A Systematic Approach to Equipment Upgrade (Part 1)
We receive many inquiries from clients wanting to improve the tone or functionality of their amps, by way of upgrades or modifications. In some cases, and for whatever reason, it seems there is a somewhat misguided conception as to the best way to achieve these goals. This first part in a series of articles is meant to address the subject by taking a systematic approach, with the emphasis on obtaining the most significant results for the least investment of both time and money, and most importantly, with the least intrusion on the original character of the equipment.
These goals, rather than diametric opposites, are actually quite symbiotic. A complex redesign of equipment will generally be quite expensive, difficult to get right the first time, and will alter the nature of the equipment to something completely different from its original design. We generally advise clients seeking this type of modification to answer a series of questions to determine why they are looking to pursue such drastic measures. The usual outcome is either that they purchased the wrong unit in the first place, or that it failed to live up to its advertised performance (or in rather rare circumstances, a product that would meet their requirements didn't exist in "off the shelf" form to begin with).
The law of diminishing returns plays a prominent role in this approach. If you implement gradual and reversible changes where they will have the greatest effect toward the end goal, this will reduce the very high costs involved in trying to obtain the last few percent improvement that many more extensive modifications attempt to deliver. The law states that you will get 90% of the performance increase with the first 10% of the money/time invested; conversely, the last 10% improvement will cost 90% of the overall investment. This is true in any high performance endeavor; from motorcycles and automobiles to guitar amps and recording or hifi electronics.
It seems that many articles, both on the web and in print, tend towards advertisement rather than education. The goal of these authors is generally to assist in the sales and marketing of a particular product by way of propagation of opinion, rather than the dissemination of technical advice based on science and fact.
We had a recent case involving a client bringing in an aftermarket output transformer for us to install in his stock condition re-issue amplifier. Though we had previously advised him that the installation of a "trick" output transformer would have a rather minimal overall effect, and that he would be better served by following the guidelines we describe below, he was adamant. He was no doubt influenced by a pseudo-scientific "adverticle" (a print equivalent to the TV "infomercial") that said this transformer would provide him with the substantial tonal improvement he desired (and was led to expect). After the installation (which ended up NOT being the simple "bolt in" described to him by the manufacturer), we noticed little, if any, change in the response of the amp.
Many clients' expectations aren't met by esoteric upgrades or modifications that had been given high praise in the media (and their advertising-driven editorial policies). Suffice it to say that it's happened frequently enough that we felt we should give some general guidelines to follow.
If you have a tube musical instrument amp that essentially pleases you and meets your needs, but isn't quite up to what you're looking for tonally, give some thought to this: the two most significant improvements you can make are new speakers and tubes, or at least a careful adjustment (bias/balance) of the tubes currently installed. Precision matched resistors, hand wound transformers and exotic capacitors all have their place (we use all of them in the amps we manufacture), however they are most effective at the end of the upgrade path, not the at the beginning. They also tend to provide the least improvement at the highest cost.
We like to use motor vehicle analogies to help our clients understand what can be rather difficult to explain in the realm of electronics and physics, as most of them have had at least some involvement or experience with motorcycles or cars. Here's an example: it wouldn't make much sense to install a high lift, long overlap cam in an otherwise stock vehicle without first making the more basic changes, such as a high flow intake and exhaust system and great tires and shocks. An exotic and expensive cam wouldn't really do much without first having been provided increased airflow, and sufficient traction to keep the rubber on the asphalt. The same logic applies to an amplification system. The speaker must allow for the desired frequencies to be transduced, or any alteration in the electronics upstream won't really be noticed. If the active devices (the tubes) aren't of good quality, or are poorly adjusted, what good can installing a $50 matched pair of coupling capacitors in the phase splitter circuit do?
A great speaker will give you the single most significant change in the tone of the amp, and the numerous varieties to choose from will allow you to alter both the frequency AND amplitude response simultaneously. To give an example and some background, consider the following scenario: your amp is a bit too loud and a bit too bright. By choosing a less efficient speaker with a rolled off high frequency response, you've solved two major problems at once and not altered the amp in any way that couldn't be easily returned to stock. You can then keep the original speaker if you decide to sell the amp later, or sell the old speaker now to recoup some of the cost of the new one. That's an improvement that's hard to beat, both in results and in your time/money investment. We're planning to post a comparison chart of speaker characteristics in the future so you can see these options represented graphically.
Let's discuss tubes next. One of the most common issues we see is the fact that most of the major manufacturers tend to use relatively poor quality, inexpensive tubes. Worse yet, they often adjust them for the longest possible life (to reduce warranty claims), rather than for the best tone or for a reasonable compromise between the two. As we've mentioned above, any improvements to the passive components (transformers, capacitors, resistors) will generally be swamped by a poor quality or misadjusted tube. There are some really good current production tubes available on the market, and still a decent stock of vastly superior NOS types available. There's no excuse, beyond profit margin, for any manufacturer to use junk tubes, though many do.
As with speakers, changing tubes can also result in alterations in both the frequency and amplitude domains. Many common musical instrument amps use the 12AX7 in most, if not all, of the preamp (small signal) circuit positions. Again, this is probably due the desire to increase profit margin by purchasing huge quantities of the same tube type, rather than using the tube that best suits the specific application in the different circuits. We recently worked at a studio session on an expensive rack mount preamp from a small production "high end" company. The player was quite unhappy with the tone, though the unit had a good reputation and was highly touted by many "name" players. What we found were eight Chinese 12AX7s (factory installed) in the unit. As we went through it and determined which function each tube performed, we selected several different types, all with compatible pin outs, to re-voice the unit more to the player's taste, and also to optimize each tube for its specific circuit task. We ended up using all NOS USA-made tubes, with a 6072 as the first gain stage, a 12AT7 as the output amplifier, 5 5751s, and a single 12AX7. The preamp sounded much better, with improved clarity, broader response and a much less congested sounding overdrive. The player was now happy, but wondered why his "high end" preamp left the factory in that configuration.
The next discussion pertains to power (large signal) tubes. These are the devices that act in conjunction with the output transformer to produce the current required to drive the speaker. Here again, we've found many new amps coming from the factory with the least expensive "generic quality" tubes installed, usually biased on the "cold" end of their operating curve. As with small signal tubes, there are some high quality, current manufactured output tubes and (for now anyway) a decent availability of NOS versions to choose from. The output tubes will tend to wear out sooner than the preamp tubes, and are also more expensive. Contrary to some opinion being circulated, it is important to use matched sets; be they pairs, quads or whatever. This allows for the power circuit to uniformly amplify both the positive and negative-going cycles of the waveform, maintaining a balance that results in a smoother tone. Tubes can be matched in about as many ways as there are shops that match them. Generally, the two most important parameters to consider are the gain factor (transconductance, the ac component) and idle current (the dc component). If the tubes have been burned in (the initial aging process) prior to matching these two parameters, and rechecked after another burn in, this will generally ensure a good match and greatly reduce the possibility of premature failure. Be wary of tube sellers and rebranders who won't describe their process of matching. There are no "trade secret" issues, it's just basic electronics.
When you hear of an amplifier requiring a bias adjustment, it generally refers to these output (large signal) tubes. Installing a new set of output tubes, but not having them biased, is essentially the same as installing new tires and not having them balanced or the air pressure correctly set. Regardless of the output circuit configuration of your amp, it is of the utmost importance to have the bias checked and adjusted when you install new output tubes. There is quite a bit of tolerance and variation, even within a manufacturer's single production run of the same tube type. Installing sets of the same "brand" of tubes will not ensure that they will all perform identically in your amp. Another point to consider is that there are only a few actual factories producing vacuum tubes. Many of the "brand name" companies just silkscreen their logos on tubes they buy from OEM manufacturers, screen out the completely defective tubes, and assign the rest a numeric or color code derived by an undisclosed method, which is therefore meaningless for comparison.
By intent, this article has been very generic in nature, and is devoid of any specific suggestions. The rationale: each amp and each player are unique individuals with different requirements; any blanket statements would be worthless and potentially misleading. We deal with every amp on a case-by-case basis to best suit the client's needs and budget.
In the next article in this series, we'll discuss passive components (resistors, capacitors and inductors), how they work with the tubes to define the gain and frequency response of each circuit, and how changing the types and values can significantly alter the gain and tonal response of the amp. As always, please email us if you have any questions or comments.