Half Dumblator

[jazzbass]

Hello everybody,

I ask for your help in trying to understand what a Dumblator is. Until now I have read and passively accepted everything that has been written on the subject but, while it is easy for me to understand the function of a reverb or a chorus pedal or a echo pedal etc. in an amp send / return, I still don't understand the dumblator and especially the half dumblator. I probably looked wrong in the TAG but, apart from the generic hints, I did not find anything that explains its function, how the sound of the amp changes, where it should be inserted in the mother board or if I can build an external one to connect to the send / return (the half Dumblator of corse), its own scheme and / or layout.
Thanks to anyone who wants to help me.
a hug
Franco

[erwin_ve]

Franco: what is it exactly you don't understand?
It contents are 1 cathode follower triode(making it low impedance) and 1 recovery triode to bring up the gain that might possibly be lost by low gain fx. That's it.

How it's done circuit wise: there are many variations and the dumblelator is not the most transparent available.
Best is to build one, experiences are subjective.

Erwin

[10thTx]

Just to echo that there are many different variations of tube FX loops:

https://el34world.com/Forum/index.php?topic=10208.0

With respect, 10thtx

[jazzbass]

Hi Erwin,

thanks to you I built the 2nd generation hybrid which is my favorite amplifier among all the ones I built (the second is the JM). Before deciding to finish the construction I tried to use the fourth hole in the chassis usable for a preamp tube to try to build a dumblator around it. The experiment was not completely successful because I knew how to connect the "send" but I didn't know where to connect the "Return" and therefore only half of it worked. I therefore decided to disassemble all the components of the Dumblator and finish the amplifier following the Normster Layout.
Remembering the sound of the amplifier with only half of the Dambulator working, I have several questions left in my head.
As I wrote, on the Dambulator I have read a lot but I have never studied in depth and I thank you for the explanation.
I'll always have a valveless socket in the chassis but it's a fantastic amp.

Hello 10thTx,
thanks for the suggestion, I copied what was contained in the articles and in the next days I will study it. I think I will finish the construction of the Dambulator started a few years ago and never finished.

A hug
Franco

[drew]

Look for posts by member aflynt if you want to see examples of amps with built-in Dumblators.

[jazzbass]

Thank You Drew.

Franco

[rootz]

Hi Franco,

I'm looking at several effect loop solutions at the moment. Ranging from tube Dumbleator to solid state Dumbleators and many variants.

Here's a couple of things that are 'wrong' (please note the quotation marks) about a Dumbleator, but in fact are a function of them as well:
- Low headroom cathode follower. The construction of the CF makes it typically easy to overdrive it. There seem to be two variants. One with a 27k cathode resistor, one with a 10k one. The latter is easier to overdrive. Turn up the master and turn down the send volume and enjoy some extra compression and harmonics added to your sound.
- A CF has low output impedance. The send pot is a250k though, making the output to the send jack rather high impedance (depending on pot setting). Combine that with cable capacitance and you can create pronounced high end roll off. Hence the bright caps to counter this effect.
- Then why not make the send pot of lower value, like 25k or 10k? Well, in that case the CF runs out of steam very fast. The tube simply cannot provide the current that a low load like 10k would need. Solution? Use a 12dw7. But then it wouldn't be a Dumbleator anymore.
- The gain recovery side is set up so it gives a slight treble boost. I'd guess again to somewhat counter act the loading of capacitive cables.

So in short: the Dumbleator is an effect in itself, an extension of the amp. If it were to provide a low driving impedance to and clean recovery from the effects, Dumble would've probably designed it different.

A Dumbleator goes between master volume and PI entrance (22n cap).

[jazzbass]

Hi Rootz,

what you write is shareable. I have read a lot about the Dumblator (this does not mean that I understand everything) and it seems to me that HAD has actually added an "effect" to his creature, perhaps inspired by some musician who would have wanted more or a sound a little different from what he already would have gotten from this fantastic project. Do you know when the first Dumblator dates back ?.
My curiosity was turned to the "half Dumblator" version mentioned by many here in TAG.

A hug,
Franco

[rootz]

Hi Franco,

Depends on which half you mean. I don't know of any amp that only uses the CF. But Two Rock used the recovery gain stage of the Dumbleator on some of their amps. It did serve as a reverb mixing stage in the Custom Reverb Sig mk1 and mk2. I've read that they (Skinner) liked only the sound of the recovery stage, but can't remember the source. Is that the half Dumbleator you mean?

No idea when the first Dumbleator was made.

[jazzbass]

Yes, thank You Rootz,

it was just my curiosity to try to understand what inspired HAD.

A hug
Franco

[bepone]

to cut the highs of the amp

[jazzbass]

Hi Bepone,

As usual, until I build the circuit and listen to the results of its insertion I do not understand .... now I know that you are right.
I have built a complete Dumblator and the scheme I have chosen does not give me satisfactory results.
The (excellent) sound of the 2nd Gen. that I finished building a few days ago became anonymous, without structure if I insert the Dumblator.
Obviously, from this moment the in-depth phase of construction and modifications begins, until I am satisfied.
I am convinced that the "Dumblator" project is a valid project, born to modify the sound of the amplifier and make it become "something else" in the good sense of the word.
A doubt arises ... that for every variation of the projects of HAD, 2nd Gen, # 102, # 124, # 183 and so on, it is necessary to "adapt" the Dumblator .... For now I try to optimize it for 2nd gen. and after we will see.

A hug
Franco

[rootz]

What are you looking for in an effects loop Franco? What exactly do you mean by anonymous/without structure? Do you miss high end? Maybe you need a more transparant solution to preserve the amp's sound?

[jazzbass]

Hi Rootz,

I don't know what to expect from an effects loop, it's the first one I build and I've never tried one.

I have read different opinions on TAG about the results obtained with and without the Dumblator and I expected a sound even richer in high frequencies, something more like the sound of a VOX AC30, a way to have two beautiful amps.

The result, on the other hand, did not live up to expectations, at least before the modifications ..... Once the Dumbulator was connected the beautiful sound of the 2nd Generation disappeared, the richness of the sound structure disappeared, it became an "anonymous amplifier" like the ones the music industry is churning out in the mid and low range.
Now, after having changed the value of the capacitors connected to the two switches, the result has changed to positive. I wasn't expecting a transparent "effect" and now I'd like to explore what I can get out of that metal box.

A hug
Franco

[Stephen1966]

Hi Franco,

Here's a nice video which walks you through the Dumbleator from an Aussie (we won't hold that against him )



Ciao,

Stephen