For over two decades, guitarists who take a hands-on approach to customizing and modding their amps have looked to Mojotone, one of industry’s leading guitar amp parts and supply companies. With a wide range of electronic parts, hardware, speakers, cabinets and just about anything else one would need for a guitar amp repair, restoration or customization project. In this interview, Mojotone founder Michael McWhorter offers his insight as your guide on repairing and restoring vintage amps, upgrading speakers, output transformers and other guitar amp parts can have a big impact on tone and aesthetics.
How would you recommend guitarists approach a DIY vintage guitar amp restoration project? How should they choose cabinets, electronic parts, speakers and hardware that may need to be repaired or replaced?
Great question. Most vintage amps are worth keeping as original as possible in both looks and sound. The best approach, when choosing replacement components of any kind, is to try getting as close as possible to the original component. If the original manufacturer is no longer around, and a lot of them aren’t, try sourcing parts that are made using the same materials.
There are a lot of people out there who wonder why you would replace a resistor, for example, with an inferior carbon composition resistor when metal film and metal oxide are so much more precise. Or, why use cloth covered wire when PVC coated wire is the standard in modern electronics.
What makes a vintage amp sound the way it does is really just a sum of all of it’s parts…including the looser tolerances of 50-year old components.
What about guitarists who want to upgrade their guitar amp, which may be a reissue or project amp of sorts? How should they evaluate and research available amp parts (speakers, output transformers, capacitors and other electronics) in order to change their amplifier’s tone to be closer to what they are looking for?
Speakers are the easiest and quickest place to start. They will give you the most bang for the buck. I would say that the speaker can affect about 30-40% of your amp’s sound. The output transformer would be next in line with maybe a change of up to 20%. Smaller electronics like capacitors and resistors can certainly have an effect but probably more things that only the player is going to notice in responsiveness and dynamics.
Let’s talk speakers. What types of speakers would you recommend for popular guitar amp models and musical styles? What do they need to be aware of when replacing speaker baffles in order to change to a different sized speaker?
There are a ton of speaker options on the market today that can make finding the right speaker a confusing process. Not only are there 50+ varieties from Celestion and Eminence, but smaller boutique manufacturers are also becoming increasingly relevant. There are also a number of amp companies who have jumped into the speaker game and contracted out to some of these smaller manufacturers to have their own speaker made.
When we talk to customers that are interested in changing speakers we first try and figure out the kind of sound they are going for as well as what amp and guitar they are using.
The new Creamback by Celestion is a really nice speaker that does well in British sounding 1×12 combos–we chose this speaker for the latest Alex Lifeson Lerxst Chi combo amp. Weber continues to make solid American-voiced speakers that we like to recommend for our reproduction Tweed amp kits. And of course you can’t discount the breadth of offerings from Eminence. They do a great job of continually trying new things instead of recycling speakers from the past.
Cabinets can have a big impact on tone as well. How should guitarists consider their options with combo and speaker cabinet sizes, woods, shapes, etc? How do they affect tone?
We mainly work with two different wood types, pine and Baltic birch. Pine was historically used for all the Tweed and Blackface cabinets and birch was used for all the original Marshall cabinets.
Pine is a lot more resonant than birch. This tends to come across with lower frequencies being more prevalent. Birch cabinets have the opposite effect with a more noticeable effect in the higher frequencies.
Upgrading a particle board cabinet to a solid pine cabinet can have a huge impact on the sound of your amp. We’ve made thousands of cabinets over the years for players that have 70’s era Fenders as well as current production Blues Jr’s.
Many guitarists also upgrade their guitar amp’s output transformer either in addition to the speaker, or as an alternative amp mod. When should guitarists consider output transformer upgrades and how should they evaluate their options?
My experience has been that most older amps’ output transformers have not held up that well after 40 years of real use. The evidence of this is that almost any transformer swap at that point brings real life back into the amp with better bottom end and more top end clarity. Occasionally this reveals a harshness in the amp or can overpower an already compromised original speaker; both are things that should be considered before making this type of modification.
Because an end user is unlikely to make several transformer swaps in the life of their amp, it certainly makes sense to use someone who has been selling replacements for vintage amps for years. In all this time, we had the added benefits of both revising our transformer line as well as gaining invaluable experience with these components. I am proud of the fact that Mojotone has been helping people for over 20 years with these modifications.
We can’t forget tubes. Replacing guitar amp tubes is one of the most common ways to customize guitar amp tone. How would you suggest guitarists choose from the various preamp tubes (12ax7, 12ay7, 12au7, 12at7, etc.) and power tubes (6V6, 6L6, EL34, EL84, KT66, etc.)? Any particular tonal nuances and tube placement tricks that guitarists should consider? What about installation and biasing?
There are so many tube options out there, and with these options comes a multitude of things to consider. Even things like EL84s, in general, not working well with highly efficient output transformers. Now, none of these “things” are rules, just possibilities.
I think the most important thing someone can learn when they are messing around with tube sets is that, only on a very rare occasion will two people have the same experience. A particular 12AX7 may have made your friend’s amp sound more bright and clear, where that same 12AX7 could inadvertently take away from your amp’s most beloved qualities.
Tubes can have different sonic effects in different amp circuits so the best thing to do is to experiment. Try some different brands and alternatives (ie. 12AU7 in place of a 12AX7) and see what effect this has on your rig.
What about capacitors and resistors? For guitarists who are good with a soldering iron, how should they choose from the various options available to them? What safety considerations do they need to take into account when replacing capacitors, in particular?
Well anyone that is playing around that deep down should already know that, when it comes to capacitors, the voltage rating is the most critical consideration; for resistors, it’s the wattage. If that is confusing at all you should really be working with someone else until you have a bit more understanding.
When you start experimenting with different types and brands, you have reached the most esoteric level of tweaking…except for grounding and layout.
For resistors, you can certainly look at things like being flameproof as a benefit that you don’t get from the vintage CCs. I think I personally have a more pronounced feeling about capacitors after cutting them open and looking at construction rather than from listening, but I wouldn’t tell anyone that capacitors don’t make a sound difference.
Let’s put tone aside and dig into aesthetics. Tolex and grill cloths are some of the most common upgrades and modifications that guitarists can make to their amps. How would you suggest guitarists evaluate their options when recovering or repairing tolex and grill cloth?
Recovering your amp is a really easy thing to do and can make your amp look new again. It’s also a great way to customize your look. We sell over 60 different types of tolex and grill cloth, so you can really go in any direction you want. Of course, if you don’t feel like getting your hands dirty, we offer professional economical recovering services.
What other guitar amp hardware and aesthetic upgrades can guitarists consider to personalize their amp and give it a custom or cleaner look?
Customized faceplates are a cool thing to do to change up your amp. We offer stock replacements or custom options for faceplates with different colors, different text, and even the option to add your own logo. We can also make name plates or band names/logos to add to your amp.
You really don’t have to be stuck with stock anything when it comes to your tone or the look of your amp. No two players are exactly the same, so why shouldn’t they be able to look unique?
In order to ensure the job gets done right, what tools will guitarists need to ensure that they properly repair or restore their guitar amp and do the job like a professional (or close to it)?
After that, you need a way to generate a signal. Most often this is a guitar, but could be (and maybe should be) a signal generator or even a cd player. Using this with a blocking cap (.1/600v) will allow you to put a signal into the amp at any point.
Then, in order to properly inject and recover signals from the amp, you will need a preamp of some sort and a powered speaker. With that, you can inject the signal at various levels or tap into the the signal path and listen through the amp to hear what the signal is doing at that point. These tools allow you to evaluate the signal path.
Then you need tools for measuring voltages. These help evaluate the power supply and the bias points of each gain stage. A nice multimeter will get the job done. I have used lesser quality multimeters as well as really nice ones; for most amp jobs using a high end meter was nice, but not necessary. An oscilloscope might be considered indispensable by some but you can certainly figure out most issues without one.
Fill in the blank. “When all else fails, ________.”
Divide and conquer. This applies to troubleshooting problems with amps in our business, but you can work it into any kind of problem solving situation. For instance, if your amp is cutting out and it has an effects loop, try taking the signal out of the “send” and put it into another amp (or the signal from another amp) and into the return. Just that single test can divide your problem in half. If both halves fail then you can look to the power supply.
Any last advice for guitarists looking to do their own amp repair, restoration, mods and upgrades?
Its addictive, run for your life.