If you follow any of my prior builds, you'll know I'm a John Mayer gear enthusiast and I appreciate his taste in expensive toys. I never really had much interest in a Dumble UltraPhonix as I thought of it as catering to the overdrive tones that the Dumble Overdrive Specials were known for. To my surprise, there are many voicings that fall under the UltraPhonix umbrella.
The Dumble UltraPhonix mod is a custom modification developed by the legendary amp builder Howard Alexander Dumble. It involves taking a classic Fender amplifier—usually a Fender Twin Reverb, Bandmaster, or similar model—and transforming it into a high-gain (sometimes), touch-sensitive, and harmonically rich amplifier. The mod enhances the amp’s natural tonal characteristics, making it more dynamic and responsive to the player’s touch.
The UltraPhonix mod is particularly revered for its smooth overdrive and clarity, allowing for both pristine cleans and creamy, saturated leads. Essentially, the Dumble UltraPhonix mod turns a standard Fender amp into a boutique-level tone machine, capable of delivering the complex, rich sounds that Dumble amps are known for. The base of the amplifier remains close to a typical Fender schematic, and then tweaked from there.
So fast forward to 2024 and Dead & Company kicks off their residency at the Sphere and a few shows in, a new amp is on stage that replaces the Dumble Small Special on stage-right.
You can clearly make out the logo says UltraPhonix and there are some other weird things about the grill cloth that initially made me skeptical that it is indeed a Dumble. Specifically, the grill cloth and Bandmaster logo blacked out with gaffer's tape. The green pilot light was new as well.
About a month into the Sphere, Emerald City Guitars posted a video on YouTube about a recent guitar safari they went on and they, low-key, picked up a Dumble. Big shout out to zmarks98 on Instagram for pointing this out to me!
Right away, all of the skepticism about the amp on stage not being a real Dumble went away. If I were to guess when this amplifier was made, it would be towards the end of Dumble's life. My reasoning for this is that Dumble used Fender chassis for Eric Clapton and Kenny Wayne Sheppard. The weird grill cloth actually was crocodile tolex and not shoddily-applied Fender black weave grill cloth. Now my head was spinning and I needed to learn more about why this is an interesting amplifier!
Research
My first stop is over at The Amp Garage for all-things Dumble circuitry. I found a post of members dissecting a '64 Bandmaster that was converted to a clean-platform UltraPhonix. This gave me chills because it made me aware of an UltraPhonix that is meant to be clean AND it was mostly documented! Not many have built it to share their experience, but large swaths of information was readily available so let's build this thing!
This specific UltraPhonix is largely the clean channel and power section of a Dumble Bluesmaster Overdrive. The Dumble Bluesmaster is a specific variant of the Dumble Overdrive Special (ODS) amplifier, designed with a focus on achieving a warmer, smoother, and more vintage-inspired tone compared to other Dumble High Gain Mod (HRM) Overdrive Specials. Here’s how it differs:
Bluesmaster Characteristics:
• Warmer, Mellow Tone: The Bluesmaster has a rounder, more midrange-focused sound, with a slightly reduced high-end compared to other Dumble ODS amps. This gives it a smoother, more “bluesy” character, making it ideal for players seeking a rich, vintage-style tone.
• Simplified HRM Circuit: The HRM (High Gain Mod/Hot Rubber Monkey) circuit in the Bluesmaster is designed to be less aggressive, offering a more organic overdrive that responds beautifully to the player’s touch. This makes it less compressed and more dynamic than the standard HRM models.
• Different Voicing: The Bluesmaster is voiced with a more pronounced midrange, which contributes to its warm, singing tone. It has less emphasis on the bright, glassy highs that some other Dumble amps are known for, making it particularly suited for blues and classic rock.
• Less Gain: Compared to other HRM Overdrive Specials, the Bluesmaster typically has a slightly lower gain structure. This allows for a cleaner, more controlled overdrive that’s easier to dial in for smooth, expressive lead tones.
Differences from Other Dumble HRM Overdrive Specials:
• High Gain Mod (HRM) Focus: While all HRM amps feature an internal “tone stack” after the overdrive circuit to shape the overdriven sound, the Bluesmaster’s HRM is tuned differently, offering less treble and a smoother, more vintage tone.
• Versatility vs. Specificity: Other Dumble HRM models are known for their versatility, capable of delivering everything from crystal-clear cleans to heavily saturated leads with a broader frequency range. The Bluesmaster, on the other hand, is more specialized, excelling in the realm of smooth, classic overdrive with a warmer tonal palette.
• Player Response: The Bluesmaster is often described as having a more forgiving and “player-friendly” response, making it easier to achieve a sweet, musical tone at various gain levels without the need for extensive tweaking.
Sourcing The Parts
I looked all over eBay and I could not find any blackface Fender Bandmaster chassis. I can find faceplates, but not the chassis to go with them. I did a little searching on Facebook marketplace, reverb, and Guitar Center and overall, it would end up costing me quite a bit extra in time and money to convert it for the right look and feel. On a whim, I checked Craigslist and I found a chassis with no head shell, no output transformer, electronics seemed mostly intact, and the price was right! I called the gentleman selling it and he was willing to ship it to me. Score!
Upon arrival, I thought of ways I could reuse the components and go for a 1:1 of that '64 that was documented. After about a week of pondering and weighing all the risks associated with using old amplifier parts, warped boards, etc. I decided to built a super-reliable amplifier using all new parts and not taking any chances. I have quite a few friends on the internet that have issues shortly after receiving a NOS-built amplifier, tubes, etc.
Keeping the build fiscally responsible too, I dug into my stash of resistors from my Deluxe Reverb build and found I had a bunch of Xicon 1W carbon film resistors left over so I decided to use them in all the spots that originally had carbon composition. Carbon film resistors offer greater stability, lower noise, and more precise resistance values compared to carbon composition resistors, which tend to drift in value over time and generate more noise. I also had some fun 2W 100K resistors that I could use wherever necessary.
When looking up output transformers, I stumbled upon a site selling NOS Fender Bassman ClassicTone output transformers! These are my favorite OTs so I was feeling real giddy to do this build. For the metal film resistors, I chose to use TE Connectivity H4P type since they are precision-grade and look cool with their black epoxy casing.
For pots, I chose to use Tokyo Cosmos epoxy-sealed potentiometers. They are 10% tolerance and since they are sealed, they should be less likely to drift and become scratchy over time.
For capacitors, I chose to use a combination of MOD cathode bypass capacitors and F&T power supply caps.
For wire, I chose 22ga Weico wire in the same color combination that Fender uses. I chose to stay away from cloth pushback wire because the cloth can actually become conductive over time. The tinned wire will prevent corrosion as well as make the wire stay in place for less chance of wires becoming microphonic through vibrations from a speaker cabinet.
The Tone Geek Touch
I was not satisfied with the specs of using old fashion vulcanized fiber board compared to modern FR4 fiber boards with much better moisture rejection properties. If I'm going through the effort of keeping this build as clean and reliable as I can, this effort is worth the time to take.
Fiberboard (Vulcanized Fiberboard) Moisture Absorption:
• Moisture Absorption Rate: Typically ranges from 5% to 15% by weight, depending on the specific type of fiberboard and environmental conditions.
• Effect of Absorption: The high moisture absorption rate can cause the fiberboard to swell, warp, and degrade over time, especially in humid environments.
FR4 PCB Moisture Absorption:
• Moisture Absorption Rate: Typically around 0.10% to 0.20% by weight, according to IPC-4101, which is the standard specification for FR4 material.
• Effect of Absorption: Due to its low moisture absorption rate, FR4 boards maintain their structural integrity and electrical properties even in high-humidity environments.
I decided I should design my own PCB so I can leverage the use of FR4 boards as well as share the design with fellow builders who are looking to build a non-reverb AB763 or this UltraPhonix! You'll notice that the font resembles the font used on the hand-drawn Fender schematics and layouts. I chose to not include silkscreen for each component so it maintains that 60's clean eyelet style. The plated holes are the same size as an eyelet and 2.0mm thick boards prevent flex and add durability when it comes to reworking the boards for capacitor changes, modifications, etc. I had made a few component mistakes when building mine and the boards held up to multiple reworks. This is the same technique I used on my JM Sig boards too.
Little tricks I also incorporated:
Added an extra smoothing supply 100uF/100v bias capacitor on the middle leg to quiet the bias.
Removed the heater center-tap from the chassis, taped it off, and added 2x 100 Ohm 1/2w metal film balance resistors.
Added a tremolo circuit disconnect switch from weber.
Added a 10k mid pot on both channels in the 2nd input spot.
Removed the corded mains wire & outlet and added an IEC in the outlet position.
Add a HT fuse in the location where the corded wire used to come out.
Removed the death cap and added in a selectable HumX circuit OR chassis ground with the ground switch.
Main speaker out is 4 ohm and EXT speaker is 8ohm OT tap.
Coax from input jacks and the grid stopper resistors directly attached to pin 2 on V1 and V2 for minimize any oscillation.
Head shell made by Guitar Cabinets Direct and modified by me for the crocodile grill tolex.
UltraPhonix custom logo scaled to 6" long made by DropLight Ind.
So the component selection mentioned above and these boards combined give it my signature "touch" for longevity and theoretical best performance for signal to noise ratios.
The Build
Using the information gathered from The Amp Garage, prior experience, and my own independent research on the Dumble Bluesmaster, I created this schematic which shows the deviation from a standard Fender AB763 Non Reverb. The full Google Photos album can be viewed here: https://photos.app.goo.gl/J1rozbLLKT71Sf5C6
As for the chassis, I did a full strip and then did some rust removal and conditioning. I also measured everything so I could create a replica chassis AutoCAD drawing in the future to make available to the market. I had the best results removing rust and polishing the surface by simply using Barkeeper's Friend cleaner that I had around for cleaning my pots and pans! lol! 60's solution for a 60's problem.
As far as the general build sequence I like the approach it like this:
Drill any locations for holding down the circuit boards or any other modification to the chassis.
Clean the chassis of all drill debris.
Populate the main signal board.
Attach wires and estimate how long to leave yourself for extra wiring on the back.
Populate the front panel.
Run wires from the front panel to an estimated drop location.
Add sockets, hardware, and back panel parts.
Watch out for long fuses that might interfere with your heater wiring later. You may need to add this near the last parts.
Add wires between the sockets that do not rely on the main board.
Populate the power supply.
Before attaching the power supply, add the screw and standoff for the signal board since it's under the filter board.
Run the filter supply wires through the chassis to general finalized locations.
Lock down the filter supply board.
Add rectifier/bias board but do not tighten down.
Loosely lock down the signal board position.
Make final cuts and solder front panel connections to the front-panel side of the board.
Make final cuts and solder the socket connections
Lock down signal board.
Install transformers.
Run wires on the back panel & finalize.
Run wires to the rectifier/bias board & finalize.
Set the bias pot to 1/2 way rotation.
DOUBLE CHECK EVERYTHING.
Startup Sequence:
No tubes installed
Use a variac or current limiter for initial startup.
Start variac at 0 volts and work your way up to ~80% of wall voltage.
This will prevent the unloaded transformers from overwhelming the filter capacitors with too much voltage.
Install preamp tubes.
Increase voltage on the variac or just run the bulb current limiter.
Check for smells or any blown fuses.
Install the power amp tubes.
Increase voltages on the variac to 100% wall voltage or just run the bulb current limiter.
Check for smells or any blown fuses.
Check bias to make sure you're not red-plating the power tubes.
Adjust bias as necessary.
Check all other voltages and plug in the guitar.
If using a current limiter, your voltages will be low. If so, shut down the amp, plug into your mains outlet, and turn it on again.
Enjoy!
Build pics.. Be sure to check out the full photo album here: https://photos.app.goo.gl/J1rozbLLKT71Sf5C6
Things I learned
Everything with this build is pretty straight forward since it closely rides the stock non-reverb AB763. The one thing that took me a while to track down is the tremolo tick fix... THE ONE THAT EVERYONE STRUGGLES WITH, even Dumble. You will notice that he added the standard "trem tick fix" cap across the roach. Oddly enough, in my build, I fixed the tremolo tick with lead dress of V3 tube and by removing the "trem tick fix" capacitor.
I also noticed that the 500k bass pot that was originally suggested by the TAG members works, but it was too sensitive for my liking. I used the online tone stack calculator and found that the bass pot does not change the tone curves but a 250k will provide a better sweep between 0-5 to dial in the tone. The amp is incredibly strong and has wonderful bass presence, so I found myself liking the tone best with the bass pot at 0. Funny enough, this is exactly how Kenny Wayne Shepherd runs his Dumble modified Bandmaster.
The top end is very sweet. Overdrive tones get into the SRV category which is lovely! This is probably the most balanced amp I have built next to SSS #004.
The last thing I learned is the back of the TOCOS pots do not connect to the front to be grounded. If you wish to ground the case of the pot, you can do so by adding another buss wire. This is wonderful because it allows you to independently ground the pot to minimize the effects of ground current running through the back of the pots, causing noise.
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