Dumble amplifiers are synonymous with the rarest finest boutique amplifiers. Alexander “Howard” Dumble started building Dumble brand amplifiers in the 60’s in southern California. Over time he made and designed many amps (most notable the Overdrive Special) that were used by some very big names in the music industry (Stevie Ray Vaughn, Eric Johnson, Carlos Santana, John Mayer, Robben Ford, Larry Carlton, Ben Harper, etc.).
All of Dumble’s amps are hand built one at a time sometimes taking years to build and tweak just right for the client. There are many stories of Alexander Dumble and the various quirks and antics surrounding his business and persona…
The Overdrive Special (ODS) is by far Dumble’s most well-known and popular amp. As a result they are Dumble’s most produced amp with approximately 250 built. With previously owned ODS’ selling for over $100,000 USD these days online they are not your average basement rockers amp.
Though many Dumble amplifiers did have their internal circuitry “gooped” with epoxy resin some people have managed to reverse engineer the amps and subsequently draw schematics. Enough ODS have been reverse engineered that it is now fairly easy to source parts and build your own ODS for much less than $100,000 USD! There are also large active communities online devoted to learning more about Dumble amps and building DIY clones.
As a result of the current wealth of knowledge on Dumble amplifiers I decided to build my own ODS. I decided to build a ODS modeled after the well documented 100W Dumble ODS #124. ODS #124 was built for a Stratocaster player apparently and sounded very good. #124 was degooped, photographed, reverse engineered and then posted online. With all this information it is fairly easy to build a replica of a prized sounding Dumble amplifier.
For more information on Dumbles:
- 1W carbon film resistors for the signal path
- 1W Dale metal film resistors for the plate’s
- Orange Drop 6PS capacitors
- F&T and Sprague electrolytic capacitors
- Alpha potentiometers
- Switchcraft jacks
- Belton octal and noval sockets
- 22 AWG PTFE Teflon wire and 20 AWG PVC wire
- Mountain and Carling switches
- Magnetic Components power and output transformer
- Hammond choke
- NOS and new production vacuum tubes
The following gallery contains pictures of the build in various stages including pictures of the building of the cabinets.
The Dumble ODS’ are generally 100W amps though 50W amps were also produced (mainly in combo format). The Dumble ODS is a two channel amp with a Clean channel and an Overdrive channel. The amps are fixed bias with solid state rectifiers generally using 6L6GC or EL34 output tubes. The preamp usually has three 12AX7 tubes. The preamp tubes are used for the Clean channel, Overdrive channel and the phase inverter.
The amp features two inputs: a normal input going straight to the first tube stage and a FET input. The FET input takes the guitar input signal and feeds it into a high impedance JFET line level booster preamplifier before entering the first tube stage (originally intended for use with acoustic guitars and other instruments with low output levels).
The first tube triode is followed by a passive tonestack with controls for Treble, Middle, Bass and Volume. On ODS #124 there are three mini toggle switches on the front panel (different Dumbles have slightly different switches).
Bright switch: bypasses the volume potentiometer with a capacitor to boost the high frequencies.
Mid switch: increases the value of the treble capacitor in the tonestack to boost the midrange frequencies.
Rock/Jazz switch: switches between a Fender-esque voicing (Rock) and a more hi-fi-type of voicing (Jazz) with less bass and gain.
The tonestack is followed by another triode stage. The resulting signal is either fed straight into the power amplifier (Clean channel) or into the Overdrive channel circuitry.
On the back of the amp there is two switches to configure the Overdrive channel and preamplifier boost (PAB). The Overdrive channel and PAB can also be activated with a footswitch connected to the back of the amp. Both the channel switch and PAB switching is done by two relays inside the amp.
The PAB provides a boost function by disconnects the treble potentiometer from the bass potentiometer in the tonstack and provides an increase in volume and a slight increase in treble content. The PAB is useful for pushing the power section, lead boost and getting a bit of extra dirt from the amp.
The Overdrive channel is two cascaded triodes being fed from the Clean channel signal. The magic and aura of the ODS comes from the Overdrive channel sound. The Overdrive channel provides a very smooth, sweet sustaining distorted tone. The Overdrive channel is carefully tuned in terms of gain and frequency response. Gain is attenuated before and after each of the two triode stages in the Overdrive channel. This limiting of gain keeps the amp smooth and prevents over the top harsh distortion tones. The Overdrive channel features controls for Ratio and Level. These two controls limit the distortion content and volume of the channel.
It should be noted that all the Clean channel controls are active and affect the Overdrive channel. For this reason it can be tricky to get a good clean and overdrive tone as the two channels are so codependent. It can be very tricky to tune a ODS and getting it to sound good.
The output section uses a standard long tail pair phase inverter utilizing negative feedback. Since there is negative feedback the phase inverter section of the amp also features a Presence control to tame the very high frequencies. The phase inverter also features a trim pot across the plate resistors. This trim pot helps balance the phase inverter and allows the builder to dial in the right balance/harmonic content depending on what 12AX7 tube is used in the phase inverter. Prior to the phase inverter there is a Master Volume control which is very effective at setting the overall volume of the amp.
As mentioned the power supply uses a diode full wave rectifier to convert the power transformer high voltage AC to high voltage DC. The ODS amps are somewhat unique in that they use a lot of filtering in the power supply. The large filter capacitors help keep the tone tight and focused which is a good thing because when you are playing a high powered amp loud you don’t want the bass farty out and being loose sounding.
Overall the Dumble ODS design is a well thought out amplifier design. The parts selection, lead dress, voltages, tube selection and speaker selection are critical in getting a good sounding amp. The design of the amps is not complex but can often be tricky to get to sound good. Building my own Dumble ODS taught me about the small things in amp building/design such as component tolerance, component composition, voltages, etc.
Overall the amp build ending up being a success. It took a long time but in the end I ended up with a very nice amp. It would be easy to build a ODS based on a schematic alone but all the time I spent researching, studying pictures, etc. lead to me building a more accurate Dumble ODS clone. As mentioned it’s the small nuances and details in this amp design that really makes it special.
I spent a long time getting the voltages on the tubes just right so they were in line with what a real ODS would have. Getting the voltages in spec involved trying different tubes as all tubes are not created equal. All tubes draw slightly different amounts of current based on their internal electrode structure therefore affecting the voltages on the tube’s electrodes (plate, cathode, grid). For example the tube used for the Overdrive channel (V2) should have plate voltages of 210V which is 10V higher than the plate voltages of the Clean channel (V1) tube. This extra 10V provides a bit of extra headroom and gain, particularly useful for a tube being overdriven. This is just one of the details in dialing in a ODS and getting it to sound good.
The output tubes were carefully set to 48mA of idle bias current which is a bit on the cool side in terms of bias with a plate voltage of 440V. ODS’ seem to like cooler bias though, the cooler bias provides a more neutral sounding output section that doesn’t provide as much distortion of its own (the majority of the unique ODS tone comes from the preamp).
In the end my tube selection was an old RCA 12AX7 for V1 (Clean channel), a Mullard reissue 12AX7 for V2 (Overdrive channel) and an unknown make 7025 I got from a Blackface Twin Reverb for V3 (phase inverter). The output tubes are Winged =C=’s. I liked these over the TAD, Ruby and NOS JAN 6L6GC’s I tried.
I did perform a few small mods to the amp to improve it. The mods do not take away from the amps tonal character but rather slightly improve on it. They are as follows:
– 50W output section
– 3 way bright switch Bright-Off-Brighter, polystyrene caps. 50pF-Off-110pF
– Audio taper on both Level and Ratio potentiometer controls for the Overdrive channel.
– 470KΩ resistor in series with 0.033µF in series with 22M PAB resistor
– 5.6KΩ NFB resistor
– 350KΩ OD trimmer set 96.9KΩ from wiper to ground.
As discussed earlier ODS #124 was a 100W amp. On my build I decided to make a 50W version. This is really not a big deal as 100W is only 3dB louder than 50W, in other words barely noticeable.
The three way bright switch just improves on the simple on-off bright switch by adding two bright options; bright and brighter. The polystyrene caps I find are smoother sounding than silver mica or ceramic caps so they works well when used as bright caps.
Changing the overdrive controls (Level and Ratio) was an obvious mod for me. Linear taper pots when used as variable voltage dividers (which is what both the Level and Ratio controls are) do not work very well. There is too much increase in volume/distortion during the pots first 1/3 of rotation then not much change afterwards. Using audio taper pots allows even response and increase in volume/distortion across the entire rotation of the pot. This was a simple mod that does not change the sound but rather helps the user better dial in tones.
I added a 470KΩ resistor in series with a 0.033µF cap that goes in parallel with 22M PAB resistor going from the treble pot to the bass pot’s wiper. This simple mod is to make the PAB sound more even and decrease the treble boost you get. The added filter brings back some bass to make the boost sound more full.
Finally since I was using a 8Ω speaker while ODS #124 used a 4Ω speaker I had to change the NFB resistor to ensure I was getting the correct amount of NFB when using a 8Ω speaker as I would if I used a 4Ω speaker. To do this I simple increased the 4.7KΩ NFB resistor to 5.6KΩ.
As mentioned in the Design portion of this article the overdrive generated by the amp is slowly built up. As a result it is very important how you set the OD trim pot. The OD trim pot is an internal trim pot mounted on the amps eyelet board. The trim pot controls how much of the clean signal is attenuated before going into the overdrive circuit. If you let too much signal in you will have gobs of distortion and the tone will be harsh and buzzy. If you let too little signal in the amp will be flat and lifeless sounding with not enough gain. As a result the setting of the OD trim pot is crucial to getting a good sounding Overdrive channel. I set my trim pot to 96.9KΩ from wiper to ground. This is generally agreed upon to be a good setting for the #124.
It took a long time but I built an accurate replicas of the Dumble ODS head cabinet and 112 ported speaker cabinet. The head cab and speaker cab were built in the same over-speced/over-built style as the real cabinets. The cabinets were built with void free Russian Baltic birch using finger joint construction techniques.
The 112 speaker cabinet uses a 12” Eminence Delta Pro which is modeled after the EV-12L (the speaker Dumble favored). The back of the speaker cabinet has an oval port. The Eminence Delta Pro is much like the EV-12L in that it is a high power speaker that provides very little coloration of the guitar signal. I decided to use the Eminence Delta Pro over the EV-12L as the Delta Pro was much cheaper, easier to source and also because it has a slight roll off in the high frequency range. What this means is the speaker isn’t as peaky or bright sounding, this leads to a smoother tone which is something I was looking for.
Both the head cabinet and the speaker cabinet were built to dimensions posted and discussed online that came from real ODS cabinets. The dimensions worked out nicely so that the head cabinet sits right on top of the speaker cabinet with no overhang.