About Our Switching Kits

Switching Kits by London Power – Human-Scale Technology

London Power‘s range of switching kits allows the player, hobbyist or builder to access amplifier channels, amp features and system configurations, using simple circuitry. All of our switching circuits are discrete so they can be repaired by anyone with average skill, and can be maintained for decades of use. There are no computers to go obsolete and no software that can be “buggy” or need updating. Just simple circuits built with noncritical components for which there will always be a compatible sub in the future.

The actual switch elements used in the audio path can be n-channel jfets, relays, BJTs, mosfets or CMOS gates. We have left out p-channel jfets as their availability and performance for switching is limited.

Most switching applications are very simple: turn a reverb or effects loop ‘on’/’off’; select a gain boost element; select an alternate ‘volume’ setting, and then there are complex requirements. For the latter, it is actually a simple matter to use jumpers, diodes or DIP switches to provide easy “programming” that can be changed at a later date. The DIP-switch option that we provide is a high-quality type rated for 10,000 cycles – ten times the usual rating for such switches.

System Voices

Preamp channels are generally used “exclusively”, meaning only one at a time. Features built into an amp, such as reverb or an effects loop, can be preset as always-on, or have channel-synchronized control. Features might be used in combination with each other, as well. The net combination of a preamp channel and other features is a “voice”.

Poorly designed switching systems tend to be of the type where each feature has its own switch along with a switch to select the channels. This is a nightmare for the performing or recording musician, as he may have to hit multiple footswitches to go from one voice to another. Ideally, each voice would have some sort of “programming” or “synchronization” of the channels and features, settable by the player. In the ideal case, hitting a single switch selects the chosen channel and chosen features all at once. Hitting a different switch would enable a different combination of channels and features, and so on.

In a guitar system, even one that is just a single amplifier and speaker, the number of “voices” will determine the number of switches required at the player’s feet. A single-channel amp has one voice and nothing to switch. Add a reverb to this amp and we have one thing to switch, so there is one footswitch to select the reverb status. The amplifier now has two voices: the dry amp sound and the amp sound with reverb.

A 2-ch amp has two voices and only requires one switch to select which voice is live. Mechanical switches have two states, being ‘open’ or ‘closed’, the same as ‘on’ and ‘off’. Those two conditions are “exclusive” as they cannot occur at the same time. Just as the reverb can only be on or off, or only one of the two channels can be on at a time, the exclusive nature of the switch allows us to use a single switch to go between two choices.

If we add reverb to the 2-ch amp, we suddenly have four choices: ch-A, ch-A with reverb, ch-B, ch-B with reverb. Having just a channel switch and a reverb switch as most designers and amp manufacturers provide is not the best approach for the musician. There will be times when the player has to hit both buttons at the same time to go from one sound to another. Here, we need a separate switch for each of the four voices. This allows direct-random-access and only one button needs to be hit to go from any one voice to any other voice.

A three-channel amp requires three switches to allow selection of any voice in a single switch transaction. From here on we invoke “direct random access” as an attribute of the switching system that makes player ergonomics simple.

Two-way Communication

The amplifier should normally be able to work independently without requiring a footswitch be plugged in. However, there are many three-channel amps on the market that cannot operate this way, where only two channels can be selected using front-panel switches. In these amps, to access all three channels the player must use the footswitch. This is not exactly good ergonomic design, and is entirely an economic choice made by the designer.

Most amps that do have footswitch capabilities and a properly capable front panel miss out on another ergonomic detail: “two-way communication”. In typical amps like this, the footswitch is configured to over-ride the panel controls. LEDs will show what has been selected at both the footswitch end and the amp end, but only the footswitch controls anything. In the rare case where the amp’s panel switches are still active with the footswitch plugged in, the LEDs on the amp may not indicate the same amp status as the LEDs at the footswitch if panel switches are used to make the selection.

Two-way communication between the amp and the footswitch assures that whatever is being indicated as being active at the footswitch end is the same indication at the amp end. Using a panel switch or a footswitch changes the indication at both ends of the system. Such a system can be expanded further to allow multiple footswitch locations, as might be required on a large stage. With 2-way communication, actuation of any switch at any location causes all locations to indicate the new system status.

None of this is rocket science, or even computer science; it was all being done before there were computers built into everything.

From the Simplest…

London Power offers two approaches for simple exclusive selection and switching: the RLY mechanical relay, and ERK Electronic Relay Kit.

RLY is simply a DPDT relay on a small board. The installer has to provide power for the relay coil and his own switch to actuate the relay. This is “stone age” technology that anyone can understand. There can be a LED on the footswitch or not, and a LED on the amp or not. There can be a panel switch or not, which can be over-ridden by the footswitch or not. You see, even with such a simple switch-plus-relay there are many options.

ERK is slightly more sophisticated using jfets wired for shunt switching as the switch elements. This means the jfets are tied to ground, which is the quietest switching form and is suitable for use with our LP-PRE 2-ch tube preamp kit, and similar circuits. ERK has options for one or two LEDs at the amp end but no footswitch LED. The optional foot switch available for ERK is a standard push-on/push-off alternate-action type with a large actuator. Players or builders can buy ERK without this switch and save half the cost of the kit providing their own.

…to Intermediate…

Many players prefer to have independent control over the functions of their system, even if this might mean that sometimes they have to hit two buttons to go from one sound to another. This depends on the playing style of the individual. For these intermediate-complexity systems, we have a sophisticated solution: DFC Dual Function Controller.

DFC uses non-latching normally-closed push buttons just like the Q-system below, so DFC can aesthetically match the Q-system. DFC uses electronic latching circuitry to allow any style of switch to be used from very small to very large – for finger tips or feet. Each half of DFC can control relays, shunt-jfets and series-jfets. The series-jfet output can also control grounded-source power mosfets as high-voltage switches for selecting tubes, for example. Each half of DFC is independent and not effected by the other, so the player has independent control over each feature of his sytem.

Each half of DFC also has double-indication of its status. This means that the nominal-off state and nominal-on state both have a LED that lights to show that condition. This is great to have when one section is used to switch between two preamp channels. One LED shows ch-1 is active while the other shows ch-2 is active. The LEDs can be individual as for this application, or they can be combined as a 2-colour 3-lead type. Of course, you can use a single LED or no LED per half of DFC, so anything up to four LEDs can be driven by the circuit – two on at a time. Current-steering allows the circuit to draw only the current that would be used by a single LED.

For players who like the large ERK-style foot switch (FSW), these can be wired for momentary or ‘pulse’ duty, as shown in our book The Ultimate Tone (TUT) fig. 9-77. These can be applied to the Qsystem as well.

For about the same cost as two ERKs, DFC provides three-times the output switching cability and more options for the actual switching devices.

…to Unlimited Sophistication

The Q-system family of switching products provides direct random access for any number of voices, as well as potential for two-way communication between two or more switch stations.

The first building block is Q-LATCH , which provides an interface for up to four switches – either panel or foot – with auto-canceling of previous selections and latching of the new selection. Multiple Q-LATCHes can be ganged “side-ways” for more than four voices to be selected, or “daisy-chained” for 2-way communication between as many stations as you need. Of course, sideways expansion and daisey-chaining can be done simultaneously just by adding more Q-LATCHes.

At the amp end, a Q-LATCH (or a few) can be used for the local voice selection. To control relays, Q-RLY is added to control up to four relay groupings. These typically control features or channels. The relay control groups can be linked to be activated by any or all of the four voice lines from the Q-LATCH. If there are more than four voice lines, the voice expansion card Q-VX is added to allow the voice control lines of the added remote Q-LATCH to access the blocks of the Q-RLY. Note that Q-VX is not needed at the amp end if there are Q-LATCHs at the amp end.

Similarly, if jfet control of features or signals is required, Q-FET has four blocks to do just this. Each block can provide the correct control voltage range for shunt or series switching. Each block can be accessed by any or all of the four voice control lines, and with Q-VX can be controlled by more than four voices. Q-FET provides a shunt control line and a series control line for each of its four blocks.

Many amps will have few things to switch, so we also offer a single relay controller as Q-SR, and a single jfet controller as Q-SF. Q-SF has a shunt control line output and a series control line.

Lighting the Way

ERK has the ability to drive one or two LEDs, or to drive a 3-lead bicolour LED. ERK is an exclusive switching system, and thus has two states, both of which have positive indication if the builder wishes to incorporate it.

RLY has no inherent or in-built LED drive or indication of its states. The builder must add his own LED driver with current limiting for the LED.

DFC has positive indication of both states for each of its two independent control blocks.

Q-LATCH has drive circuitry for local LEDs to indicate which voice is active. LEDs can be wired directly to the board, or the Q-LED board can be used, most often in combination with Q-SW as a panel selector.

Q-RLY and Q-FET also have voice line LED driver circuits. Where one might decide to have the “smart footswitch” with Q-LATCH and a “dumb amp” with no panel switches, it is still desired to have LEDs on the amp to indicate which channel has been selected even though there are LEDs on the footswitch.

For simpler systems, Q-LDV LED driver can be used to control the amp’s panel LEDs to display the voice selected. If one chooses, the LEDs driven by Q-SR and Q-SF show that the feature is active. When the voice indication “overlaps”, as in the case of a 2-channel preamp with one extra feature, two of the four voices engage ch-1 while the other two voices engage ch-2. It is typical here to place a LED beside each channel’s controls to show which is active. An additional LED can be placed by the FX loop controls to show when it is active. There are two ways to deal with this situation as the downloadable PDF describes.

Q-RLY, Q-FET, Q-SR and Q-SF also have “function” LED drivers, as the control blocks do not necessarily relate to voices. For example, one control block might be for channel-1, another for channel-2, another for reverb and the fourth for an effects loop. We might wish to have a LED light when the reverb is active and one for when the loop is active, or for whatever the function is.

The Q-system accommodates whatever one might need, and can be easily reconfigured if you change the amp or other parts of the system.

For more information, download our informative PDF: London Power Switching Kits Selection & Application (135 kB, 22 pages).

Cathode-Bias/Fixed-Bias Switching in Tube Power Amps

 ~The following is an excerpt from our book, The Ultimate Tone – Vol. 2 (TUT2) by Kevin O’Connor

Vintage guitar amps often produce a tone that is described as “round” or “mellow,” and they can produce a desirable overdrive sound. Some of these amps are low powered, and others have lower supply voltages than later models, but many use “cathode bias” for the output tubes instead of the more efficient “fixed bias” method. Cathode-biased output tube tone can be “round” if no bypass capacitor is used, or it can be “crisp” if a cap is used – sounding quite similar to a fixed-bias output stage.

Modern players have the advantage of an extremely wide “acceptable” tone range to experiment within. They are not limited to the relatively clean sounds that were demanded in the fifties. A greater variety of basic amp tones allows the musician to keep his system simple, but occasionally places him or her in the position of requiring more than one guitar amp for a range of music styles. Depending on individual requirements, it is often desirable to be able to have both cathode-biased and fixed-bias tones available from one amp, at the flip of a switch.

Bias switching is a simple task, but demands the use of reliable components – if you buy surplus parts, confirm their integrity with an ohmmeter. One consideration that is not critical for every player, or on every amp, is whether a tremolo feature is present (and used). If a tremolo is used, then we must not disable the fixed-bias supply completely, as it is tied into the tremolo as a “mute” control. In this case, we simply want to disconnect the bias supply from the output stage when cathode bias is desired.

To accomplish the bias switch in the simplest way, a double-pole double-throw (DPDT) switch is required. This should be CSA or UL approved, to assure the quality of the device. It can be a miniature type, though, as the voltage and current stresses are negligible.

In Fig.1, we see a typical Fender bias-balance supply with the new cathode-bias option and switching added. To add the cathode-bias components, you must unsolder the ground straps from pin-8 of each output tube socket. Two 1-ohm 1/2 watt resistors are used to tie the cathodes together, and to the bias resistor (Rk). This resistor will be a high-power wirewound – 1W to 5W, try 500 ohms for a pair of 6L6s, or 800 ohms for a pair of 6V6s. The bypass capacitor (Ck) can be plastic or electrolytic, depending on the value you choose. Its voltage rating should be twice the value of the anticipated DC bias. For electrolytics, make sure that the positive lead is attached to the 1-ohm resistor bridge, and that the negative lead is tied to ground. This cap should be kept well away from Rk, as Rk will generate a lot of heat.

Note that Ck can be switchable also. Place a 10k half-watt resistor in series with the negative lead of the cap, and connect a second mini-switch across the resistor. With the resistor shunted by the switch, the cap allows full output power, and a loud, bright tone. With the resistor isolating the cap (switch open), the power is restricted but the tone is mellower and breaks up sooner.

To switch the cathode-biasing components in and out of circuit, contact-A of the DPDT is wired as in Fig. 1. The other pole isolates the fixed-bias supply from the grid-leak network for the output tubes, as appropriate. The second pole is series connected between the raw bias supply and the bias pot. Lift the wire that goes to the tremolo off the bias pot, and move it to the supply side of the switch contact.

For fixed-bias operation, the switch is up. Contact-A is shunting out Rk and Ck so that the 1-ohm resistors are tied to ground. Contact-B connects the bias supply to the grid leak network, for stock, fixed-bias operation.

Flipping the switch down allows Rk and Ck to come into play, where the current flowing through the tubes generates its own bias voltage across Rk. Contact-A shorts the tap of the bias pot, and thus the grid-leak network, to ground, so that the cathode-bias arrangement will be complete. Contact-B opens to isolate this newly grounded point from the bias supply. The supply maintains its usual level, so that the tremolo can still be turned on and off at will.

An alternative method using a single-pole double-throw (SPDT) switch and a transistor is given in Fig. 2. The SPDT performs the functions of contact-A in the previous circuit, while the transistor performs contact-B’s function.

In cases where there is both no tremolo and the bias supply is plate-derived (not from its own winding), then an SPDT can be implemented as in Fig. 3. Here, a very large resistance – typically 150k – protects the bias supply diode and source winding from excessive dissipation when the bias supply is shunted to ground.

If the bypass capacitor is used, its value can be determined using the capacitive reactance formula*, and the value of Rk. You must decide what the frequency response of the cathode-bypassed circuit will be. The formula tells you where the response is down by 3db. Maximally flat response occurs starting at a frequency ten times higher. For guitar amps, response to 70Hz is typical, although if response is down at 70Hz, the perceived crispness of low notes will be enhanced.

*f = 1/(2pi x Ck x Rk) and Ck = 1/(2pi x Rk x f)
As an example, for a 500-ohm cathode resistance and a 70Hz 3db down point, Ck is under 5 microfarads. For flatter response, a value of 50 microfarads or even 500 microfarads should be considered.

**Note that the Rk values recommended above are conservative compared to what is typically used in cathode-biased amplifiers. This extends tube life and helps make the tone difference between fixed-bias and cathode-bias a bit wider.

IMPORTANT: Screen stops should be a minimum of 1k-1W flame-proof for all tubes and this is especially important in cathode-biased amps.