An Introduction to Amp Modding
The following guest article is by Joe Gillespie. Joe runs a great blog over at www.joesjamblog.com about all things Spider Jam (as in Line6). Joe has been building and modding amps for quite a while (check out his ‘Little Tweedie‘)
There’s a line in the Dire Straits song “Sultans of Swing” about “Guitar George” that goes “Yes, and an old guitar is all he can afford.” That has always amused me because it really should have been, “A new guitar is all he can afford.” There is something about old guitars and amplifiers that makes them so desirable – and expensive. It can’t all be nostalgia. Is a late ’50s Les Paul or Stratocaster really better than their modern counterparts? Considering all the advances in technology, can an old Vox or Fender amp honestly sound better?
Well, as far as amplifiers are concerned, old amps certainly do sound better than new ones. They were better made from better quality components. If you want an amplifier of 1960s build quality today, you have to pay a lot of money for a ’boutique’ hand-built model – or make it yourself!
Modern amps are built to a price. They use mass production techniques, usually outsourced to the Far East. The components are cheap and mounted on printed circuit boards. They work, they are affordable, but they are shamefully characterless and don’t suck the notes from your fingertips the way that an old amp can.
It might surprise you to know that valve amplifier design has changed very little in the last fifty years. Forgetting all the transistor designs and modern modeling amps that attempt to capture that vintage character, valve-based guitar amplifiers still use exactly the same circuitry now as they did back then. Where the rest of the electronics industry has embraced modern, hi-tech, integrated circuits and the resulting miniaturization, valve guitar amplifiers are essentially frozen in time. There have been little or no advances in valve technology in fifty years and the valves themselves are now only made by a handful of companies in Russia and China – because they were slower to catch up with modern electronics.
There is a movement afoot today where people are trying to regain some of this character and tone. It is now possible to download vintage amplifier schematics from the Web, source the components, and put together a modern clone with a few basic tools. There are companies who specialize in kits of components of many vintage models that you can build yourself without any expert knowledge – but higher quality kits do tend to be more expensive than modern off-the-shelf models.
Before going any further, I have to make an extremely important point. Valves require very high voltages to work. These voltages have to be derived from your mains supply where they are stepped-up to 400 volts or more using a power transformer. These voltages are lethal to humans. Working on valve amplifiers without due care can kill you dead in an instant! Where a regular AC (alternating current) voltage will give you a shock and you quickly pull your hand away, a DC (direct current) does not throw you back, it makes you hold on. If the voltage traverses your heart – on its way from you hand to your grounded other hand or foot – your heart will stop. If this prospect terrifies you, then good. It is supposed to! Furthermore, these lethal voltages persist for quite some time after the amplifier is switched off, if you have not properly discharged the capacitors. I’m not going to explain how to do this here, there are many other resources on the internet that will explain how to make working with valve amplifiers relatively safe. Find them!
If you want to explore the possibilities of valve amplifier modification, rather than start with an expensive kit, you can buy a cheap amp and set about seeing how you can improve it. A prime candidate for such experimentation is the Epiphone Valve Junior or one of its clones e.g. The Harley Benton GA5 and Grainger 5W. These little amps are all made in the same factory in China with the same circuit boards and only differ in the cosmetics of the cabinets and have some minor component variations. There are combo versions, with 8″ speakers or head varieties that can power whatever speaker cabinet you have at hand.
As valve amps go, these are about as basic as you can get using a ’single ended’ design with a single output valve, an EL84. There’s only one other valve, a 12AX7, which is really two valves in one in one package giving two stages of pre-amplification. The EL84 provides 5 watts output power, which doesn’t sound like much, but you will be very surprised how loud these little amps are. The relationship between perceived loudness and wattage is logarithmic, so you would need fifty watts to go twice as loud as five.
The Epiphone Valve Junior only has a single knob – a volume control. The Harley Benton version adds a single tone control, but it is virtually useless and should be removed. The entire amplifier is built upon a small printed circuit board. This allows a certain degree of modification to be done but if you do a lot of work on it, the tracks can come away from the board and are tricky to repair.
The metal chassis and cabinet are of reasonable quality and can be kept. The output transformer is small and feeble and is one of the first things that people change to get a meatier sound.
Having told you how cheap and basic these amps are, you will be surprised at how amazing they sound straight out of the box. If you have previously been used to valve amp simulations from pedal or modeling amplifiers, a Valve Junior is like a breath of fresh air. Good as it is, it can be made a lot better with a few fairly simple modifications. With a little extra effort, you can get great vintage Fender, Vox and Marshall tones for very little outlay.
The modifications, apart from the previously mentioned output transformer upgrade, are mainly to ‘purify’ the signal path. Key here, is the quality and type of the interstage coupling capacitors where the nondescript originals are changed for high quality Sprague or Mallory caps. These capacitors along with subtle changes to resistor values in the preamp stages provide the ‘voicing’ going from clean, chimey Fender tones through to classic Marshall crunch.
The resistors on the input affect the touch dynamics of the amplifier. The original amp is not too good in this respect and just a couple of resistor value changes can make a world of difference and give that long, signing sustain craved by blues and rock guitarists.
Tone controls, or ’stacks’ as they are known, effect the tonal color of the amp but they do so by removing certain frequencies. This has the unfortunate side effect of reducing the overall gain. It is best to keep tone controls to very simple treble-cut designs such as used in early Fender designs. If you want a full Treble, Middle and Bass tone stack, you have to add an extra valve stage to compensate for the substantial loss in gain but then you also have the problem of where to put the potentiometers without resorting to a new, custom chassis.
I’m not going to go into any great detail about modding a Valve Junior here as the subject is extensively covered elsewhere on the Web. There is an excellent forum at http://www.sewatt.com/vj-mods. The FAQs sticky at the top of the forum page gives information about all the possible modifications, schematics, tips and tricks and is a very good place to start.
If you get bitten by the amp modding bug, you will inevitably want to do your own ’scratch’ design. Traditionally, valve amplifiers were built using tag or turret board to mount the resistors and capacitors, which are then wired to the valve bases, transformers and potentiometers. Hand wiring an amplifier like this is time consuming and expensive commercially but is still the best approach. For the modder, it offers simplicity and the minimum of tools and equipment. All you will need is a multimeter, a soldering iron (preferably one that is temperature controlled) and a few basic tools for cutting and stripping wire and cutting holes.
Individual components are not expensive. A new speaker or output transformer are probably the most expensive items. Some valves can be expensive but the law of diminishing returns comes into play where you have to pay an awful lot more for minimal difference in quality. There are purists who will only use cloth-covered wire and vintage capacitors. Well, good luck to them. If I can’t hear the difference, I don’t care.
More experienced modders will attempt to improve just about any amp. They may be looking for more crunch, more sustain, more vintage sounds – there are many reasons. Popular modifications are ones which retrograde modern Fender amps to something more like the classic ‘Tweed’ and ‘Blackface’ models. Technical improvements in modern Fender Bassman amps are often removed to regain the ‘old’ sound which sags under load. I have modded my Peavey Classic 30 beyond recognition tonally, although I wouldn’t recommend this operation to the uninitiated because of its very fragile ‘U’ shaped printed circuit board.
For those of us that can only dream of owning a real vintage amplifier but want those same classic tones, modding offers an excellent solution. Use your search engine and you will most likely find that what you want has already been done and documented by others on the World Wide Web. Whether you want to capture the vibe of some long gone model or to make an ‘ideal’ amplifier that was never actually made, just turn on your soldering iron, roll up your sleeves and get stuck in – but be careful!
Useful modding links.
AX84 – http://www.ax84.com/ – custom amp designs for beginners
Epiphone Forum – http://forums.epiphone.com/Default.aspx?g=topics&f=4 – modding discussion
Hoffman Amps – http://www.el34world.com/schematics.htm – schematics, kits and parts
The Blue Guitar – http://blueguitar.org/articles.htm – modding info
Sewatt – http://www.sewatt.com/vj-mods – amp modding forum
Torres Amps – http://www.torresamps.com/ – kits
Turretboard – http://www.turretboards.com/ – kits and parts
Weber Amp Kits – https://taweber.powweb.com/store/kits.htm – kits and parts
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Hi, good post. I have been wondering about this issue,so thanks for posting. I’ll definitely be coming back to your site.