Anyone familiar with Dumble amp?

[big_teee]

Larry Carlton, Robben Ford and a host other Guitar Gods play them.

Why do they play Dumbles and not something else?

See it this way: why would anybody go to a good tailor to have a suit custom made, while there are millons of cloth suppliers?

Is it worth U$50000 ? ..... looks that for some it does.

Did the Guitar Gods pay U$50000 for theirs?

I'd guess, say, somewhere around U$5000 .

Larry Carlton & Robben Ford - Live In Tokyo - YouTube

enjoy

The Vid and Amps sounded good!
But, Not $50,000 Good!

[Alan0354]

You can skip the various tone shaping switches if you want, but look at the gain setting resistors across the stages, there are quite a bit of difference. LNFB = local negative feedback, it's on the second gain stage. Second set of tonestack was added to some later generations of ODS, there were also versions with trimmers mounted inside the chassis, they allowed tweaking the OD channel's tone, it's up to you whether you want to include it or not. My point is that you should try the circuit as is, since you are trying to find out "what's so special about the Dumble", why modify anything at all before you even hear it?

Here is the marked up schematic of the Humble Overdrive with the values of ODS in blue.



As you can see, there is very little difference between the two.

1) 330p to 390p of the Teble cap in the first tone stack.
2) The cap from 390p to 500p across the grid stop of the second tube is very insignificant from experience.
3) The 100K to 220K at the input of the third stage can easily be compensate by adjusting the 100K OD Trim.
4) The rest are slightly different idle current on the last two stages. ODS use slightly lower, but the gain is bumped up by high value plate resistor, so it is literally pushed, no difference.

I'll try the LNFB and some of the changes after I get the original working, I truly don't believe in this case will make much of a difference.

[Alan0354]

The Vid and Amps sounded good!
But, Not $50,000 Good!

I think I can rap it up and test tonight or tomorrow. We'll see whether it has a million dollar sound!!! But my grandson is coming, that will slow things down.

[g1]

My understanding is you will need at least four of them for the million dollar sound.

[Alan0354]

Got it wired up, voltage is still too high, but I did got it to play. I can only play in low volume as it's late. But I can tell it sounds very different from the typical Marshall 2204 et al. It is more like the fuzzy type distortion for the 80s and metal. It sounds pretty good, but I really need to lower the voltage and crank it up to tell.

From observation, I have to adjust the first tone stack way off the norm to get good distortion. But again, it is at low volume, really need to crank it up tomorrow.

EDIT:

I don't know why some of the Dumble have the extra FET stage at the very front. One thing I notice, this preamp has a lot......a lot of gain. I have to turn each volume down to prevent it from too much distortion. It a stark difference from the 2204 front end.

[jazbo8]

As you can see, there is very little difference between the two.

I'll try the LNFB and some of the changes after I get the original working, I truly don't believe in this case will make much of a difference.

Really? Not much difference? Why bother then? I will leave you to your experiment...

[MarkusBass]

It seems to me that there are some mistakes on the schematic (BTW, have you got it in full?). The bright caps should be connected to the wiper of the VOLUME pot. Currently they are not connected at all.
I'm not sure what you mean by NFB effect. Dumble used NFB in his amps but it was rather implemented as big resistor (10-40 MOhms) and 5-50nF capacitor between anode and grid of a tube. This (together with grid resistor) forms a negative feedback loop. It decreases distortion of the tube stage and makes the frequency response of the stage wider. I don't see anything like this on the schematic you posted. Can you clarify this? Do you mean the capacitors between anode and cathode of the V2 tube?

Mark

[Alan0354]

No, that is only the preamp section as I only build this part. The LNFB is the 25p from the plate to the cathode on the 3rd and the 4th tubes. But actually it's a positive feedback.

[Alan0354]

You can skip the various tone shaping switches if you want, but look at the gain setting resistors across the stages, there are quite a bit of difference. LNFB = local negative feedback, it's on the second gain stage. Second set of tonestack was added to some later generations of ODS, there were also versions with trimmers mounted inside the chassis, they allowed tweaking the OD channel's tone, it's up to you whether you want to include it or not. My point is that you should try the circuit as is, since you are trying to find out "what's so special about the Dumble", why modify anything at all before you even hear it?

I took a second look, the 25p is not LNFB, it is positive feedback!!!

The circuit actually introduce a slight +ve feedback. NFB is going from plate to grid, going from plate to cathode is +ve feedback. That I have to try tomorrow.

[Mick Bailey]

I have an old magazine which list the ODS as an "insane" $3500.

A key to the sound is to look who's playing them; you're not hearing an average player trying one in a music store, or a gigging musician in a function band playing one at a wedding. I don't think that Robben Ford sounds that much different through a hired-in Fender.

[Alan0354]

To me, the more important question is 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? Most of the amps follow JCM800 2204 configuration. If Dumble is so sort after, why didn't more amps follow that configuration?

Money is more about marketing and luck. You have one famous player use the amp, words spread and attract more big name players.

[MarkusBass]

I took a second look, the 25p is not LNFB, it is positive feedback!!!
The circuit actually introduce a slight +ve feedback. NFB is going from plate to grid, going from plate to cathode is +ve feedback. That I have to try tomorrow.

I think that the capacitor is 27pF and it forms (together with output impedance of the tube stage) a low-pass filter. But the differences start at 20 kHz so I doubt that you'll hear it. It seems to me that the capacitor is added just to increase the stability of the amp at over-acoustic frequencies (prevent HF oscillations). Please try it and let us know whether you can hear the difference.

Mark

[Alan0354]

I think that the capacitor is 27pF and it forms (together with output impedance of the tube stage) a low-pass filter. But the differences start at 20 kHz so I doubt that you'll hear it. It seems to me that the capacitor is added just to increase the stability of the amp at over-acoustic frequencies (prevent HF oscillations). Please try it and let us know whether you can hear the difference.

Mark

It is +ve feedback, it cause slight peaking. Negative feed back is going from plate to grid, going to cathode is +ve feedback.

But the feedback is very small as the 25p or 27p form a voltage divider with the 5uF cathode cap. My first guess is it ready do nothing because the signal is divided by 5uF/25p=188,000. So it's almost no signal feeding back. but I am going to try it tomorrow.

[MarkusBass]

It is +ve feedback, it cause slight peaking. Negative feed back is going from plate to grid, going to cathode is +ve feedback.

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).

but I am going to try it tomorrow.

Please do. This would prove error in the simulation software.

Mark

[Dave H]

But the feedback is very small as the 25p or 27p form a voltage divider with the 5uF cathode cap. My first guess is it ready do nothing because the signal is divided by 5uF/25p=188,000. So it's almost no signal feeding back. but I am going to try it tomorrow.

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.