This month is part four in our  series on tube guitar amps. I want to continue with a discussion on an amp's power supply. I invite you to the Metro Music Mayhem Archives to view earlier articles in this series.

What is the power supply? I always find it easier when analyzing to break the guitar amp up into separate subsections. Even though they are all interrelated in the production of the final sound, it makes it easier to understand by looking at the different sections. I divide it up into preamp, power amp, effects, switching control and the power supply.

The power supply does just what its name says. It takes the 120 volt AC that comes out of your wall outlet and converts it to several AC and DC voltages that power the various functions in the amp. Although there are many new technological permutations of the power supply we are going to discuss only the classic linear style power supply used in most tube amps. This power supply consists of a transformer, rectifier and filter caps. Some of the classic amps  include a device called a choke and I am also inclined to include the output transformer as part of the tube amp power supply for reasons I will get to.

Here is an explanation of how the power supply operates:

The 120 volt  AC power from the wall is input to the primary of the power transformer. On its output (the secondary) we derive the raw AC voltages we need throughout the amp. Typically you will find the 120 volts AC stepped up to 300-500 volts to power the tube plates, stepped down to 5-6 volts to power the tube heaters and other voltages in between to create bias and control voltages. Next those voltages that need to be converted from AC to DC are run through a rectifier circuit. The rectifier can be a tube or a silicon diode, an important point we will come back to. The rectifiers convert the AC power coming from the transformer to DC by letting only part of the AC  pass. Alternating Current means that each cycle of power has both a positive and negative component. It is a safer and more efficient way for the power company to transmit power to you as opposed to Direct Current. but an amplifier needs DC to operate so we must convert (rectify) it. The rectifiers only do part of the job of creating DC. They create DC but it is in pulsed form. As the rectifiers turn on and off with the alternating power there are gaps between each cycle.  The filter caps are fed this pulsed power. They charge up when the rectifier conducts power to them and release the power during the time the rectifiers are off filling in the gap and creating smooth DC power. This occurs 60 times a second and is why if the filter caps are not doing there job properly the 60 HZ A/C can mix into the audio path and modulate the guitar signal with the 60 HZ making it sound like there is a ghost note between Bb and B playing along with every note that is played

The design of the supply has many variables that have an influence on the sound of the amp. It's ability to deliver power under heavy load determines how loud the amp can play regardless of the tubes installed. We all think that four output tubes equals 100 watts and two tubes 50, but that is only if the design of the power supply can meet the demands of the tubes to create that power.The filter caps since they store energy have an influence over bass response and if lacking in capacitance through age or design under heavy load will affect the low frequency response and as we said above possibly cause ghost notes to appear.The output transformer also plays a part in the power supply. The high voltage that the tubes use to create audio is directly input into the output transformer on its way to the tubes. An output transformer that can't handle the power given to it from the power supply will not let the amp achieve the power level it is capable of. So it can be said that the set comprised of the power and output transformer are crucial in determining what the tubes can do and become part of the tone of the amp by virtue of the fact that if they can pass the needed power the amp can play loud and clean at least up to what the tubes can do or if underpowered will cause the amp to sag even before the tubes do.

I mentioned before that you will see a filter choke in some amps and there is much mojo surrounding this part. In the early days it was hard to get the filter caps small enough in size at a high enough voltage and capacitance to create clean DC power. A choke can be used instead to smooth out the DC coming from the rectifiers in lieu of more capacitance. Due to modern manufacturing methods giving us less expensive caps in smaller packages the choke has been replaced by a resistor, falling out of favor because of its expense and propensity to create hum. How much influence does a choke really have in adding to amp's tone? It is really dependent upon the circuit it is used in so there is no real answer for this.

Now let's discuss the rectifier.

In the early days we had one choice for the rectifier- a tube. A tube rectifier is an inefficient device dropping typically 40-60 volts from its input to output. Similar to what we discussed last month with cathode biasing, this voltage drop increases under load causing the amp to sag and compress. As time went on the solid state diode was developed. it is way more efficient and drops less than a volt from input to output even under load conditions. So just like when we compared cathode biasing to fixed biasing we have two different possibilities, sag with the tube or louder and cleaner and more bass with the   solid state rectifier. Due to the economics of manufacturing and demand for higher power outputs the tube rectifier is not used much anymore except in reproductions of vintage amps or boutique amps looking for a vintage vibe.

There is one exception to this - The Mesa Boogie Dual Rectifier amps. As we discussed in earlier articles when you are playing an amp and deriving your distortion from the preamp adding  additional compression from the power amp or power supply  just serves to muddy things up. The Dual Rectifier has one of the highest gain preamps on the market. The amp also contains two or three tube rectifiers. You don't have to use the tube rectifiers, there is a switch to select silicon diode rectifiers and bypass the tubes. The problem I see happen is most players are not aware of the mud they are adding to their tone especially at high volumes when using the tube rectifiers. They bought an amp that is called a Dual Rectifier so they think the tube rectifiers are something they should be using. As with all the other marketing hype bells and whistles Boogie puts on their amps the average player doesn't understand how to use it. I have been servicing Rectifier amps since the mid nineties and advise every heavy style player to switch to the silicon diodes giving him more headroom, tighter bass response and less sag. Invariably I get a phone call back the next week to the effect of "Dude, this amp never sounded this good"  You could say that the Rectifier amps can be considered extra versatile inasmuch as you have the ability to turn the preamp gain down and use the rectifier tubes to cause sag to play blues giving an extra versatility to the amp, but not once have I met someone who realized they could do this and who wants a 100 watt blues amp anyway. There are certainly better choices. I view the whole Dual Rectifier thing as marketing hype.

Another component in the power supply subject to much hype are the filter caps. An electrolytic filter cap is designed for a life of 10-15 years. Obviously we have plenty of amps in service that are 40-50 years old. I would venture a guess that most of these have had new caps installed by now although I still sometimes get them in for repair with the original caps.This brings up the question of changing the tone of the amp by replacing them. I can assure you that if you have a sixties or seventies amp with the original filters it does not sound as good as it did in the seventies. If you are a collector you can leave them alone for aesthetic sake, but if you are gigging I would change them out to improve bass response, reduce ghost noting and to prevent them from going up in smoke during a gig and taking out your irreplaceable vintage output transformer. And by the way, the fact that you left the amp in the closet for twenty years doesn't help. Caps need a certain amount of voltage on them or they lose what is called their form. Taking the unused amp out of the closet  and plugging it in will result in almost certain destruction. Additionally in many of the older amps especially the lower priced models the manufacturers for the sake of economy used multi section can caps. These cans contained two to sometimes five filter caps in one package and were cheaper to implement than individual caps. This was a poor design element, because even when the caps were new they were prone to creating ground loops which means hum. It is always better to take these multi section cans out and replace them with individual caps. Lastly, there is also much hype over boutique and audiophile(audiofool) brands of caps. Is there a difference and is it worth the higher price? In my experience I don't think so.

Next month we are going to delve into common failure modes of a tube amp and their remedies. Feel free to send me your questions and suggestions. See you next month.