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
281-636-8626
