Dumble Amplifiers

From Wikipedia, the free encyclopedia

Tom McGrath's Dumble ODS 50W Combo Converted to Head and Cabinet by Doug Stalters of Vintone Circuits
Tom McGrath's Dumble ODS 50W Combo Converted to Head and Cabinet by Doug Stalters of Vintone Circuits

Dumble musical instrument amplifiers are custom manufactured in very limited numbers by Alexander ("Howard") Dumble of Los Angeles,California. Dumble began making amplifiers in the late 1960s in Santa Cruz. There are reportedly fewer than 300 in total.

Alexander Dumble … December 1, 1993 writes, "I still custom manufacture all of my products “one-at-a-time” with extreme care and attention. So each amplifier and speaker enclosure can very much be considered a prototype device suitable as a model for mass manufacturing, copying procedures. Hence, this is the reason for the security agreement. The emphasis here as always is making the highest quality product capable of resplendent, impressive performance."

 This article or section is written like an advertisement.
Please help rewrite this article from a neutral point of view.
Mark blatant advertising which would require a fundamental rewrite in order to become encyclopedic for speedy deletion, using {{db-spam}}. (June 2008)

The amplifiers are famed for their sound that is very responsive to the touch and feel of the player and for their characteristic overdrive distortion tone. Their clean sound, too, is clear, defined, and well-balanced, offering good presentation of all the frequencies with coherent fundamentals. Dynamic response maintains 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, Ben Harper, Lowell George, Henry Kaiser, John Mayer, Kirk Hammett, Todd Sharp, Steve Ferris, Steve Lukather, Sonny Landreth, Rick Vito, Tom McGrath, Steve Kimock, Jackson Browne, David Lindley, Dean Parks and Todd Epstein.

Contents

[edit] Influence

Several companies have produced amplifiers somewhat based on the Dumble Overdrive Special. Some DIY-amplifier building hobbyists have also started to build Dumble-influenced amplifiers from un-official schematics and inside chassis photographs circulating on the Internet although many have been voluntarily taken down following requests from Mr. Dumble. Amp makers who openly promote their "Dumble Clones" include the Tony Bruno Super 100 and Andy Fuchs' line of Overdrive Supreme amps. Dumble's German importer in the '80s made a clone called the Kitty Hawk Standard. Since the 1980s, Mr. Dumble has covered the preamp circuitry of his amps with a thick layer of usually opaque epoxy, protecting his schematic's exact design, although several amplifiers are known to have been "de-gooped" and their component values measured.

[edit] Circuitry

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
  • the 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 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] Models

[edit] Overdrive Special

50W ODS Combo
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.

[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). The 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. Power output was usually 100W, although Stevie Ray Vaughan's famous "King Tone Consoul" SSS was rated at 150W. It was a "clean", loud amp with no overdrive section. The standard model used 4 x 7025 high-mu twin triodes; 1 x 5751 high-mu twin triode; 4 x 6L6 power tubes and 12AX7 preamplifier tubes. The amp used a standard Dumble tonestack with an optional tone filter.

[edit] 150W Overdrive Special

Back Panel of 150W Dumble ODS
Back Panel of 150W Dumble ODS

In 1987, Dumble made two otherwise unique amplifiers for Todd Sharp at 150W utilizing signature components from several of his other famous models. The power supply is similar to that of the 150W Dumbleland amplifier, but using 7025 tubes as in the Steel String singer. It is switchable from 150W to 75W. Preamp tubes are: a)6 x 7025, b)1 x 7247, c)1 x 5751 while the poweramp uses 4 x 6550A tubes. The ampifier also uses the SSS 3-spring reverb. Dumble also included a tremelo. The back panel includes controls for adjusting the bias and dynamic balance which are believed to be unique to this model; speaker outputs are switchable at 2,4 or 8 ohms. Inputs include normal and FET as with most Dumble amplifiers. The footswitch permits turning on and off the overdrive, the pre-amp boost, the tremelo and reverb. The intention according to Dumble was to combine the best features of the Overdrive Reverb and the Steel String Singer with the distinctive overdrive circuit of the Overdrive Special. One of Sharp's amplifiers was stolen and has not been recovered. The other, pictured here, is in Los Angeles, reportedly in a recording studio. The power circuit has been described in Aspen Pittman's "The Tube Amp Book."

[edit] Winterland

The Winterland appears to have been designed primarily as a PA system. It is rated at 450W and was named after the San Francisco concert hall.

[edit] Dumbleator

Dumble also occasionally makes a tube-buffered external effects loop called a Dumbleator. A few Dumble amps have a Dumbleator circuit built into them, but most do not. Many users insist that a Dumbletor is required to obtain the optimal sound when using effects boxes and pedals. The Dumbleator has separate "Send" and "Receive" circuit for the effects; a gain control, "Bright Switch" and a return level. Dumbleators are even more rare than Dumble amplifiers.


Dumblelator II

Alexander Dumble ... December 7, 1993 writes, "The Dumblelator is mono-in with mono-out while the Dumblelator II is mono-in with stereo-out."

Alexander Dumble ... December 11, 1993 writes, "The Dumblelator II just allows for the second signal half to be amplified to the correct gain point where the resulting signal can…"

[edit] External links