This wonderful old Selmer-era Ampeg bass head was pulled out of its retirement in the closet and put back into service. But it had a few issues to address, so that it could reliably pump out the tones that is the Ampeg Experience.
This unit appears to be absolutely factory stock. The Houston humidity has had an effect on the aluminum faceplate.
Taking a tour of the rear panel, we see a bracket upon which the line cord may be wrapped.
The convenience outlet is a three-prong unit, which is nice. The hum balance control adjusts the bias current in the output tubes to be the same.
Magnavox owned both Selmer and Ampeg for a while, if I recall correctly. Note the tube layout information.
Here is the other bracket for the cord, and the output jacks and impedance switching.
Dude, are you still smoking?
This is a nice intersection between hand wiring and the use of an etched circuit board.
This cap and the bleeder resistors are slated for replacement.
Yes, you can still get multi-section capacitors if you shop diligently.
The prongs of the new capacitor need to fit in the slots in the chassis.
There is plenty of height inside the chassis, but it doesn’t hurt to document what we have.
Likewise, we’re documenting what we have.
Here is the new multi-sectioned filter capacitor and the hole where it goes in the background.
The outer can of all of the capacitors is isolated from the chassis, so these green fiberglass spacers are used under the capacitor.
I think we’re done here!
The new cap looks nice on the top of the chassis.
The axial filter capacitor will be replaced with this part. I am forming the leads to appear in a manner similar to the original part, seen above. A little Teflon insulating tubing helps keep the electricity under control.
Wires will be attached to the terminals, so the leads are formed into a loop to accept the wires.
The wire bending is done with a hand-tool called “chain nose pliers.”
The original part has a mounting ring around it. We will need to recycle this mounting scheme to maintain originality.
The ring is off! I was a little concerned that I would mess it up, but a little heat was all it took.
Here is the original mounting ring applied to the new capacitor.
The ring can slide around just a little bit to give us a nice-looking mounting solution.
And here we are, all wired up and ready to go back to work.
These guys have been replaced recently, and they check out as new. So, they will remain in service.
Some of the front panel slide switches were dirty, so some cleaner and lubricant were sprayed into them.
Now that the caps are changed out, let’s look at the top of the chassis. The output transformer and output tubes are on the left side of the chassis.
The preamp tubes and power transformer are at the right end of the chassis.
The open areas around the tube sockets are a nice touch. The chassis is steel and very stiff, even with the relief.
More low and medium voltage goodness at the other end of the slot. Nearly every schematic test point is accessible from the top of the unit without turning it over on the bench.
This 6K11 Compactron tube tests very good, with each of the sections closely matched to the others. Good News!
The input jacks were corroded, so these were changed out with new Amphenol units.
Here is the inside-the-chassis view of the new jacks.
The neon indicator for the AC power was functional, but the indicator for the high voltage was not.
So, this neon indicator will take its place. The mounting hole is the same size, but the new part is chrome. What to do?
We scrubbed the chrome ring with steel wool, then applied several coats of black polyurethane paint to the bezel.
Here are both indicators. The high voltage is amber, and the AC indicator is red, as it was when the amp came from the factory. The colors are a bit messed-up because of the jpeg processing in the camera… looks good in Real Life!
Another happy customer picks up his finished bass head!
Thanks for reading all the way to the end!
CONTACT – David Latchaw EE
281-636-8626
Robin’s 50 watt combo was serviced earlier, in this blog post: https://www.unbrokenstring.com/crate-vintage-club-50-amp-repair/
This is a 30 watt unit, with very similar construction. Look at all that St. Louis Music goodness!
The loudspeaker is 100% and made in U.S.A.
This pic just documents where the wires go.
These wires go to the reverb tank. They are marked with a magic marker so that they can be correctly re-installed.
The tubes need to come out as we are removing the printed circuit board from the chassis.
These numbers correspond to the tube numbers on the schematic.
While we’re here, we’ll make a check of the condition of each of the tubes.
Restoring the washer stack is essential to keep the strains on the printed circuit board to a minimum when reassembled.
So we are now able to remove the main circuit board.
As we did with the 50 watt unit, we are taking this opportunity to clean up the front panel.
Sure enough, the capacitors have reached End Of Life.
This circuit board is now recapped! Pretty!
After reassembly, this quick check shows us that we can drive 30 watts continuously into eight ohms with no problem!
This unit has an easy bias setting arrangement. Here, we’re using the 4 1/2 digit Fluke meter to measure current. The AC power is supplied by the Variac on the shelf.
Some of the cabinet screws were cross-threaded at one time in the past. So, we can chase them with a die.
The captive nuts are cleaned up with a matching tap. We’re good for Final Assembly!
Another fine combo amp is ready for the next million miles!
