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JP-50 (2003)

The JP-50 is considered a “stage use” single-channel guitar amplifier capable of louder volumes than the JP-10 amplifier, with higher headroom. However by using the Output Drive, High/Low Power and Damping controls, it’s reasonable to expect the JP-50 to sound full and lively at volumes suitable for home practice use.

Front panel controls

Low, Mid, High – EQ controls function as expected, and have an additional mini boost switch associated with each knob. The Low Boost adds more depth/chunk and helps give definition/separation to the low frequencies. The Mid Boost helps to get a more singing lead tone and cut through the mix. The High Boost is useful for times when you want to add some “sharp edge” to the treble for a more aggressive overdrive sound or more defined/touch sensitive clean sound. This differs from the presence control in that it adds a more “aggressive” upper treble character rather than adding more brightness. It works in the uppermost frequencies only.

Presence – A way of adjusting the treble frequencies beyond the EQ’s “high” control.

Damping – Controls the overall damping level of the poweramp for either a more aggressive/glassy character or a smoother/warmer character. On some 2004 models this dial has an extra function: with the knob pulled out (Depth mode), it thickens the low frequencies and expand them beyond the range of the EQ’s “Low” dial. (This control is marked as “Resonance” on some models, depending on the faceplate style.)

Gain – sets the overall gain/distortion level.

The Master Volume knob controls the overall volume level.

The Voicing switch selects one of four overall gain voicings:

Voicing 1 – The voicing with the lowest gain and distortion. Capable of achieving lower-gain classic rock sounds or with the Gain turned down, some bluesy/cleaner sounds. NOTE: this is not a “clean channel”. If you’re considering buying a JP-50 second-hand, be aware it was not intended to be very good at getting clean sounds.

Voicing 2 – Takes Voicing 1 and adds some more gain, snarl, and growl. Sounds reminiscent of late 70’s to 80’s rock can be achieved.

Voicing 3 – The “mid to high gain” voicing. Very much like Voicing 2 but with more gain and distortion, a bit more full and rich sounding. (This Voicing 3 was changed for 2004. The original 2003 Voicing 3 was “moved” to become the current Voicing 4. To attain the original 2003 version’s Voicing 4 sound with a 2004 model, use the 2004 model’s Voicing 4 and the gain boost together.)

Voicing 4 – Another gain stage is engaged, and Voicing 4 starts where Voicing 3 leaves off. As the gain is increased, a transition to a more thick “high gain” sound occurs. This voicing has qualities found in “classic overdrive” amps while having plenty of overdrive suited to modern hard rock music.

Gain Boost (on some 2004 models)- A miniature switch (above the Gain control) gives extra tightness and subtle increase in midrange focus to any of the voicings. (Left = off, right = on.)

Rear panel controls

Output Drive – This switch controls the level of gain at the last gain stage of the preamp. Using higher settings makes for a punchier sound, which can be very useful at lower volumes or medium gain settings. Cleaner and classic rock sounds become deep and punchy. Using lower settings helps even out the dynamics, which is quite useful for very high gain settings especially at higher volumes. It’s not recommended to use higher settings of this control and higher gain/volume settings. The amp’s own poweramp distortion takes over at high volumes and provides more punch and distortion, so lower Output Drive settings are more practical. Excessive feedback is more easily avoided this way too.

Loop – This is a series preamp out / poweramp in type of effects loop best suited to rack effects. Some pedals will still function well in this loop, depending on their design (generally speaking, this loop is not suitable for most pedals for anything other than lower to moderate volume levels).

Speaker Outputs – There are four jacks to be used as Speaker Outputs, generally meant to be used separately. Use either the 4 ohm jack, the 16 ohm jack, or one of the two 8 ohm jacks. When connecting two 16 ohm cabinets, both 8 ohm jacks can be used for a proper impedance match.

Attack – This switch is meant to allow for a different overall feel of the amp. Shallow has a more contained feel and Deep feels deeper overall.

Features on the top/back of the chassis

High/Low Power (some 2004 models only) – Switches between pentode operation (full power) and triode operation (low power) of the output section. Power is reduced to about 40% when in Low Power mode and the amp takes on a somewhat smoother/rounder character.

Response (Vintage/Modern) (some 2004 models only) – When set to Vintage, the amp responds and sounds as though it has a tube rectifier. More bounce and “sag” are introduced, making for a more classic touch-responsive feel. When set to Modern, the amp’s response is more focused and forward-sounding.

Output tube bias pot / test points – Changing output tubes is easy for the JP-50 amplifiers. There are bias test points and an externally accessible bias pot on the back of each amp—opening the amp is not required to set the output tube bias. A small screwdriver and a regular volt meter are the only things required. The poweramp of the JP-50 can accept various tube types (see next section).


Four 12AX7 preamp tubes are used for the preamp of the JP-50. (One is for the phase splitter section, the one closest to the output tubes.)

The JP-50 shipped with EL34 output tubes, but can accept a variety of different output tube types.

When changing output tubes, bias the amplifier as per the “biasing your Peters amplifier” instructions.

Fuses/lamp bulb

JP-50 fuse ratings

Mains = 2.5A Slo Blo

HT = 300mA (1/3A) fast-acting

JP-100 fuse ratings

Mains = 3A Slo Blo

HT = 750mA (3/4A) fast-acting

All fuses should be rated for 250V.

Early units had a separate power lamp instead of an internally illuminated power switch. The bulb for that lamp is rated for 6.3V / 150mA (0.95W).

Removable protective bar

On earlier units, the metal bar on the back of the amp is a quick-release type. Just pull the bar and it unhinges from a couple clamps that hold it in place. To replace, push until it snaps into place.

Later models had thumbscrews to hold the metal bar in place.

Note: This document is meant as a reference for JP-50 and JP-100 models only. These amplifiers have different features, functionality and sound from the JP-10 model.