Can I operate my amp without a speaker plugged in?

If you have a tube Amp then no, you can’t. This can result in a tube failure and, even worse, a blown output transformer. If you turn your amp on, plug in your guitar, start playing and there’s no sound, turn it off immediately and check your leads, ensuring the speaker is correctly plugged in.

Can I use a speaker with any impedance (OHM’s) with my amp?

Always match the speaker impedance with the amp’s output impedance. Unmatched impedances will result in power loss, premature tube wear and at worst, tube failure. Also, another important factor is to ensure that the speaker’s power handling capacity is greater than the amp’s wattage output.

Can I move my tube amp when it’s hot or do I need to let it cool down?

Yes, but only if you are gentle with the amp, otherwise allow your amp to cool down after playing before moving it. The internals of the tubes heat up to very high temperatures and are very fragile. Knocks and vibration can damage and shorten the life of tubes when they’re hot.

Can I install the output tubes on my amp myself?

A qualified technician should do this. Incorrectly biased output tubes will either run too hot, which will greatly reduce the life of your tubes, or too cold resulting in distortion at any level. Don’t use the amp, get the bias adjusted before you cook your tubes. Pre-amp tubes don’t require bias adjustments but may need selecting.

Will installing new tubes in my amp make it sound better?

Yes, if your currently installed tubes are old and worn, or of an inferior quality, a new set of tubes correctly installed and biased will restore the amp’s sound back to its original glory. Tubes don’t last forever, and start slowly wearing out over time, degrading the sound quality of your amp. It’s a gradual process, and at some stage you notice that your amp just doesn't’t sound that good anymore. In addition to worn tubes, internal electronic connections (pots, sockets, connectors, etc.) start to corrode and deteriorate over time and need to be cleaned as part of an Amp service. 

So the question should be, “where is the humming, buzzing noise coming from in my system?” To simplify this problem, it’s a good idea to break the system down by isolating the various components so you can find the source of the noise.

Checking The Guitar For Noise
Most likely, the culprit is your guitar, so check that first. Unplug your pedals or bypass them and go straight to the amp, turn up the volume on the guitar and listen for the noise. If it’s there, turn down the volume and if the noise goes away, it’s your pickups.

Single coil pickups are renowned for picking up noise, and manufacturers go to great lengths to design low noise or noiseless pickups due to this problem. Switching to a Humbucking pickup will reduce the noise, confirming again that the guitars pickups are the cause.

However, there is an acceptable level of noise tolerated from pickups and if this level is reasonably low then you will have to live with it. If it’s excessive, you may want to get a Luthier to offer some advice on reducing those noise levels.

Leads And Pedals
Guitar leads generally work or don’t work, so if your lead hums it probably won’t work either. Get it repaired if it’s a good quality one, or use another.

A lot could go wrong with pedals, due to the multitude of electronics in so many little boxes, each with connecting cables and usually a common power supply operating all of them at once.

Noise from your pedals may be coming from an over-burdened or poorly regulated power supply, introducing hum into the system. If this is the case, check the power supply’s capacity to see if it can handle the total power requirements of all your pedals.

Your pedal itself may be faulty (check by physically removing each pedal from the system one at a time) or it may be one of the connecting leads that is causing the issue. It’s a good idea to occasionally push connecting leads in and out of each pedal. This will give them a clean in case there is some corrosion build up in the pedal sockets. Other noise problems with pedals could be put down to poor pedal location, i.e. hum sensitive pedals like Wah or distortion pedals being located too close to a noisy power supply, etc.

Checking The Amplifier For Noise
This procedure will systematically allow you to check the various section’s of the amplifier to locate the source of the noise.

The first thing to do is to remove the guitar lead going into the input of the amp. This will allow you to check the amp for noise in isolation.

Turn the amps Master Volume and Gain controls to off or zero with the speaker plugged in and then turn the amp on and listen for the noise.

If the noise is there it’s being made by the power section of the amplifier, possibly by a bad tube, transistor or a power supply component like a capacitor. If the noise is not there with the volume and gains turn down, turn the master volume up and if the noise appears, then there is a fault with the preamp section of the amp, possibly a noisy valve.

If there is still no noise when you turn the Master Volume up, leave it up and now turn the Gain control up. If the noise appears, the problem is to do with the input section of the amp, possibly the first valve or even the input socket. Also, if the amp has reverb, turn its control up and if the noise appears then there may be a problem with the reverb unit (or the connections to it).

So why use P2P if there is no significant advantage in sound? Well, we are dealing with tubes that are, let’s face it, outdated, inefficient, oversized and power hungry, and require an external power source just to turn them on! But, they just happen to sound fantastic, and they look good too - so, we keep using them. Tubes need to be plugged into sockets mounted onto a chassis along with usually three, rather large transformers, some pots, sockets, and switches. Components like resistors, capacitors, diodes etc. can be mounted on simple tag board as used in earlier amplifiers and boutique amps today. Compared to solid-state amplifiers that have a much higher component count, constructing them on tag boards would be too laborious and costly so a PCB would be a better choice.

P2P is a very reliable construction method, and also figuring out the schematic for repair is a lot easier because the circuit is pretty much laid out in front of you. However, in construction, each component has to be individually placed on the board and hand soldered which is time-consuming. One disadvantage of some P2P tube amps’ designs is from an electrical point. You have a circuit (wires, components, etc.) that runs back and forth all over the chassis. Therefore, careful wiring and placement of components is critical. Having many stages like this can pick up hum and noise and could also be prone to self- oscillating if correct wiring techniques are not used. Therefore, PCB designs have an advantage over this type of P2P construction due to the circuitry being located in a smaller area. The tighter construction of a PCB decreases the need to run wires all over the chassis, therefore, keeping all connections to and from the tube and other comps as short as possible. The result is a circuit that sounds great with low noise and is very stable and not prone to oscillation. I use PCBs when constructing complex multi-stage and multi-channel amplifiers that have a greatly increased component count, and for simpler amps I use P2P turret boards.

We often hear of the unreliability of PCB designs, and this is true only to a certain extent. Single-sided PCBs, (tracks on one side only) that are wave soldered are the worst offenders. It is common for the wave soldering machine to skimp on the amount of solder required for larger components, and over time these solder joints break and the amp fails due to the lost connection between that component. Double-sided PCBs have tracks on both sides of the board that connects from side to side with plating through the holes where the component leg is inserted. Because of this, when soldered correctly over the same wave-soldering machine the PCB will have very reliable solder joints. One of the main reasons manufacturers started to use PCBs was to build a more cost effective product because PCBs by and large are much easier to assemble. However, as part of the cost-saving measures, the qualities of the other components in the amplifiers were compromised. This cost saving measure resulted in an overall lack of quality compared to the earlier style amps where the emphasis was more about tone than cost, and many of them also happen to be P2P wired. 

There are basically two main sections to a guitar amplifier, the preamp, and the power amp; both using tubes designed for these sections. Preamp tubes are the smaller tubes and are mainly dual triodes such as 12AX7, 12AT7, and 12AU7.  These preamp tubes are voltage-amplifying tubes and form the gain stages, and are used to amplify the small signal from the guitars pickups to something more useful. The gain control is usually placed after the first gain stage and the signal is then shaped with the tone control circuit. 

The power stage consists of much larger tubes and are usually power pentodes or tetrodes such as EL34, 6L6, 6550, 5881, EL84. They receive the signal from the phase’s splitter and amplify it to produce enough power to drive a speaker.

There is also another type of tube called a rectifier tube used in the power supply of some amps and it's used to convert the AC voltage into DC. Common types are 5AR4, 5U4G, 5Y3, GZ34. Modern amps use silicon diodes, but the softer sound obtained from tube rectifiers is still preferred by some amp makers and guitarist. 

ALL TUBES ARE NOT MADE EQUAL

A tube is a delicate, intricately constructed mechanical device. Although made under strict design and manufacturing rules each tube turns out slightly different to the next one, even though they are all the same types. So you end up with the same tube type, an EL34 for example, but with a spread in their characteristic (the electrical differences in the same type of tubes). To compensate for the differences, we need to adjust bias voltages to suit the individual tube or select the tube for the bias voltages. 

TUBE GRADING OR RATING

As we spoke before of the spread in tube characteristics, it is possible to use this to our advantage. Tubes at one end of this range or spread, will conduct a current quicker and require a lower bias voltage than tubes at the other end of the range, which are slower to conduct and require higher bias voltages. The ones that require lower bias voltages will sound dirtier with less headroom and more saturation, great for power amp distortion. At the other end of the range with higher bias voltages required, these tubes will sound the cleanest with maximum headroom and minimum distortion. Some tube suppliers use a scale from 1-12 or colored dots or just the current value marked on the tube. 

BIAS ADJUSTMENT

Amps are made with either a fixed bias voltage or an adjustable bias voltage, usually with a trim pot operating in a small range. Amps with a fixed bias voltage, i.e. Mesa Boogie, need the tubes to be selected for the amp. This is done so that the amp sounds the way its manufacturer wants it to sound both before and after a re-tube. To re-tube this amp properly, the same grade tubes are needed.  Amps with adjustable bias voltages can use a range of graded tubes without too much change in sound after a re-tube. Using a Marshall as an example, you would select a matched set in its bias range, and then adjust them for the correct current. If you want the amp to sound the same after a re-tube, use the same grading and bias settings as used with the old set.