Many guitar magazine articles have been written stating that if you want to get the vintage sounds from your tube guitar amplifier, you must use vintage parts. Fortunately for us this is never 100% true. In almost all cases, manufacturers used parts that were selected from those simply available to them at the time and in mass quantities at the cheapest price. The transformers and speakers were typically 'light duty', as most amplifiers of the early years had no more than 40-watts RMS of output power. In fact, most (if not all) vintage guitar amplifier speakers were typically rated for 10 or 15-watts power handling (be sure to check out the section on speakers for an understanding of how these differences really affect the tone). When the challenge for more power was issued, certain parts had to meet different needs. Transformers and speakers had to be 'beefed-up' and the tone changed as these new parts were harder to punish into giving us the sound we liked. Another casualty in the quest for more output power was a simple overlooked fact; the power supply (B+) was increased to help achieve more output wattage from the same pair of 6L6's. Aside from also giving more headroom, here's how that idea works out a little further; by simply following the schematic for coupling capacitor values, yet increasing the B+, the frequency response changes. One guru-written book mistakenly states that tubes pass high frequencies 'more easily' at higher voltages. This is flat-out wrong; in reality the plate resistance for any tube decreases when the plate voltage increases. The facts are that while the amplification factor (mu) is constant throughout a wide range of plate voltages and currents, the transconductance and the plate resistance usually show considerable variations1. But let's not just accept that theorem from respected books such as Basic Theory and Application of Electron Tubes (Department of The Army Technical Manual). Let's look at a chart for a pair of common dual triodes from the early days of audio, and do a little intuitive rationalization on our own.
By using the above chart for the dual triode tube 6SC7 (taken from the RCA tube manual RC-17), we can see that with keeping the bias steady at an arbitrary value of -2VDC on the grid, a plate voltage of 200VDC yields approximately 1.2mA of plate current (as seen above in the lower red highlight ). With a calculator we can derive at a DC plate resistance of 166,666 ohms. By 'bumping up' the plate voltage to 275VDC, we have a 'new' plate current of 2.5mA along with a decreased DC plate resistance of 110K (as seen above in the upper red highlight). This is a reduction in plate resistance of about 34%, a significant number. Other tubes can exhibit a wider plate resistance swing, as we'll see below.
The above chart for the 6SN7 (again from RC-17) demonstrates that with an arbitrary bias of -4VDC, a plate voltage of 150VDC yields a plate current of 6.5mA, or a DC plate resistance of 23K. By bumping up the plate voltage to 250VDC we derive a new plate current of 22.5mA, and a DC plate resistance of 11K. This is a reduction of about 52%, and is a tremendous difference. Varying the bias has the same effect, as we see below, staying with our 6SN7 triode.
Here we have kept the plate voltage at 200VDC, but varied the bias voltage. With a bias voltage of -6VDC, we acquire a plate current of 7.5mA, and a DC plate resistance of about 27K (seen above in the lower red highlight). Now by lowering the bias voltage to -4VDC (seen above in the upper dark blue highlight) we get a plate current of 13.5mA, and a DC plate resistance of about 15K. This is a reduction of almost 45%. With a tube manual, a pencil and a calculator, you can find out for yourself that every tube exhibits some plate resistance changes with a change in either plate voltage or grid bias voltage.
Now carrying that idea further; we might know that by decreasing the input impedance of a tube, we must increase the coupling capacitor to achieve exactly the same low frequency cut-off point. This can be demonstrated in certain Fender amplifiers. Observe the schematics of those with a phase inverter input grid resistor value of 1Meg and an input coupling capacitor of .001uf (Twin Reverb AB763, for example). Now compare that to a grid input resistor of 330K and a coupling capacitor of .01uF (Twin Reverb AA270). This was done to keep the same frequency response of the amplifier. Fender did leave the coupling capacitor at .001uF with 330K grid resistors on the AA769 Twin Reverb, but changed over right away, in speculation to counter complaints that their Twin Reverb may have sounded too 'thin'. The same effect can be replicated by increasing the plate voltage, with some tubes exhibiting a greater variation of plate resistance than others. This is now the time where Mr. Thevenin comes in and reminds us all that the input impedance of the stage will be a mathematical exercise, so let's avoid that and just say these examples are used for demonstrating a point and not a design lesson. And I know there is a difference between AC plate resistance and DC plate resistance (the AC plate resistance is defined as the ratio of the change in plate voltage to the resulting change in plate current, with the grid voltage held constant; the DC plate resistance is simply the static plate voltage divided by the static plate current), as well as static versus dynamic loads. All we need to remember for now is that AC plate resistance is lower than DC plate resistance. And remember our book Basic Theory and Application of Electron Tubes (Department of The Army Technical Manual), which states..... "the higher the applied voltage, the lower is the a-c plate resistance. Likewise, a change of the negative grid voltage in the positive direction causes a lower plate resistance." This statement is repeated for all tube types, and seems pretty straight forward. In our first example above, the 6SC7 triode's AC plate resistance would be 75 volts (the 'bumping up') divided by 1.25mA (the resulting increased plate current), or 60K. However, the AC plate resistance is anything but constant. The whole point to that previous ramble was to demonstrate that the frequency response of a tube circuit would change with a change in plate voltage and the same circuit values held throughout. Most 'guru' types miss that fact, and claim the coupling capacitor type or resistor type made the tone 'warmer'. It did not, nor did the capacitor type do anything to the tone. An old, 'dried up' paper capacitor can affect the tone, as we'll see later.
To reiterate; most 'guru' types put the credit of the vintage amplifier tone on certain parts; resistors and capacitors as examples, and not the resulting frequency response changes (along with having more headroom) of increasing the B+ or simply crediting 'dried up' parts. Here we come to another point where my own opinions and theories are the only proof that I have to offer. I have always been a purveyor of nostalgic advertising, especially as it pertains to the electronics industry and guitar amplifiers. I also spent many years in the consumer electronics servicing industry and made a hindsight observation that no capacitor or resistor was ever marketed or advertised to the electronics/audio industry as having better tone! They may been marketed as having less inductance, or having better value tolerance, but that's about it. I would like to think that if it made even a 1% 'improvement' in sound, the advertising department would have a field day, coining catchy slogans that would send 'Put A Tiger In Your Tank' out to pasture. I catch a lot of criticism on the point that any AC signal (including, but not exclusive to, sound) cannot, and does not physically travel through any capacitor (or tube) God has invented thus far! A simple understanding of the simplest definition of exactly what constitutes a capacitor should be all the proof the average person requires. However, this is not the case, and I'll just clarify my understanding of the definition and leave you to draw your own conclusions.
In its neutral state, both plates of a capacitor have an equal number of 'free electrons', as shown below left. When the capacitor is connected to a DC voltage source through a resistor, as shown below right, electrons (negative charge) are removed from plate A, and an equal number are deposited on plate B. As plate A loses electrons and plate B gains electrons, plate A becomes positive with respect to plate B. During this charging process, electrons flow only through the connecting leads and the source. The movement of electrons ceases immediately when the voltage across the capacitor is equal to the applied voltage. If the capacitor is then disconnected from the source, it retains the stored charge for a period of time dependant on the type of capacitor. Now, intuitively, if the source is an AC signal (as in audio work) the charging/discharging process is repeated.
Most electronics theory books, even those aimed at high school students, make a point-blank statement similar to the following, in an attempt to establish a basic 'fact' about capacitors.
The capacitor has the capability to store the charge as an electric field on the plates, to be released as a current into the circuit of which the capacitor is a part4. The 'insulator' in your capacitor can be air, oil, ceramic, paraffined paper, Teflon, mica, glass, and other materials. It could be argued that since no insulator is perfect, leakage (including, but not exclusive to, inductance) may be substantial enough to introduce a multitude of distortions (phase shift, linearity, etc.) to the waveform. As an example, the old paper and wax capacitors will not have as high of an insulation resistance as the modern plastic types, while other capacitor types can have a sloppy tolerance, poor temperature coefficiency, drift, leakage inductance, etc. However, at guitar frequencies capacitor types won't make much of a difference that the average to above-average person can hear. These distortions would most likely affect the overall frequency response and/or phase shift of the capacitor's linearity in the ultra-sonic range, so don't worry if the wave form looks any different on your oscilloscope. That is what we in the business refer to as 'listening with your eyes', and is a very remarkable feat. What you can hear is your 'old' paper capacitor drying out, and causing a high-frequency roll-off (increased inductance), and a slight loss in signal level. Of course a 'new' capacitor will sound different; the original tone has returned to your amplifier! The rolled-off high-frequency response always gets confused with sounding 'warmer', because the kids today that espouse certain capacitor types obviously have little to no experience with building or restoring old-time tube audio equipment. I repeat, finding an old-time audio jockey in your area is important. They have much more to teach you than any book written by some new-kid-on-the-block that gets basic theory wrong.
I have recently 'discovered' a few writings from 'vintage' electronics magazines that may help shed some light on the topic of capacitors. Although each 'idea' may seem disparate from the other, by sifting through each writing, and amalgamating through the 'facts', an answer could be at hand. You will to do further investigating on your own. After you have digested this information, you may supplement it with a visit to a website that explains things a little better than I can ever hope to. Rather than plagiarize the writings, I will merely suggest you read them yourself by CLICKING HERE.
Unfortunately, an AC signal (including, but not exclusive to, 'sound') can and does have to physically travel through a resistance. DC flows through a resistance quite well, also. This lead to the popular myth that carbon composition resistance have 'the tone'. And, while it might be hard to scientifically argue that myth, what most people who experiment with carbon composition resistors find out is that they are noisy. Any 'magical' enhancements are washed away in a sea of white noise in extreme cases (not all that uncommon, either). If resistors do have a 'sound', it is akin to bacon frying or someone crumpling up a piece of paper over and over again, but at sporadic intervals. How does this happen? The stability of carbon composition resistors leaves a lot to be desired, with the possibility of a permanent increase in resistance (up to +3%) from the simple act of soldering the resistor into the circuit. Other composition maladies include; a 2.5% decrease in resistance from applied voltage, a 10% resistance increase due to high humidity conditions, and resistance decrease at higher frequencies due to the capacitance of the resistor (dependant on the original resistance value and the applied frequency)5. Combine that with a resistor that has a sloppy tolerance, and you have a recipe for trouble, my friend. However, most of these mentioned woes seem to occur at higher RF signal frequencies. The problem of 'thermal drift' is still the one common trouble area in most vintage amplifiers. By simply changing any high value resistance (such as plate loads), we can alleviate the 'frying bacon' sound. Otherwise, resistor types don't really matter to the audio frequency range.
Vacuum tube history is a story so vast and unbelievable, that it would take far too long to tell you everything you may want to know. In a nutshell, many years ago vacuum tube manufacturing was done by only a few real factories. RCA would have GE 'make' RCA tubes, but RCA also bought smaller amounts of tubes from Sylvania as well as Tung-Sol and Westinghouse. To appear as if the consumer had a choice in tube brand purchases, the same tube would be labeled RCA, Radiotron (in the Eastern U.S.), or Cunningham (in the Western U.S.), and still have been manufactured by GE! The lesson here is that you shouldn't be hung up on using only RCA 'Black Plate' 6L6's in your Super Reverb! These tubes could have been made by GE, and also sold to anybody else who asked for them! You need to develop a trained eye and be able to discern the actual origin of any tube you examine. Vacuum tube manufacturing today can be so full of poor quality control that this makes tube 'matching' a necessary evil. Having said that, there are still a number of manufacturers making excellent quality tubes today that are not the same as the 'oldies', but they can hold their own with these classics. It is very true that in some amplifiers older Russian and Chinese output tubes simply cannot hold up. One explanation is that these tubes are put in amplifiers that were designed 'on the edge' and run any set of tubes very hard. 99% of the time it is the screen grid that is the 'culprit'. Common offenders include early Marshall and HiWatt heads. The whole mania surrounding NOS 6L6's started when Sovtek decided to market a 'fake' 5881. In reality, it was a tube intended for the Russian military. Manufactured by the Reflektor Corporation, of Saratov, these are simply a 'close' relation to the 6L6 your Grandfather knew. They were not identical in pin arrangement, nor were they even called '6L6', but had other numbers like 6P3C or 6P3CE. When they were first brought over to North America, they were rebased, and labeled 5881! They do work as a 6L6 type, but have to be treated as a separate tube entirely. When wannabe technicians bias these tubes exactly as they would a 'real' 6L6, the results are disappointing. Many aspiring writers post their 'taste tests' of tube types, and make a statement similar to '...All tubes were biased to 40mA.' They are just wasting their time, and yours. With careful circuit (re)design, and proper biasing, an astute technician can make virtually every tube sound very acceptable and last a reasonable length of time. Today it seems the Russian manufacturers are quality-conscious as they have never been before, and the tubes seem much better. If you can learn to work with them, they perform well, and can sound fine in most amplifiers. The real damage to the Sovtek reputation was the 6V6, or more properly the 6Pi6C or the 6N6C. These 'not really all that close' 6V6's were rated for a plate voltage of only +250VDC! Of course they didn't fare well in a Deluxe Reverb, and the reputation of Russian tubes is forever changed.
It has taken some time, but I am slowly getting together the 'real' spec sheets for the original Russian tubes brought over to North America and branded Sovtek. So far, I can tell you that the 6P3C is supposed to be the equivalent of a North American 6L6GT and the 6P3CE is purposted to be the equivalent to a North American 6L6GC or the 5881. The specs would absolutely be different. To compare just how different, CLICK HERE! Be sure to read my notes at the bottom of each page!
Please don't say the NOS tube has a 'sound', because to repeat again for your edification; AC signal cannot and does not pass through any tube6. Remember that the term 'AC signal' includes (but is not exclusive to) any sound source, such as an electric guitar. In the early pages of the massive tome Radiotron Designer's Handbook, we are cautioned "..... it must be remembered that it is the plate supply which, in reality, supplies the power - the valve merely controls the current by its varying d.c. plate resistance." Then the remaining 1,452 pages fill us up with formulas, load lines, and equivalent circuits, which are admittedly..... "valid under limited conditions..... only a convenient fiction..... merely a device to produce in the load the same a.c. currents and voltages which are produced by the valve when alternating voltages are applied to the grid." Oh, well. At least they didn't spring that on me at the end of the book. And that little mention of the varying plate resistance seems to have flown over the heads of people who criticized my 'opinions' mentioned earlier. It ain't easy pointing out that the Emperor isn't wearing any clothes, and it's disheartening to have basic tube theory 'facts' ridiculed by kids who've been in 'the tube business' since 9:00 AM last Wednesday.
Tube 'matching' is a phenomenon that refuses to die. Older tubes were manufactured under much tighter tolerances, making matching unheard of and unnecessary. When many surplus outlets appeared after WWII, manufacturers worried just a little (they lost a quite few 'new' sales). When Japanese tubes came into the US at a retail cost of 25% less than American tubes, manufacturers worried a lot! Enter the bright idea that we need 'matched' tubes. Replacing advertisements stating 'Only the very best goes into every box!' were ads stating that these tubes were factory 'matched' to a strict tolerance (as opposed to what before?) to ensure minimal distortion! All you need to read here is any old electronics magazine like Radio Electronics to see the hype machine in full force! Tube manufacturers asked retailers to display placards offering 'Free Professional Tube Testing On The Latest Electronic Equipment' in an attempt to coax customers into their establishment, where they could be convinced that they needed to purchase a new set of tubes for their TV or radio! This was the start of the stereotype radio/TV repairman as a 'crook'. 'Callbacks', or repairs that needed to be redone a short time after the initial repair, were the bane of early radio/TV repairmen, and part and parcel of the 'crook' label. Advertisements promised to cut 'callbacks' to a minimum by using a certain brand of tube or capacitor, the most common part to fail. Nothing was promised about having better tone, though. The Canadian electronics magazine Electron featured an article on how the Canadian tube industry was feebly trying to offset the imported tube challenge. To see that article, CLICK HERE.
In the interest of sharing everything I have 'discovered', regardless of how disparate it may seem now, you can read a small sampling from a few papers Mullard published looking at tubes in a way no one else seems to be able to.
HEY! Take a look at my own modest collection of classic tube boxes, and see the advertising packages of yesteryears' tubes! CLICK HERE!
Transformers and speakers have attained a cult status of their own that would rival L. Ron Hubbard, and with good reason. My personal observation and opinion is that these parts do have the most drastic effect on your final tone. But not for the reasons written in books and magazines. 'Interleaved winding output transformers on a paper bobbin have better tone!' is a popular myth started by one Texas-based tube amplifier guru. He states that the paper bobbin puts the winding closer together, and is more efficient. You are then told that by doing a redundant test of lowering the pickups on your electric guitar 1/4" and hearing the volume loss you can prove that the modern day plastic bobbin transformer is equally as inefficient! What is omitted from the equation are a few simple facts that should be evident to someone with experienced eyes.
If you compare the physical size of electrically identical power transformers from yesterday and today, you'll get a much better idea of what I am talking about. The same 'improved' technology was passed on to output transformers. This is why I don't like Hammond output transformers; they're too good for what I personally like to hear in a guitar amplifier! I want an underdesigned, flimsy output transformer! Many 'new' technicians are flabbergasted when they see the teeny little output transformers Marshall put in their early amplifiers. Kevin O'Connor, in his excellent book The Ultimate Tone, comments that he compared it to the same physical size as a Fender Deluxe Reverb! End of point. Other myths of vintage output transformers; in a nutshell, interleaving the windings of the output transformer will increase the high frequency response7. BIG DEAL! Guitar frequencies drop like a three-foot putt after about 4kHz. What I would worry about is the amount of iron in the core and the primary winding impedance. More iron increases the low frequency response and the transformer core will saturate a little less. Some transformers were advertised as having a nickel core that offered less distortion! Primary impedance influences power output and frequency response. Higher impedances yield narrower bandwidth, less distortion, and less output. Since most early manufactures were trying to avoid distortion, they opted for a higher primary impedance, and interleaved the winding to bring the high frequency response back to what it may have been at a lower primary impedance. They sacrificed high output for fidelity. You had to figure a happy balance of these when selecting an output transformer. Remember, early manufacturers had to use either radio or PA transformers in their amplifiers until the guitar market was seen as a viable medium and create its' own parts 'industry'. These transformers were never designed for 'tone'.
While most tube gurus espouse the gospel as written in their 'Bible', otherwise know as 'Radiotron Designer's Handbook', few have obviously read it. For if you have attempted to even read one page of the 250 pages devoted to transformers, you would be driven absolutely mad. As much as I admire the book, these pages are clearly written for nuclear physicists, and I just can't imagine Leo Fender asking about the flux density/saturation point of the steel used in a transformer he was ordering. Speakers are a whole different ball of wax; too much to detail here, so they'll get their own 'lesson' (see SPEAKERS). For a glimpse into the world of audio transformer manufacturing, check out.....
Recommended reading for this lesson includes two books listed below that deal with vintage advertising. Any product, no matter how much it influences our lives today, started out as an untried and unproven commodity. I will also suggest checking back issues of Vacuum Tube Valley, available from Antique Electronic Supply (see LINKS). Ignore articles like 'Capacitor Listening Tests' and go right to the great stories about the history of certain tubes. Each issue deals with one tube, and so far have included the 12AX7, 6SN7, 6L6, 6V6, 6550, EL34, and 6BQ5. You will be very surprised to learn the how and why behind certain tube developments. I also would like to suggest a small test: Call up a few manufacturers and suppliers, suggesting you are a boutique guitar amplifier maker. Ask about getting a chassis made, and find out the difference in cost between 16-gauge steel, 18-gauge steel, and aluminum. Ask about getting the faceplate silk-screened. Check out resistor prices. Check out capacitor prices, as they are the part with the widest price range. Remember that bigger value capacitors cost more than the smaller value capacitors for the same maker/type/voltage! Check out the prices on filter capacitors, and decide if you are going to use FP's or not. As you add up the prices you'll learn how manufactures did it 50 years ago; cut a few corners here, use smaller filters there, etc. It all added up. This is the reality of economics and business. The most interesting case of 'snake-oil salesmanship' I heard about capacitors having a 'sound' had to involve a repair column in Vintage Guitar Newsletter. The article stated that LCR filter capacitors had a 'British' sound! Ask any supplier if they have any 'British' sounding capacitors!
Remember, through advertising we've been told 'Nothing runs like a Deer', 'Nobody does it like Hoover', and 'It isn't clean until it's Lysol clean'. Makes you wonder why anybody else even bothered.
1) Electron Tubes, R.G. Kloeffler
2) Electronic Fundamentals, Thomas L. Floyd.
3) Electronic Fundamentals, Thomas L. Floyd.
4) Basic Radio, Volume 2, Marvin Tepper.
5) Radiotron Designer's Handbook, 4th Edition, F.Langford Smith.
6) The ARRL Handbook, 42nd Edition, 1965 (page 62)
7) Radiotron Designer's Handbook, 4th Edition, F. Langford-Smith.
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