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!
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