After building a 50W Dumble Overdrive Special (ODS) the next logically thing was to build a louder, fancier Dumble amp. Dumble Overdrive Special’s come in two flavors, HRM and non-HRM. HRM stands for Hot Rubber Monkey (though often called Hot Rodded Marshall) and refers to a Marshall tonestack placed at the end of the Overdrive channel. This added internal trim pot based tonstack allows the player to have better control over the Overdrive channel tone.
Since the PAB disables the tonestack in the Clean channel you can see how using the PAB and the Overdrive channel simultaneously allows you to have a dedicated clean tone and rely less on the Clean channel tone. In other words you don’t have to compromise your Clean channel tone in order to get a good Overdrive channel tone and vice versa with HRM ODS’.
Though there are many similarities between the HRM ODS and the non-HRM ODS this 100W HRM build and my 50W non-HRM ODS build are quite different sounding. I enjoy the complexity, tweaking and sound of Dumble amplifiers so I decided to take on this build for these reasons.
For more information on Dumble amplifiers and their background, controls, etc. see my 50W non-HRM ODS build.
- 1W carbon film resistors for the signal path
- 1W Dale metal film resistors for the plate’s
- Orange Drop 6PS capacitors
- F&T and Nichicon electrolytic capacitors
- Alpha potentiometers
- Switchcraft jacks
- Belton noval sockets
- 20 and 22 AWG PVC wire
- Mountain and Carling switches
- Hammond, power, choke and output transformer
- NOS and new production vacuum tubes
The HRM version of the ODS evolved to improve on the non-HRM ODS. Early Dumble ODS amps were non-HRM while more recently built amps are HRM. With non-HRM amps there was no Overdrive channel tonestack (treble, middle and bass controls). Once you got a good clean channel tone you liked you had to tweak the Overdrive channel so it sounded just right. With non-HRM the Clean channels controls such as Treble, Middle and Bass were active when the Overdrive channel was on.
The HRM ODS adds a post Overdrive channel tonestack so the player has more control over their overdrive sound. With HRM amps the PAB should be used in conjunction with the Overdrive channel. The PAB disables the Clean channel tonestack. So if you engage the PAB and Overdrive channel you only have the Overdrive channel tonestack in the circuit. This allows the player to better dial in their ideal overdriven tone. Of course you can use the Overdrive channel without the PAB engaged too; there is many different flavors of tones to be had.
The design of the ODS HRM amps is very similar to the design of the non-HRM ODS amps. The main difference between HRM vs. non-HRM is the attenuation/filter network leading into the Overdrive channel and the passive three band EQ at the end of the Overdrive channel. There is some difference in component values but the generally topology and structure of the amps are the same.
The attenuation/filter network leading into the Overdrive channel is different than a non-HRM’s because there is less gain available in HRMs because of the tonestack added at the end (the tonestack is very lossy). To compensate for the gain lost by the tonestack a bit more signal is let into the Overdrive channel. There is also a bit less filtering done on the input of the HRM’s Overdrive channel. This is because the tonestack at the end of the Overdrive channel should take care of filtering/tone sculpting.
The three band EQ (tonestack) at the end of the Overdrive channel is nothing more than a modified Marshall tonestack featuring treble, middle and bass controls. It is import to note though that the Overdrive tonestack uses trim pots so the controls are not adjustable once the amp is in its head cabinet. The idea behind this was most likely that these amps are hard to tune so once you find that sweet spot you don’t want to deviate from it.
It should also be noted that with the HRM the channel switching is done a bit different than with non-HRMs. With non-HRMs when the Overdrive channel is engaged the clean Volume, Master Volume are in the circuit as well as the typical Level and Ration controls associated with the Overdrive channel. With the HRM the circuit is set up so when on the Clean channel the clean Volume control and the Master Volume controls are active. When on the Overdrive channel the clean Volume, Level and Ratio controls are active. What this does is lets the player have better control of the overall volume of both channels.
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I only performed one modification on the amp. I always though the idea of the HRM tonestack was a neat idea but wouldn’t it be neat if you could tweak the tone without opening the amp up to get inside to the trim pots.
Making the three internal trim pots external potentiometers would be the obvious solution. The problem with though is that there is really no where to put three extra pots on the front or back panel of the amp. Adding extra controls would also ruin the look of the ODS in my opinion.
What I decided to do was add a relay inside the amp. What this relay does is allows the user to switch between the stock HRM tonestack (which is a Marshall tonestack) and a second tonestack which is that of a Fender amp. This switching can be done remotely with a footswitch so you can select between a Marshall Overdrive type sound and a Fender Overdrive type sound.
I have the Fender tonestack set up for a bit more clarity and high end while the Marshall tonestack is set up to be smoother with more mids (the classic ODS sound). The HRM tonestack switching mod adds a lot of extra tones and versatility to the amp. You can set two unique Overdrive tones and toggle between them on the fly.
Possible configurations include:
Clean with PAB
Overdrive with Marshall tonestack
Overdrive with Fender tonestack
Overdrive with Marshall tonestack and PAB
Overdrive with Fender tonestack and PAB
The three button footswitch I built has footswitches for PAB, Overdrive/Clean channel and HRM. The HRM switch toggles between the two different overdrive tonestacks. The HRM LED only lights up when the Overdrive channel is active. The LED is red for Marshall overdrive mode and green for Fender overdrive mod.
The footswitch has a regulated filtered 12V DC coming to it from the amp. This 12V is to power the LEDs inside the footswitch. I decided to add a DC jack to the back of the footswitch and tap of the 12V and hook it up to the DC jack. This allows the player to hook up their pedals to the footswitch and have them powered of it (well actually the amp is the one supplying the power).
This Dumble ODS build featured a head cabinet and a 212 speaker cabinet. Both the head cabinet and the extension speaker cabinet were built to Dumble specs using the same dimensions and materials when possible. I decided to use Fender blonde style tolex for the cabinets, Fender rough blonde for the extension speaker cabinet and Fender smooth blonde for the head cabinet. The grill cloth on both cabinets is Fender oxblood and is a dark brown/red. I tried to emulate the look of Larry Carlton’s Dumble cabinets as I thought they looked very classy.
Both cabinets use Russian Baltic birch wood and utilize finger joint construction. The cabinets are very solid and sturdy, over designed and over built.
The 212 extension speaker cabinet houses two 12” speakers as the name suggests. The cabinet is slightly oversized compared to a typical 212 speaker cabinet but that’s what the actually Dumble ODS cabinets are like so I did the same. The extra space creates a bit more low end and volume and allows space for the unique oval port on the back panel.
I ended up using two Eminence speakers, a Swamp Thang and a Tonker. Both speakers are high powered ceramic speakers. Dumble amps are very particular when it comes to speakers and which sound good and which don’t. Generally speaking they like high powered ceramic speakers with a pretty flat frequency response.
Through much research and listening I found out about the Swamp Thang and Tonker combo. The Swamp Thang is known for its big bottom end and midrange with a slightly rolled off high end. The Tonker is the opposite with a tight bottom end and lots of high end sparkle. Together the two create a very harmonically rich pleasing combo.