Hi Guys

In an audio system there has to be a point in the signal path where the signal can be made smaller - all the way to zero - allowing the listener to regulate how loud the sound is. A voltage divider made from fixed resistors with the tap points selected by a switch will accomplish this goal, but not very gracefully. We can add more resistors in the divider string, creating more tap points and add more positions to the switch, for smaller loudness increments. Integrated forms of this feature highly in many hifi offerings.

We can expand this idea to as many points as we wish and there are many hifi systems that use relays to select the corresponding taps in a stereo system. The simplest use the panel switch to control the relays directly, while others replace the switch with a vector control and digital interface. Integrated circuits are available as 'electronic volume controls' where the resistive elements are selected by CMOS switches during the signal zero-cross to reduce switching noise.

A variable resistance will do this much more easily and with the potential for infinite resolution, i.e. equivalent to an infinite number of resistive taps and switch positions, and thus infinite loudness levels.

The potentiometer has a resistive track with a total specified resistance from end to end. There will be a tolerance for the value, which is usually 20%. This means a 10,000-ohm pot can be 8,000-ohms up to 12,000-ohms and still be within specification. Since the pot is the voltage divider in its entirety, this sloppiness of production does not matter, at least until you want to change more than one thing simultaneously.

A mechanical moveable contact can be wiped from one end of the track to the other. This contact is creatively called the 'wiper'.

The rate of change of resistance versus the rotation of the wiper is called the 'taper'. If the resistance varies in direct proportion with the percentage of wiper sweep, the taper is 'linear', denoted by a raised-letter 'B' in the value. A linear 10k pot is denoted as either B10k, or as 10kB.

For the loudness change to feel linear to our ear, the power has to change logarithmically. For it to seem half as loud, power must go down by ten times. A simple reduction by half would only feel "a little bit quieter'. Therefore, we need pots with a log taper, meaning that the signal must change very quickly in amplitude over the sweep of the pot. Where a linear pot set half way will have half the input voltage at its wiper, a log taper pot will have one-tenth. Log taper is denoted by a raised-letter 'A' as A10k or 10kA, for a 10k pot.

It is very difficult to lay down a logarithmic resistive track, so instead, small linear sections are laid down and the hope is that the transition points are not too audible.

In a mono audio system, the issues of pot value tolerance and taper irregularities can be minimized, and are generally non-problematic.The taper of the pot is selected for each pot position in the circuit, based on the circuit itself, and how the sonic result should be over the sweep of the pot. The basic division by circuit is 'passive' or 'active'.

Passive controls work freely comprised of resistors, capacitors and maybe chokes, but are otherwise independent of gain elements. All of these pots will be log taper, for Volume, level, and tone controls.

When a pot is inside the feedback loop of a gain stage, we may use linear or reverse audio, taper 'C' to achieve a linear loudness or effect change.

In a stereo system, or one with yet more audio channels, ganged pots with multiple sections are used. Each section is specified as for a single-section pot, but now we have a new parameter called 'tracking'. If you feed the same signal into all of the pot sections, will all the wipers have the same output voltage? For conventional pots, the answer will be 'no. Even for expensive pots of this type tracking will be within 3dB, which is quite noticeable.

ALPS from Japan has their famous 27mm stepped pots which are 42-position switches with resistors between the positions. Using 1% resistors allows these pots to track very well, within 1dB which is the nominal limit for Human perception of a loudness change. despite the opportunity to calculate the steps very accurately, many of these pots have a wide overall tolerance but do divide the signal properly. For modern equipment made ever smaller, this pot option is discarded.

The physical size of the pot has a relationship to its expected life time, measured as 'cycles', end-to-end wiper sweeps. Tiny pots like 6mm and 9mm diameter, may be rated for 1,000 to 10,000 cycles, or have no rating. 16mm are usually 15,000, as are average 24mm pots. 28mm mil-spec pots might be rated from 15,000 to 25,000. This is for carbon tracks.

Pots can be made with conductive plastic tracks and these generally have 100,000-cycle life ratings, at least down to 16mm.

All of the above are panel pots, with shafts that accept fixing a knob to, that can be easily manipulated by the equipment user and hopefully which matches the aesthetic of the equipment. A single rotation from CCW to CW is generally 300-degrees of the circle.

Wire-wound pots are made using a wire coil of resistance wire with a wiper that sweeps across one side of the coil. These or most often designed to handle high power, although there are precision units made by Bourns and Spectrol used for fine-setting of machine and instrument parameters. These are typically 10-turn pots, where the pot shaft is rotated ten times in the same direction for the wiper to move fully from one end of the track to the other. Life cycle is usually 1,000,000, but keep in mind it takes 10 turns to go end-to-end. These pots are not something that you make broad adjustments with, so they are unlikely to be used by hobbyists except for a bench power supply.

The 10-turn pot is also a panel-mount and requires a Vernier dial to be useful and for the operator to know where along the track the wiper is situated. A vernier dial shows which revolution the pot is at and the infinite division of that rotation. A small plate is secured under the nut that secures the pot to the panel; the plate has a tab that lines up with a slot in the knob and a set screw secures the knob to the shaft. These controls are surprisingly small, typically about 17mm diameter, not including the solder tab connections.

In an audio amp or musical instrument amp, even one with tubes, the signal voltages across the pots are generally small, less than 10V or so. This is good, because the pots used in these devices are often limited to 50-250Vdc. Linear pots have higher voltage ratings than log pots, where, for example, 16mm linear is rated at 250V and 16mm log is only 50V. This rating is based on the physical construction of the pot and has nothing to do with the resistive value. Moving up to 24mm pots gives more leeway at 500Vdc for linear and 100V for log.

Voltage ratings on pots is something that nearly all amp designers are unaware of. Even for those who know about it, they often make poor choices or assumptions for specific applications, such as when adding Power Scaling in MI amplifiers. In the basic form the pot sustains high voltage and must be of sturdy design, explaining why London Power used a mil-spec pot rated at 1,100V. This rating applied even to log taper pots, which turned out to be better for the application than linear pots. Copyists skimped and used standard 24mm pots with dire results. The present kits use less expensive 16mm pots that fit into smaller spaces, but they see much reduced voltage and will last their normal life.

Power ratings for pots are often routinely ignored. Again, linear pots have higher ratings than same-size same-value log pots due to the segmentation of the log pot resistive track. In most applications the power rating is not a limiting factor provided the resistive value is high. In guitar amps the lowest values might be 10k for a bias pot, or 25k for a midrange frequency control, with all others 100k or higher; majority values are 250k to 1M.

A 16mm linear pot is typically rated for 125mW and log is 60mW. If this is a 250k pot, the voltage across the track is limited to 176V and 122V, respectively, so not a big concern in common tube amp circuits. Hifi amps may use down to 1k pots, with 10k being the most common these days (and nights). Voltage limited by power is now 35V and just under 8V, respectively for linear and log at 1k.

In tube amplifiers that are fixed-biased, the bias pots should be not too high in value. London Power uses 25kB as standard.

Hi Guys

Capacitors are everywhere in audio circuits and their power supplies.

A capacitor is formed when two conductors are separated by an insulator. That insulation is called the dielectric, and can be air (not very useful, but the cause of 'parasitic' capacitance), paper, oil, plastic, ceramic, mica, electrolytic materials, polymers, or organics. The conductors are called 'plates' and are effectively large flat metal surfaces with leads attached. The entire assembly is covered in a suitable coating to keep the environment out.

Capacitance value is in Farads, which is a huge value for an audio circuit, or even for a power supply, although small "super caps" of a few Farads were used in lieu of a battery as backup power in VCRs and similar to support memory circuits. In audio, we see values from picoFarads (pF) up to milliFarads (mF). Note that microfarads should be denoted as uF. Ancient schematics will show uuF instead of pF.

Man-made capacitors are imperfect with the following flaws in various extents:
DA - dielectric absorption is the inability of the dielectric to release built up charge
DF - dissipation factor is the inability of the dielectric to follow fast changing charge/discharge cycling
ESR - equivalent-series-resistance is a fixed internal resistance not the impedance based on frequency

Remember: the power supply is the other half of the signal path so the caps used here make a difference that is audible.

Ceramic caps
These are made from various rare earths and historically were single layer, where the dielectric is between two parallel plates, as the iconic capacitor model represents. Discs with radial leads are typical, and the value accuracy and stability depend on the specific materials used. The best ceramics for audio are C0G or NP0 rated, and are good up to values of 100pF.

Distortion is otherwise quite high with higher values and adds "grit" to the sound. In a guitar amp, this may be welcome, but in hifi it adds distortion that smears the sound.

MLCC Caps
Multi-layer ceramic capacitors are used in switching power supplies and have some quite terrible characteristics. Their capacitance varies with the voltage across the cap. The capacitance varies with frequency of the current through the cap.Their voltage rating is typically unusable, with half the rating being a common safe maximum. Even though they are designed to nominally handle high switching currents, this rating is often overstated. Unsuitable for audio.

Mica Caps
Mica is a natural material that can be shaved extremely thin. It has high voltage withstand and can be cut precisely for reasonable capacitance accuracy. Distortion is much lower than with ceramic and higher values can be used, although anything above a few nF may be expensive and impractical.

Plastic Caps
There are a few common plastics used as dielectrics in capacitors.

Polyester has a few trade names, such as Mylar, and is the most common plastic cap type. It has an unfortunate attribute of adding odd-order distortion to series audio signals. The higher the signal voltage is across the cap, the higher the THD. Compared to ceramic, a polyester cap will sound cleaner in a guitar amp, but it is best to use other polys for critical applications. placing caps in series can reduce the individual capacitor distortion but the composite effect may be not enough to justify the use of multiple capacitors.

Polyethylene is about equal to polyester.

Polycarbonate was favored for a while, but has its own distortion mechanism. Lower THD than polyester. Not as low as others.

Polypropylene is distortion-free! The best poly cap and its reach and breadth of available sizes, voltages and values expands every day. Although safety caps of the X and Y types use PP for its self-healing characteristic, the caps use impure PP and are unsuitable for audio.

Polystyrene is also distortion free but only available in very low values and at voltages <100V or so.

Paper Caps
paper dielectric is ancient obsolete technology that is entirely unsuitable for audio use. Yes, vintage equipment contains these caps, which decades ago were already dried out and behaving more like inductors than capacitors. Replace them with any type of poly cap and the amp will be loud and clear again.

Oil-filled Caps
There are usually meant for motor-start applications and are typically in a metal can with faston-style connections. Not suitable for audio.

Electrolytic Caps
Electrolytic capacitors have a dielectric that is formed using voltage and they require regular application of voltage to remain healthy. If you install a high-voltage cap in a low-voltage circuit, the cap will eventually reform to the lower voltage and not be able to meet its rating unless you slowly reform it by carefully applying higher voltages to it. In general, electrolytic caps have just about the worst combination of DF, DA and ESR possible, but this depends very much on manufacturing quality.

The rated voltage of an electrolytic capacitor is also its WORKING voltage, which means that you can use the cap at the rated voltage without concern.

Life expectancy is listed in 1,000s of hours at the rated maximum temperature and voltage. Realistic life doubles with each 10-degree drop in temperature, so that a 3,000-hr cap rated for 85C should be within spec for 64,000-hrs at 25C. If this were a 105C rated cap, its life would stretch out four times longer. For high-fidelity audio, capacitor life is considered over at 14 years, or when DF begins to go crazy (scientific, ya?)

High-values of capacitance are achievable and electrolytics are the mainstay in power supplies of all (conventional) types. Because of their poor audio qualities and wide value tolerance, electrolytic caps should never be used to define frequency roll-offs, such as in the feedback loop of a solid-state power amp.

Polymer Caps
This is a newer dielectric that is a sort of "enhanced electrolytic". Current surges are handled better and reverse voltages withstood better. They work well in stacked power supply situations, for example, full-wave voltage multipliers.

Tantalum Caps
tantalum capacitors have good value stability and can be made to tighter tolerances than electrolytics.king them an upgrade in old-style security systems that relied on fixed-frequency oscillators. On every other count, they are unsuitable for use in audio circuits.

Hi Guys

You cannot build a linear circuit without resistors, and there are some less known parameters that need to be considered when selecting these essential devices. For our purposes building audio circuits, we tend to use axial-lead carbon-film, metal-film, metal-oxide, wire-wound, types in different circuit locations or to achieve different sonic results.

Resistors in small-signal areas will be carbon-film or metal-film and may be 1/8W up to 600mW. Power resistors from 1W to 3W tend to be metal-oxide, with 5W+ being wire-wound. London Power does not use carbon resistors.

Resistance & Tolerance
The resistance value is the most important parameter and you will always see this on schematics. Resistance is in 'ohms', denoted by the Greek letter Omega. There are historic ranges of values that seem odd today, with newer ranges popularised as manufacturing methods improved allowing greater accuracy in achieving the expected value.

The value charts tell you how many values there are per decade. The decade is denoted as 'E' and the number of values plainly as a number. For example, E12 has 12 values per decade - not very useful. E24 has 24 values per decade allowing the huge gaps of E12 to be filled in a bit better. E96 offers the best overlap and complete coverage of common and less-common values. Within each chart, the basic values are given and these can be multiplied by any exponent (hence E) of 10 over a range that covers fractions of an ohm to millions of ohms.

With each of these value ranges, the resistance tolerances improves, becomes tighter and is represented by an ever-smaller percentage.This percentage represents how far off the value can be. For example, a 1k resistor with a 10% tolerance can be as high as 1,100-ohms or as low as 900-ohms. If its tolerance is 1%, then its value can be between 990 and 1,010 ohms.

E12 is 20%
E24 is 10%
E48 is 5%
E96 is 1%

Material
Resistors can be made from various materials that give them unique characteristics. 

Cracked-carbon resistors have the worst characteristics, including: poor tolerance, high noise, high distortion, high voltage sensitivity causing distortion with varying voltage.

Carbon composition has finer particles and all of the problems of cracked carbon to a lesser extent.

Carbon-film takes the carbon issues down another level.

Metal-film has low-noise, very good temperature stability and low-distortion for varying voltage.

Metal-foil is slightly better than metal-film all around but with higher price and greater bulk.

Metal-oxide has higher power capability than metal film but is slightly noisier.

Thin-film and thick-film resistors have characteristics between carbon-film and metal-film.

Wire-wound uses a metallic element usually covered in ceramic.

Power Rating
Self-explanatory, the power rating tells how much heat the resistor body can dissipate in free air. Like most devices, most of the heat is dissipated by the component leads. Smaller body sizes will have a higher surface temperature for a given power than will a larger body size.

Some ranges of resistors with power rating of 1W+ may have shock thermal ratings allowing their continuous power rating to be exceeded briefly.

Temperature Stability
Resistors generate a small amount of heat internally, but their actual temperature rise depends also upon ambient temperature and the ability to remove the heat. the stability is listed in ppm (parts per million) per Celsius degree © or Kelvin degree (K), which are equivalent. Power resistors will tend to have poorer temperature stability than low-wattage devices, but there are exceptions at both extremes.

Voltage Rating
Yes, resistors have a voltage rating. Resistors have two leads and are constructed either axially or radially. The space between the leads has a voltage limit based on the exterior coating of the resistor and the shape of the body. Ridges on the body can increase the effective distance (creepage distance) between the leads, providing better voltage withstand.

Package
Through-hole resistors have leads arranged either axially or radially from the resistor body. The leads provide support for the device and the through-hole mounting on a PCB provides large solder connections. power devices should be elevated from the board by 6mm.

Through-hole resistors are the historic format used in hand-wired assemblies, on circuit cards, on turret boards, and on most printed circuit boards even today.

There is a type of power resistor that has its own heat sink with solder-lug leads and attachment holes. The smallest type is rated for 20W provided it is bolted to a proper sized heat sink.

Surface-mounted devices allow smaller overall circuit assemblies and are very difficult to handle manually, and thus are the mainstay of highly-automated assembly, particularly for computers, monitors, cell phones, portable devices, and generally every consumer electronic item considered to be disposable. It is an irony of our times that such sophistication is so devalued.

Hi Guys

We use power mosfets in various applications in tube amps and in hifi amps. These are LINEAR applications, so we need robust devices that can withstand heat with a modest heat sink arrangement. As TUTs detail, mosfets are generally designed for switching applications where the device is fully 'on' while conducting current, or fully 'off' while withstanding voltage. In both states, the heat dissipated by the mosfet is ideally zero.

The most common packages for mosfets are:
TO-220, good for up to 50W
TO-3P, TO-247, good for up to 150W
TO-264, good for up to 300W
There are many other cases that are minor variations of the above and generally identical in power dissipation.

You will see mosfets with massive power ratings over 1kW, but have the TO-247 or TO-264 case, and this tells you right away that the devices are for switching only. A look at the safe-operating-area graph displays plots of various pulse times, but NOT 100ms, which is considered by some designers to be equal to DC operation - which it is not. When looking at one of these graphs, the scale is log on both axis, current on the y-axis and voltage on the x-axis. You will see that the vertical space between the 1ms line and 10ms line is the same as for the 100us line and the 1ms line. From this you can use a scale (ruler) to see where the 100ms line would be, or further, where the DC line would be. At DC, the 1kW+ device is now less than a couple of hundred watts.

Another thing that is assumed for those super-high power ratings is water cooling.

In general, because of the optimisation for switching, a given mosfet will have the smallest die that can handle the rated current and voltage. For linear and DC applications the mosfet will be 'on' all the time, conducting varying current depending on the load. For this we have to go to larger die sizes and to safely achieve that we go to larger package sizes. The default for linear use of mosfets is to use the largest package size that is reasonable, meaning up to TO-247. There are large modules with screw terminal connections, but these are for switching use in very high-power inverters.

Whether we are building an amp from scratch or modifying an amp, the physical arrangement often limits us to using the chassis as a heat sink, or maybe to the use of a real but small-ish heat sink. A fan may be necessary in some applications.

ON-resistance will limit mosfet performance at high currents and low voltages. This will be relevant for audio power amps, or for a low-voltage regulator, where a regular bipolar-junction-transistor would be a better choice.

Many mosfets have built-in gate protection zener diodes. However, it is safer to use external gate zeners to avoid having to replace the mosfet should its own zener diodes be damaged. Generally, a single 12V zener oriented correctly from source to gate will provide protection against static discharge. A gate-stop resistor should be placed directly at the mosfet gate.

Mosfets come in two forms, as n-channel and p-channel, complementary materials with complementary characteristics, similar to NPN and PNP BJTs. P-channel mosfets are inherently linear-rated since the process to manufacture them corresponds to linear requirements. Most applications we encounter use only n-channel devices, which are easier to produce with voltage ratings up to 4kV and current ratings exceeding hundreds of amperes, neither extreme applying to what we do.

Despite their impressive ratings, mosfets can be rather delicate when it comes to heat management. Use large-package devices. Assure free air flow over the mounting area. Add a fan if possible. Add a heat sink with the fins oriented correctly. These recommendations apply to all of the uses below.

Power Supply Regulator
To use a mosfet as the pass element of a supply regulator it must be able to withstand the incoming voltage and the maximum current load. This product could well exceed the capability of a single device, warranting the use of parallel devices. Of course, there are ways to alleviate this situation by limiting load current until the regulator output voltage has risen to its rated value, and thus reducing the voltage across the device before load current begins.

In a high-voltage regulator, the ON-resistance is not an issue and the device may exhibit a few ohms; current ratings will likely far exceed the load current; voltage rating 50V or higher than the unloaded input value is sufficient. It is often the case that a much higher-voltage rate device is not much more expensive than the adequate device.

Active Hum Filter
The same caveats apply here as for the power supply regulator. Under normal circumstances, the active hum filter application is a bit easier on the mosfet as the voltage across the mosfet will settle to be equal to the gate-to-source voltage required to turn the device 'on'. Output voltage inherently rises slowly, which will control the load current until maximum output is reached. Large package sizes are preferred for best performance. In a tube amp the use of a TO-220 here is a mistake and will impair tone, for factors imposed by the small die within the small case.

Power Scaler
In Power Scaling applications for tube amplifiers, we need high voltage devices with moderate current capabilities. Excess voltage rating is not too important as long as there is at least 50V above the highest voltage in the circuit that the mosfet is used in. Because there are high ambient temperatures created by the tubes themselves, the larger mosfet package sizes should be used exclusively.

Audio Power Amplifier
A pure solid-state power amplifier works at voltages to accommodate the peak voltage required by the speaker.For example, an 8-ohm load driven to 100Wrms  needs a peak power of 200W, corresponding to 40Vpk at 5Apk. This requires +/-40Vdc for ideal loss-less nonexistent devices. Most output stages are voltage followers which place the turn-on voltage of the output device in series with the load, requiring that the loaded supply voltage be higher than this sum. Using mosfets, we should have at least 5-6V more than the peak load voltage, so +/-46V as a minimum loaded value. Generally, designs aim for +/-55V to +/-65V.

Waste heat is based on the signal shape and the load characteristics. For a resistive load and sine wave, the ideal maximum waste heat is 25% of the load power. Real world conditions drive this to 35% or so. if one biases the amp for class-A operation then waste heat approaches 100% or even higher.

Because of the nature of mosfets compared to BJTs, the latter perform much better with less complicated circuitry and/or compensation. Mosfets benefit more from feed-forward techniques and complex frequency compensation, unless the goal is to experience the mosfet's unique distortion characteristics.

This is the only application where mosfets require back-to-back zener gate protection. The usual practice is to have a series-pair of zeners for the upper circuit half (n-mosfets) and a second zener pair for the lower circuit half (p-mosfets).

Tube Selection
In a tube power amp with multiple tubes, individual tubes and/or pairs of tubes can be turned 'on' and 'off' using cathode switching. This is generally achieved using mechanical switches, but TUT shows that mosfets can be used here quite easily. This is one application where a small package like TO-220 is ample, since this is a pure switching application.

Cathode Follower Replacement
A few tube circuits use a cathode-follower directly tied to a common-cathode gain stage. The CF can be replaced by a mosfet source-follower if the triode is to be reconfigured for gain. In this case, the mosfet can be a TO-220 of nearly any current rating as long as the voltage rating is sufficient for the unloaded supply voltage. it is typical that the circuit resistances stay the same and the mosfet will conduct 1mA or so, and does not require a heat sink.

Mains Current Limit
Back-to-back n-channel mosfets can be used to control mains voltage, either as a switch, or linearly to control current. the latter is useful when a DC mains-blocker is used. The mosfets must be able to withstand the peak mains voltage, preferably with some margin if the unit is used where the mains can become excessive.

Is there a way to safely test the max filament current on a PT? I have a garnet revolution 2 with 6L6's, wondering if it could handle kt66 or el34? PT #'s 6K2952 DGD7

Hi Kevin et al.

On the Custom Special schematic in TUT3 there is a zener diode (100v, 1w) straddling Vb and ground.

My amp building experience is limited, I'm used to seeing resistors here.
I believe it's purpose is to provide a stable fixed bias voltage relative to ground.
So if the plate voltage varies for whatever reason, the bias remains the same.

My question is - Why is this better than the bias following the ups and downs of the plate voltage?

The amp I am building will hopefully be gigged.
It has a universal power transformer, wired as Kevin suggested at the back of TUT3.
So different countries, different mains voltages and hopefully lots of festivals with dodgy generators and clubs with terrible mains supplies.

Regards.

Hi Kevin et al.

I'm laying out the eyelet board for my TUT3 Custom Special build (again).  Great fun.

Checking the suggested layout against the improved schematic in TUT3 I see a few discrepencies.
Some of them I can work out, some of them I can't.

Is this a good place to ask about them?

Regards.

Hi all.

I am part way through a build of Kevin's Custom Special amplifier from the TUT3 book.  Great circuit.

Checking my current needs for the 6.3VAC secondary, I calculate 4x 1.5 A for the 6CA7 valves and 3x .3 A for the 12AX7 valves.
Which comes to 6.9 Amperes.

The rating for the specified Hammond 278CX transformer is 6 Amperes on the 6.3 volt secondary.

15% over the rated capacity seems a lot to me.  Am I being overly conservative?

Hi all!


I am plannng to build the Hood Amp, it is in Tonnes of Tone.

Is it necessary to use the hefty diode bridge or can discrete diodes be used?



Warm regards,

Strelok

Hi Guys

In the Precision Power Scale thread I discussed using a "build-out" resistor to improve the control resolution at the quiet end of the Power Scale sweep. This method can be applied to all previous Power Scale installations. Ideally, we want the "audible transition" of sound-to-silence to occur exactly at the end of the PS sweep. The value required depends on the supply voltage in the amp, the kit type, and how quiet you wish the minimum loudness to be.

Classic-PS and the SB-series both have a build-out resistor already. The main purpose of the inclusion of this resistance was to keep the mosfets 'on' at the full-CCW end of the Power Scale pot sweep. However, if the sound disappears before full-CCW, then adding a bit more resistance in series with the pot-0 lead will be effective.  In these Power Scale forms, it may only take a few kiloOhms, maybe up to 10k, to achieve this AT alignment with the pot sweep.

There were quite a few DC-PSK variations, and then the SF-series. The DC-PSK may require a build-out resistor up to 33k.

The Super Value SV-series followed, with many variations as I tried to use component values within the kit to eliminate dead spots of the pot sweep. Build-out resistor values of 10k to 33k are typical. Dead sweep at the CW or CCW pot ends shift with the supply voltage, requiring a tweak of two resistor values in the kit to optimise it for a given B+.

The Precision Power Scale Zh also needs a build-out resistor of 10k to 33k. I was too quick ordering these Zh has no loud-end dead spot but will have a quiet-end dead spot without the build-out R.

The Zp Precision Power Scale circuit has an on-board AT pot allowing the installer to set the audible transition EXACTLY. Zp has no dead spots and I believe it represents the ultimate performance in Power Scale kits.

The Precision Power Scale kits are still labeled as SVn but with a Zh or Zp suffix, as SVn-Zh and SVn-Zp. The SV2-Z forms incorporate VCK, since VCK is always needed in Power Scaled cathode-biased amplifiers. VCK is still available separately as it has other applications.