Welcome to Harris Audio Electronics! I’ll be using this site to post about projects I’m working on, circuits I find cool, or anything else that strikes me, to save my friends and family from being talked at endlessly about things they don’t find interesting.
Feel free to drop a comment in this or any future post, I love chatting about guitar electronics.
Finally, the amp is complete! Since the last log, I received my head shell from Modulus. I had them design it to aesthetically match the Peavey 1×12 I’m using with it, and I think it really turned out great.
I also finished the footswitch. I reached out to Amplified Nation to ask if I could purchase a footswitch from them, since they use the same form factor plugs, and I did not want to drill my own enclosure. They were kind enough to sell me a pre-drilled enclosure and I was able to build my own for very low cost.
The above picture is the final build, or close to it. It’s a little more rats-nest-y than I’d have liked around the power supply and tube sockets, but it’s a quiet and functional design, so I’m leaving it alone. One thing of note is that I ran B+ voltages with the blue wires on top of the board rather than under the board with every other route. I liked the idea of being able to quickly visually identify all my high voltage runs.
The only real mistake on this board after first pass build was that I landed some of the filter caps on the right side of the board in the wrong through-holes. Quick and easy fix, and it’s working great now.
Modulus made a fantastic head cab to perfectly fit their chassis. They gave me some options for custom finishing, and I went with black tolex, black grill cloth, and white piping–simply to match the cab I was planning to use.
They did a wonderful job. Everything fit together perfectly and easily.
I’m really pleased with how the amp sounds. The clean channel is very Fender-y if you want, but has a lot more mids available if you prefer. It can be a very unforgiving tone, but one that cuts through a mix extremely well.
The overdrive tone is exactly what I was hoping for. I did an A/B with my vintage Super Reverb cranked–perhaps my favorite tone on the planet–and it was impressively similar, but because the power amp isn’t being crushed to achieve that tone, it takes things like fuzz pedals a lot better.
My primary complaint is that I wish the amp had built in spring reverb. For now, I am using a Wampler Reflection in the effects loop, and it’s working great without a Dumbleator. It’s pretty much scratching the itch, but I may one day build or buy a standalone, tube-driven spring reverb unit.
I recorded a sound sample of the ODS using a Lewitt LCT 440 Pure close mic’d on the Eminence Texas Heat in the speaker cab. You’ll have to excuse the playing and lack of mixing/mastering. The rhythm guitar is the clean channel with all the preamp options off, the lead tone is the OD channel with the Deep switch on. The only effect is the reverb pedal on in the loop of the amp.
ODS Sound Sample
Overall, I couldn’t be much happier with this amp. I’m thrilled with how it turned out aesthetically, and I love how it sounds. This will absolutely be my main amp moving forward.
The Dumble ODS is not my first amp build, so I had some idea of what I was getting into when I set out to build it, but up until now, I had built exclusive Fender amps–a Tweed Champ and a Super Reverb specifically.
Because those are ubiquitous, kits and vintage style replacement parts are easy to come by, so very little actual planning needed to go into it. It was more of an exercise in just making sure I ordered everything I needed.
This build was a little more complex. A few versions of the Dumble ODS are quite well documented, but there’s not as much of a clone build market around it compared to Fender and Marshall, so things needed to be sourced a bit more carefully.
I broke the planning down into 3 stages: The Guts, The Transformers, and The Hardware.
There’s relatively a lot going on in a Dumble, and I knew from my experience building a Super Reverb that doing a complex amp fully point-to-point was a pretty time-consuming and error-prone adventure.
This is where AA Electronics comes in. They make PCBs for amp clones, including the Dumble ODS, which allows you to use one board to build 102, 124, or 183. They also provide schematic, layout, and a bill of materials. No excuses now!
As I have done in my previous amp builds, for resistors, I picked 1%, low tempco, metal film everywhere I could. Metal film is less noisy compared to vintage style carbon comp, which is often a popular choice in Fender-style amps. I also overrated them in terms of power requirements versus what you might see in a Fender Twin. It turns out, the AA Electronics BOM recommends metal film and overrated power specs, too, so that was nice to see.
For capacitors, I tended towards Orange Drops for signal path and F&T for Electrolytics. There were a few spots where other brands were chosen, but in my experience those two brands are reliable and consistent, so I stuck with them where I could.
Once I got the board and the parts order in, I populated the board:
If you look closely, you might notice that I landed some of the smaller filter caps in the wrong terminals. Luckily I caught this before firing the amp up for the first time. I’d say this was the most challenging part of the build–all 3 versions of the ODS had different power filter sections, and the board accommodates them all, plus accommodates radial and axial caps. So in some spots, it’s easy to make a mistake.
Choke and output transformer were an easy choice. Fender Twin Reverb reissue transformers fit the spec wonderfully, and are easy to get. I already had a brand in mind: I have had fantastic luck with Hammond transformers. They sound great in my Super Reverb build, and they have taken a beating being dragged around and abused at band rehearsals and gigs.
The power transformer was one of the more challenging decisions to make in the build process. A Twin Reverb power transformer is a good choice for the high voltage supply, power tube bias, and heaters, but the ODS uses 12VDC relays to engage the overdrive channel and PAB, and that isn’t readily available on a Twin Reverb PT.
The AA board includes a section of the board to take a 6VAC signal and runs it through a voltage doubling rectifier circuit and a 12VDC regulator to provide 12VDC to the relays. But nothing that I could find in Hammond’s lineup had an auxiliary 6V secondary tap in addition to the taps listed above.
I had a few options. 1) Mercury Magnetics makes a PT for the Ceriatone ODS clone which has a 6V auxiliary rail tap because Ceriatone also uses a voltage doubler. So that would drop in cleanly. It would also cost like twice as much as a Hammond.
2) I could try to power the voltage doubler with the heater tap–roughly 6VAC. As long as the heater tap was properly rated in terms of current consumption, this likely would have worked. But I hadn’t seen anyone else doing it, and I wondered if there was an impact on noise or something I was overlooking. Not a risk I wanted to take just to have to tear the amp apart and start over with a new PT.
3) Twin Reverb PT + standalone stepdown transformer. The third option I saw was to get a cheap, small stepdown transformer, wire it up in parallel with the big power transformer, and power the voltage doubler that way. This was the most cost effective choice, and lower risk than option 2, so this is what I did.
I found a 120V to 12V stepdown on Amazon with a center tap. I wired the primary side up in parallel with the Twin PT. On the secondary side I used one leg of the 12V and the CT (so, 6VAC) to power the voltage doubler circuit, and it worked perfectly.
With this stepdown transformer, I also had the option of turning the voltage doubler into a bridge rectifier and powering the regulator with all 12VAC rectified, or running the relays off 12VAC with no rectification. I decided to use the board as designed ultimately.
These Twin Reverb transformers are massive and heavy (diet coke for scale, and for the required caffeine to build an amp).
Hardware is the part of building an amp I’m worst at. I’m sure some people fabricate their own chassis, build their own cabinets, etc. I don’t have the wherewithal to do this, so I wanted some off-the-shelf options. There were a few choices, but I ended up going with chassis, faceplate, backplate, and a cabinet from Modulus Amplification.
The chassis is heavy gauge and feels very sturdy, the plates were a flexible material instead of being like a hard acrylic, which I found quite nice to work with. There were a few pitfalls here. Some of the switch types specified by AA Electronics were different from the ones pre-drilled for by Modulus, so I had to make sure I ordered the right things. The Modulus naming scheme is also slightly different from the AA naming scheme (“Mid ” instead of “Deep”, for example) but that was extremely straightforward to figure out. Additionally, the Hammond transformers didn’t perfectly drop in to the pre-drilled holes, so I had to drill some holes to accommodate them in spots–this is about the extent to which I was willing to work on the mechanical part of the build, apparently.
All in all, everything fit together quite well so far. I am still waiting on the Cabinet to be built and delivered, but I have no real concerns about fit since I ordered it from the chassis manufacturer.
With all these decisions made and orders placed, I was able to complete the amp build, and I currently have a working Dumble ODS sitting up on blocks. Once the head cabinet arrives, I’ll make a new post of completed build pics and some tone clips.
This will be the final circuit analysis article for the ODS #102, and it will cover everything after the tone stack but before the phase inverter. In addition to the tone stack, this is the place where Dumble’s circuit diverged most heavily from the Fender Twin Reverb, adding an overdrive channel and master volume.
Because the tone stack and master volume are passive, they tend to load the input gain stages. For that reason, it’s extremely common in tube amp designs to put another gain stage or two after the tone stack to aid in gain recovery or simply provide a high input impedance/low output impedance buffer for the tone stack.
In Fender’s AB763 amps with reverb and vibrato* (like the Twin Reverb), the normal channel has 1 gain stage after the tone stack but before the phase inverter. The vibrato channel has a 2nd gain stage that acts as a recovery for the reverb circuit. It doesn’t introduce a ton more gain, and the ODS is based more on the normal channel in this respect, so I’ll ignore it for now.
The schematics are as follows:
The Fender Twin schematic is on the left and ODS on the right. The first thing to note is the Fender cathode resistor, Rc3. I have it listed as 1.6k ohms, but if you look at a schematic, you may note that it’s 820 ohms. Both channels’ post-tone stack gain stages share the cathode resistor, so each stage effectively sees 1640 ohms in parallel. In order to get the correct bias, I used 1.6k.
Also of note, the ODS has a couple of additions. There’s the parallel RC made up of R1 and C6 on the grid and there’s the RCR feeding back from the output to the grid made up of R2, R3, and C5. I’ll skip the detailed circuit analysis and show what they do:
The Twin Reverb gain stage, shown in red, is straightforward, 36dB of gain across the audio spectrum just starting to roll off at 20Hz. The ODS gain stage is lower gain overall with a low-mid shelf and a more pronounced bass rolloff. The ODS has slightly more gain before the tone stack and the tone stack loads it slightly less. There’s also a lot more midrange available in the tone stack of the ODS, so you can see why these adjustments were made.
Not pictured in the schematic, but of note, the ODS also has a master volume after the clean gain stage. That means if you turn the input volume all the way up but the master volume down, you’ll get all the gain in the preamp without making the power amp incredibly loud–like you might on a Fender Twin.
*This is what Fender calls it. It’s actually tremolo. Meanwhile, the “tremolo” they installed on Stratocasters is actually vibrato. Go figure.
Things getting interesting when you engage the Dumble’s overdrive channel. The overdrive channel is not actually an independent channel–rather it keeps the clean channel and EQ fully in tact, and introduces additional gain stages after the one seen above, like so:
The first stage is the one I looked at above from the clean channel, the drive channel adds what I have labeled as Stage 2 and Stage 3.
Stage 2 is slightly lower gain than the stages before it, and is biased slightly cold (but not JCM800 or SLO cold-clipper level), but it still provides more gain to the circuit.
Stage 3 provides yet another high gain stage, and when everything is turned up, the gain after tone stack to the input of the master volume is profoundly higher than the clean channel or twin reverb:
There’s 68dB of gain compared to 36 dB and 34 dB from the Twin and ODS Clean channel respectively. It’s somewhat mid focused, rolling off at around 100 Hz and 5 kHz–apocryphally trying to mimic how a cranked Super Reverb sounds.
This is enough gain that if we were to look at a transient simulation you’d see a significant amount of clipping on the output signal going into the PI at this point. Again not pictured–the Master Volume moves to the output of Stage 3 with this channel is engaged. The fact that you can then dial back some of the signal without losing the clipping using the master volume before it hits the phase inverter is indeed what makes this an overdrive stage. You can get lots of distortion without all the volume.
But what’s particularly interesting about Stage 3 is the potentiometers at the input (Drive) and output (Ratio). It’s common for high gain amps like this to use a resistor divider to dump some signal between stages so the gain doesn’t turn into a fuzzy mess. The Dumble ODS lets you have access to these controls as external pots. If you don’t want the 3rd stage to contribute much more clipping, you can turn down the Drive. If you want clipping from the third stage, you can turn up drive, and set ratio to control the volume relative to your clean channel, since master volume impacts both “channels.”
Having done this analysis, I can now see that when I’ve built this amp, I’ll likely set up Volume and Master Volume on the clean channel to achieve a clean tone I like, and then I will treat the Drive and Ratio almost as their own pseudo Volume and Master Volume within the OD channel.
I can see why guitarists who are very discerning about their overdrive tone like this amp. The EQ is flexible, and the amp offers a lot of control over the gain characteristics.
This wraps up the circuit deep dives for the ODS 102 compared to the Twin Reverb. I basically only covered the preamp, which leaves the power amp and power supply overlooked.
In both cases, Dumble made changes to the typical Fender design, but they were not significant departures. The Twin has a beefy solid state rectified power supply, a long-tail phase inverter, and a high headroom 4x6L6 power amp. This version of the ODS keeps those same features at a high level, and makes changes to double down on the clean headroom after the preamp–the goal here is to get your gain from the preamp.
So, future build logs will actually be documenting the build, which is currently ongoing.
H.A. Dumble’s most significant departures from the Twin Reverb when designing the ODS were undoubtedly the tone stack and following preamp gain stages. Dumble took a fairly straightforward and standard tone stack, completely re-voiced it, added a bunch of tone shaping options, and optionally added additional preamp gain stages if you engage the overdrive “channel.” This is where the character of the ODS comes from. First, the tone stacks.
All the Fender AB763s had practically the same tone stack. A value would vary here and there, and sometimes there was a middle control instead of a fixed midrange. The following is the version that made it into the Twin.
This is a simple passive filter based 3 band EQ with a bright switch on the volume knob. V1 in the schematic is functioning as the voltage taken from the first gain stage. Each capacitor-potentiometer combination makes a high pass filter set at different values, and R1 and the capacitors create low pass filters.
Below is the magnitude response of the Twin Reverb tone stack with all controls turned to 10 and the bright cap off.
Technical Note: For the Fender tone stack simulations, I inadvertently had the volume pot turned down. This does not impact any of the conclusions or waveforms except that the magnitudes are shifted down by 20dB.
There is a low-mid scoop at about 300Hz. Treble is most prominent, at 6 dB higher than bass frequencies and 20 dB higher than the low point of the mid scoop. This is a very bright tone stack even without the bright cap.
Below is the magnitude response with bass and treble turned all the way down, but mids left all the way up. This is an attempt to “correct” for the mid scoop and see how flat the tone stack can be.
The scoop is much less pronounced, but still present and moved up to about 700-800Hz. There’s now 5 dB of difference between mid scoop and treble and bass. Still not quite flat.
This mid scoop is why people love mid-boosting overdrives like the Ibanez Tubescreamer with Fender amps.
Fundamentally, the Dumble tone stack is a similar structure to the Fender tone stack, but with altered values and a few added features: A Deep switch, a Rock/Jazz switch, and a preamp bypass (PAB) switch.
With Rock/Jazz set to Jazz, Deep switched turned off, and Preamp Bypass disengaged, the tone stack schematic is as follows.
Like the Fender tone stack, the 3 bands are made up of stacked high pass filters with R1 creating low pass filters with the middle and bass pots. With all values turned all the way up, the tone stack is still quite treble heavy compared to bass (+8dB) but with no mid scoop:
The Deep switch shorts around C4, and functions as a mid boost:
The treble rolloff -3dB point moves down from 1.5kHz to 300Hz, providing a lot more midrange content.
The Rock/Jazz switch set to Rock reconfigures the tone stack:
And the magnitude response changes thusly:
This reintroduces a mid scoop, but still one that is pretty slight compared to Fender. I can’t discern what makes a mid scoop rock compared to jazz, and would be fascinated to learn how Dumble settled on those names.
I didn’t simulate bright switches, but they do what you expect–boost treble frequencies by shorting them around the volume pot. This feature has an interplay with the volume knob–it is least effective if Volume is on 10 because all frequencies from the tone stack are passing in full. At low volumes when you’re dumping most of the signal to ground, the bright cap will short a lot of treble around the volume pot.
I also didn’t simulate the Dumble preamp boost. PAB bypasses the tone stack so you’re not losing any signal in the tone stack, but from a circuit analysis perspective, it’s pretty boring.
Next up, post-tone stack preamp stages, and build pictures!
Now that I have a few amp builds and repairs under my belt, I have decided my next amp project will be a Dumble ODS clone. I’ve always loved the recorded Dumble tones, and whether or not it’s apocryphal, the story is that Dumble was trying to replicate the tone of an overdriven Super Reverb when he set out to build the first ODS. Considering the Super Reverb is my all-time favorite amp, all the more reason to add an ODS clone to the collection. Specifically, I’ve decided to build ODS #102, simply because it’s an amp owned by Robben Ford, and he has one of the quintessential Dumble tones.
Before actually building the amp, I wanted to take a look at the circuit and figure out what makes it tick. The early ODS amps were likely built from or based on black panel Fender Twin Reverbs. The 4x6L6 output, the solid state rectifier, and the transformers of the early models were all very similar to the Twin Reverb. And since I am intimately familiar with the AB763 Fender circuit having built 2 and repaired others, I think the AB763 Twin Reverb is a great frame of reference for comparison to the ODS.
The Twin Reverb has two channels, with two inputs. But the two channels have identical input gain stages, and the two inputs per channel feed a single common cathode gain stage, so there really is only one circuit to consider.
The Dumble ODS has a single channel with two inputs–Normal and FET. More on the FET input later. Assuming plugging into the #1 input of the Twin Reverb and the Normal input of the ODS, the circuits are as follows:
Topologically, the two input stages are nearly identical. When plugging into input 1 of the Twin Reverb, you see Rg1 and Rg2 in parallel, resulting in a grid resistor of 34k, practically identical to grid resistor of the ODS. The load and cathode resistors are both larger for the ODS, resulting in less DC current and higher gain in the ODS. But both cathode resistors are bypassed with large capacitors, so both should be pretty high gain.
The bypass capacitor and cathode resistors in parallel create a high pass filter that can theoretically impact the bass response. The RC for the Twin rolls off at 4Hz, the RC for the ODS rolls off at 9Hz. Functionally, both amps’ input stages have a low frequency roll off well below the range of human hearing, let alone what a guitar can provide.
The above plot is the simulation result of the gains magnitudes across frequency (Vout/Vin) of the input stages of the Twin (red) and ODS (blue). As expected, the Twin has a slightly lower frequency bass rolloff, both amps have high mid band gain, but the ODS’s gain is slightly higher (~38dB vs. 36dB).
The ODS has a second input, called the FET input. This is not a second channel in the way that the Vibrato and Normal channels are two different signal paths in a Twin, rather the FET input places a JFET-based common source amplifier between the guitar and the common cathode amplifier, which you’d normally drive directly, like so:
R1, R2, and C3 form a lower voltage rail off the high voltage power supply, R3 is the load resistor, R6 and C4 are the source resistor for biasing and the bypass capacitor for high gain. C6 is the AC coupling cap, and the Trim Pot is used as a resistor divider to trim out some of the added gain.
I used a generic 2N3819 N-channel JFET in simulation, because I had the model for it handy. Depending on the particular JFET choice, the circuit can have up to 20dB of mid band gain pre-trim.
The above sim results are the gain magnitude of the JFET stage pre-trimming (red), post-trimming (dark blue), output gain of the tube common cathode stage when driven by the FET stage (green), and for reference, the same magnitude plot from above of the commong cathode stage driven directly (light blue).
The FET stage alone provides about 16 dB of gain midband, and because I have the trim pot set to divide the output signal in half, about 10 dB of gain post trim. Notably, it has a bass roll off between 10-100Hz, comparable to the tube stage, but does not roll off any treble at all.
When stacked with the common cathode gain stage and compared to the standalone common cathode gain stage, the JFET input has more mid band gain and a more exaggerated low end roll off. Because of the way it’s biased, I also expect it to have plenty of headroom. My guess is, this circuit will cut some bass and sound brighter, and depending on how the trimmer is set, could provide a substantial boost, as if you were using a clean boost pedal in front of the amp.
So, into the ODS normal input, the input stage looks very much like a conventional, Fender-style input gain stage. But the FET input adds a significant departure from the 60s-era Fenders.
For the next build log, I plan to do a circuit analysis of the tone stack and following gain stages, where I expect I will see the ODS significantly split from vintage Fenders.
