Dumble Amplifiers
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Dumble musical instrument amplifiers were custom manufactured in very limited numbers by Alexander ("Howard") Dumble of California.
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[edit] DUMBLE AMPLIFIER MODELS
The amplifiers are famed for their sound that is very responsive to the touch and feel of the player and in those amps having an overdrive channel, characteristic overdrive distortion tone. Clean sound is clear,defined well balanced presentation of all the frequencies with really coherent fundamental. Dynamic response maintain all the subtle nuances of playing from top to bottom voices in both clean and overdriven tones.Amplifier models have included: Overdrive Special(ODS), Overdrive reverb, Steel String Singer(SSS); Dumbleland and Winterland.
Musicians who have used Dumble amplifiers include :
Carlos Santana, Robben Ford, Larry Carlton, Eric Johnson, Stevie Ray Vaughan, Lowell George, Henry Kaiser, John Mayer, Steve Lukather, Sonny Landreth, Rick Vito, Tom McGrath, Steve Kimock, Jackson Browne, David Lindley, Michael Husser (musician) Kim (Kimbo) Davis of Blackbird and Point Blank.
Several companies have produced amplifiers based on the Dumble Overdrive Special . Some DIY-amplifier building hobbyists have also started to build Dumble influenced amplifiers from un-official schematics and photographs circulating on the Internet . Since the 1980s,mr. Dumble has covered the preamp circuitry of his amps with a thick layer of opaque epoxy, protecting his schematic's exact design, although several amplifiers have been "de-gooped" and their component values measured.
The circuitry varies somewhat between individual amplifiers, because each one was built and voiced for a specific player. Amplifiers built around the same time tend to share some of the same circuitry and component values. However, several amplifiers were later sent back to Dumble to have their circuitry updated.
Part of the characteristic sound of Dumble amplifiers comes from dumble's choice of parts: - the type of capacitors used in the tonestack and in the overdrive stages , type of plate resistors, the output transformer, the lead dress, and the use of shielded wire. Dumble amplifiers are built on eyelet boards . The chassis is made of aluminum.
All Dumble amplifiers operate using vacuum tubes. Overdrive Specials use 12AX7 tubes in the preamp, 6L6 tubes in the power amp in older models, and (EL34) tubes used at in the power amplifier section in some newer models. A separate Dumblelator buffered effects loop (a cathode-follower output with a triode gain stage for recovery) is recommended for using effects processors with a Dumble amplifier.
[edit] Overdrive Special
There are two inputs: a "normal" input and FET input which 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.
[edit] Overdrive Reverb
Much of the circuitry in the Overdrive Reverb is similar to that of the Overdrive Special. The reverb circuit in one known specimen uses three tubes ( two tube configurations are commonly used). First tube is used to amplify the input signal, which is then sent to the second tube that drives the reverb tank through a transformer in a configuration similar to that used in Fender amplifiers. The third tube mixes the clean and the reverberated signals . Both reverb "send" (the signal level sent to the reverb tank) and reverb "return" (how much of the signal returned from the reverb tank is mixed with the dry signal) are individually controllable.
[edit] Steel String Singer
This model was made in very limited numbers . It was a "clean", loud amp (100 or 150 W) with no overdrive section .Standard model used 12 AX7 preamplifier tubes and 6L6 power amplifier tubes. Standard Dumble tonestack with optional tonefilter. Reverb was similar than that found on Overdrive reverb.
[edit] External links
- Amp Treasures - a collection of photographs of Dumble amplifiers, some with descriptions
- Vintone Circuits - Detailed photos of Dumble Overdrive Special Cabinetry and Faceplate
- The Amp Garage - Discussion forums for people interested in Dumble and Trainwreck amplifiers
- Rob Livesey's Page - A collection of photographs, datasheets, and other information about Dumble amplifiers
