Dumble Overdrive Special
From JMgear
John frequently uses the Dumble ODS in conjunction with a few Two Rocks that all run simultaneously. The Dumble tone is representative of the SRV side of the tone that John gets when he plays. This is one of the rarest and most desired amplifiers amongst vintage collectors. They have been said to sell for up to $30,000 dollars from private sellers and the price is rising.
Photo of the Dumble Overdrive Special[1]
From wikipedia.com:
Overdrive Special 50W ODS Combo
There are two inputs: a "normal" input and FET input where the signal from the guitar goes to a J-FET line level booster pre-amplifier before entering the first tube stage (originally intended for use with acoustic guitars and other instruments with low output levels).
The first tube is followed by a passive tonestack with adjustments for treble, middle, and bass. Typical switching options available in the different models include:
* Bright (bypasses the volume potentiometer with a capacitor to boost the high frequencies) * Deep (changes the voicing of the tonestack for a warmer tone) * Mid (increases the value of the treble capacitor to boost the midrange frequencies) * Boost (disconnects the treble potentiometer from the bass potentiometer for a preamplifier level boost) * Rock/Jazz (switches between a Fender-esque voicing and a more "hi-fi-type" of voicing).
The tonestack is followed by another clean preamplifier stage. The resulting signal is either fed straight into the power amplifier ( clean channel), or into the (two-stage) overdrive section. The power amplifier uses a standard long-tail pair phase inverter with negative feedback. It will break up , but only at very high volume levels, or when the preamplifier "boost" switch is engaged.
In some newer models (1990s to present), the overdrive stages are followed by a passive tonestack (treble , middle and bass), which is adjustable using trimmer potentiometers inside the amplifier. This is known as the "Hot Rubber Monkey" (HRM) modification (these amplifiers are voiced for the overdrive channel to be used with the preamplifier boost engaged). Most players use three tones: clean, clean with boost on, and overdrive with boost on.
The "Skyliner" EQ refers to a newer (mid-1980s to present) way of voicing the tonestack. It is said to provide a better clean tone than the earlier voicing of the tonestack. Some lower serial number amps have been retrofitted with this EQ.
The so called "Robben Ford Mod" (terminology used by DIY-hobbyists) refers to using a power resistor instead of a choke in the power supply in order to induce more "sag" in the amplifier.
