Power Scaling Kit FAQ by Kevin O'Connor

These are questions you might have before or after a PSK Power Scaling Kit is purchased.

Q: How do I know which PSK to use in my amp?

A: You can use the PSK-1, PSKpc and PS-BOX with all amps.

The PSKpc is just the PSK-1 with a printed circuit board included. This is the same PCB as used in the pre-assembled PS-BOX. This allows an installer to make his own "Power Scale Box".

The PS-BOX is intended for use with high-value and/or vintage amps where a less extensive mod to the amp is preferred. This means the Power Scale control is around the rear of the amp, less ergonomic for players but easier for the installer.

You can save some money and construction time by using the PSK-2 if your amp is cathode-biased and you have no immediate intention of providing a fixed-bias option. This covers amps such as:

Ampeg: AC12, B12/B/N/NF, EJ12/A, ET1/B*, ET2/B* B15/B/N/NF, G12, GS12R, GU12, J12/A/B/D/R, M12/A, M18, R12/A/R/RB, R15R
Fender: Champ, Bassman, Deluxe 5C3, 5E3, Harvard 6G10, Princeton 5C2, 5D2, 5E2, 5F2/A Pro 5C5, 5D5, 5E5 (not 5E5A), Super 5C4, 5D4, Tremolux 5E9A, Twin 5C8, 5D8
Gibson: BA-15RV, EH-150, GA6, GA40, GA79RVT*, Hawk
Leslie: 122*
Magnavox: 260A,280A*, 401, 411, 421, 431, M2, M4, M6, M7, M8
Marshall: some 20W models
Trainwreck: Rocket, Liverpool
Traynor: Guitarmate, Studiomate
Vox: AC15, AC30

*one PSK-2 per channel, or as needed
If your amp is fixed-biased and is 600V or less, then use the PSK-1. This includes almost every amp model ever made, with a partial list as follows:

Acoustic: most models
Ampeg: all models not listed above except SVT and V9
Bogner: all models
Crate: all models but think twice
Dumble: all models
Egnater: all models
Fender: all reverb models and those not listed above except PS-series
Gibson: all models not listed above
Hiwatt: all models except DR/STA 200/400-series
Jim Kelly: all models
Marshall: all models not listed above except 200, 2001
Mesa-Boogie: all models
Peavey: all models
Randall: all models
Soldano: all models
Trace Elliot: all except VA-series
Trainwreck: Comet, Express
Traynor: all models not listed above except Super Custom Special
Vox: all models not listed above

If your amp is very high power and/or the screen supply is independently derived - not from the plate supply - then use PSK-1 combined with the THS. Models this applies to include:

Ampeg: SVT and V9
Fender: PS-series
Marshall: 2000, 2001, newer 400W bass amps
Sovtek: MIG-60
Trace Elliot: later 400W bass amps, VA-series
Traynor: Super Custom Special
Univox: all models

For models not listed anywhere above, check the actual amp wiring.
Q: How much space do the PSK cards take up? How big is the PS-BOX?
A: It depends on how you make them. A suggested layout for the eyelet board is included with the kit, and is usable directly in nearly all Hiwatts, Marshalls and Traynors because they all have spacious chassis. In many Fender amps, the layout and card will have to be split in two to accommodate the available space.

The PSKpc includes a printed circuit board. The PCB is 4.25"(L) x 3.5"(W) x 1"(H).This will fit into most Marshall chassis but not Fender.

The PS-BOX is 6"(over flanges) x 5.25"(over heatsink) x 2"(to clear knob). This mounts inside the amp sleeve in heads and combos.

In all cases, there is an additional card that mounts inside the amp chassis, approximately 3"(L) x 1.5"(W) x 1.25"(H), which can be oriented as required.

Q: Is the card included?
A: Only for the PSKpc.

The PSK-1 and PSK-2 include only the electronic components, schematic, layout and installation instructions. The card materials, tools and instructions are part of the CCK Circuit Card Kit. The CCK has enough material for you to build many PSKs, or a complete amp with the Power Scaling built in. You should have material left over for other projects, but you only have to buy replacement eyelets and cards as they are used up.

 

Q: Is there a lot of heat from the circuit?

A: Only on some amps and only over a portion of the Power Scale control range. The warmest circuit range for the PSK B+ regulator is between full power and half-voltage. At less than half-voltage output, the B+ regulator runs cool. This is only when tubes are in place and biased properly and handling audio signal.

Heatsinking is easiest to achieve by bolting the regulating device to the chassis using the supplied hardware. However, the best performance is achieved when the regulating device is isolated from the chassis, i.e., use a bolt-on heatsink and cool with a fan if necessary. Using the bolt-on heatsink, a fan is only required in amps over 60W, or if they are biased very hot, as with cathode-biased amps.

The bias regulator in the PSK-1, PSKpc and PS-BOX runs cool all the time.

Q: How much experience do I need to install a Power Scaling Kit?
A: These kits are recommended for experienced techs and hobbyists. You should be able to look into any amp chassis and recognize the rectification method, bias and plate supply components and output stage grid circuits. If you cannot do this by sight without a schematic of the specific model, then you might want someone better qualified to do the installation.
For those wishing for less construction time, we offer the pre-assembled Power Scaling Box. However, one must still be able to recognize the above circuits.
Q: Does the Power Scale control have to go on the front panel?
A: It does not have to, but once you have the circuit operational you will probably want it on the front. You will probably want to have the 'Drive Compensation' control on the front panel as well, if you wire the amp in a way that requires its use.
Q: Why do I need drive compensation?
A: As you make the output stage look smaller, if you do not reduce the drive signal from the splitter, then the output stage will distort more than when it looked large. The 'Drive Compensation' control is essentially a post-phase-inverter master volume. However, unlike the usual post-PI MV, we're using this control to reduce distortion instead of increase it. This is a refinement that is not needed in every situation, but does allow true power scaling of the output stage and allows the use of your "usual" settings for all of the other amp controls.
Q: Does this mean that the post-PI MV is really doing all the work?
A: Not at all. It merely allows the stock overdrive level of the output stage to be retained at lower power levels while also retaining the contributions to tone and overdrive from every stage up to the splitter.
Q: Can I eliminate the Drive Compensation control and have single-knob power control?

A: Yes. By Power Scaling the splitter as well as the output stage, automatic limiting of output stage overdrive will occur. Any tweaking of this can be accomplished using a conventional MV wired at the power amp input, but generally this approach achieves single-knob power control.

The PSK notes explain these options.

Q: Can the Power Scale control go into an unused jack position?
A: If this is on the face plate, then it is STRONGLY recommended that the controls and circuits for the PSK be placed at the power supply end of the chassis. The voltages on the Power Scale control itself are very noisy and should not be run past the EQ, Volume, Reverb, and Gain controls or near the input jacks.
If you are thinking about using the second speaker jack location for the Power Scale control, that is okay. Again, the wiring to this control must be tightly twisted or shielded per the instructions with the kit. However, the rear panel location may prove inconvenient once you start using the control. On Marshalls, you might want to relocate the Presence control to the rear panel with the Power Scale taking its place on the front-panel. On older Fenders, you might want to swap a tremolo control to the rear panel with the Power Scale taking the right-most location on the chassis.
Q: How do I control bias?
A: The appropriate BMK Bias Mod Kit must be added to the PSK-1 or PS-BOX to have fine control over tube idle conditions. Both PS-kits  provide tracking bias voltages in proper proportion with the variable plate supply, but in fixed-biased amps you must still fine tune it for your particular tubes. A BMK2 will provide individual control for each tube in a two-tube output stage, or it can provide control over each push and pull circuit half when more power tubes are present.
Q: What about the bias controls - where should they be mounted?
A: These are panel-mountable controls and physically smaller than the Power Scale control. They can be mounted as internally-accessible adjustments to avoid drilling holes in the chassis. This may prove inconvenient, since most amps have external tubes, the bias adjusts should be accessible externally, too. This would mean mounting the pots so they are not easily bumped - set them back on the chassis so you have to reach in. Or, mount them on a bracket so they are recessed from the rear panel and can be turned with a flat screwdriver.
Q: How do I set the bias once I have the pots mounted?
A: Cathode current sense resistors are included with all BMKs. These are installed per the kit instructions and the Dissipation bias procedure in "The Ultimate Tone vol.2" is followed. The meter tip-pin jacks are installed in the rear panel of the chassis. Always set bias with the Power Scale control set fully clockwise (full power). If the SCK Sag Control Kit is also fitted, set the sag control for minimum sag.
NOTICE -
PSK-1, PS-BOX and TBS generate their own raw bias voltage. Their operation is independent of the stock bias circuit in the amp.
Q: How will the PSK effect tube and capacitor life?
A: When the PSK is set to any setting less than maximum, tube life is extended. Your filter caps will be protected by the current limiting incorporated into all of the PSKs.
An amp equipped with Power Scaling can actually be used intermittently without the usual temperamentalness of electrolytic caps. If after a long storage period, the amp is powered up with the Power Scale set to its lowest setting (fully counter-clockwise), then low voltage is applied to the caps. Gradually increase the setting towards maximum over about a half-hour to one-hour period. This will reform the oxide layer that is the dielectric for the cap. So, you can use Power Scaling to reform filter caps.
Q: Can I use the FTC Fixed-to-Cathode bias kit on any amp?
A: Yes, it will work on its own. There are notes on our site, in the FAQ and in TUT2 that describe how to have switchable fixed-bias and cathode-bias.
Q: Can I use the SEM with a PSK?
A: Of course. The SEM Single Ended Mod can be applied to any push-pull amp regardless of the installation of Power Scaling. It is something I've had on my own amps since the 1980s. You can use it with any of our power amp kits, too.
Q: Can I use the Sag Control Kit with the Power Scaling Kit?
A: Certainly, but we recommend you install the PSK first. Once you have that feature and use it, you may find the need for "sag" to be inconsequential.
Also, the SCK responds to dynamic current, i.e., current changes, so it works well with typical class-AB fixed-bias amps. Hot-running amps like the AC-30, any SE amp, or very high biased fixed-bias amps have little or no dynamic current. Therefore, the SCK will not provide any useful sag effect.
If you do combine the SCK with the PSK, you will find the SCK helps increase sustain and has a smoothening effect on the sound. With PS set to maximum, and the 'sag' control set to maximum, a squishy sound may result, but makes an interesting effect. At less extreme combinations of the controls, the SCK becomes very useful in achieving a "singing" amp sound, and sustaining notes that easily roll over into feedback at lower volumes.
Q: How do I decide whether to buy the VCK or VRK?
A: The VRK Voltage Restoration Kit is used when you want the output voltage to be a proportion of the input voltage, and an exact maximum value is unimportant. This is typical when you have taken out a tube rectifier. A solid-state rectifier usually produces more voltage due to smaller diode losses, and this can be harmful to your filter caps. The VRK allows the voltage to be restored to its previous value. As the supply varies with loading - all supplies are like this - the voltage is maintained at the proportion you select using resistors included with the kit.
The VCK Voltage Clamp Kit is used when there is a specific maximum voltage that the supply should never exceed. Zener diodes are used to provide incremental voltage adjustments that are thermally stable. If the input voltage drops below the clamp voltage, then the output voltage merely tracks it downward, but when input voltage rises above the clamp voltage, the output voltage remains at the desired maximum voltage.
Both the VCK and VRK include current limiting to protect the filter caps, transformer and tubes.
Q: Will the voltage clamping and restoration kits affect the sound of the amp?
A: They do not effect the sound of the amp. They are essentially transparent just like the PSKs.
Q: I'm not sure if I'm skilled enough to install a PSK in my amp. Do you offer this service?
A: No, we don't, but you can contact any of the recommended PSK installers listed on our Links page.
Q: How about the "big" question: Does Power Scaling really work as well as you say?
A: Yes, it does - but you expected us to say that. Power Scaling has been a feature on our amps for a long time. Players who have used Power Scaling simply cannot go back to a fixed-power amp. Readers of our books and our amp customers know we do not make any claims we cannot meet, and that we do not settle for compromise. Attenuators are a compromise.
Q: If I want two Power Scale controls, do I just buy the PS-DUAL, or do I need a PSK as well?

A: The PS-DUAL is an add-on that can be used with any of the PSK-1/2 kits. The PS-DUAL provides a second completely independent Power Scale control, the footswitch circuitry and fast-transition circuitry. The PS-DUAL can also switch your preamp channels with extra shunt-connected jfets.

We usually recommend that a player wishing to go all out and install dual Power Scale controls on his amp first try a single control. This will let him try out the effect with the preamp channels, to see which combinations work together and what the preferred sounds are. Gaining this experience often simplifies the final decision and saves a lot of money.

Q: Where would I use the Power Scale Boost instead of the PS-DUAL?
A: The PSB is just that, a boost setting for the existing Power Scale control of a PSK. Again, the PSB is an add-on kit to an existing PSK-1/2. The amount of boost depends on the setting of the PS control. The most extreme boost occurs with PS at minimum, yielding 10dB boost. The PSB can be wired without the boost control, so the transition is from the PS setting to full power. In this case, the maximum boost ratio is 40dB. However, such a transition is not likely to be very musical.
Q: How can I get the PSK regulator to run cooler if I am using just moderate reductions from maximum power?
A: A fan will move the heat out, and a low-speed fan running at reduced voltage is ample for this task. Power can be derived from the heater supply, as shown in TUT. The PSKpc has this provision built onto its PCB.
Q: Can I combine a PS-DUAL with a PSB to have separate Power Scale controls for each channel, but a boost for my lead channel?
A: You could combine, them but there would be complications. A simpler approach would effectively be a PS-TRIPLE. The power control circuitry is identical to that in the PS-DUAL, but expanded with a third section and third independent control. This much at least is very straightforward. The issue of selecting between three controls is identical to having three preamp channels. To be ergonomic in a performance, you need direct random access from each sound to any other sound, requiring a device such as the 40174 chip and momentary switches. The chip can control four independent controls/channels, leading naturally to a PS-QUAD.


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