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D-PRE BUILD ( mini marshmallow preamp)
Looking to share build experiences. I am about to start one and connect it to a low watt amp.
(02-21-2024, 04:14 PM)coco Wrote: Looking to share build experiences. I am about to start one and connect it to a low watt amp.
(02-21-2024, 04:14 PM)coco Wrote: Looking to share build experiences. I am about to start one and connect it to a low watt amp.

I built the D-PRE and connected it to a 6CM6  small pentode 9 pin power tube, single ended. I designed a   power supply to drive both. Runs at about 250 V B+
As built it, I measured every component before installing onto the board so I know the board was built correctly.
 I added a Master Volume after the channels are relay switched so essentially there are three level controls. I didn't use the tiny PCBs for volume control and jacks.  At full power getting about 5 watts rms which is about what I was after.
It is housed in an 11” X 4” X 2.5” chassis so very compact.

I have built and heard many full size  Dumble #124 style amps so I can make some comparisons.

It drives the 6CM6  pentode power tube very well. 
There is decent cleans when using the clean channel
It is decently touch sensitive. 
The Mid boost and Jazz/Rock do what they’re intended to do.
The Treble and Mid controls are extremely subtle. 
The Mid control is hardly noticeable. Such that I confirmed it was working using my o’scope. 
The Early Drive (AKA Trim) does as expected but it goes into OD very quickly. Not much range at all. Same with the Late drive (AKA  Level) , goes into OD quickly, not much range.
Between the Early and Late OD Drive though, you can find a nice saturated OD that resembles a Dumble.
Having an OD level and Master Volume allows the player to adjust the OD to match the clean channel which is very useful.
I used one relay for channel switching and for muting the second OD triode. Muting the 1st OD triode is suggested but space was limited and I didn't want the added complexity of a second relay.
KOC was helpful in a couple of emails. Some parts were missing and he confirmed my substitutions.

The PCB is small and parts are very small. Service and mods will be challenging.
This is not kit for a beginner. There is minimal and only basic  documentation.
FWIW, if I was designing this, the board would be larger, less compact, use 1/2 W metal film res. and fewer radial caps. I would probably try to put the pots on board.

This was an experiment to see how close I could get to my 6L6 #124 at much lower volume levels. I will continue to work on the project (Fx loop) and also see what other players think of it. There is still some work to do but it was a decent start.
Hi Guys

Resistors of the same power rating come in all different sizes.

If you look at the guide on our site: "About Our Kits", you will see that we use only metal-film resistors, except where significant power must be dissipated, in which case we use metal-oxide or wire-wound. The small metal films are 600mW and are tech that has been around since the 1960s. These are 25-50ppm temp coefficient, 1% tolerance, so very stable.

The caps are generally plastic films, the red ones are polypropylene. Electros are long-life types.

Having the pots on a common board with each other, let alone with the rest of the circuitry, is not a good choice for a kit unless a drilled chassis is also available. We keep things flexible so hobbyists can customise the layout to suite their own aesthetic and requirements. We chose to have the tube sockets on a common carrier with the Rs and Cs to control parasitic elements and have unit-to-unit consistency, despite variability of wiring to the pots and jacks from one build to another. We can also follow Galactic Grounding methods for lowest-noise and best note articulation.

D-PRE is based on Howard Dumble's topology, shared with the Mesa Mk1. TUT6 explains how this arrangement evolved from Fender reverb amps and the basic form is straight-forward. There are Dumble fanatics that have catalogued as many available variants of values as they can find, naming them, numbering them, etc. keep in mind that Mr. Dumble tweaked each amp for the specific customer. If that player later sold it, then it is the luck of the draw if the next owner is getting exactly the tone he expected?

That means that D-PRE is like all the other D-samples out there and all the various clones inasmuch as it is just a foundation and one can tinker with it how one sees fit. This is the case with every tube circuit ever built for musicians. D-PRE comes with a set of values that may or may not have been catalogued, but its tone is in the range as reported above.

The original circuit did not include as many user Level controls as D-PRE does, as half the examples of HDs were modified fenders and Marshalls. TUT6 illustrates this and discusses ways to modify the circuit. The topology has limitations in its basic form as the Clean channel has to be set up before the OD section. As is typical of early-EQ circuits, the EQ is more effective for the clean output than for the OD output. In later assemblies, Howard added EQ just for the OD, which goes a long way towards voicing the OD for tones suitable for rock.

Have fun

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