Selecting a Power Scale Kit

OLD "CLASSIC" PSKs

Sold until the end of 2007, our "classic" PSK-1, PSK-2, PSKPC and PS-BOX used switching regulators to achieve Power Scaling. Although this technique helps to keep waste heat in the regulator itself to a low level, installing the kits requires a very experienced tech. Any ground problems existing in the amp are brought to the forefront and must be corrected. Not all techs have the knowledge to correct such problems.

As switching regulators, the PSKs are inserted into the power supply ahead of the filter capacitors. This order requires current-limiting protection be built into the Power Scaling circuit. Circuity of this kind is very simple, but is relatively uncommon in guitar amps, so most techs do not know how to adjust it for individual applications. This order also means that dialing power up is instantaneous, but dialing down depends on the audio circuit to discharge the caps.

Over-all, the classic-PSKs are good for single channel amplifiers, but a tech experienced with Power Scaling and ground-flaw correction is required. PSK-1, PSKPC and PS-BOX were for fixed-bias amps; PSK-2 was for cathode-bias amps.

NEW DC PSKs

The new generation of Power Scaling Kits is DC. The kits require proper heat-sinking but allow much better performance in multi-voice amplifiers. Installation is also simpler, so less skill is required for successful installation. Assembly is made even quicker as all the new kits have printed circuit boards.

The new DC PSKs are split into two groups: "Super Flexible" and "Super Budget". Use the "-1" version of either for fixed-bias amps and the "-2" version for cathode-bias.

"Super Flexible" DC-PSK-1 & DC-PSK-2

The DC-PSK-1 is used for any fixed-bias amp up to 500W and/or with up to 700Vdc supplies. For cathode-biased amps up to 120W and/or with plate supplies up to 525Vdc, use the DC-PSK-2. For either kit, the plate and screen supplies can be widely different or similar. Any type of pot can be used for the Power Scale (PS) pot. A DRIVE COMPENSATION control is included, as is a bias pot in the DC-PSK-1.

The DC-PSKs are "lossless" regulators. You do not lose any B+ when PS is all the way up. The DC-PSK-1 uses London Power's "optical" regulator, while the DC-PSK-2 uses CFP regulators. Both are highly stable and extremely responsive. Power output changes as quickly as you can sweep the PS pot or switch between two different PS pots.

The PS pot provided gives a nearly linear loudness change versus control sweep. But... the installer can use any style, shape, size, rating and value of pot desired. Adding extra PS pots is as simple as adding the pot and a means to switch between them, using any low-level switching method, including: jfets, CMOS, BJTs, LDRs or relays. It is possible to implement a remote PS function using an expression or volume pedal. For this, a means of automatic drive compensation must be configured (see TUT6).

"Super Budget" SB-1 & SB-2

If you simply have a "typical" single-channel amplifier and are working on a budget, the SB-1 and SB-2 provide highly affordable yet high performance Power Scaling solutions. Use the SB-1 in fixed-bias amps and the SB-2 in cathode-bias amps. A "typical" amp is one in which the screen voltage is derived from the plate supply through a choke or resistors, and thus both voltages are very similar.

Both Super Budget kits rely on a 28mm mil-spec PS pot, which allows the circuitry to be very simple. The PCBs can be broken into two or three pieces (SB-1), or into two pieces (SB-2), to accommodate the chassis space available. To add a second or third PS pot, requires the use of identical 28mm mil-spec pots, and the use of relays for the selection.

DC-SUS