D-TONE AMPS D-Style sound
Dumble Overdrive Special amp history

*Original Dumble Overdrive Special amp no. #023 from auction on Reverb.com

Dumble amp history

Alexander "Howard" Dumble began working on the Overdrive Special in the 70s. As we wrote on this page about Mr. Dumble biography , at the beginning he modified mainly Fender Tweed like Bassman model and Blackface era guitar amplifiers (like Deluxe and Super Reverb models), which is why Dumble amp circuit is largely based on early Fender schematic ("American sound") and is colloquially called non HRM (non Hot Rodded Marshall). Later the Overdrive Special Dumble amp, were produced in the 90s, were closer to the Marshall amps schematic, ("British sound") and were called HRM (Hot Rodded Marshall). Basic difference between non HRM and HRM is that non HRM (Fender circuit) have TMB (Treble/Middle/Bass) i.e. tone stack on the first gain stage - on preamp, while HRM (Marshall layout) have TMB after the second gain stage - after preamp. The placement of tone stack is one of the key elements that affects amplifier sound. Differences in the operation of TMB between Fender and Marshall also result from slightly different filtration values ​​of the RC circuit. TMB Fender has larger capacitor capacitances and a higher resistance value responsible for slope of curve, thanks to which its equalization "cuts out" more mid frequencies. Marshall TMB corrects band to a lesser extent, which makes Marshall amplifiers sound more "linear", and therefore have much more midrange. Another very important difference between the "British/American sound" is power tubes. Both Fender and Dumble used 6L6 tubes, while Marshall always used EL34 tubes. Some sources say that, he modified some Dumble ODS amp for EL34 tubes, but this was a definite minority of his amplifiers. 6L6 tubes are characterized by a much more aggressive sound than EL34. They also have more saturation in lower band. Unfortunately, they are "slower" than EL34, so when playing we have impression that any articulation is always a bit "delayed". Of course, these are purely subjective feelings and have nothing to do with latency. 

Fender Bassman TMB tone stack

Fender tone stack TMB

Marshall Plexi TMB tone stack

Marshall tone stack TMB

Why does Dumble Overdrive Special amp sound different than Fender / Marshall?

Let's focus first on CLEAN channel - this is first amplification stage (both triodes of ECC83 tube). It is characterized by a crystalline and very dynamic sound with a lot of "headroom". To a large extent, it refers to the sounds of Fender amplifiers of the Blackface era. Between halves of ECC83 tube there is a TMB, and cold band switches: BRIGHT (high frequency boost); MID (midrange boost); JAZZ / ROCK and PAB (preamp boost). While MID and BRIGHT are basically standard EQ switches, the JAZZ / ROCK and PAB switches aren't. And so we come to another non-standard solution of the Dumble ODS amp, which greatly affects its characteristic sound. We will not describe from the electronics side how this switch works, because this knowledge is not needed for guitarists - let's focus on the sound. In the JAZZ position, frequency response of the preamp is basically "flat" - and as the name suggests, it is ideal for playing JAZZ music. In the ROCK position is the complete opposite - we get a lot of low and high frequency, with a slight undercut of the mid frequencies. Robben Ford  use this setting in his Dumble guitar amps. The PAB switch (located on back faceplate and on the footswitch) boosts volume by approx. 10 dB and enhances the midrange. This isn't a classic booster, so we won't get more gain, which is a very good solution, because we don't interfere with distortion of clean signal. On other half of ECC83 tube, Mr. Dumble also used a rather non-standard solution: he placed a capacitor between anode and tube grid, and a high-impedance resistor on each side of it. This filtration also greatly affects sound of CLEAN channel. OVERDRIVE channel is the most characteristic sound of this amplifier called "Dumble sound". He achieved them, among others, by cascading the signal known from the Mesa / Boogie MKII amplifiers. While Fender and Marshall "stuck" with one channel amps in the 60s-80s, Mr. Dumble and Randall Smith (founder of Mesa / Boogie) "went a step further" and built an amplifier with two independent channels. What is the cascade? Signal from Clean channel (first amplification stage) goes to the second amplification stage, i.e. both triodes of ECC83 tube of OVERDIRVE channel, undergoes a large clipping, and then "leaks" to CLEAN channel to be amplified again. Thanks to this solution, both Mesa / Boogie and Dumble amps had much more gain at that time! 

Dumble Overdrive Special amp history

*Original Dumble Overdrive Special amp no. #023 from auction on Reverb.com

Mr. Dumble also used another, very important, non-standard solution on the second amplification stage. Between anode and cathode of ECC83 tube (both triodes of ECC83 tube) he added a capacitor that filters out high frequencies and basically cuts out characteristic "bzzz" band, known from Marshall amplifiers. This filtration largely allows for a creamy and soft overdrive sound - and that's what's famous "Dumble sound". Inverter and power amplifier are basically a standard layout characteristic of the AB class amp (push-pull). There is negative feedback here responsible for PRESENCE and power reduction by half, consisting in changing the operation of power tubes from a pentode to a triode.

D-TONE AMPS Dumble style amp kit

Signal filtering on first and second tube stage in Overdrive Specialist by D-TONE AMPS, based on Dumble amp schematic

Last characteristic solution of the Dumble amp Overdive Special is an additional FET channel. This is a separate FET input, in which the signal is additionally amplified and then directed back to NORMAL input, i.e. to first amplification stage. This is simply layout based on one J-FET transistor. Name of channel comes from its type. It doesn't distort signal, but only saturates it with a "fat" sound. While in the case of guitars with HB pickups, FET channel can even "spoil" sound of the amplifier, it harmonizes perfectly with the Fender Stratocaster! We get the typical "Texas blues sound" known from Stevie Ray Vaughan solos. Mr. Dumble also used a FET circuit in the Steel String Singer model. 

D-TONE AMPS Dumble style amp kit

FET channel in Overdrive Specialist by D-TONE AMPS, based on Dumble amp circuit

To sum up: the Dumble Overdrive Special head / combo amp, is definitely closer to the sounds of Fender than Marshall with a certain "but". CLEAN channel is definitely an "American sound", but OVERDRIVE channel in our opinion, it is between "American and British sound". Many sources compare this distortion to a Fender Bassman amp with an Ibanez Tube Screamer, and there is indeed a lot of truth to it. Let's not forget that the first Marshall is a copied Bassman... On this page we describe about Dumble amp clone. However, Dumble guitar amp has a nicer frequency response than diode-transistor TS-808 / TS-9 overdrive, and therefore more versatile and useful. In our opinion, every guitarist in his career should play and appreciate 3 amplifiers: Fender - for clean tone, Marshall - for crunch distortion, Dumble - for combining and developing the sounds of both of these amplifiers. 

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