My Beer Has A Head – B52 AT100 Head

Chris of Shadow Council told me that his amp head just didn’t sound right after he spilled some “moisture” on top of it. Could the Unbrokenstring Crew take a look and return this amp head to its former metal glory?

Our patient works OK but sounds weak and distorted. Distorted is part of the program, but weak is not.

I think the warranty is out by now, don’t you think?  But I still have these tags on my seat cushions that say “Do not remove under penalty of law.”

A perfect sine wave goes in, and this comes out of the clean channel. We have an issue at or just before the phase inverter circuit.

Let’s look around inside the amp.  At this end of the front panel, we have the power switches and pilot light.

From left to right are the master reverb and master volume controls, and the controls for the CLEAN channel.

The yellow rectangular parts are relays for channel and mode switching.

There are a lot of controls for the OVERDRIVE channel.

I really like the separate circuit board for the input jacks.  This makes servicing the jacks very easy.

Shifting our attention to the rear of the chassis, we see the power amp circuit board at the bottom of the picture.  At the top of the picture is the effects loop send/return jacks and level controls.

At the top of this picture is the rest of of the effects send/return jacks circuit board, with level controls and the foot switch jacks at this end.

The circuit assembly on the left is a ‘Line Out’ jack with its own level control.  The cluster of wires in the middle go to a loudspeaker cabinet impedance selector switch.

Another nice feature is that the output jacks are on their own circuit board, making service easy.  Can you spot the overheated resistor in this picture or in the previous picture?

On the left is the Rectifier Selector switch.  A half-wave tube rectifier circuit, a full wave tube rectifier circuit, or a solid state rectifier circuit can be selected here.  The AC line cord is on the right.

Note that the black plug for the reverb tank goes to the red jack, and the red plug goes in the white jack.  Think about it.

The chassis is out of the case.  Let’s do some tests!

The final amp circuit board has been pivoted up out of the way in order to check circuit wave forms.  Fortunately, the tubes don’t need to be ALL the way in the socket in order to do some tests.  This tube had a bent pin, and I didn’t force it into the socket.  Easy to fix with the power off.

The problem is somewhere around the third socket from the left.  I wonder what’s going on with the contamination?  Beer?  Whiskey?  Probably not vodka.

A pink rag is catching the junk floated off the circuit board with this pressurized circuit board de-fluxer product.

A vacuum tube voltmeter showed that portions of the high impedance circuits around this tube were upset by gunk under the tube socket.

The socket has plenty of gunk of its own and will get cleaned up with solvent and a brush.

Here we see the circuit board underneath the socket.  Some of the residue is conductive and has upset the circuit operating voltages.  But look to the left at the base of the next socket.  See anything?

This socket has been removed as well.  Can you see it?

The Xacto knife points to a trace in the circuit board that has disappeared.

The entire board was scrubbed, sockets were cleaned, and reassembled to the circuit board.  The blue wire bypasses the open-circuit section of the burned printed circuit board we spotted earlier.  This section of the tube amplifies a LOT better when the plate terminal is hooked up to the following stage.

A pair of low value resistors serve as fuses in the high voltage plate circuit.  Component RX is toast.

While we have the amp apart, we can fix the loose knob cover on the “Rectifier Select” switch.

Old glue residue was removed from these parts.  Super Glue is just the ticket for this sort of thing.

If you don’t get it on your fingers, the Krazy Glue works well.  Don’t ask me about the fingers, please.

Now that everything is working, a protective coating will be sprayed over the top of the circuit board to keep ‘moisture’ away from the live circuits.  Each tube socket (and anything else we don’t want coated) is masked off.

This circuit assembly is masked and ready for coating.

Today we will use this commercial/industrial silicone conformal coating.

The spray pattern looks about like this.  I think we’re ready to go.

The circuit board gets four coats.  Each coat is sprayed from a different direction (top, bottom, left, right) to build up a uniform protective coating.  The coating ‘conforms’ to the shape of the circuit board, thus the name “conformal coat.”

I let this cure for a couple of days.  Now bring on whatever “moisture” you want to spill into the top of the cabinet!!

As you can see here, we’re peeling off the masking tape.

Red on white, black on red…  or something like that.

The tube base clamps were cleaned up, ever-so-gently bent back into position, and installed.  These machine screws hold the circuit board to the bottom of the chassis.

All the wiring is reinstalled where it came off.  I take a lot of pictures because my memory may not be perfect.

Just about done with reassembly.

The tubes were in excellent electrical condition. They all go through the tube pin straightener before they are re-installed.

You saw these jacks in an earlier picture.  I took the opportunity to clean and lubricate all the jacks while the amp was apart.

Ah, that’s more like it.  This is a 400Hz sine wave amplified to about 110 watts into a non-inductive high-power resistor.  The bias current to establish Class AB1 operation is just about perfect, judging by the zero-crossing points on this waveform.  Attached to any decent cabinet, this amp would be melting our teeth at this power level.

Support this local band!

Thanks for reading all the way to the end!

Contact – David Latchaw EE
Cell – 281-636-8626