Why Should I Match My Tubes? A technician's explanation
Power tubes are electrically matched so each tube will have essentially the same amount of idle plate current and amplification characteristics when plugged into an amplifier. This is done so the tubes can be biased together in an amp for optimal sound quality and tube longevity. Apex® matching also provides extra quality checks to help reduce the possibility of receiving a faulty power tube that may have slipped by the manufacturer.
Signal Amplification: Plate Current and Transconductance
Most power tube circuits work by applying an input voltage signal to the tube's control grid which creates a corresponding output current signal at the tube's plate. The linear amplifier example of Figure 1 works the same way. The negative grid voltage is set for a bias point at the middle of the linear range of operation and the signal stays within the linear range of operation.
When biased at the middle of the linear range of operation, a tube operates in "class A," meaning plate current keeps flowing through it for the entire signal cycle, both positive and negative phases. If the negative grid voltage is set for a very "hot" (high idle plate current) bias point as in Figure 2, the positive phase of the input signal may swing into the saturation region where the grid voltage is no longer in control of plate current flow and the output signal becomes distorted from lack of headroom.
If the negative grid voltage is set for a very "cold" (low idle plate current) bias point as in Figure 3, the negative phase of the input signal may swing into the cutoff region where the grid voltage is so negative that plate current cannot flow and the output signal becomes distorted from lack of leg room.
Power tubes are biased near cutoff when operating in "class AB" so that the negative phase of the input signal will put the tube(s) on one end of the output transformer in cutoff (no plate current flow) while the tube(s) on the other end of the output transformer take over the amplification duties of that phase. This arrangement, known as "push-pull," allows for an effective doubling of the linear range of operation for the input signal. If the amplitude of the input signal is increased so that both negative and positive phases of the signal swing beyond the linear range of operation as in Figure 4, overdrive distortion will occur.
In a guitar amp with a fixed bias, the bias point is set by adjusting the bias pot which changes the control grid voltage. If the bias point is set too cold, the sound can be thin and uninspiring. If the bias point is set too hot, the tube will be overheating and the sound can be lacking in definition. Setting the bias involves finding a bias point where the amp sounds the best and the tubes are not overheating. A unique feature of the linear amplifier example is the transconductance is the same for every bias point in the linear range of operation. Trasconductance in the case of a linear amplifier is the slope of the line shown in the transfer characteristics (see Figure 4). The same amount of change in control grid voltage (∆V) will result in the same amount of change in plate current (∆I) anywhere along the linear range of operation. By contrast, the transfer characteristics for tubes are not linear from cutoff to saturation. They are curved and the transconductance changes at different bias points. The transconductance for a tube's transfer curve is represented by the slope of the line tangent to the curve at each point along the curve. Figure 5 illustrates the difference in transconductance between two different bias points along a tube's transfer curve.
Since tubes of the same brand and type have variance in their transfer characteristics, with some tubes running a little "hotter" (more plate current) and others running a little "cooler" (less plate current) in the same amp, the objective of matching is to group tubes into sets having reasonably similar transfer characteristics so they can be biased at the same operating point in an amp.
A Real World Example
Let's take a look at matched and unmatched tubes in a very common tube guitar amplifier, the Fender® Blues Deluxe®. Figures 6 and 7 show schematics of the original fixed bias/non-adjustable Blues Deluxe® power amp circuit with current and voltage measurements taken from a matched pair of tubes that run a little cooler and a matched pair of tubes that run a little hotter in the same amp.
The "cool" matched pair of tubes drew 21 mA(DC) per side and had plate and screen voltages of 436 VDC for an idle plate dissipation = (.021 A) x (436 V) = 9.2 Watts. The "hot" matched pair of tubes drew 35 mA(DC) per side and had plate and screen voltages of 424 VDC for an idle plate dissipation = (.035 A) x (424 V) = 14.8 Watts. The "cool" pair sounded thinner with a grainier breakup in overdrive. The "hot" pair sounded fuller with a smoother breakup in overdrive. This example shows there is a difference in the tone and electrical characteristics between a "hot" matched pair and a "cool" matched pair of power tubes in the same fixed bias/non-adjustable guitar amp.
Figure 8 shows measurements taken in the same amp with one of the tubes from our "cool" matched pair in V4 and one of the tubes from our "hot" pair in V5 to make an unmatched pair. The cool tube drew 20 mA and had plate and screen voltages of 426 VDC for an idle plate dissipation = (.020 A) x (426 V) = 8.5 Watts. The hot tube drew 39 mA and had plate and screen voltages of 425 VDC for an idle plate dissipation = (.039 A) x (425 V) = 16.6 Watts. Since the "hot" tube had nearly twice the plate dissipation of the "cool" tube, it would likely wear out quicker in the same amp over time. The sound of this unmatched pair was not terrible. There was an overall fuller sound than with the "cool" matched pair, but still a grainier breakup in overdrive distortion than with the "hot" matched pair. The Blues Deluxe® has a fixed bias, class AB output stage with two 6L6GC tubes operating in "push-pull." This arrangement is one of the most common circuits for electric guitar amplification. Most amps with this type of circuit allow for bias adjustments by way of a bias pot. One common biasing rule of thumb for 6L6GCs running in class AB in this type of amp is to set the bias pot for a corresponding idle plate current measurement of 35 mA. Let us modify the same Blues Deluxe® amp by adding a bias pot, set the bias according to this rule of thumb and see how the tone and electrical measurements compare.
While drawing 35 mA per side, the "cool" pair had plate and screen voltages of 423 VDC for an idle plate dissipation = (.035 A) x (423 V) = 14.8 Watts. This is the same plate dissipation as the "hot" pair biased at 35 mA, but there was a definite difference in tone. The "cool" matched pair of tubes really came to life when biased for 35 mA in this amp. The overall tone, which was thin and grainy at 20 mA, really filled out and warmed up at 35 mA. When cranked to full blast with the volume control, the overdrive break-up was smooth, easy and thicker than with the "hot" pair. What happens when we adjust the bias on our unmatched pair of tubes? In Figure 10, the bias was set for the "cool" tube in V4 at 26 mA (11.2 Watts of plate dissipation) and the "hot" tube in V5 at 44 mA (18.8 Watts). In Figure 11, the bias was set with the "cool" tube in V4 at 35 mA (14.5 Watts) and the "hot" tube V5 at 56 mA (23.2 Watts). It was difficult to hear a difference in tone between these two bias settings for the unmatched pair. They didn’t sound terrible, but they didn’t sound as smooth as the two matched pairs did when biased for 35 mA. With regard to the electrical measurements, the plate dissipation calculations show a significant mismatch in plate dissipation between the tube in V4 and the tube in V5 at both bias settings.
What if we modify the amp for two separate bias pots, one for each power tube and set their bias for 35 mA separately?
With two bias pots as shown in Figure 12, a bias voltage of -41.1 VDC on the "cool" tube resulted in a plate current of 35 mA and a plate voltage of 425 VDC for a plate dissipation of 14.9 W. A bias voltage of -46.3 VDC on the "hot" tube resulted in a plate current of 35 mA and a plate voltage of 424 VDC for a plate dissipation of 14.8 W. This setup actually sounded good, but still wasn’t quite as smooth and rich in overdrive distortion as the "cool" pair biased at 35 mA. This two bias pot method might be the way to go if you want to get the most out of a pair of NOS tubes that aren’t available in matched sets, but one bias pot and a matched set of tubes makes it easier to get to that sweet spot where your amp sounds the best. Matching allows you to become familiar with the electrical characteristics of power tubes and what those characteristics mean for your amplifier’s tone. Once you find the electrical characteristics that sound best in a particular amp, you have a way to consistently find that sound again and again. Why matched tubes? They put you in control of your tone.