Anyone familiar with Dumble amp?

[tedmich]

These amps sound neat, I'm going to keep my eyes open on Clist to pick one of these up!

[JoeM]

Larry Carlton with his ... Twin Reverbs

[Alan0354]

This is not true. Without the cathode capacitor the signal on cathode is in the same phase as on the grid. And the signal on the anode is inverted. For positive feedback you would need signal on anode to be in the same phase as on cathode.
I have just simulated this in SPICE and this is simple low-pass filter. At 20 kHz I get 0.4 dB attenuation and 2 dB @100 kHz (for 1 stage).
Please do. This would prove error in the simulation software.

Mark

It is +ve feedback. Think of it this way, let's hold the grid constant, if the plate goes negative, the 27p cap will try to pull the cathode lower. If the grid is constant, then the Vgk will be reduce ( less negative) WHICH in turn cause the tube to conduct more current.....WHICH will cause the plate to go even lower........It's a +ve feedback.

You cannot look at the cathode by driving the grid here. You have to look at it as a COMMON GRID stage that you drive from the cathode( which is the case here that the 27p cap driving the cathode). When you pull cathode low, you cause more current to conduct and cause the plate to go low. It's +ve feedback.

Try reduce the 5uF cap to 1000p cap, you should see it peaking.

[Alan0354]

I think you are correct. There is virtually no feedback. It's a low pass filter as Mark said. The 27p could be connected from plate to ground and it would be the same but it's academic as its effect will be masked by the much lower low pass filter formed by the 180k grid resistor and Miller capacitance.

27p is just too small to do anything. The cathode is at virtual ground with respect of the 27p cap. I think this is just RF suppression........Or more like putting in just for the sake of putting in. There is not enough gain at this late stages to cause any problem, RF is more a problem at the first stage. I experiment a lot on caps to ground, you really need at least over 100p to really do anything in OD channel.

[Alan0354]

[ATTACH=CONFIG]30518[/ATTACH]

Larry Carlton with his ... Twin Reverbs

I am not familiar with Dumble's history, just amazed by the price. Is the twin supposedly modified by Dumble? I don't look at who play the amp, I only go by listening and playing on it myself. Good player can make the crappiest amp sounds great. Not to mention the effect, recording play a major role.

Case in point, Carlos Santana was my idol in my playing days, I went to 5 of his concert. He was known to have beautiful guitar sound, silkily smooth sustain. One time I was right up front and heard his playing directly through the amp instead of through the PA, it was fuzzy, raw and bassy. Nothing like if you move back farther away and hear his guitar sound through the PA. Through the PA, then is sound just like in the record. The sound from his Mesa did not sound that good without the mic.

There are so many video clips on the amazing sound of the Marshall 2204, but if you go to the guitar store and plug your guitar in dry, you sound nothing like in the videos!!!!

My grandson is still sleeping right now ( from gaming till late last night). So I still have not crank it up and listen. I am not going to say one way or the other. I was very careful in building the front end, measure the value of most of the components before putting it in, adjust the plate voltage to within 10V from the schematic. We'll see. I'll report back after I crank it up.

[JoeM]

No, that amp is not modified by Dumble. My point is a player of Larry Carlton's caliber can play through anything and sound great. Most recent concert a friend of mine saw Larry he was playing through a BF Super Reverb.

[MarkusBass]

It is +ve feedback. Think of it this way, let's hold the grid constant, if the plate goes negative, the 27p cap will try to pull the cathode lower. If the grid is constant, then the Vgk will be reduce ( less negative) WHICH in turn cause the tube to conduct more current.....WHICH will cause the plate to go even lower........It's a +ve feedback.
You cannot look at the cathode by driving the grid here. You have to look at it as a COMMON GRID stage that you drive from the cathode( which is the case here that the 27p cap driving the cathode). When you pull cathode low, you cause more current to conduct and cause the plate to go low. It's +ve feedback.
Try reduce the 5uF cap to 1000p cap, you should see it peaking.

I tried with the 5uF cap reduced to 1000p. Only the gain has changed (as expected). Please take a look at frequency response of this stage (in a range from 10kHz to 200kHz). The green trace is without the cap, and the blue trace is with the cap. As you can see this is low pass filter. The change starts at 30 kHz and it's only -0.4dB. At 200 kHz there is 1.6 dB attenuation.
I suggest that you feed the stage with 30kHz frequency and check the results (output signal) with 27pF cap and without it.

Mark

[Alan0354]

OK, finally grandson awake and I have a chance to crank it up to 1/4. It sounded quite nice. Rich OD because of the 4 stages. It is nice to have the pre OD tone stack to shape the sound. I have a fixed tone stack on my KMD, but I put it after the second stage, the result is not as good.

I compare all three preamps in the Bassman, 2204, My own and Dumble, Dumble definitely sounds smoother and liquidy. But I miss the bite of the crank up 3 stages. That to me is the biggest difference, that you need 4 stages with attenuation in between to get the liquid smooth OD.

Far as touch sensitive, being able to turn the guitar volume down to clean it up, I don't see any advantage on the Dumble, in fact, 3 stages tends to clean up better. On the Dumble preamp, I tried to lower the volume of the first stage and turn down the trim of the second stage to lower the gain, I still cannot get the superior touch sensitive that were shown in the videos.

All in all, I like it a little better than the 2204 preamp, the main thing is the lack of gain of the 2204. I already have to do tricks to up the gain of the 2204 and still not quite there. 2204 really need a booster at the front to bring out the glory. Where as the Dumble has plenty of gain.

That said, if I have a optional booster in front of the amp, I still lean towards the 2204 as I can clean up better by turning the booster off, with the Dumble, I don't have that option.



I am sure Dumble has his secret recipe on how to place the wiring to get some magic interaction to get the sound. I cannot read his mind, so I just try to layout the circuit for minimal cross talk and let the circuit alone make the sound, no magic, just clinical.

[Alan0354]

I tried with the 5uF cap reduced to 1000p. Only the gain has changed (as expected). Please take a look at frequency response of this stage (in a range from 10kHz to 200kHz). The green trace is without the cap, and the blue trace is with the cap. As you can see this is low pass filter. The change starts at 30 kHz and it's only -0.4dB. At 200 kHz there is 1.6 dB attenuation.
I suggest that you feed the stage with 30kHz frequency and check the results (output signal) with 27pF cap and without it.

Mark

[ATTACH=CONFIG]30522[/ATTACH]

I can only tell you what I said is standard theory. That in the feedback, it is consider a common grid stage. Go through the explanation in my post how the polarity of the signal goes. If you look at transistors stuff, these feedback is quite common and well explained.

[Alan0354]

No, that amp is not modified by Dumble. My point is a player of Larry Carlton's caliber can play through anything and sound great. Most recent concert a friend of mine saw Larry he was playing through a BF Super Reverb.

That I can totally agree. I thought you were talking about the magic of a Dumble modified Twin!!! It's the player, the recording and the effect pedal together with the amp that make the sound. With emphasis the player is the most important factor in the equation.

I am not going to say Dumbles don't worth the money, it's in the eye of the beholder. I cannot even say about the real Dumble because he might have some magic through the interaction of the wiring. BUT far as the circuit concern, I build the preamp, I even measure the values of the components before putting them in. My amp is EL34. I am sure I did very good job in avoiding any interaction of the wires so the sound is from the circuit, not magic(or evil) from the wires. It sound good in it's own way. But the 2204 and my own design sounds good too, particular I have option of high gain with my design. I don't think there is a clear winner, just different taste.

For me, it worth $2000 as a good boutique amp, not the obscene price they are asking for.

[pdf64]

'why no big name amp use Dumble's configuration that put a pre-distorted tone stack right after the first stage and have the second tone stack after the final stage?'

Mesa Boogie made their name using that arrangement in the Mk series amps, eg http://schems.com/manu/mesaboogie/boogie_mkiv.pdf
The graphic eq facilitates post OD tone shaping.
Pete

[Alan0354]

'why no big name amp use Dumble's configuration that put a pre-distorted tone stack right after the first stage and have the second tone stack after the final stage?'

Mesa Boogie made their name using that arrangement in the Mk series amps, eg http://schems.com/manu/mesaboogie/boogie_mkiv.pdf
The graphic eq facilitates post OD tone shaping.
Pete

But most Mesa don't have post eq or tone stack. You really need both.

Mesa amps are my least favorite. Because Carlos Santana was my idol, I really tried hard in liking Mesa, I tested so many of them and I just could not put my credit card out. I tested out a MKIV a few months ago, it did not change my mind. Something about the Marshall or Fender tone stack, you cannot use a Graphic Eq to replace that tone stack.

I tested quite a few amps lately including the very expensive PRS. The only one I like is the Fender Super Sonic. The Dumble preamp reminds me a little of the Fender Super Sonic. The other that comes very close is the Marshall DSL41C, only the clean sound is not as good as the Super Sonic. I am absolute surprised that my two most favorite ones are cheap amps.

[Audiotexan]

For me, it worth $2000 as a good boutique amp, not the obscene price they are asking for.

In a limited sense..true enough.

But consider any high $ car. Same thing.
For instance a Ferrari isn't worth 250k (or even half that) to me as a car, but I can't deny the fact that there's an intrinsic value attached due to sheer collectability and limited supply (inflated or not).

Same for the amps. So there might be what? A couple hundred max, made thus far? Versus how many 'rich' players (major signed acts), and even hundreds more 'rich' collectors (many of whom don't even play)...What's 'obscene' to you and I is pocket change to some folks. Basic supply and demand.

Worse for me in particular? I don't think there's a tube amp worth $2k. Boutique or not. Not knowing what the components cost.

Do I think folks should make a decent living doing an honest job? Absolutely!! ESPECIALLY those that produce a physical item (as they have even higher overhead). But am I going to pay 'designer label' prices. I'm not that...person (to be kind).

[Alan0354]

I absolutely agree, there is a collectable value or the name value. But this is a technical forum, I just call it as it is. We all been around the block a few times and I hope we don't go by hype one way or the other. I just make an honest assessment by my taste. I think I gave it a really good assessment for $2000. This is not like a 60s vintage strat, this is still electronics after all. No matter what kind of mojo, there are still theory behind every bit of the sound ( whether we understand it or not.).

As for cars, it's very different, it's for show, but for amps, it's still about the sound in the recording or on stage. You can parade with your Ferrari Testarosa, knowing that the cheap Mustang of today will run circles around you. Even the higher end SUV will shame that!!!! But it's for show as you don't go on the track. If you have an amp that does not resonate with you, or people think it's a dog, it does not matter what name on the amp, it's still a dog.

As for $2k, believe me, I have played and listened to amps more than $2k, they can be bad. Just because one idiot with name use it does not mean anything. At least I cannot say anything bad about this front end. It does sound very good, not amazingly good. I can make the 2204 front end work and I like my design just as much. That, is a lot more I can say about most of the other amps I tested.

[Loogie]

My point is a player of Larry Carlton's caliber can play through anything and sound great.

That's it.

That Dumble/Mesa sound is ok, but unless the player bends a few notes you can't really tell its being produced by a guitar. The sound of the string interacting with the pickup is buried beneath layers of processing. Its a cool sound, but I wonder how much of Carlos Santana's brilliance we've missed because he wasn't playing through something else. I don't mean that it has to be dead-nutz clean, and I think that dark fuzzy sound can be nice in small doses, but, I dunno, there's something very interesting about the sound of an electric guitar string. And maybe I haven't seen them all, but it seems like just about all the demo sounds you hear from guys who have built their own Dumble style amps are accompanied by a cheesy backing track. Great builds, great players, but there just aren't enough examples playing the thing straight into a mic.