The more this amp was played, the stranger it sounded and the worse the noise from the speaker became. What’s up with that? The Unbrokenstring Crew to the rescue!
A quick walk-around the amp revealed a nice nameplate with ‘just the right amount’ of road wear and mojo. See the mojo?
Removing the covers reveals a straight-ahead circuit board layout, with the tube sockets on another circuit board. The input jacks had been replaced along with a few other components.
Removing the knobs and potentiometer nuts allows us to pivot the main circuit board downward so that we can work on it. The first order of business is to look for any workmanship issues, because this amp was built during the time when the factory transitioned from an assembly process using conventional solder to one using lead-free solder.
This is not factory soldering, but some later repair work. This solder joint will get reworked.
Flexing the circuit board in the vicinity of this ribbon cable results in a terrible racket from the loudspeaker. Something’s wrong here!
Following the cable, we found an unsoldered joint on one of the tube sockets. This is a factory workmanship problem.
After the missing solder joint was restored, the amp was still noisy. I captured some of the noise waveform on the oscilloscope display here.
Further troubleshooting revealed that the tone caps were noisy. Here are some new ones, right from Mouser. The Orange Drop capacitors on the left are the correct value. The darker capacitors on the right are epoxy dipped silver mica, very stable and will withstand the full working plate voltage in that part of the circuit.
C7 is the capacitor associated with the treble control.
C6 is part of the mid-range tone control. C5 is part of the bass tone control.
C18 is in series with the volume control, was noisy, was the wrong value, and is now replaced. This amp is VERY well behaved now.
Jacob jacks the Epi Les Paul into the amp to run it through its paces. This is an amazing combo amplifier!
Jeff picks up his amp wearing his special T-shirt. Only musicians will get it.
Thanks for reading all the way to the bottom!
CONTACT – David Latchaw EE