|
For information on the older "classic" PSK products (e.g., PSK-1, PS-BOX), see the bottom of this page.
London Power's Power Scaling Kits:
The Cutting Edge of Power Scaling Technology
From the Inventors of Power Scaling
The new generation of Power Scaling Kits is DC, whereas the "classic" kits used switching regulators. DC operation affords high stability and extreme responsiveness - power output will change as quickly as you can sweep the PS pot or switch between two different PS pots. DC operation also allows much better performance in multi-voice amplifiers. Installation is simpler, so less skill is required for success. Assembly is even quicker because all the new kits come with printed circuit boards.
For more information, TUT4 provides a complete tutorial on Power Scaling methods, while TUT6 expands on the discussion of DC Power Scaling.
The new Power Scale kits are split into two groups: "Super Budget" (the SB-1, SB-2 and SB-84) and "Super Flexible" (the SF-1 and SF-2) . The majority of guitar and bass amplifiers (>99%) have nearly identical plate (Va) and screen (Vs) voltages, and most operate at <540Vdc - and, most of these are below 500V. The "Super Budget" Power Scale kits are designed to accommodate the most popular voltage range of <500V. The "Super Flexible" Power Scale kits cover this range but also handle much higher voltage and large differences between plate and screen voltage. The SB-84 is especially adapted to small amps using EL-84 output tubes. All kits require proper heat-sinking, usually accomplished by mounting the pass elements to the chassis. All kits use a Drive Compensation control to assist in keeping tone exactly where the player needs it, and also allowing four operating modes of the amp.
HIGHLIGHTS: Which to Buy?
The first decision: plate voltage (Va)
Va less than 500V
The "Super Budget" kits are ideal for the majority of amplifiers on the market, from the Valve Junior to the latest Marshall. Save money and space. The SBs are a truly fantastic value! Above 500V, even the mil-spec PS pot may arc during rotation.
Va = 500V to 900V
The "Super Flexible" kits accommodate higher voltages and are aimed at installers who desire precise control over voltage proportioning. This is more of an aesthetic concern but is an easy goal to achieve. These kits are more amenable to remote control and other modifications. Of course, the Super Flexible kits can be used with any lower supply voltage.
The second decision: bias method
Tube amps are either cathode-biased or fixed-biased.
Cathode-bias is also called "self" bias, and uses a passive resistor for the tube to work against and find its idle point. The SB-2, SB-84 and SF-2 are designed to work with cathode-biased amplifiers.
Fixed-biasing uses a negative-voltage supply to control the tube grids, and thus requires more circuitry for proper Power Scaling of such amplifiers. The SB-1 and SF-1 accommodate all fixed-biased amps.
Hybrid amplifiers using BJTs to drive the cathode of the output tubes are Power Scaled as if they were cathode-biased. In most such amps, the supply voltages are quite high and require the SF-2. Applicable amps include: MusicMan's RD-series and Peavey's VTX-series.
Note that proper operation of the SB-1 and SF-1 is assured by addition of the RBX (Raw Bias Auxiliary supply ) kit. Most stock bias supplies are of too high and impedance and too low a voltage to allow full performance of the Power Scale kits.
Third decision: Vs much less than Va
In most amps Va and Vs are within 5V of each other, as Vs is derived from Va using a low-value resistor or through a choke. All of the Power Scale kits accommodate this condition.
Historic amps using EL-84s have a much larger dropping resistor and thus a much lower voltage for the screen than for the plate. This includes many old Gibson amps, the current Valve Junior, some old Vox amps, and other lost brands. For these, the SB-84 is ideal.
For high-voltage amps, SF-1 or SF-2 will be required. The Hiwatt and Trace Eliott 400W amps are directly accommodated, where Univox and the Fender PS-series require a minor modification to the kit.
Fourth decision: switchable bias method
If the amp can be switched from fixed-bias to cathode-bias, then the Power Scale kit required is based on the fixed-bias condition. By their nature, the fixed-bias-capable kits inherently accommodate cathode-biasing.
Fifth decision: tube type
In general, the tube types used in amp have no bearing on which Power Scale kit is used.
The SB-84 was designed specifically for the Valve Junior amp, which happens to use an EL-84 output tube. This is a cathode-biased amp where Vs is 20-30V lower than Va, and use of an SB-2 would introduce a performance compromise. Current-production EL-84 amps are a mixed bag of cloning and economy, with most being compatible with the SB-2 and a few benefitting from use of the SB-84.
Sixth decision: multiple preamp channels
Power Scaling can be used "globally" like the 'presence' control on vintage amps, or be synchronized with preamp channel selection. If the latter is the ultimate goal, then the switching methods required for each Power Scale kit should be considered.
All SB-kits require the use of series relay switching to select between multiple PS pots.
All SF-kits can use series or shunt switching to select between multiple PS pots. Shunt switching allows the use of jfets, which is likely the method already in use for the preamp voice selection. The fact that the PS pot is ground-referenced in the SF-1 and SF-2 makes these kits the preferred Power Scaling solution for multi-voice amps.
Seventh decision: Multiple power amp channels
Power Scaling multiple power amplifiers simultaneously can be achieved with all Power Scale kits.
The only potential complication here is with the 'Drive Compensation' control. In single-PA applications, DrvCmp controls two audio paths simultaneously. In a stereo PA, there may be four audio lines to control, depending on the topology of the circuit; then six paths in a three-PA amp, and so on. Multi-section pots become expensive at four sections, so alternate techniques must be used for drive compensation, specifically automated drive tracking of the PS setting.
Detailed Differences
SB-1: <500V; fixed/cathode bias; PS 28mm; DrvCmp 16mm; bias 16mm; series switching only
SB-2: <500V; cathode bias; PS 28mm; DrvCmp 16mm; bias 16mm; series switching only
SB-84: <500V; cathode bias; PS 28mm; DrvCmp 16mm; bias 16mm; series switching only
SF-1: <900V; fixed/cathode bias; PS 16mm; DrvCmp 16mm; bias 16mm; shunt/series switching
SF-2: <900V; cathode bias; PS 16mm; DrvCmp 16mm; bias 16mm; shunt/seres switching
About Our Sustain & Sag Kits
This section has moved - please see our page: Selecting a Sag or Sustain 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
are not familiar with 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.
Overall,
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.
|
