Quick Output Jack Tune-up On Martin X Custom

Near the top of the heap of all acoustic guitars available today (yes, I know, I’m a Gibson Guy) is the Martin X Custom.

This beautiful specimen had a loose output jack and was suffering from a low battery.  Let’s take a look!

The gang at Jealous Creatures use this guitar for songwriting and performing.  Check their Website at www.jealouscreatures.com  !

The output jack masquerades as a strap pin.  The loose nut is underneath this trim bushing.  Here we go!

The strings need to come off so that we can get inside the guitar.  While we’re there, we will replace the battery.

With the trim ring out of the way, we can see the actual nut that needs to be tightened.  However, there is another nut inside the guitar.  I want to pull the jack and readjust the number of exposed threads so that everything is secure.

If the inside of the guitar is to the left, this is the washer stack-up.  The inside nut is still threaded on the jack.

Here is a fresh commercial/industrial battery, placed inside a Velcro bag.  This sticks inside the body of the guitar where another piece of Velcro is fastened.  Note that we need to do a little ‘lead dress’ to get all the wires out of sight!

The piezo pickup lives under the saddle, inside the copper braid that you ‘might’ see down in the bottom of the slot.

Re-assembled, squared away, restrung, and tuned to pitch. This is a beautiful guitar and sounds great!

Speaking of sounding great, check Jealous Creatures at their Web site and on Facebook !


Contact: David Latchaw  EE


Marshall MG15CD Input Jack Fix

Someone had attempted to repair this little practice amp, and now it was still not fixed and there were parts missing to boot.  The Unbrokenstring Crew To The Rescue!

Finally, something that is not made in China!  I’ve worked in South Korea in loved it.  Wonderful people.

The input jack is supposed to be soldered to the circuit board, but the solder joints failed completely.

Strictly speaking, it’s poor practice to use electrical connections for mechanical attachments, but once we moved from point-to-point-wired amplifiers to circuit-board-centric amplifiers, jacks and potentiometers were suddenly used as mechanical mounting devices as well as electrical components.

So let’s start by removing all the old solder and cleaning out the holes.  You can see here that these are not plated through, which works in our favor.

The other half of the joint is the component leg.  Oops, these have failed in the mechanical aspect.

So let’s add our own new legs.  Prosthetic leads for input jacks?  Maybe someone will write a song about it…

We’re ready to go back into the circuit, with fresh copper leads which should work as well as the old component legs.

We need to be quick when soldering these new legs to the circuit board, or we’ll melt the work we’ve already done.

I think we’re ready to begin reassembly.  The circuit board is entirely supported by what you see here.

The screws that held the chassis to the top of the amp enclosure were missing.  We used some extra screws from the shop to replace the missing mounting screws.  New ‘Tinnerman’ nuts were located and pressed into service.  I think this is going to work!

I think Erika is happy with her repaired amplifier!


Thanks for reading all the way to the bottom.

David Latchaw  EE


Peavey ValveKing 100 Head Million Mile Checkup

This well-worn road warrior was back home and ready for a fresh set of new output tubes and an annual checkup.

With the front and rear grilles removed, we can get to the tubes.  Perhaps we would remove the chassis and clean off a year’s layer of foreign object debris from dusty roads and smoky honky-tonks, while we’re at it.

We pulled the output tubes and cleaned up the chassis.  This unit is still like new underneath the surface grime.

Click on the picture to get a hi-res close-up of the underside of this unit.  By The Way, if this picture is distorted, then you need to upgrade to a browser from the 21st century.  Just sayin’…

Here is a close-up of the top side of the circuit board where the output power tubes live.  Nothing to see here.

On the other side of the circuit board, we see that the sockets were hand-soldered in place at the factory.  The smokey areas are just the fumes from the solder rosin.  However, we will need to clean up the carbonized flux before it becomes conductive.  Solder flux does not conduct electricity, but the dust and moisture it attracts does.  And there’s hundreds of volts to be found here, so this contamination has to go before it becomes an issue in the future.

Here is a close-up of another hand-soldered socket.  This one is in a little better shape but will still be cleaned.  The solder bridge between the two pins in the foreground is supposed to be there, but I checked the schematic to make sure, just in case.  This amp was working before the re-tube, so there was very little chance that this solder bridge was ever an issue.

I am using an acid brush with the bristles cut almost all away as a stiff scrubber.  Straight rubbing alcohol is a safe solvent for this sort of clean-up.  An inexpensive source of lint-free, absorptive paper is a coffee filter, as you see here.  Using an absorptive wipe gives the ‘junk’ some place to go besides somewhere else on your circuit board.

The tube socket site to the right is good-to-go.  Three more and we’re done here.

A fresh 12AX7 will perform phase splitter duties.

These old soldiers were all barely functional, suffering from a build-up of gas.  The interior vacuum inside an operational tube is not perfect.  Most of the oxygen and water are captured by the silver ‘getter’ seen at the top of these tubes.  Over time, heat will drive the moisture and some oxygen out of the getter region and back into the interior of the tube, where it interferes with the electron beam between plate and cathode.  Time for a well-deserved retirement!

More porn for all you tube nerds,  Here is a matched quartet of 6L6 tubes.  Ruby brand was the OEM choice for Peavey amplifiers from this era.

Oops.  I did it again.  Guitar Tech Porn!

Almost there!

We’re warming up, using the purely resistive loads which you see on top of the cabinet.  An oscilloscope samples the output from the amplifier, and a sinewave generator is connected to the input jack.  A total harmonic distortion meter (THD meter) is paralleled across the resistive load and monitored separately.

All crossover distortion is gone.  With four output tubes, the bias check is a little tricky.  There was a little bit more tweaking done after this picture was taken, in order to get the middle a little fatter and the total THD on the clean channel as low as I could get it.

Here’s the back side view of a cool-running amp after it’s million mile checkup.  We’re ready for another million miles!

Thanks for reading all the way to the bottom!

Contact information : David Latchaw EE