Onslow Custom Modifications

  • Completely original designs

  • 35 years of modding experience

  • Proven reliability

  • Used by top professional musicians

Onslow Amp Designs

When I started modding amps I set out to capture the tones of Carlos Santana, and Jimi Hendrix. It was mid-Seventies and Marshall was just releasing their first Master Volume model. The first Boogie hit the market, basically a more compact Fender Twin design with a single extra gain stage, and no channel switching. Hiwatt had their Harry Joyce preamp mod and that was pretty much it.

That year I bought a 1969, 200 watt Marshal Major head from my buddy for $150. It was a PA head and I wanted a Lead model, so, I went to the distributer and bought some schematics. One mod followed the other and soon I was experimenting with channel-switching and FX loops.

The sound of Rock started to change. AC-DC, Metal and Eddie Van-Halen. I worked for 10 years until I found that tone. Still more change ...

Today I've been there and back. From searingly intense drive levels before stacks of Marshalls to soft moaning Blues and screaming Albert Lee country licks. I now offer many different gain levels to suit an ever growing variety of styles. Rich fat clean tones on Class A modded amp or a Stevie-Ray Blues solo tone on a Bluesman Modded Combo, to a full Hendrix 100 watt Marshall tone,,, that's what I do

FX Loop

An FX loop is not for every type of device. Overdrive pedals, wah-wahs, phase-shifters, and compressor pedals sound better when used before the amp rather than in a loop. If however, you use a high-gain amp and wish to add a digital processor for reverb or delay then a loop becomes indispensable.

An FX loop has many advantages. By inserting a digital processor in an FX loop that is placed after the preamp's overdrive you are able to :

  • Prevent distortion of digital reverb FX processors when using the preamps overdrive channel. Using reverb before your amp is the same as plugging the reverb into an overdrive pedal. When used in the loop instead, you can set the master volume of the overdrive channel to match the clean channel volume. This will deliver a more consistent reverb sound.

  • Prevent the compression of delay effects. If you use delay before an amp the effect will seem to compress when you push the preamp, even if you are not using much drive. This will cause the delay to get louder compared to the dry sound. Use of an FX loop eliminates this problem

  • EQ the guitar and not the reverb. Since the preamp's EQ is now before the reverb you will not be thinning out the sound of the reverb when you boost the highs of your amp

  • Reduce digital processor noise to a great extent. Since the amps EQ is before the loop, boosting highs on the preamp will not increase the noise from the processor

  • Use a volume pedal that will act as a master volume, since it is after the preamp

  • Use other preamps such as a JMP1 and simply plug into the loop return

  • Use stereo setups. By using either an additional power amp, or another head that is also equipped with an fx loop, you can use one preamp to control both sides of a stereo setup

  • Send the amp signal to a digital recorder for later re-amping at louder volumes. This way you can record at low volumes, then feed the signal back to the fx loop return the next day, crank the amp, and take as long as you want to get the mic positioning, etc

FX Loop Features:

Most modern amp manufactures cut production costs by using solid-state integrated circuits in both the Send and Return circuitry of the loop, even in their most popular "all-tube" amplifiers. This means that your painstakingly achieved tube tone is brought down to guitar level and re-amplified all the way back to power amp level by transistorized integrated circuits! And, to make matters worse, your guitar signal must go through this circuit even when you are not using the loop!

I use an all tube design for the Send and Return loop circuits and install an additional 12ax7 tube to ensure that your tone in not altered. You can drive the FX level as hard as you want without worrying about transistorized clipping. I add an FX Return volume control so that you can drive the loop to whatever level is best suited for your FX and still have total control over the final volume of your amp.

  • Low impedance loop Send will easily drive line inputs as low as 10k ohms without signal degradation or loss of high frequencies.

  • High impedance FX Return suited for FX pedals

  • FX Return Volume Control* acts as final Master Volume of your amp

  • -10dbm operation - compatible with both home audio semi-pro effects and stomp box pedals.

  • Not sensitive to RF interference

Multiple FX Loop:

For even more control I offer the option of separate series FX Loops for Clean and Overdrive channels. No need to switch effects when changing channels anymore, simply use different effects for each! Master override loop when not using individual loops still works in all channels when you don't want to use separate effects.

Consult the section that deals with your amp to see if the FX loop is available for your model

Solo Boost

The Solo Boost came about in response to requests for a slight increase in volume when doing solos, just a slight boost that could be controlled by a foot-switch. So that's what this is, an easy to control variable volume boost with no tonal change that operates on all channels.

Set the control to minimum for a barely noticeable volume increase. Raising the boost control will give you a very gradual increase in volume. With the control open to full the amp will max out at about twice as loud. This is great when you are on a gig and have to set the boost level without listening to it. Set the pot to midway and you can't go wrong! (Requires FX loop mod.)

  • Precision controlled volume boost

  • Foot-Switchable

  • Both clean and overdrive channels

  • No Tonal change

Radio Frequency Elimination

The air is saturated with radio frequency signals today. Cell phones, wireless internet, dimmers, neon lights all generate potential noise for your amp if it is susceptible to RF pickup. RF does not always manifest as an radio station, sometimes its a rushing white noise or a ground-like humming.

Try this test:

  1. Turn the volume on the guitar all the way off

  2. Plug into your amp and turn up the amp volume

  3. Move the guitar cord up and down and left and right in the air.

Now If you get a loud noise that grows or fade as you move the cord or a radio station cut in and out then you a victim of RF pickup and your guitar cord is acting as the antenna. With bad cases you get a loud steady ground hum that never goes away

This noise can be completely eliminated! All my overdrive and FX loop mods are thoroughly insulated from RF noise. RF elimination does not affect your tone in any way but will really make things quieter.

Power Tremolo

My favorite Tremolo effect is the Fender Princeton design. The tremolo is applied directly to the output tubes effectively switching the tubes on and off to generate the effect. Even when driving the amp into hard saturation the tremolo effect is not compressed or reduced.

The more powerful Fender amps had two channels and the first channel was not intended for any effects and so Fender used a different tremolo design that incorporated a photocell and neon light-bulb pair. This circuit is bad for many reasons:

  • In order to provide a smooth boost and cut effect, the whole circuit is biased to be set mid volume when the tremolo is off

  • The tremolo effect is produced by shunting (shorting) the signal to ground. This excessive loading greatly affects the tone

  • The effect is lopsided or unsymmetrical due to non-linearities of the whole light-bulb photocell mechanism

  • The tremolo effect is diminished when the amp is driven hard and the preamp becomes compressed.

Power Tremolo Features:

Transform your later Fender photocell design and gain in tone and depth:
  • Smooth symmetrical Tremolo

  • No signal shunting or loading therefore better tone

  • No diminishing of effect when amp is driven hard

Power Tremolo works just as well on a Marshall. Since it operates by modulating the negative bias to the power tubes there is no additional circuitry in the signal path that can alter the tone of your amp

Cathode Biased Class A Power

Everybody Loves this one. Convert your amp to Class A Cathode-Bias operation similar to a Vox AC30. Sounds fat and sweet, and more compressed, just beautiful with a Fender guitar. Want the original sound again? No problem, the ground switch is now the Class selector so flick it back to the amp's original Class AB Fixed Bias mode.

100 watt amps really benefit from this mod and will gain new life and bounce. No more need for a compressor on the clean sound.


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