A preamp tube does the job of amplifying the tiny signal from your guitar pickup and making it bigger. Most guitar amps have anywhere from one to six signal tubes depending on the number of features the amp incorporates (channels, send return loops, gain stages, tremolo, reverb pan, etc). The V1 position, the first gain stage, is often cited as the critical tube to substitute in guitar amps. The reasoning behind this is that if the first gain stage is adversely noisy, that is, the tube is generating hiss, picking up electrical hum or sensitive to external vibration (microphonic), then this unwanted noise will contaminate your guitar signal. In fact, with an extremely bad tube in the V1 position the noise can be higher in level than the guitar signal the tube is supposed to be amplifying. At best this is off-putting in a gig; at worst, really objectionable in a studio situation.
A good N.O.S. tube, such as Mullard ECC83 can work wonders in the V1 position to ensure low microphony, hiss and hum. European tube manufacturers utilised a high grade magnesium-nickel alloy (see BS 3504:1962), which was even better than ‘Cathaloy A30’, to make their cathodes. This alloy was specifically developed to be beneficial to the barium/strontium/calcium oxide layer to maximise the electron emissivity and operational life of the cathode and this coupled with Mullard’s precision electrode geometry meant that there’s never been a tube quieter than a Mullard ECC83. There are other tonal benefits to be had by swapping the V1 tube out too, in guitar amps and Effectrode pedals. Any changes in tonal characteristics can be attributed to the following factors:
- Capacitance This is not so much a case of long plate tubes having greater capacitance than short plate tubes—a quick look at the datasheets for a 1962 RCA 12AX7 and modern JJ ECC83 reveal the plate to grid capacitance for these long and short plate tubes are both exactly the same at 1.7pF—but more of case of computer tubes exhibiting greater capacitance than audio tubes. For example the 12AV7 has a plate to grid capacitance of 1.9pF and a 5965 comes in at almost double the capacitance of a 12AX7 at 3.0pF. Tubes with longer plates have higher capacitance and depending on how the tube circuitry is implemented, substituting a short plate tube with a long plate variety can give a gentle and welcome rounding off of the top end yielding a thicker, more bluesy sound. Technically what is happening is that the capacitance of the tube forms a low-pass filter with grid resistor in the circuit to filter out high frequencies at -6dB/octave. Incidentally, as a general rule tubes with longer plates exhibit greater microphony than tubes with shorter plates, however Mullard had their manufacturing process nailed—inter-electrode tolerances were as close to perfection as anyone ever got—and electrical tests indicate their Blackburn made long plate ECC83 is a great deal less sensitive to external vibration pickup than any modern long or short plate tube.
- Gain Factor Substituting a 12AX7 for a tube with another of lower gain factor, such as a 12AY7 will lower gain in the preamp section. Again, the amount of gain reduction depends on the circuit implementation. In a typical tube preamp circuit where the plate resistor is chosen so that the 12AX7 is operating close to its full 33dB gain, a 5751 tube will reduce gain by a few dB and a 12AY7 substitution will knock about -15dB off this to open up the sweet spot for more bluesy sounds. The tube in the V2 position on the Effectrode ‘Blackbird’ and ‘Tube Drive’ pedals spot can also be changed out for lower gain tube types to increase the amount of headroom for a bigger, bolder sound, an approach adopted by Stevie Ray Vaughan’s amp tech—he substituted the stock 12AX7 tube with a 5751 in the V2 spot on SRV’s Vibroverb and Super Reverb amps.
- Bias Point In tube textbooks such as the RCA Receiving Tube Manual you’ll find recommended biasing arrangements for different tube types. The idea is to set bias voltage with the cathode resistor to obtain maximum linearity before onset of clipping on the negative and positive signal excursions. However, being guitarists, that’s not we’re usually after and invariably we’re seeking some additional distortion to add texture and complexity (in a word, ‘hair’) to the straight sound of the guitar. If a 12AX7 tube is substituted with a 12AY7 then the biasing point will shift and this alters not only gain but the sensitivity therefore altering the biting point before onset of clipping. So, to summarise, bias can be adjusted by either changing the resistance of the cathode resistor or the tube itself.
Modern tubes can also be utilised in the V1 and V2 positions to alter capacitance, gain and bias conditions as described above and therefore affect tone, but the space-grade materials and construction of good quality N.O.S. tubes, manufactured by the likes of Sylvania, Tung-Sol and Mullard, are almost always an assurance that the N.O.S. tube will exhibit superior performance in terms of reliability, longevity and noise over a modern tube. Some would go even further to suggest N.O.S. tubes impart other kinds of indefinable sonic ‘magic’ to tone. This is a contentious subject amongst hard core tube heads (I freely confess to being one myself) where religious dogma clashes with cherry-picked scientific factoids, inevitably generating friction—and where there’s friction there’s heat, and often fire—these debates are normally best avoided.
However, there are many subtleties in human auditory perception and often overlooked flaws in electronic amplification systems which do have real effects on tone. For instance, the fact that all tubes are microphonic to a greater or lesser extent. At higher volume levels this inherent defect of an electron tube means that any tube will pick up ambient noise or vibration (almost certainly the amplified guitar sound) to some degree and feed some of this component back into the direct signal adding more depth, dimension and body to it—this non-linear effect is one of the reasons it’s proven so challenging to replicate certain aspects of the ‘tube sound’ with analogue solid-state (transistor, op amp or FET) circuitry or various emerging digital technologies. Perhaps this explains why some audiophiles prefer long plate tubes—although higher in microphony, this microphony in small doses imparts a euphonic effect on tone. This sensitivity to external vibration also adds sustain to the guitar sound and in more extreme cases feedback howl. This is considered adverse microphony, however can be useful in certain special situations where it can be used as an effect in its own right.