What's the difference between a Series & Parallel effects loop?
Up until just recently, almost all guitar amps that had effects loops, had series effects loops. A series loop interrupts the signal path between the preamp and the power amp and inserts the effect processor signal into that path. This means the entire signal from the preamp travels through the processor and re-enters the power stage. It's basically a one-lane road going from one place to another.

Parallel effects loops have just recently begun to surface. A parallel loop offers two paths from the preamp to the power amp. One path is a direct connection from the preamp to the power amp as if the amp had no loop at all. The other path sends the preamp signal to the effect processor (via the loop) and then routes it back to the power amp, mixing it with the direct (dry) signal. Most amps that offer a parallel effects loop have a variable mix knob, so that you can control how much of the effect you want mixed in with the dry signal.

How do you choose one over the other? In a series loop, many modern, high quality effect processors can be used effectively without any problems because the sound quality will not be degraded when traveling through the processor. Additionally, there is a mix control that allows the user to adjust the dry and wet signal within the processor itself. A series loop works fine in this case, and though the entire signal is being routed through the effects unit, the tone is still coming out of it uncompromised.

The parallel loop is useful when using vintage effects and other effects that don't have any kind of mix function and that sometimes suffer from bad signal-to-noise-ratios, which can lead to tone degradation. Lately it seems that there's been more of a return to vintage effects and stomp boxes, which has probably caused more of an interest in the parallel loop.

I think that a parallel effect loop is a good thing, and the ability to be able to adjust the mix is nice. However, they don't work well if you are using effects that change the volume of the signal (such as tremolo, compression, or noise gates), or when mixing the wet and dry signals causes an out of phase situation. Technically, if you turn the mix to 100% in a parallel effect loop, it should operate exactly like a series loop, although this is not the case with all amplifiers on the market.

In my opinion, if you don't need a parallel loop, don't bother. It's generally a lot more circuitry, and can end up costing more money. However, if you do use older effects or stomp boxes, I highly recommend a parallel loop for its flexibility.

Point to Point vs Printed Circuit Boards
In the early days before circuit boards were designed, technicians relied on various "anchor points" to connect wires together. Primitive layouts consisted of terminal strips screwed to the chassis or even pieces of cardboard fitted with brass eyelets. Components and wiring were then soldered, point to point , to these terminals or eyelets to create the circuit. As time went on, clever technicians figured they could etch the wiring patterns into copper foil laminated to a strong and rigid phenolic board. The components could then be soldered directly to these copper "traces". This was the beginnings of modern printed circuit (PC) boards. In today's boards, epoxy reinforced glass fiber replaces phenolic and more than one layer of copper is now possible. When using PC boards, precise parts placement and consistent wiring is guaranteed.

I feel that point-to-point wiring is still very useful when designing an amplifier prototype or building a custom one-of-a-kind amp. It's quicker, easier, and cheaper to do than a one-off printed circuit board, and it's generally easier to make circuit modifications when using this method. Well done point to point wiring is also very nice to look at - if you've ever seen the inside an early Hiwatt, you'll know what I mean.

In my opinion, however, it is far more cost effective and less labor intensive to use PC technology in a production environment. To you the consumer, this means a better amp for less money. I believe that an amp built with well designed circuit boards is easier to work with, is far more consistent and reliable, and more rugged mechanically than a point to point wired amp. Another advantage is that, since the circuitry is clearly mapped out on the board, PC boards are also quicker and easier to service.

In closing this discussion, one should bear in mind that there is absolutely no sonic difference between point to point and printed circuit board wiring. Detractors of PC boards have argued that they are less reliable due to cracked solder joints or failure prone do to burned traces - neither of these complaints are even an issue with a properly designed board.

Tube vs Solid State Rectifiers
The job of the rectifier is to convert alternating current (AC) which comes out of the wall socket and subsequently the power transformer, into direct current (DC), which is needed to operate all of the circuitry in the amplifier. This process takes place through a device called a diode. Diodes act like a one way valve, allowing current only to flow in one direction.

The diodes in the days of old were vacuum tubes. Today's diodes are made out of silicon.

The tube diode was the first diode ever invented (in fact it was the first tube ever invented). For many years it worked quite dependably, and it was the only choice for rectifying current for high voltage power supplies. When solid state technology was developed in the 1950's, it was found that solid state diodes could do the job of tube diodes, only better.

A tube rectifier has internal resistance. The more current that travels through a tube rectifier, the more the voltage drops. When the voltage drops, the power of the amplifier also drops. The tube rectifier has the drawback of not being able to provide a consistent voltage when it's under load. The other drawback is that the tubes themselves run hot, and can be relatively short lived. Unfortunately, modern day sources for rectifier tubes are not very reliable, and even in their prime these tubes were usually the weak link in most amplifiers.

An amp with a tube rectifier tends to sound much "spongier" in the bottom end. Low frequency notes take more current through the power tubes to reproduce. This increased current causes a voltage drop in the rectifier tube and the amp loses power. So, when more power is actually needed, the amp gives less. Because of this, a tube rectifier amp will sound spongy and more distorted at high volumes. This, probably more than anything, is what gives a vintage amp its sound and color.

A solid state rectifier has no internal resistance whatsoever. It has a very consistent fixed voltage drop that occurs whether there's no current or full current - approximately .7 volts. When an amplifier needs power at low frequencies, there will be no limit to the current that travels through the rectifier. This results in an amp with more headroom that is punchier, more articulate, and able to deliver the goods in the bottom end.

In my opinion, all amps should have solid state rectifiers. I don't believe there are any really good rectifier tubes on today's market and, even if there were, why use them? The technology is obsolete, they are horribly inefficient, and far more expensive and troublesome to build into an amp. These tubes, no matter how good, will routinely need replacing, adding to your maintenance expenses. Besides that, tube rectifiers kill the headroom of an amplifier. If you want that spongy, vintage sound, there are other ways to do it. I have successfully designed and built amps that have replicated that soggy bottom, vintage tube rectifier sound using solid state rectifiers and various circuit modifications.

Of course if you have a vintage amp that uses a tube rectifier, by all means keep it that way! That's what makes it sound the way it does. But if you're contemplating getting a new amp, I recommend avoiding future headaches by staying away from tube rectifiers.

Class A and Class A/B
Class A and Class A/B describe how the power tubes work within the power section. To properly explain the technical differences between these classes of operation would be an entire article in itself. However, the characteristic differences can be summarized as follows:

Class A/B amps tend to have greater dynamics, sounding punchier, tighter, and cleaner, and have cooler running tubes. The Class A amp sounds more vintage and squishy, because it's compressing and distorting more. Tubes in a Class A amp tend to run hotter as well. For the same given tube compliment, Class A/B will produce two to three times as much power as Class A. An example would be an amplifier with two 6L6s in the power section. Operating in Class A, the maximum power we could expect would be around 20 watts, while operating in Class A/B would easily yield 50 watts.

Just for tube life alone, I believe Class A/B is the way to design any amp. The amp will run more efficiently with more power, and you'll enjoy not having to replace power tubes as often. If the tonal characteristics of a Class A are desired, an A/B amp can be carefully designed to do that (the Soldano Atomic and Astroverb are good examples of such a design).

In Conclusion
I hope you've found the information I've provided here interesting and useful. If you're shopping around for a new or used amp, I'm hoping this article will help you better understand what you're looking at. And remember, the opinions stated here are simply that - opinions. Use the facts I've provided to draw your own conclusions.

Now for one final thought. It is an indisputable fact that the finest guitar in the world played through a garbage amp will sound like garbage, yet I see so many guitarists that spend no more time (and hardly anymore money) buying an amp than they would buying a cheeseburger. If you want your sound to be the best it can be, don't sell yourself short - spend some time, look around, and invest in a quality amplifier!

Michael John Soldano Jr.
July 12,1998