High Desert Sonic Institute

Mod Blog

Follow the process of converting a 1950’s Bell and Howell Filmosound 179 tube amp into a gnarly guitar amp, with pictures and detailed descriptions.

Hot Rod Deluxe Rebuild

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Fender’s Hot Rod series of amps brought classic Fender tone into the age of PCBs, and tho they sound great they typically don’t last as long as the vintage gear Fender made their name with. Leo Fender had a rule about designing and building: every component should be overrated by %20. This mentality permeated everything Fender made for the first decade or so of the company’s production. That’s why 60 year old Fender amps are still around today. Nowadays, these amps utilize similar circuits that still have that classic Fender sound, but are executed with PCBs and other components that fall short of Leo’s %20 rule. So, when they do fail, I like to rebuild them utilizing the same techniques Leo would’ve done back in 1962. This particular amplifier was brought in to the Institute for resurrection by Keegan of Jet Dread Stone, an awesome local band. You can check them out on Bandcamp or catch them around the high desert.

Here are a few shots of the original innards as they came to me. You can see the 470 ohm screen resistors have been replaced, the originals most likely burned out. This is fairly common in tube amps, especially old Fenders that have the screen resistors mounted right onto the tube sockets. They can pass enough current to develop some heat, so when you place them right onto a tube socket and add the heat of the tube itself, it’s only a matter of time before they fail. But here, we see that the current draw of those resistors was actually too much for the traces on the PCB itself! Someone has tried to remake the current pathway with wire leads and gobs of solder, but this is no way to run a tube amp and is prone to further failure.

You can also see that this amp has several silicon-based chips in it. These are there to provide extra gain, as well as for the reverb circuit. In rebuilding this amp, we’ll eliminate not only the problematic PCB but also all these solid-state additions that inhibit that natural tube tone. Talking the project over with the customer, we decided to go with a single-channel version of a classic AB763 circuit, a Blackface Fender Vibroverb, and modify it to utilize the existing controls on this amp. To match the circuit to the faceplate, I needed to add a Mids control as well as some sort of “Drive” control to the preamp, a master volume and presence control, and bright and boost switches. All of these additional controls are affective mods that can be added to any classic tube circuit, and they simply enhance the player’s control over the nuances of their tone. Bellow you’ll see the Fender schematic I based the new build off, and the modified schematic below it with the voltages I eventually measured from the finished amp (as well as a few post-build circuit changes) marked in red.

(Despite the note in the workbook, I didn’t “Try to wiggle just the dry” though I think it could produce interesting results!)

I placed the master volume just before the Phase Inverter in a pretty standard spot. To achieve the Drive control function, I placed a 100K ohm pot in between the tone stack and ground, a modification often referred to as a “Raw” control. It effectively removes the tone stack from the signal path, creating a full frequency boost that goes beyond simply turning up the tone controls to maximum. As the Drive knob is increased, the effectiveness of the tone controls diminishes, but the amp gets grittier and more touch sensitive with more preamp overdrive.

There were several minor modifications beyond the added controls that were for either tonal or stability considerations. For instance, the player mentioned they wanted a very clean amplifier, so I removed the cathode bypass capacitor on the second gain stage. Though it isn’t illustrated in my schematic, I did install a boost switch that introduces the standard Fender 25mF/25V cathode bypass cap at this point in the circuit for a full-frequency boost. Similarly a very standard bright switch was added around the first volume control. I also separated the cathodes of the 3rd gain stage and the reverb recovery stage for stability concerns, and used a smaller cathode bypass cap on the 3rd gain stage to keep the low end tight. You may also notice a beefed up power supply for a quieter amp with tighter response. In part due to the added power filtering, this amp is dead quiet! I actually thought something was wrong when I first powered it up it was so quiet!

Once the circuit is decided upon, the next step is to gut the PCBs and existing components and fit the tube sockets and transformers. In addition to those classic Fender cleans, Keegan wanted tube-driven reverb and tremolo, which required adding two more tubes and another transformer that the chassis didn’t have already in it’s layout. I located the additional reverb driver tube down amidst the existing preamp tubes, but tucked the tremolo tube up near the power transformer. Since this is simply an oscillator and a buffer, it’s not as susceptible to adding electrical interference into the signal path and this ended up being the most convenient spot for the tube. It’s right up next to the Speed and Intensity controls, which I ended up installing in the unused Preamp Out and Power Amp In jack spots. Here you can see the empty chassis with transformers, the filament run tightly twisted and the power section mostly wired up. Like laying down coats of Gesso onto a canvas, proper power supply and filament wiring is the foundational work that allows the true artistry of amplifier design to flourish.

You can see in the photos above the first power supply node mounted on terminal strips between the power transformer and the rectifier tube socket. Below is a close up of the tremolo circuit, with the phase network mounted in between the tube socket and the controls. Keeping components close to the other components they work with is a method derived from point-to-point style wiring that minimizes wire in the amp and helps cut down on unwanted noise. Every inch of wire you put in the amp is another inch of antenna ready to pick up electro-magnetic interference, and it’s also another small amount of resistance for your signal to pass through. While excellent amps can be packed full of wiring, minimizing wire length in the amp results in better signal-to-noise ratio (SNR) and better tone.

The preamp wiring is mounted on a turret board, and crucially I place the filter capacitors close to the components they power instead of using can caps. This allows for more efficient wiring, using less wire, and also allows for a star grounding system that helps prevent unwanted noise and ground loops. You’ll also see here lots of twisted pairs. Twisting certain sets of wires together can help minimize inducing signals into other nearby wiring. It’s good practice to twist the Cathode, Grid and Anode leads from the SAME TRIODE section together - not from different triodes tho.

Aaaaaaand here’s the completed electronics! We still need to swap out this speaker for a Weber 12F150 for sweet vintage Blackface Fender tones, but thus far this beauty plays great. It has a wide range of sounds available with all the controls, and as intended it can be extremely clean, clear and bright at gigging volumes. With the drive control up and the boost switch on, you can dial back the master volume and get a very dirty sound at even bedroom levels of volume. The tremolo and reverb have that classic Fender depth and smoothness. This puppy is ready for the dog show. Stay tuned for some clips and feedback from Keegan!

Jacob Erwin