Aubrey's Musings:

There are a lot of myths and misinformation out there about amplifiers. Let me offer my two cent's worth on some of these issues.

Myth #1: Amplifiers made in the 50s and 60s are far superior to the currently available vacuum tube amplifiers. This is just wrong. While the early amplifiers are valued by collectors and musicians alike, there is no solid technical reason why they are superior to a well built amplifier of today. I'm not a big believer of magic and I don't believe that old amplifiers are magic. All things engineering can be explained, measured, reproduced and even improved on.

Some history is in order. Early amplifiers prior to the 59 Bassman were not well designed. Designers simply took suggested circuits from the RCA Receiving tube manual appendix or licensed designs from Western Electric. These were not very well performing circuits. I have in mind a Fender Pro and a Gibson BR-9 for example. The amplifiers were also very cheaply made. They have very poor frequency response, cheap output transformers and very high distortion and not the good kind either. The art of Guitar Amplifier design reached it's technical pinnacle when Leo Fender introduced the 1959 Bassman 5F6A with 4 10 inch speakers. This design was a true breakthrough in design with touches of brilliance. A screen bias scheme was introduced for example that changed the characteristic curve of the 6L6 output tubes in response to the grid drive. This had the effect of greatly linearizing the characteristic curve and improving amplifier linearity and thus lowering distortion. This basic design remains the basis for all Fender tube amplifier designs today and was copied by Jim Marshall in England to become the first Marshall amplifiers. But since Jim Marshall utilized WW2 surplus parts to make the early amplifiers, the Marshall sound was unique. Jim Marshall continued to evolve his designs thus creating the unique crunchy British sound that we know. The development of the 6L6GC beam forming pentode tube in the US and the conventional EL34 vacuum tubes in the UK further separated the two camps of design US versus UK. On both sides of the pond, these tubes were breakthroughs but the 6L6GC was inherently more linear this making the Fender amplifiers cleaner by design. A piece of history here. Jim Marshall copied the Fender Bassman 5F6A almost exactly for the JTM-45 except he used surplus KT66 tubes in the output stage instead of American 6L6 types. The KT66 tubes were a high gain true pentode tube whereas the 6L6 was a beam power tetrode and had inherently lower gain. So the JTM45 has excessive grid drive because of the KT66's higher gain. Jim missed this fact. He could have made simple design changes to correct this but didn't. This inadvertently gave the JTM45 a crunchy overdrive performance characteristic that became part of the Marshall sound.

With modern components it is possible to copy a schematic of an early amplifier and the end result sound the same or better. All components are readily available (but sometimes difficult to locate). If an amplifier of today were to be placed along side a new off the shelf 1950s, 1960s or 1970s amplifier (using a time machine of course) you either could not tell the difference or if the modern amplifier were made with superior components, it would sound better. However, and here is the rub, the older amplifiers of today are antiques and have degraded. They are fragile and the component values have shifted. There are those that argue that this is a good thing and that the old amplifiers age like a good wine. Bull! The components used in that era were standard commercial parts of the standard quality grade available at the time. The highest quality components available were not used for Guitar Amplifier use any more than they were used for TV or Stereo use. It simply made no commercial sense. Standard grade component of today are far superior to the available parts of the earlier era.

Myth #2: The components used in the day are superior to components of today. Lets examine commonly used components one at a time. Note that resistors and capacitors make up 95% of all electronics components.

First, resistors. A resistor is simply a device that opposes the flow of an electric current. Resistors are the most common of all electronic components. They are used to establish bias points, couple signals, establish gain values, and form necessary electrical circuits. At the time of the golden age of the electrified guitar sound, the prevailing commercial resistors used were carbon composition types. These were the standard part for 40 years. They are manufactured by binding together carbon particles forming a hard substance that is placed between two contacts that are attached to leads. The density of the carbon particles determines the effective resistance. They are usually brown cylindrical devices marked with color codes. They were cheap, readily available and met the requirements. I was around then, designing and building electronic instruments, and I never questioned the use of carbon composition resistors. We used them for most applications. However they were noisy and contributed to a background noise in audio equipment. What we did not realize is that they were also hygroscopic. They absorb and retain moisture. Over the years this causes them to drift, an increase in value over time. Resistors from the 1960s are as much as 40% out of tolerance across the board and sometimes as much as 100% out of tolerance. This is just not isolated resistors but all of them. So an amplifier of the day is not operating as designed because all the resistors have drifted shifting bias points and shifting gains. This does affect performance and in unpredictable ways. There are those who say that the carbon composition types had properties that made them superior for guitar amplifier use. That they added color to the sound. There are indeed some side effects of carbon composition resistors that are unusual. They change their resistance slightly in response to changing voltages across them. Do they change value in such a way as to distort the signal in a pleasing even enhancing way? In a stretch, maybe I could imagine a single resistor somewhere in an amplifier that could change the sound with this effect but given the hundred or so resistors in an average amplifier, there is no conceivable way that this effect could be beneficial in all cases and in fact defies logic that this would be a good thing. Consider that what little I have read about this effect, it is in the order of a 1% shift in value over a large voltage swing. Likely the resistor itself had drifted over 40% in an older amplifier, this would seem to be the larger effect. The standard resistor of today is either metal film or metal oxide (for higher power uses). These resistors are near perfect compared to the carbon ones.

I utilize metal film resistors for all signal applications. These resistors are very quiet, very stable over time, temperature and applied voltage and very accurate. They are the standard preferred type today. I typically use 5% tolerance resistors versus 10% tolerance carbon types that were standard in the day. When I test metal film resistors, I typically find that they are within 1% of the marked value even though they are 5% parts. In fact, there is little to differentiate a "precision" 1% resistor from a commodity 5% one. I found out lately that one of my preferred vendors, from which I buy a lot of parts, has been buying 1% resistors from a major wholesale supplier and re-packaging them as 5% parts and selling them. In quantity, 1% parts are cheaper than 5% ones so he was taking advantage of this. After I discovered this, I have started buying 1% MF resistors from a wholesale supplier as well as the 5% ones from my favorite supplier depending on other buying decisions. Carbon resistors are still available (but not common). All that I have seen are made in China. I know of no US manufacturers of carbon resistors.

Next let's examine capacitors.

There are two basic types of capacitors. Filter capacitors, used to filter out the power supply ripple and coupling capacitors that couple signals from one amplifier stage to another. Filter capacitors of the era were not very reliable and were the most common component to fail in TVs, radios and amplifiers of the era. They tended to explode, leak and open up. Have you ever heard a radio that buzzes or hums? This is as result of a failing filter capacitor. Filter capacitors have rated voltage that is often very close to that actual design operating voltage. However with today's 120V power mains versus the 110V, 115V, or 117V (depending on the era), all filter capacitors are either already defective or will become so the next time the amplifier is plugged in. Crossfire amplifiers are designed with higher values of capacitance and working voltages in the power supplies and are de-rated by 40% to 50%, thus significantly increasing the reliability. Commercial amplifiers utilized the minimum amount of capacitance required because larger capacitors were more expensive. The capacitor manufacturing technology used today is superior to those of the era. This results in lower ripple which results in lower hum. It also improves impulse response of the amplifier and lessens power supply sag which results in muddy notes.

Next, let's talk about coupling capacitors. Depending on when the amplifier was made, it likely contains paper capacitors insulated with bees wax. These leak and degrade over time. If they do not become too leaky to operate, they likely have drifted in value. Coupling capacitor failure is a very common occurrence. I almost always encounter defective coupling capacitors when servicing an older amplifier. I utilize polystyrene capacitors for coupling applications. These types have almost immeasurable leakage, low inductance and precise values. There are very expensive "audio" capacitors available today that cost 100 times what a commercial part costs. I will not mention a brand but I know of two. These capacitors (coupling type) make outrageous claims about superior sound qualities. As I read about them, I find that they are made of the same materials and constructed in the same manner as the Famous Mallory 150 type (whose modern version is my preferred type). One of the properties of a capacitor that would limit high frequencies is inherent inductance. There is nothing in the way these expensive capacitors are made that would limit or change that. I have studied the effect of inherent capacitor inductance and it has little if any effect in the audio range. Remember that guitar amplifiers do not have tweeters in their speaker cabinets and speaker frequency response (example, Celestion G12 Blue) rolls off quickly at about 8kHz. Any effect occurring at 30kHz, for example, clearly does not effect tone. I have studied available literature and advertisements in trade magazines from the heyday of vacuum tubes. I have seen many ads for capacitors as each manufacturer tries to make his case about why their product is or was superior to their competition. I have never seen an ad claiming that their product "sounds" better. If they could have found a way to make that claim, they would have.

Let's talk about transformers, particularly output transformers. The output transformer is an important component in determining the amplifier's tone and it is the single most expensive component in the amplifier. Many believe that the transformers made in the 50s and 60s are better than those of today. Well, there is some truth to that. Some background here. The most important part of the OT is the core. The core is made of Iron laminations. The metallurgy of these laminations is critical to the tone, efficiency and overall performance of your amplifier. Some of the best Iron, for transformer use, was made in the 1950s by accident of history. Some of the best transformers were made by Triad and Stancor in the USA, Radiospares and Partridge in the UK. Prototype and very early production models of Marshall and Vox amplifiers used these high quality transformers. However, in later production, cheaper transformers with poorer materials and build quality were substituted for cost savings reason. The readily available sheet iron of today does not have the great magnetic properties that made some of the old transformers so great. So it is likely that the transformer of your recently purchased reproduction tube amplifier does not sound as great as an original 1950s or 60s amplifier did. However, there are modern companies that know metallurgy and transformer design as well or better than the original designers. These companies source custom Iron and materials and build a superior transformer. I use these higher quality transformers in the Crossfire designs. They are very expensive and that is one reason why a Crossfire Boutique amplifier costs more than a store bought amplifier. This over simplifies the very complex subject but great transformers are available today that are as good as any in the day. Many are much better.

So I see a trend here, older components from the magical age of musical amplifiers are all sonically superior, right? That is the myth. So by that argument, building a custom amplifier today is a wasted effort as it cannot possibly achieve that magic mojo of yesteryear. Listen yourself, call me and play a Crossfire. You will hear the difference. A modern tube type amplifier, built with superior components and hand wired will impress even the most diehard of musicians.