Dumble ODS: 50W #124

17 July 2009

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:

Dumble Wiki: the Wikipedia article on Dumble amplifiers; a good general overview.
Amp Garage: vast wealth of Dumble information, schematics, layout and discussion.
Dumble Sound Clip:
Dumble Pictures:

Parts:

  • 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

Build:

The following gallery contains pictures of the build in various stages including pictures of the building of the cabinets.

Design:

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.

Schematic:

ods-124-schematic

Results:

Impressions:
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.

Mods:
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.

Cabinets:
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.

{ 16 comments… read them below or add one }

Ed Goforth May 8, 2013 at 18 h 59 min

“470KΩ resistor in series with 0.033µF in series with 22M PAB resistor”

Hello, which 22m Meg resister did you put the filter network in series with? One 22M from the PAB relay ties the Bass wiper to the top Bass lug, and the other 22M ties the bottom Treble lug to the top Bass lug. I am assuming you connected them between the Treble and Bass point (22M). So you wire the 470k-.033uf in series with the 22M, correct? When looking at the schematic, it would seem that one would parallel the network (470k-.033) across the 22M, so that there would be the 470k-.033 between the treble/bass lugs for a boost effect. Am I looking at this wrong, please explain, my brain is not getting it today… :^)

Thanks for your help!

Reply

cmoir May 8, 2013 at 20 h 31 min

Hi Ed,

Slight typo on my part (good catch, will fix). The 470KΩ resistor in series with a0.033µF capacitor is in parallel with the 22MΩ resistor that goes from the treble pot to the bass pots wiper. This brings the overall resistance of that 22MΩ while adding a bit of capacitance in parallel. Result is less harsh boost fuller sound.

You can see this in picture #8 under Results.

Reply

Ed Goforth May 8, 2013 at 22 h 35 min

It’s all good, I just wanted to be sure I understood correctly :^) Thanks again for sharing. I’m always for getting improved sonics. I love my #124 all tube Clone, but I am always looking for ways to improve something, I have never totally been happy with the PAB the way it is stock.

Reply

Ed Goforth May 8, 2013 at 22 h 01 min

Ok, I see it now, thanks! It makes total sense to me, I built my clone per Gil Ayans notes but also like to experiment till I am happy with the sound. I will try this mod too since I am currently using a 1-12 EVM-12L 8Ω speaker and a Dumble inspired 1-12 cab using a 4 ohm output transformer, it does lose some headroom.

“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”

I also have a Twin replacement 4 ohm output transformer and will try this mod. It also has a Marshall 100 watt power transformer that provides about 470-475 vdc to the plates so my bias might need to be a little lower… It is at 30 ma each tube now :^) What do you recommend? I have a set of JJ 6L6 in there now, had a set of Ruby Mcmaster 6L6’s that sounded good till a screen grid resistor went south and took a 6L6 with it. I liked the Winged “C” 6L6 but now they are quite pricey. Have been using a Tung-Sol reissue 12ax7 in V1, a old Mullard ecc83, sometimes a Chinese 12ax7 or a JJ 803S or old Mullard in V2 and have an old Mullard PI tubes V3, I used to have a GE 6681 Made in great Britain, also a Mullard that sounded the best in the PI till the glass nipple came up missing one day…. I hear that the Sovtek 12AX7 LPS sounds good in the PI. I like the #124 best of all the various Dumble versions I have heard so far. Can’t wait to try your mods on it now! thank you so much for sharing! It would be nice to get back some lows when using the PAB, as it is now, I have also implemented the Mid boost as a pull boost on my Middle control and have it pulled for the PAB to sound any good, if it’s pushed in, the PAB sucks!. I read at the Maverick site (the one with Steve Farris from Mr. Mister, taling about his 90K+ Dumble, a low plate classic from the 80’s if I remember correctly) that some ODS, most likely the HRM’s, had the Mid Boost engaged when using the PAB, that was how there were used. I like your idea with the 470k-.033 filter across the 22M between the Treble/Bass lugs. I currently have a 100k OD trim set at about 40 to ground and a 100K linear Level pot, as it has a very sweet overdrive curve, but thinking about going back to the 250kA Level and 350k OD trim at your setting of 96.6k?
Ed

Reply

cmoir May 9, 2013 at 10 h 42 min

Hey Ed,

Oh I see you know Gil, he did some great work bringing the #124 information to the net.
The PAB mod I mention is actually from Gary from Glaswreck Amplification.

If running the plates around 470V DC then your bias should be around 40mA (44mA for 70% plate dissipation but Dumble’s like a cooler bias setting).

Long plate structured tubes sound great in the PI. Don’t forget to do some careful listening and tune in the PI trim pot.

HRM’s with PAB and mid boost at the same time is something I have never heard of. Doesn’t seem right. The effect of the mid boost is negated with PAB engaged. It is very common to put the mid boost on a footswitch.

Reply

Ed Goforth May 9, 2013 at 13 h 34 min

Yes, Gil has been very helpful indeed! I look forward to Gary’s mod, I don’t like the PAB on my amp, it sounds too thin, so this mod should be a good improvement :^) I usually set the PI about 10 volts higher on the second side to start and fine tune by ear from there. I have my bias at 30 ma per 6L6, maybe I will get a little more headroom at 40 ma per tube. I have a 1 ohm resistor from the cathodes to ground on each socket. It is a 100 watt amp with a 4 ohm Twin replacement output TF, do you think it make any difference with a 5.6k FB resistor since mine is 100 watts? and I run a EVM-12L single 12 cab. When using the PAB, I am usually in the bridge PU, in the neck it can get a little muddy especially with Humbuckers. I love the Neck PU sound a lot and don’t use much Mid Boost in that setting. I appreciate all your suggestions and will give the PAB mod a shot! :^)
Thanks!

Reply

Joel October 30, 2013 at 2 h 42 min

Hey man! I actually just ordered the chassis for an ODS clone based off of the 124 circuit. The only thing I’m having issues with is finding a build of materials for this amp. Would you be able to help me out? I appreciate it!

Reply

Chris Moir October 31, 2013 at 0 h 19 min

Hey Joel,

I do not have a BOM for my personal amp but here is a good one from Amp Garage.
http://ampgarage.com/forum/viewtopic.php?p=159980#159980

You can search for more BOMs there. Hope this helps!

Reply

Joel October 31, 2013 at 18 h 24 min

Thanks man!!!!!! I’m super pumped to build this and hope that it turns out great! I have built pedals for years and want to move on up to something more interesting!

Thanks so much!

Reply

Davide Borra July 10, 2014 at 6 h 00 min

hello, good information but it is possible to know the exact dimensions of the cabinet, thanks a lot!

Reply

mochatheca November 25, 2015 at 5 h 32 min

Hi there
i am in the process of planning a 50w Ceriatone Overtone Special build.
You state Magnetic transfos and Hammond Choke, but the pix look different.
Did i see it wrong.
Can you provide me the tranny models you used?
thanks you

Reply

cmoir November 25, 2015 at 23 h 12 min

Hi, Hammond choke and power transformer with a Magnetic Components output transformer. Since the output transformer is coupling your entire guitar signal from the high voltage tubes to the low voltage speaker you want a good quality transformer in this position. Of the three peices of “iron” in this amp this is where I would recommend spending your money.

Hammond 290FX – PT
Hammond 194B – Choke
Magnetic Components – Bassman OT (40-18011)

Reply

Mike Travis. May 15, 2016 at 16 h 14 min

Hi, All very good but why would you use noisy old carbon film resistors. ?. They fry. Quality metal film resistors should be used surely. Used as pre-amp plate feeds Carbon resistors certainly are noisy. First thing I used to do in “frying” amps was change all the 100K etc: plate feeds for metal film. Quiet silent amp now. And be careful where you ground the pre-amp cathodes for zero hum especially the 1st. stage.

Reply

mike February 14, 2017 at 14 h 59 min

Hi, Have you experimented with Mercury Magnetics transformers? They are way pricey compared to others, wonder if the extra $$ makes a difference in some amps.

Reply

cmoir February 14, 2017 at 22 h 50 min

My personal opinion are they are not worth the hefty price tag for the small sonic improvement they can bring to some amps. I like Heyboer or Classictone (Magnetic Component) transformers.

Reply

mike February 15, 2017 at 19 h 53 min

Thanks! Ive heard of Magnetic Components but haven’t heard of Heyboer, will look them up.

Reply

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