The finished BYOC Classic Delay kit (Photo 1).
Tools You’ll Need for This Project
- Soldering Iron
- Solder
- #2 Phillips screwdriver
- Wire strippers
- 8 mm (5/16") nut driver/wrench
- 10 mm (25/64") nut driver/wrench
- 1/2" nut driver/wrench
- 14 mm (9/16") wrench
- Flush cutters
- Small needle-nosed pliers
- Third hands
These tools are available via a variety of suppliers, including StewMac, Allparts, and Amazon.
Step 1: Review the Instructions
The kit as it arrives (Photo 2).
Each BYOC kit comes with a detailed set of instructions in the form of a PDF that can easily be printed out. The Classic Delay’s instructions can be found at http://www.byocelectronics.com/classicdelayinstructions.pdf. They are 32 pages long, and I suggest following their steps as you read this article. Here, I will refer to specific page numbers that correspond with the steps. I’ve also taken photos to coincide with the steps. Within the instructions, we find a table of contents, pictures of the fully assembled pedal, a list of parts included, and step-by-step instructions from internal population and soldering to external assembly. With any kit, always read the instructions carefully before you start building.
Step 2: PCB set-up
Photo 3
A lot of pedal builders use PCB jigs that are specifically designed to hold several printed circuit boards so that they can be easily populated and soldered. These jigs are great tools. However, they’re not something a casual DIY enthusiast will often have. Essentially, all we really need to do is elevate the PCB off of the working surface so that the leads of the components can pass through the bottom side of the PCB. This is most helpful with components that can be soldered from the top side of the PCB. “Third hands” or “helping hands” (Photo 3) are a good tool for this job. So, what can we do to elevate the PCB if we don’t have access to third hands? Simply turn the enclosure upside down and place the PCB perpendicular (Photo 4). This will elevate the PCB enough to allow the component leads to easily pass through.
Photo 4
Step 3: Populating Resistors and Diodes (instructions page 9)
Photo 5
When populating PC boards, we typically like to work lowest to tallest in regards to the seating. Seating is how far above the PC board the component rests. Resistors and diodes sit pretty low to the PC board, so populating them first makes sense.
Resistors: Remove the resistor from the paper ribbon, bend the leads (Photo 5), and place them through the corresponding pads (Photo 6) as directed. (A pad, by the way, is the term for the designated surface area of a component’s electric contact point.) A good practice is to populate all resistors of the same value before moving on to the next value. Populate all 1k resistors, then all 10k resistors, and so on.
Photo 6
Reading resistor color bands can be confusing, so don’t forget the reference guide on page 7 of the instructions. Here, you will find a detailed breakdown of each value and its corresponding 5-band reading (i.e. 1k = brown/black/black/brown/brown). Once all the resistors are placed, we can go on to the diode.
Diode: Next, we need to place the diode. Just like resistors, remove the diode from the paper ribbon, bend the leads, and place them through the corresponding pads. Diodes are polarized and need to be oriented a certain way (Photo 7). Be sure to match the diode to the outline on the PCB, as shown on page 10 of the instructions.
Photo 7
Step 4: Solder Resistors and Diode
Photo 8
Fire up the soldering iron! Once the iron is up to temperature (650 to 750 degrees Fahrenheit, depending on how fast you work), it’s time to get to work. Resistors and diodes can easily be soldered from the top side. So no need to flip the PCB over. Place the solder tip to the pad and feed the end of the tip a bit of solder. What we’re looking for here is solder that covers the entire pad, encapsulates the lead, and has a nice shine to it. Think of it like a shiny Hershey's Kiss shape (Photo 8). Repeat this step for every part. If you’re new to soldering, you should consult our concise guide to soldering, online or in PG’s October 2015 issue.
Soldering tip: Shut the iron off between population steps. Use a small, tabletop fan to blow the solder fumes away during soldering steps. Quick tutorial videos on YouTube are also of benefit here, for builders new to soldering.
Step 5: Trim the Leads
Photo 9
Next, place the PCB upside down to expose the leads (Photo 9). Using flush cutters, trim the excess leads (Photo 10).
Photo 10
Step 6: Soldering IC Sockets (page 11)
Photo 11
Place the 8-pin and 16-pin IC sockets into the corresponding pads on the top side of the PCB. We need to solder the IC socket pins from the bottom side. This means flipping the PCB over without having the IC sockets fall out. For this, I like to use a small piece of foam (Photo 11).
Photo 12
I place the foam on top of the PCB (Photo 12), then flip the foam and PCB simultaneously so the foam is below the PCB and the bottom side of the PCB is facing up (Photo 13).
Photo 13
For this soldering step, we’ll want to solder ONLY 1 pad and then flip the board over to ensure that the IC sockets are sitting flush. If the IC sockets are not flush, go back to the bottom side and reflow that solder pad while pressing the PCB downward and keeping it parallel to the work surface. Solder the remaining pads (Photo 14).
Photo 14
Step 7: Soldering the Voltage Regulator (page 12)
Photo 15
Place the voltage regulator into the three corresponding pads, while keeping mindful of the orientation. The flat side needs to match the outline on the PCB. Now, we’re ready to solder. This can be done from either side. Flush-cut the excess lead and we’re done here (Photo 15).
Step 8: Soldering the Trim Pot (page 13)
Photo 16
Place the trim pot (Photo 16) into its corresponding pads. For the trim pot provided, I found it easiest to solder the two exposed legs from the top side (Photo 17) and then the remaining leg from the bottom side.
Photo 17
Step 9: Soldering Ceramic and Film Caps (page 14)
Photo 18
Place all ceramic (pill-shaped) and film (red) caps into their respective locations. Again, use page 7 of the instructions as a reference for identifying the correct capacitors (i.e. 100n film cap, which may say “104” or “.1” or “u1” on the body). We’ll need to solder the pads from the bottom side. So, once again, the piece-of-foam trick can be your friend here. Flush-cut the excess lead and we’re done (Photo 18).
Tip: An alternate method would be to bend the leads away from each other so they stay in place when the PCB is flipped over (Photo 19).
Photo 19
Step 10: Soldering Electrolytic Caps (page 15)
Photo 20
Let’s do the same thing here as we just did in Step 9. One thing to note is that electrolytic capacitors are polarized and need to be populated in the correct orientation. This is denoted on both the PCB and the component itself. On the PCB, the positive pad is denoted by the square pad. On the electrolytic capacitor, the positive lead is the longer leg. Additionally, the negative lead of the cap also corresponds to the white strip on the body (Photo 20).
So, let’s go ahead and populate the caps and solder them with either the foam trick or by bending the leads. Flush-cut the excess lead and we’re done (Photo 21).
Photo 21
Step 11: Battery Snap and Hook-Up Wire
Photo 22
This DIY kit offers a battery option for those that feel so inclined. (See page 17 of the instructions.) For this step, we’ll need to connect the battery snap to the PCB. Before soldering, we’ll want to first feed the two wires through the holes directly below the solder pads. These holes act as strain relief for the battery snap and have + and - signs next to them (Photo 22). Then, simply solder the red wire to the positive (+) pad and the black wire to the negative (-) pad (Photo 23).
Photo 23
Next, we need to cut and strip the included wire (page 18). We need four 2.5" pieces of wire, and one 1.5" piece (Photo 24).
Photo 24
Place the 2.5" wires into the top side of the solder pads for in, out, and the two courtesy grounds—all handily marked. Then, solder from the bottom side. Do the same for the 1.5" wire going to the ring pad (Photo 25).
Tip: If painting the enclosure is desired, this is the last chance you’ll have to do it. After this, there will be components mounted to the chassis.
Photo 25
Step 12: Install DC Jack (page 19)
Photo 26
Place the DC jack into the large hole on the back heel of the enclosure and tighten the nut using a 14 mm (9/16") wrench (Photo 26).
Step 13: Potentiometers and Status LED (pages 20 and 21)
Photo 27
The included instructions for this next part have you mate the PCB to the inside of the enclosure. This will help line up the pots and LED. However, afterwards it will make soldering the remaining wire more difficult. Here’s a trick we can do that gives us the benefits of using the enclosure to help with pot alignment without needing to take the PCB out afterwards to solder the remaining wires. What we’ll want to do is simply use the face of the enclosure to hold the pots and set the LED height. Additionally, we can use the flush cutters to help balance the PCB (Photo 27). Be sure to have the long lead of the LED mate with the square pad, then solder away!
Step 14: Final Hook-Up Wire
Photo 28
My steps continue to vary slightly from the instructions throughout the rest of the build, so you might want to do a side-by-side comparison. Let’s finish the wire for the DC jack next. Cut and strip three 1.5" pieces. These need to be placed into the three power pads at the top of the PCB marked -, +, + , as in Photo 28.
Photo 29
Next, cut and strip five more 1.5" pieces. Place these in the footswitch pads at the bottom of the PCB marked 1, 2, 5, 7, 8 as shown in Photo 29.
Photo 30
Last wire! Cut one 1.5" piece. Only for this one, strip half an inch off of one side (Photo 30). Place the short-stripped side into the footswitch pad labeled 4.
Step 15: Footswitch prep
Photo 31
For the footswitch, we need to jump lugs 3 and 6. To do this, we’ll use the remaining wire. Place the footswitch into the correct hole on the face of the enclosure. Then, cut a 1" piece of wire and strip half of it. Feed the exposed wire through lug 3 and into lug 6 and then solder both (Photo 31). Cut the excess wire.
Step 16: Insert the IC
Photo 32
Now, we’re ready to insert the integrated circuits into their respective sockets. These need to be placed in the correct way, and there are two ways to identify them. Pin 1 is the top left leg of the IC—which is the rectangular black object in Photo 32. These legs are marked with a small dot in the top left corner or the upside of the IC is marked with a debossed half circle.
Step 17: Mounting and Final Soldering
Photo 33
PCB: Now for the real fun! We’re ready to start inserting chassis-mounted components. Let’s start with the populated PCB. Place the PCB with the three pots lining up with the drilled holes, place the pot washers on the shafts, and then tighten the pot nuts using a 10 mm (25/64") nut driver/wrench, for the results in Photo 33.
Photo 34
Footswitch: Now that the PCB is securely tightened, let’s go ahead and do the same for the footswitch. Remove all hardware from the footswitch bushing except for one nut and the lock washer (Photo 34).
Photo 35
Feed the footswitch through the footswitch hole, making sure that the footswitch has the two poles that we soldered together facing the bottom left (Photo 35).
Photo 36
Next, place the plastic washer onto the bushing (optional), and tighten the hex nut with a 14 mm (9/16") wrench (Photo 36).
Photo 37
Lets go ahead and solder the footswitch wires to their respective footswitch poles (Photo 37). See the instructions’ page 26 for pole-numbering reference.
Tip: Solder from the top row down—i.e. 1, 4, 7, then 2, 5, 8.
Another tip: Remember that the wire for pole 4 also connects to pole 9.Step 18: DC Jack
Photo 38
Next, we’ll need to solder the three wires that go to the DC jack, as explained in page 22 of the instructions. Start with the middle wire, since it sits the lowest and will be easier to get at without the other two wires in the way (Photo 38).
Step 19: Audio Jacks
Photo 39
The last chassis-mounted components are the two audio jacks. Let’s do the stereo input jack first. Go to page 24 of the instructions to see how these jacks are oriented. The lock washer goes on the bushing first, then gets placed into the hole to the left of the DC jack. Then, place the washer onto the bushing and tighten the hex nut with a 1/2" nut driver/wrench.
Follow the same steps for the mono output jack. And then, it’s the final soldering step: Solder the wires to the appropriate lugs on the audio jacks (Photo 39), as on instructions page 28. Then, finally, place the knobs on, tighten them down, and we’re done!
Jens’ two-microphone setup for resonator guitar and his Fender Super Reverb.
In a resonator, the metal cone underneath the front plate functions as a base for the bridge saddle and vibrates like a loudspeaker to project sound. Because of the instrument’s directional character and ease of feeding back, resonator guitars are difficult to use in live bands with loud stage volume. The goal is to get enough volume resonator onstage and in the room without feedback. Since it’s an important part of the instrument’s character, it’s necessary to capture some of that metal-cone voice within an overall balanced guitar tone that sits well in the mix. And you need to be able to achieve this in a way simple enough to reproduce the same tone night after night, and focus on your music, not technical problems.
The easiest way to mic a resonator guitar is to place a microphone on a stand about a foot in front of the guitar and run the signal into the mixer. But this pure acoustic route only works well in low volume situations—due to feedback—and requires you to play sitting down or standing still.
“Because of the instrument’s directional character and ease of feeding back, resonator guitars are difficult to use in live bands with loud stage volume.”
You can also use the type of clip-on microphone (often used for violin) made by Neumann and DPA, among others, to make you mobile, but if you get too close to the monitors or mains, feedback ensues. I recommend the microphones that come with a preamp and volume dial, to give you the most control. If you use one of these, point the mic directly toward the metal cone. Finding a nice tone will require some experimentation.
Another option is a piezo pickup. Some newer resonator guitars come with factory-installed piezos. It’s not very difficult to install a passive piezo yourself. You need to file and sand the bridge piece, detach the metal cone for wiring and soldering work, and, finally, drill a hole in the guitar where the jack plug goes in through the strap button. Getting the string tension correct over the entire length of the bridge saddle is the key to success. Lately, I have used hard oak as bridge material, which creates a smooth and mellow tone with bronze strings.
The author’s resonator “kit.”
Piezo pickups are less prone to feedback than acoustic microphones, so they are better for high-volume stage scenarios. But piezo pickups alone will not fully capture the tone of the resonator cone, so if you’re looking for more of that, add an acoustic clip-on microphone. That’s what I do. Both the piezo and the clip-on go to my Fender Super Reverb, and I use the normal channel for the feedback-sensitive acoustic microphone while the piezo goes to the vibrato channel with reverb. For dirty tones, I sometimes use an overdrive pedal for the piezo pickup. There are independent EQ and volume settings on both channels.
The third and last microphone technique I have experimented with is a humbucker. I bought one from 12 Bar Blues Pickups that is only 6 mm tall and built specifically for resonators. It fits nicely under the strings with enough clearance. The advantage with a passive magnetic pickup is obviously the simplicity. The installation process is easy if you simply tape or glue the pickup to your guitar and let the wire run externally on top of your resonator’s body. The kit I bought contains a small volume and tone box with jack input, and requires no battery. The result is, essentially, a hollow-body electric guitar suited for pedals and regular amps. It is also more resistant to feedback than both piezo and acoustic microphones. When I want to add some of the “bluegrass” flavor of the resonator cone, I add the acoustic microphone as mentioned earlier.
One drawback with conventional magnetic pickup technology is that bronze strings have a lower output, since bronze is less responsive to magnetic fields than nickel. I have kept the bronze strings on my resonators because of the great tone. To get even output on the bass strings, the bridge height measures a little higher on the bass side than the treble side.
So, there we have several different techniques for capturing modern resonator guitar tones, including a few options with guitar amps. Now, go experiment!
What exactly is bias? Bias is prejudice in favor of or against … oh wait, wrong kind of bias. I think he wanted me to write about bias in a tube amplifier, which is far less polarizing.
Bias, as defined in the RCA Radiotron Designers Handbook, is “voltage applied to the grid [of a tube] to obtain a desired operating point.” Well, that is the most basic explanation, but for the most part it is good enough and pertains to the majority of tube output stages in our favorite tube guitar amps.
Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased.
Besides “applying” a voltage to a vacuum tube, however, biasing can occur in another way as well. There are quite a few amplifiers, such as a Vox AC15 and AC30, any of my Budda and EAST designs, and even most of the early, low-wattage amplifiers of the tweed era that use what’s known as a “cathode bias” design. This is where the current flowing through the tube (which attains the aforementioned “desired operating point”) is not set by the voltage “applied” to the grid of the tube, but is instead set by the resistor in the cathode leg of the tube. It’s a bit more complicated than that, but the result is an amplifier whose output stage is “self-biasing.”
Most amplification devices, including transistors and even preamp tubes, need to be “biased” in order to perform properly, but this type of biasing is fixed in the design parameters of the circuit. In the case of the preamp tubes in your guitar amp, bias is based on the value of the cathode resistor, among other things. But that’s enough design theory for today. Let’s get back to the core task of biasing the output tubes in most guitar amplifiers.
First, the bias voltages you see listed on many schematics, such as 52V on a black-panel Fender Twin Reverb or 51V on a Marshall 100W Super Lead schematic, are merely approximations of the voltages that should be expected in that area of the circuit. Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased. Tube bias is also dependent on the high voltage (or B+) applied to the plate of the output tube, which can vary within tolerances of the transformers as well as in the AC line voltage fed to the amp. (This is why amps can sometimes sound better in one room or club than others.)
But even more important to understand is that tubes produced in different factories across the globe will bias up differently! What I mean by this is, if you properly bias a set of output tubes—let’s say 6L6s made in Russia—and then you swap them out with a set made in China, in the same amplifier without changing the setting of the bias control, the end result will almost always be a different bias reading. This is why it’s always best to have checked and reset the bias whenever output tubes are replaced. Now, how do we do that?
The Preferred Method
Fig. 1
There are several different ways to measure output-tube bias current at idle. The safest method is to use what is commonly called a bias probe (Fig. 1). This is a device that is inserted between an output tube and its socket. (I typically make my own bias probes, but if you simply search “bias probe” online, you’ll find plenty to choose from. If you already own a multimeter, you can simply purchase the probes, but there are also options to purchase a full system with either a digital or analog meter, should you need it.) This device breaks the connection between the cathode (which is the metallic electrode from which electrons are emitted into the tube) of the tube and its ground connection, and inserts a small value resistor in between. It then allows the voltage across the resistor to be read. The resistor is typically 1 ohm and the resulting voltage drop across it is in millivolts (mV), so no chance of shock here. This provides a true and accurate measurement of the actual current flowing through one tube. Then, you set your bias and you’re done!
But even more important to understand is that tubes produced in different factories across the globe will bias up differently!
Ah, but wait! How do you set your bias? Let’s learn a bit more. Most tube amplifiers, if they are not cathode-biased designs, have some way to adjust the output-tube bias. One longstanding exception to this are most Mesa/Boogie amps. The bias voltage in these amps is not adjustable, which is why Mesa suggests only purchasing their tubes for their amps, because they are designed to fall within the acceptable bias range for their amps. This adds a certain degree of confidence for owner servicing, although, of course, it limits your options.
Let’s take a look, however, at a typical Fender or Marshall bias control. Most older Fenders have a pot with a slot for a screwdriver mounted to the chassis in the area of the power or mains transformer, while most older Marshalls have their bias pot mounted on the circuit board. (You might want to go online to look at schematics for your amp to help you find it.) Either way, this is where you’ll make your adjustment.
To get started, you’ll most likely need to pull the chassis and place it in a stable work environment. Insert the bias probe device between one of the tubes and the socket (Fig. 2). Make sure all the volume controls are set to zero, turn the amp on, and let the tubes warm up. It’s also good to try to have a load on the speaker jack—whether a speaker or an appropriate resistor or load box. This is not 100 percent necessary for just setting the bias to a particular number, but sound checking is one of the ways I like to make the final adjustments, so being able to connect the speaker to the chassis while it’s on the bench is certainly a necessity for me.
Now, where to set the numbers? There are certainly more than a few opinions floating around on the interwebs about what optimal bias settings are. Some engineering types will tout 50 percent maximum plate dissipation or 70 percent maximum dissipation, and while it may look good or make sense on paper, I’ve heard the result of guitar amplifiers designed by the book to optimal specifications … and to me they sound, well, less than optimal. It may work in the hi-fi world, where perfect sound reproduction is the goal, but guitar amplifiers are in the sound production business, so it’s a bit different. (In the most basic terms, maximum plate dissipation is the amount of power the plate of the tube is designed to deliver.)
Different types of output tubes have their own acceptable range of bias current. There are so many variables at play that there is no “correct” number. The plate voltage in the amplifier, the output transformer’s primary impedance, and the country of origin of a tube all factor into how it interacts with the voltage and output transformer to define what the optimal bias current will be. Below are the average ranges for some typical octal output tubes:
• 6L6: 25–35 mA
• EL34: 30–40 mA
• 6V6: 18–25 mA
• 6550: 35–45 mA
• KT66: 30–40 mA
Fig. 3
These should be the ranges in which these tubes will perform and sound the best, and they can be accurately measured with a digital multimeter. The best way for you to decide what setting is best for you is a combination of the reading on the meter and your ears! Using the bias control, set the bias to somewhere in the ranges given above (Fig. 3) and play the amp. Note: Some amps will act funny and develop horrible noises (parasitic oscillations) when a bias probe is in place while the amp is being played. If this is the case, you’ll need to remove the bias probe each time you play the amp.)
Move the setting a couple mA in one direction or the other and play again. Don’t expect extreme changes; that’s not what we’re looking for. Listen for subtle differences. Is one setting a little more or less harsh? Is the bottom end too soft or flubby? Is the amp as clean as you want it? Sometimes these little subtleties are what make one amp sound and feel better than another!
Most older Fenders have a pot with a slot for a screwdriver mounted to the chassis in the area of the power or mains transformer, while most older Marshalls have their bias pot mounted on the circuit board.
Also, you should be doing this at the volume you would typically use onstage or in the studio. You may not notice much change if your volume is at 1, but you want to optimize the amp for the way you will be using it.
Eyes Wide Open
Fig. 4
Knowing the ballpark bias numbers is good, and adding your ears is even better, but I also like to see what I’m hearing, so I always incorporate an oscilloscope when I’m setting the bias on an amp. I mentioned crossover distortion above, and when it comes to setting up amps for today’s pedal-hungry players, I find that setting the bias to where there is just a hint of crossover distortion at full output is what works best. Fig. 4 is what that looks like on the oscilloscope. This keeps the amp very clean and makes most pedal users happy.
By the way, here’s a mini primer in crossover distortion. In a push-pull output stage, which is found in most amplifiers with two or more output tubes, each tube (or pair of tubes) is responsible for amplifying at least half of the audio signal. If the tubes are not biased properly, one tube (or pair) will stop amplifying before the other tube (or pair) start amplifying. This will create crossover distortion. Proper biasing will allow the two halves to interact correctly. It’s like a nice firm handshake between both halves.
Beware These Old-School Methods
Let’s look at a couple popular methods that I do not recommend, but are worth discussing because they are, nonetheless, common. The first is: With the amp off and output tubes removed, use a multimeter to measure the resistance of each half of the primary side of the output transformer. This would typically be from the center tap to each side of the primary winding.
In the most basic terms, a transformer is a bunch of wire wound around a steel core. On the primary side of an output transformer, the center tap is the electrical “middle” of this long length of wire. This is typically where the high voltage is applied. The ends of this length of wire are connected to the plates of the tube, thereby applying the high voltage to the tubes. As an example, typically in most Fender amps, the center tap is red, and the ends of the primary windings are blue and brown.
Fig. 5
Next, install the output tubes, turn the amp on, and measure the voltage drop across each half of the output transformer with the amp at idle in operational mode (Fig. 5). Voltage divided by resistance will give you the DC current through the tubes. For example, 1.17V / 15.8R = 0.074, or 74 mA. The numbers I used here were actual measurements in one side (one half) of a 100W amp using four output tubes (two per side). So, divide the 74 mA by two, and you get an average of 37 mA per tube.
Next, you can try the shunt method. This requires a multimeter that can read DC current in milliamps (mA). Connect one meter lead to the center tap of the output transformer and the other lead to the output transformer’s primary side. Typically, in most amps using octal tubes (6L6, 6V6, EL34, 6550, KT88, etc.), this will be pin 3 on any output tube socket. Turn the amp on and, in operating mode at idle (i.e., volume off), measure the current across that half of the output transformer. For example, if your measurement is 72 mA and it’s an amp that utilizes four output tubes, the current measured is for two of those tubes, so once again divide by two to arrive at 36 mA per tube.
I’ve heard the result of guitar amplifiers designed by the book to optimal specifications … and to me they sound, well, less than optimal.
Both of those methods are very old school and still in practice, but I wouldn’t use either for two reasons: 1) I don’t believe they’re very accurate, and 2) they’re dangerous! You’re probing around inside the high voltage area of the amp, and one slip will either take out a fuse, take out a tube, take out your meter, or, worse case, let you know exactly what 450V DC feels like! So, although these methods are used, let’s just say no here.
Some Personal Insights
I’d also like to add a little personal experience to this procedure, based on decades in the biz. Back in the day, when I began servicing and modifying gear, guitarists were regularly playing 50- and 100-watt amps. (Everybody looked at me like I had three heads when I came out with the 18-watt Budda Twinmaster, but that’s a whole other story.) There were some overdrive and distortion pedals around (now all vintage), but certainly not the pedal proliferation we have now, so players were pretty much guitar, cable, amp … go! In these situations, I would most times run the tubes with a pretty hot bias so the amp would be fatter and overdrive a bit earlier and easier, as a decent percentage of the overdrive was developed by pushing the output tubes. As time went on, output attenuators became more popular, so amps could be pushed hard, but at more manageable volume levels. That was still a good scenario for a hotter bias of the output tubes in high-power amps. Eventually, players started playing lower-power amps, so they could open them up and get great output-tube distortion at lesser volumes. The problem is that hotter-biased low-power amps tend to get mushy and have less definition when pushed hard, so a more moderate bias setting is preferred here—just enough so there is no crossover distortion. Move up to today’s scenario and you’ll find that almost all overdrive and/or distortion is typically coming from a pedal. In that case, an amp is nothing more than an amplification device for pedals.
So, that’s what I’ve learned about tube-biasing from my decades of experience. But the bottom line is, there is no absolute right or wrong settings when it comes to biasing an amp. Keep your ears open and go with what sounds best to you.