The call came in from an individual who had ‘this old guitar amp’ that didn’t work. They wondered if The Unbroken String Crew could get it playing for the grandchildren. I was not ready for what I saw when they dropped it off.
This Fender Bassman was absolutely original. I could not tell that anyone had ever worked on it except for a couple of new tubes.
Everything was original, with some signs of wear and tear as you might expect to find after fifty years. Only the speaker cable was aftermarket.
Nothing came on when power was applied. This may be Pandora’s Box.
Let’s take a look inside. Not many people have ventured to this point in guitar amplifier space and time. Come join me as we go exploring!
The tube chart is obviously original, with the serial number hand-stamped as you see here. WOW!
Let’s take a look at that fuse.
What do we have here? I’ve not seen a fuse like this since I left Oklahoma.
This is vintage Wrigley’s Chewing Gum aluminum foil, wrapped around a stick. One end of the aluminum has burned away. Mojo Toan… NOT!
The original two-wire line cord has been VERY hot. This line cord will be replaced with a three wire IEC cord set to bring this amp up to code, as it were.
This is a good reason to have a fuse in your AC power. This penny was rolling around inside the amp chassis. Don’t you just love grandkids?
Let’s take a chassis tour and enjoy a bit of history. Here is the power transformer, power tubes, a bias balance potentiometer (more on this later) and a separate turret board that holds the solid state rectifiers and a separate power supply for tube bias. The pilot light is visible in the upper left hand corner of this picture.
On this end of the chassis, we see the preamp tube sockets and the entire turret board. The brown paper capacitors are dual section capacitors, as they have three legs. These are over fifty years old, but will be changed out on general principle.
Here is a closer look at the solid state rectifiers, bias power supply, and pilot light.
These front panel controls are the bass, treble, and volume for the normal channel, and a slide switch labelled “BRIGHT.” The control mounted on the pan of the chassis is a ‘bias balance’ control, which is set for minimum hum. No, this does not adjust the bias per se. We will adjust this after the new tubes are installed.
The two normal input jacks are in the center of the pic. This is a good shot of the traditional Fender brass grounding strip found behind the control panel.
These are the controls for the bass channel, the DEEP slide switch, and the bass input jacks.
Not much on the rear panel behind the preamp tubes, but you can take a good look here at the nice workmanship and component quality shown throughout this amp.
From this view, you can see the output jacks, the power switch, and the standby switch.
To the right of the fuse holder, you see the ground switch and a convenience outlet. The ground switch will be problematic with the three-wire-cord conversion, because only the green wire ground in the power cord can go to the chassis, whereas with the two prong cord system, the AC hot wire can be connected to the chassis through this switch and a single capacitor. If the capacitor fails, people die from electrocution. I’ll show you a solution if you keep reading.
To remove the old power cord, the strain relief grommet is compressed with some heavy pliers.
We will keep this grommet for reuse with the new IEC cord set.
The old cord gets now gets cut away from the convenience outlet.
To simplify rewiring the AC circuit, I’ll pull the convenience outlet.
The wire used in this amp is bare wire, with a yellow ‘push-back’ insulation. I’m saving all the original wiring and will reuse it in the new AC circuit.
Now I’m removing the ground switch, to facilitate rewiring it into a ground-lift switch.
Everything is out where I can work on it.
The black wire is the ‘hot’ lead to the power transformer and the white wire is reserved for the neutral leg. Can you see the error? Black wires should be on brass screws. In the early days of residential AC power, either prong of the power cord could be hot. But in this century, the brass screw has been reserved for hot and the silver screw has been reserved for neutral. This is easy enough to correct.
The Fender Death Cap is connected to the chassis here. It needs to come out, because if it shorts, the chassis becomes electrically hot and WILL electrocute anyone touching it or connected to it (read: guitarist.) Not Cool!
The Fender Death Cap is hidden down in the wiring. Out it comes!
This is a better view of how it is attached directly to the chassis. I would have just cut the wire, but I want to reuse this hole and connection point for the green wire ground connection.
You don’t see this sort of thing anymore! Not made is USA, but made in the U.S. ‘Cuz this is ‘Murica!
Now, we are ready for the new line cord, sporting the recycled strain relief grommet. We’re about done here.
If you were sharp-eyed, you probably noticed the cathode bias resistors attached to the bottom of the tube sockets. Here is a good view.
Do you see the cooked resistor on the socket of the tube? That was the cathode bias resistor. Somewhere along the line, a tube failed and overheated this resistor.
The burned resistor measured about two hundred ohms. This really upsets the operating point of whatever tube works out of this socket. Tough duty!
I will change out both parts so that both circuits can stay matched. A pair of new resistors will have the same initial value, will behave the same over temperature, and will age in similar manner.
These new flame-proof resistors are considerably tougher than the carbon composition resistors they replace.
The high voltage filter capacitors are located under this pan.
The outer cardboard tubes have been rotated to verify the values and voltages of each component.
The capacitor on the left has vented. The bulges on the other capacitors show that they are near end of life.
They are all ready to pop and make a mess.
These are German-made high temperature, long-life capacitors. I consider these to be equal or better than the originals.
With the new caps installed, a power-up test shows a healthy 279 volts (unloaded) for the tube plates.
I think our work here is done. The external appearance of the amp is not altered, so substituting those after-market high voltage capacitors will be our little secret, OK?
One last repair… Every control is scratchy. Those little slots you see on these controls is (1) an opportunity for dirt to get inside the control, and (2) an access point to squirt some tuner cleaner. After squirting some cleaner in these controls, they were still scratchy. I don’t want to change out the controls because the new stuff is not nearly as good as these 50 year old parts, and I don’t want to diminish the value of this amp. Time for surgery!
Each control was de-soldered, removed from the front panel, and disassembled. This old stuff is service-able, unlike the mass-produced controls built by the ChiComs today.
The shaft is grounded to the mounting bushing, and the back cover is grounded to the chassis because the shaft rubs on a spot in the middle of the back, as shown. Old grease and oxidation has been removed, and a tiny drop of synthetic lubricant will be smeared at this point of the rear cover.
The shaft jammed inside the bushing. A little judicious filing burnished the shaft where it had been scarred by the knob set screws.
Here again, we find fifty years of oxidized grease and dirt.
You can see several sliding contact points between the actual resistive element and the outside world. On the right, the dark ring is the actual resistor. The thin copper ring in the center forms a connection between the resistor slider that floats over the resistive element and the middle (wiper) terminal of the potentiometer assembly. On the right, we see the brass slider, fastened to the shaft and knob. All the metal components are now burnished. The resistive element in this style of potentiometer is fairly tough, so it will get a burnishing with a cotton swab. DO NOT ATTEMPT to touch the resistive element in any modern potentiometers, as they are thin films and irreparably damaged by the smallest unseen scratch or sleek. You Have Been Warned.
Everything gets reassembled with synthetic electronic spray lubricant and reassembled. Here, we’re making a quick check of the operation of the control before it gets reinstalled (and we go on to the next one…)
Each control is reconditioned in sequence. This is going to be a long night!
Earlier, I showed pictures of a ‘bias balance’ control. This control does not set the bias point of both tubes, but rather balances the idling current through both tubes. The factory adjustment literally says, “Adjust the control for minimum hum.” And it works. For the Tubeheads out there, the minimum hum point in this particular amp leaves each tube idling at 35.2mA of plate current. This is also why the cathode resistors need to be matched. But the tubes characteristics can vary a bit, and the balance control allows the tech to compensate for that variability. See my tube bias jig under one of the output tubes?
Before we reassemble this guy, let’s take one last look at the definition of hand-crafted 1960’s American electronics. Just gorgeous!
Not only will the grandkids have access to a nice amp, but this bit of history will hold its value for many years to come. Thanks for the opportunity to refurbish this wonderful bit of history.
And thank you for reading all the way to the end!
CONTACT – David Latchaw EE
281-636-8626
