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Nuts & Volts Magazine (August 2017)

Easing Into Tube

By Bryan Bergeron    View In Digital Edition  


Want a break from microcontrollers and MEMS devices? Try tubes. They’re deceptively simple to work with, but — as with most things — take years to fully master. If you’re a guitarist or know of a guitarist who plays with tube amplifiers, then the easiest way to work with tubes is to perform a modification or mod. There are dozens of guitar amp mod shops on the web; some offering individual component upgrades and others offering extensive kits.

Mercury Magnetics is my go-to source for transformer and choke mods. They’re not the cheapest option on the Web, but I’ve modded four amps with their power, output, and choke upgrades with great success. By upgrade, I mean that the replacement transformer is beefier, has a higher quality core, more current carrying capacity, and — in general — results in a better sound. More affordable sources for tube amp magnetics are available on the Web, but cheap rarely means upgrade.

More invasive modding involves changing components in the amp — especially the power supply electrolytic filter capacitors, the capacitors in the primary signal path, and the tube bias circuitry. For power supply capacitors, the goal is usually to provide greater capacitance, lower internal resistance to AC, and increased longevity. This usually means going from a $3 capacitor to a $5 or $6 electrolytic capacitor. Of course, you could opt for a gold plated component with a super audiophile rating, but that would be a waste of $20.

Modding the capacitors in the signal chain of a tube amp is more subtle. Generally, the component values remain the same, but the type of capacitor is changed; for example, from a disc ceramic to a silver mica. The result is a change in tone that may or may not be better than the original. The decision about what capacitor technology to use for a given tone is simplified by vendors that sell basic mod kits and advertise a certain tone, such as “Fender Tweed” or “Bluesy” that they’re targeting.

I just finished modding my Fender Blues Junior amp with a capacitor kit from Fromel Electronics (www.fromelElectronics.com) and it was easy. This mod — together with swapping out a Mercury Magnetics for the output transformer — essentially gave me a new amplifier. A basic capacitor kit from Fromel or similar suppliers is about $35. I’ve also used a basic tube amp mod kit from http://billmaudio.com: a one-man shop specializing in Fender amps. The basic mod sells for about the same price as the Fromel kit.

Of course, you can also mod a tube amp by changing the speaker, installing a different brand of tubes, and by moving the entire assembly to a high quality cabinet made of real wood. (I’m just focusing on the circuitry mods here.)

The next step in a tube journey is a full tube amp kit. The most popular guitar amp kits are patterned after the Fender 5F1 and 5E3 amps. The 5F1 is a small 5W bedroom amp with a single output tube, and is usually paired with an 8” or 10” speaker. The 5E3 is a full practice amp, with 15W output to a 12” speaker. There are at least a half dozen domestic sources for these kits; my favorite being Boot Hill Amps (www.boothillamps.com). Expect to pay about $130 for the components, circuit board, and chassis for a 5F1 kit, and $200 for the 5E3. You’ll have to add magnetics, a speaker, and a cabinet. These three items can easily overshadow the basic amplifier kits in cost.

Even so, the 5F1 and 5E3 kits are simple to build. The schematics are easy to locate on the Web, and they sound great when completed. Moreover, most of the kits out there use the old school point-to-point wiring found in boutique amplifiers. I can’t hear a difference between an amp with components mounted on a printed circuit board vs. one with components dangling in space, but many players swear by the design.

Although I don’t have first-hand experience with full kits that contain everything from a cabinet, tubes, speaker, and components, this is usually more economical than cherry picking from several suppliers and then paying shipping fees for each group of components. I’ve heard good things in the forums about the 5E3 guitar amp kit from Tube Depot (www.tubedepot.com). At $600, however, the cost begins to overlap with current off-the-shelf Fender tube amps — not to mention amps on the secondary market.

When it comes to DIY electronics — whether tube or silicon-based — it’s impossible to compete on price alone. You’re paying for the experience and satisfaction of building it yourself. A complete 5F1 amp from Monoprice sells for $99, which is about $250 less than what you would pay for the parts in kit form. However, when it’s time to replace a blown capacitor or tube on the Monoprice, you won’t know where to begin to look. With a DIY tube amp, you know every wire, component, and solder joint.

Please consider sharing your tube journeys with others in the Reader Feedback section.  NV



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