Dumble Amplifiers Dumble musical instrument amplifiers are built by hand in very limited numbers by Howard Alexander Dumble of California. The amplifiers are famed for their singing, "clean" overdrive channel that is very responsive to the touch of the player. They produce a tone somewhere between a Fender and a Marshall, with a strong midrange and a balanced voice. Models include: Overdrive Special (ODS) Overdrive Reverb Steel String Singer (SSS) Small Special Odyssey Winterland Dumbleland Mega PLX Slidewinder Modifications to other amplifiers, including the Ultra-Phonix modification to the Fender Bandmaster and Bassman 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), and Kim (Kimbo) Davis of Blackbird and Point Blank Because only about 250 Dumble amplifiers have ever been built, and Mr. Dumble only builds amps for certain players, they command extraordinarily high prices on the used market, upwards of thirty thousand dollars. Several companies, such as Fuchs and Two-Rock, produce amplifiers based on the Dumble Overdrive Special. Some people have built clones of Dumble amplifiers from schematics and photos circulating on the Internet. Since the 1980s, 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. Contents 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 for interstage coupling, in the tonestack, and from plate to cathode in the overdrive stages has a discernable impact on the tone of the amplifier, as do the type of plate resistors, the output transformer, the lead dress, and the use of shielded wire. Dumble amplifiers are built on eyelet boards made out of either G10 FR-4 Garolite or a Formica-like wood-grain material. The chassis is 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 in the power amp in some newer models. Other models have used tubes such as 12AT7 (reverb driver, phase inverter), 7247 (in the overdrive stages), KT88, and 6550. 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.  Overdrive Special There are two inputs: a normal input which goes directly to the first preamp stage, and a FET input which goes to a FET preamp which was originally intended for use with acoustic guitars and other instruments with low output levels. Most guitarists plug into the normal input. Larry Carlton's volume pedal has been fitted with a FET preamp by Dumble in order to counteract the effects of pickup loading and cable capacitance. The first preamp stage is followed by a passive tonestack with adjustments for treble, middle, and bass. Switching options available in the different models include: Bright (bypasses the volume pot 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 cap to boost the midrange frequencies) Boost (disconnects the treble pot from the bass pot for a preamp level boost) Rock/Jazz (switches between a Fender-esque voicing and a hi-fi-type voicing). The tonestack is followed by another clean preamp stage, which in some models employs a local negative feedback loop. The resulting signal is either fed into the power amplifier (for the clean channel), or into the two-stage overdrive section. Contrary to some rumors, the overdrive is not created by a "Tubescreamer encased in epoxy" -- aside from the optional FET preamp, there are no solid-state components in the signal path. The power amplifier uses a standard long-tail pair phase inverter with negative feedback. It will break up, but only at high volume levels, or when the preamp boost is engaged. In some newer models (1990s to present), the overdrive stages are followed by a tonestack which is adjustable using trimpots inside the amplifier. This is known as the Hot Rubber Monkey (HRM) mod. These amplifiers are voiced for the overdrive channel to be used with the preamp 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 "Robben Ford Mod" refers to using a power resistor instead of a choke in the power supply in order to induce more "sag" in the amplifier.  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. Half of the first tube (a dual triode) is used to amplify the input signal, which is then sent to the second tube, a dual triode with both sections parallelled that drives the reverb tank through a transformer in a configuration similar to that used by Fender. The reverb recovery is handled by the other half of the first tube, and the third tube, also a dual triode, mixes the clean and the reverberated signals using a configuration similar to one shown in the Radiotron Design Handbook. 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.  Steel String Singer The preamp is very similar to the clean preamp of an Overdrive Special, with very similar tone circuitry. Some models use two EF86 pentodes (the first one configured as a triode) rather than a single 12AX7. The reverb topology is identical to that used in the Overdrive Reverb, with separate Send and Return controls. Some models have a tremolo with rate and modulation controls. Little information has surfaced about the high and low filter controls used on some models. The power amp uses a long-tailed pair phase inverter. Four 6550 tubes are driven by a 12BH7 operating as a direct-coupled cathode follower.