Octal Tube Base Replacement Experiment

This fine old 6V6 tube tested good, but the base was mechanically loose from the glass envelope.  Will the Unbrokenstring Crew be able to salvage this fine old Made In U.S.A. specimen?  Let’s find out!

Let’s unsolder the internal wires from the tube base.  These foam pads will protect the tube from the jaws of my vise.

To get the wires back in the right pins, I added an index mark on the socket and on the glass envelope.  The permanent marker is not permanent on the glass nor on the bakelite base, so we need to be careful not to wipe it off.

Once the solder is removed from inside the pins of the tube base, the envelope is easily separated.  Here you can see the glass neck that allows the air to be removed from the inside of the glass envelope, at which point the glass is heated and the neck is sealed.  The silver inside the glass envelope is from flashing the ‘getter’ to remove the rest of the oxygen and water from the tube envelope.

The glue used ‘back in the day’ varied by manufacturer.  It consisted of partially organic, varnish, rosin, and ground glass as a filler.  The finish on Stradavarius violins was not much different!

This chemical soup is pretty good stuff, as you can see that it is still adhering to the glass after half a century.  Not many modern adhesives can attest to that sort of performance, particularly where the difference in thermal expansion of the adhered materials is so different.

In this picture, I have soldered temporary wires which shall serve as extensions of the leads of the tube.  This will permit each lead to be aligned with its pin while the tube socket is reassembled to the glass envelope.

This is a little better view of how the extensions work.  Each blue wire is threaded into the corresponding pin as the tube is partially reassembled.

From 1950 organic adhesive, rosin, and ground glass, we progress to the 21st century.  This high temp gasket maker is designed for operation to 600 degrees F and low electrical leakage, suitable for automotive sensors.

The plan is to ensure glass-to-socket coverage of red RTV, with some additional red RTV inside the tube socket for mechanical strength.  I don’t want to get a lot of red RTV around the delicate glass neck, either.

Now we are ready for the big Red Squish!

We should have good red RTV coverage now!  The mess can be cleaned up later.

I left a fillet of red RTV around the top of the tube base.  Does everything look straight to you?

Our victim is left to cure overnight in a yoga position, to assure that the red RTV is intimate with the chakra.

We’re back in the clamp again, this time to remove the blue wires and resolder the original leads inside the tube pins.

The extensions are 30AWG wire wrap wire;  so small that just a touch of the soldering iron is enough to remove them.

Each pin is partially filled with solder with enough heat to assure that the internal leads are well-secured.

The transconductance of this tube is identical to what was measured before the base was replaced.  No electrical leakage current was measured from isolated pin to isolated pin using a capacitor leakage checker of the Heathkit IT-28 Capacitor Checker.  Then I installed this tube in my Gibson GA-30 for a final test.  Sounds good!  Takeaway from all this:  Don’t pull on the glass envelope of a tube when removing it from a socket!  Pull on the tube base.

Thanks for reading all the way to the end!

Contact: David Latchaw  EE


University Sound Speaker Columns

I picked up a pair of these loudspeakers on eBay a few years ago.  I was intrigued by the line array concept, plus these were architectural loudspeakers that could be used out-of-doors if kept out of direct rainfall.  Just the thing for the sound at outdoor shows for my dancing group, The Rhythm Cloggers of Houston, Texas!  However, the foam behind the metal grille was shot.  New foam was ordered.  With the company Christmas party coming up, it was high time to get these loudspeaker cabinets squared away!

The grilles are held on at each end with these U-shaped aluminum corners.  Off they come.

Each end of each loudspeaker is identical.

Yuck!  The old foam had oxidized and crumbled away to nothing.  This won’t keep the rain out!

This is something that should NOT be done on the carpet.  I took these out to the shop because the concrete floor is easier to clean.

Removing the old foam from the grille is trivial.  However, a respirator is needed unless you want to cough for a month.

What attracted me to these line arrays is that the loudspeakers themselves are tilted in the same manner as a Fresnel lens focuses light, although the lens itself is physically flat.  The idea is to ‘focus’ sound in a horizontal plane, rather than let it spread up and down in a spherical manner in accordance with the inverse square law.

In a mobile DJ setup, I wanted to use Neutrik Speak-On connectors.  These screw terminals are fine for fixed, architectural applications, but were not what I wanted to gig with.

The internal wires are retained, and the old terminals are just snipped off.

New plates are fashioned from standard electrical junction box plates.  The hole for the Neutrik Speak-On is already drilled.

Here is a close-up of the new connector.  It’s really hard to confuse one of these with a quarter inch phone jack, which is a real positive when setting up in a hurry under less-than-ideal conditions.

The backsides of the grilles were sprayed with clear automotive headliner adhesive, then the new foam was smoothed into place.  The new foam was cut over-sized from a roll, so there was plenty of leftovers that required trimming.

A sharp Exacto knife works well to trim the foam.  This is the same foam I used in an earlier post to replace the wind screen in a Shure microphone.  This stuff is handy to have around!

While I had the units apart, I checked the wiring to learn a little bit more about how this line array works.

Here’s the new audio jack installed in the electrical junction box cover, now painted black.

The old wiring is now soldered to the jack from the back.  “Steel City” for heavy metal music, maybe?

A little heat shrink tubing adds physical support to the solder joint, as well as adding some electrical insulation.

The edges of the grilles are supported by some aluminum channel.  When we reinstall the U-shaped aluminum ends, we’re done here. Let’s try it out!

The University Sound Speaker Columns are hard at work behind the scenes at the Spectrum Scoreboards Christmas Party.  These ran all afternoon, driven by a one hundred watt powered mixer that you can see to the right of the fire extinguisher.

Thanks for reading all the way to the bottom!

Contact: David Latchaw EE

DigiTech Screamin’ Blues Pedal Refurb

Jim purchased this used pedal from a national retailer.  It worked at the store but not at home.  He took it back and the national retailer wanted him to buy a new pedal.  Could the Unbrokenstring Crew help?  Check It Out!

This pedal is absolutely mint;  No sign that it had ever been used.  We hooked it up and it was non-functional on internal battery power, but worked perfectly using an external pedal power source.  Well at least the electronics work!

Let’s put a new battery in it.  Oops, when we pulled the battery, this corrosion was all we found.

Everything is made in China, but could we at least use a name-brand battery that wasn’t dented, Mr. National Retailer?

We loosened up the nuts on the jacks and removed the case screws.  Fortunately, the electrolyte did not leak very far.

Here is a view of the top of the circuit board.  We can easily replace the battery clip leads and get this back on the air!

While we’re here, let’s indulge in a little Tech Porn.  The printed circuit assembly is a nice mix of modern surface mount parts and quality film capacitors, with some small electrolytic caps where it makes sense to use them.

The circuit side of the board is uninteresting unless you are into this sort of thing.  Those are some tiny vias!

The red and black wires on the left are the battery leads, which will be de-soldered.  The long potentiometer shafts reach all the way through the top of the unit and out where the user can reach the knobs.

Another view of the top-side printed circuit board assembly.  These battery leads are coming off now.

The battery leads are unsoldered.  This unit was built with high temperature lead-free solder, but poses no problems for rework such as we are doing here.

Meet the new battery clip.  The case of the pedal has been clam-shelled around the circuit board assembly and we’re installing the screws and connector hardware.

The nuts on the jacks are tightened by hand with a socket.  A layer of felt glued to the socket prevents damage to the finish of the pedal.

The knobs press on, no setscrews.

The foot pedal lid was reinstalled and the unit tested fine with the battery.  Moral of the story: Buy name-brand batteries and don’t leave them in the unit and forget about them!


Thanks for reading all the way to the end!

CONTACT: David Latchaw  EE