why a warm humid gig caused it to fail after so many below 0 load outs and bouncing around in the back of the car.

And after resoldering the joint the amp is quiet while playing.
I notice that this Bassman 50 looks a little different than the one I had years ago. Gone are most if the paper disc capacitors.

I’m guessing that this is why the amp has been much quieter than my previous 50 watt Bassman.

Questions:

1 - There is a cut wire here. Any idea what the wire used to be for?

2 - The ends of the grid resistors are a little noisy when I tap them. Nothing broken, and quiet while playing bass, but tapping with the sharpie caused some crackling... Is this normal? Maybe a dirty tube socket?

Thanks!

BassmanPaul · #2 03-30-2011, 11:13 AM

If you check the other end of the "cut" wire you will see it's soldered to the chassis. By wrapping this wire around the bundle of wiring it keeps the bundle in shape and retained AND acts like a shield. Quite normal.

Sorry wrong wire.

That one i would guess is another tap on the OPT. Can't really see where the other wire is going.

pbagley · #3 03-31-2011, 09:50 PM

Quote:

Originally Posted by BassmanPaul

If you check the other end of the "cut" wire you will see it's soldered to the chassis. By wrapping this wire around the bundle of wiring it keeps the bundle in shape and retained AND acts like a shield. Quite normal.

Sorry wrong wire.

That one i would guess is another tap on the OPT. Can't really see where the other wire is going.

You are correct. The uncut wire coming out of that hole is soldered to the chassis. I was wondering about the cut wire, disappearing down into that hole. I guess I'll have to pull the cover off the filter caps one of these days... Of course my definition of one of these days is probably in 2012 or so.

Thanks!

- Paul