Guitar Amplifiers – Treat them right

Amplifier storage

Your amplifier is a bit like you, in that it does not like to be left in an inhospitable environment. We are referring to climate of course, and in particular, large variations in temperature and humidity. Dampness and condensation are enemies of electronic equipment, and exposure to atmospheres that promote either is to be avoided.

If leaving your equipment in storage spaces bereft of climate control is unavoidable, good quality flight cases give decent protection against the elements, and to a certain extent, a snug-fitting, premium quality multi-layered fabric cover will too. A simple dust cover is not a satisfactory defence.

Protecting your amplifier and keeping it clean

An obvious choice for maintaining your amp’s brand new condition, is of course to keep it in a flight case. However, this is not an option for everyone, as they can be relatively expensive and add unwieldy bulk if transporting equipment by car.

A decent set of quality fabric covers will add negligible weight, and afford reasonable protection against knocks and bumps that would result in spoiling the amp’s cosmetic appearance. These can also incur moderate cost, but what price to keep your machine clean?

A dust cover will keep the dust off but is a poor protector against ‘roadie abuse’.

Never use any aggressive cleaning agents on your amp’s covering or panels. A damp cloth will usually more than suffice. If in doubt, search in the user manual or contact the manufacturer for advice.

Correct connection of a speaker cab or cabs to an amplifier

Very often a perplexing topic for the layman to get to grips with. Let us deal with valve (tube) amps first, and then we’ll look at their solid state cousins.

Valve power amplifiers are designed to interface with the ‘load’ (your speaker cab or cabs), via an output transformer. This transformer is designed to optimally transfer the maximum available power, from the power valves, into a nominal matched impedance, i.e. 16 ohms, 8 ohms, etc. The closer this matching is, the more efficient is the payload delivery.

So, when using a 16 ohm cab, set the impedance selector on the amp to 16 ohms too, or use the dedicated 16 ohm output. Likewise, when using an 8 ohm cab, ensure the switch on the amp is set to 8 ohms, or use a dedicated 8 ohm output. The same rule applies for using 4 ohm cabs.

Solid state power amplifiers are different, in that they have no such interfacing circuit element as an output transformer, although in bygone days this was not always the case. However, it is extremely rare nowadays.

Instead, the manufacturer will declare a minimum safe impedance that the amplifier is designed to connect to. This is usually 8 or 4 ohms, sometimes 2, and is more often than not stated on the rear panel next to the speaker outputs themselves.
The amp will deliver its maximum payload into the declared minimum impedance. It will not deliver its maximum payload into higher impedances but will quite happily drive them.

So, for example, say a 300W amp is rated to deliver its full power into a minimum impedance of 4 ohms, it will then only deliver maybe 220W into 8 ohms. 2 ohms is NOT an option unless you like the smell of burnt out power transistors!

On modern amps there are usually protection circuits to prevent catastrophic failure. Hopefully these never have to be triggered.
Always consult the amp’s user manual for speaker connectivity, especially if using multiple cabs which results in a change to the total impedance.

The inputs and outputs commonly found on amplifiers

There are a number of ‘ports’ on a guitar amplifier and they handle various signal levels, depending on what they are designed to connect to.

The input where you plug your guitar is only expecting to receive a fraction of a volt. It does not deal in power so it is not handling any appreciable current.

Send and Return sockets on FX loops usually cater for signals ranging from guitar level, as in the input mentioned above, to line level which is a little hotter at 1V to 2V. Again, power is not a factor here.

-20dBV or -10dBV is guitar level for connecting stomp box pedals in the FX loop. 0dBV or +4dBu is line level for connecting to studio grade rack effects and some power amps.

Footswitch sockets will usually handle DC voltages, generally no more than 15V max.

Speaker outputs deliver peak signal voltages of typically 50 to 60VAC, but also have significant current capability as they do deal with power. Beware!

Playing and overdriving amplifiers at full volume

A common question we get asked is ~ Does it harm an amplifier to play it at full volume and with overdrive? The short answer is no, at least not if the amplifier is designed properly. There’s more chance of it harming your ears, or the audience’s.

That said, there’s a good chance you may have to replace the power valves more regularly as you thrash them to their limits, and thus wear them out faster.

Biasing of amplifiers

This is not really a job for the end user, unless they are particularly competent with electronic circuits in power amplifiers and are familiar with the use of Voltmeters and their measurement ranges.

In valve amps there are dangerously high voltages lurking therein and they take no prisoners when the probing hand becomes careless.
The nominal bias setting declared by the manufacturer is their recommendation for maximising both tone and reliability of the power stage. If the bias is set too high, there is a danger that the power devices will overheat and possibly destroy themselves due to passing excessive current, even when the amp is not being played.

Pulling two valves from a 100W amplifier to reduce output power

I certainly would not recommend this being done without first verifying it is safe to do so. This would involve a tech checking that the inevitable increase in voltage levels within the amp were not causing excessive stress to any circuit elements. For example, the anode voltages of the remaining two power valves, which could arc over if too high.

Stock fuse values can also become redundant with the change in configuration. The output transformer will no longer be optimised for the power amp or matching to the load.

Scheduled maintenance for amplifiers

During its lifetime, your amp is subject to wear and tear. This is normal and is the result of several cycles of heating up and cooling down, handling power, high voltage stress and high current stress. Component values drift with age which in turn can affect other parts of the circuit adversely.

Just like cars, amps need a maintenance schedule to monitor any degradations, and to correct any deviation from the allowed tolerances before they cause more serious issues.

An annual service is usually sufficient, unless the amp is running for ten hours a day, every day, in a recording studio or similar.
End user safety is a prime concern during the lifetime of an amp and to this end, Root Two runs a PAT safety test on every amp we service to ensure the customer is not in danger of harm from electrical shock.

The soundness of mechanical fixings can also be compromised over time so also need to be checked.

A service can be thought of as an MOT for your amp, keeping it roadworthy and sounding its best

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