Dumble ODS: 100W HRM

29 May 2010

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.




ODS HRM Main Schematic


ODS HRM Fender-Marshall Tonestack Switch PCB










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
  • 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.

{ 18 comments… read them below or add one }

Keven May 8, 2013 at 1 h 03 min


Did you made the head cabinet by yourself or did you bought one? If it’s the first case, is there a way you send me or share the plans for the head cabinet? I got the plan for the extension or speaker cabinet but I can’t find the plans for the head.



cmoir May 9, 2013 at 10 h 46 min

Hey Kevin,

Dumble amps have come in various shapes and sizes over the years as such the clone chassis also come in varying sizes. There is some plans available but you do have to tailor them slightly for your chassis. As long as you get the esthetics right and the scaling of the cabinet right though you should have a great cab.



Keven May 9, 2013 at 21 h 33 min

Oh Thanks!

I searched for weeks and haven’t found a plan. I know I got to adjust the chassis but it was the cosmetic parts that pissed me off ^^ Anyway thank you!


Mario November 2, 2013 at 23 h 37 min

Maybe I overlooked something but where did you get the chassis ? Thank you for posting and sharing.


Chris Moir November 3, 2013 at 9 h 53 min

Hi Mario,

Chassis came from http://cepedals.com/Dumble-Style-Chassis.html. These guys do runs of these really nice chassis from time to time.


Greg March 2, 2015 at 21 h 21 min

Chris, Great in depth info re: the HRM amp. I have some questions for you.
Where did the chassis and amp case come from?
Were the xfmrs part of a kit or specced by you?
Is PAB, pre amp A/B switching?
How does that amp sound?
Can I hear this amp on Youtube?
The PAB relay has no connections in it’s other mode. Why?
Is this an actual Dumble schem or one you modded up?
Tell me when to stop lol…..
Everything else makes sense, the Rock/Jazz switch on the first tone stack is a little confusing but I’m not a stack expert. What is it accomplishing?


cmoir March 10, 2015 at 22 h 54 min


– Chassis came from here http://cepedals.com/Dumble-Style-Chassis.html
– No kit here, all parts speced by me. Transformers are Hammond.
– PAB (pre-amp boost). This function in both clean and overdrive modes. It disconnects a portion of the tone stack and frees up some gain giving a boost! Since the overdrive mode has a tonestack already (that’s what makes a HRM amp a HRM amp) it makes sense to use the PAB when in the overdrive mode. That way there is not two tone stacks in the signal path.
– Google/YouTube Dumble amps, there is tons info/soundclips on them.


Greg March 11, 2015 at 1 h 17 min

Nice…good PAB idea and good work and info. I just read about guys doing a stack and neg feedback disconnects for alternate sounds. I like your idea better. I will be adding a variable NF circuit as I’m doing some work for a friend on a Fender so thx! Keep posting ideas and amp completions.


Pedro Martinez April 18, 2015 at 19 h 11 min

I plan on making this amp myself, and I’m curious as to what dimensions/specifications you used on the turret board. Also, do you think this amp will sound good running through Celestion heritage G12h-75hz and G12k100 combo?


cmoir April 18, 2015 at 21 h 23 min

Hey there, dimensions vary but do a quick search of the Amp Garage forum to see the size of the layouts/turret boards documented there.

Those speakers are not usually paired with Dumble type amps but are both nice speakers. I think it would be worth trying them!


rene February 2, 2016 at 19 h 33 min

Hello nice site you have and very nice schematics. I see that in de HRM schematic you didn’t add the anode and supply voltages. would it be possible to inform me about them?

Could you say that with the HRM tone controls (on the overdrive channel) bypassed you have a similar sounding amp as the Skyliner version?

I’m trying to decide which one to build. (but maybe that choice is easier when i can just add or leave out the HRM tonestack on both amps)

Would you think the would benefit from a better OT. Such as a Hammond audio OT with greater more bandwidth?


Peter Wysocki August 2, 2016 at 17 h 50 min

Would you please help with model/part number for transformers and choke.
Kindly thank you for your article, pics and guidance.


cmoir August 4, 2016 at 3 h 20 min

Hammond 290FX for the PT
Hammond 1750U for the OT
Hammond 194B for the choke


Peter Wysocki August 5, 2016 at 1 h 05 min

Thank you for a swift response and info. I will build HRM 50W model replica. Would you consider the above listed transformers fully suitable for that build? I am sure other brands could be used but, would that provide better results sound wise? Kindly please, advise.
Thank you


cmoir August 7, 2016 at 16 h 22 min

The model numbers I listed are for a 100W amp. The PT and choke will still work but will put out slightly higher voltages than you may want due to the lighter loading of 50W vs. 100W. The OT will not work, you need a OT with double the primary impedance.


jigibao December 4, 2016 at 22 h 21 min

Hallo and gretengs from Belgium. Im interested in bilding these amp but would like in same time to bild the Dumble Overdrive reverb version, but cant find the shema nowhere?? could you help? regards from Sonny..


Dragos February 7, 2017 at 12 h 17 min

Hi there,
I am trying to build this amp and I could use a specific parts list and actual measurements on the schematics.

Can you help pls ?


cmoir February 10, 2017 at 10 h 12 min

Detailed BOMs and voltage measurements can be found on the Amp Garage forum. Best of luck!


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