Madamp G3 for Jazz: Adding a James Tonestack and Reverb

  • A year ago, I showed a suggestion in this forum on how to get a somewhat warmer sound out of the G3 by replacing the tone control with a James Tonestack. I was actually very happy with that the whole time, the Madamp is absolutely suitable for jazz! The only thing I was missing was a reverb. Now I finally got around to it and added one to my little Jazz combo.

    But I couldn't do it without somewhat breaking the style. A reverb tank with springs would have been more appropriate, and of course it would have to be driven by tubes. But I wouldn't have been able to fit it into the cabinet, and the effort would have been very great. In addition, I simply don't like the sound of the springs as much as the sound of the modern digital versions. In the end, I decided to use the "Accutronics BTDR-2H-M Reverb", which is available at http://www.musikding.de.

    In the attachment you will find the circuit diagram. As you can see, I grabbed the signal behind the tone control at the volume pot. Originally I wanted to tap the signal behind the V2a tube, which brings the signal for the phase inverter back to higher levels. However, these levels were much too high for the 5V technology of the reverb module. Taming the signal with a voltage divider was of course possible, but then I would have had to amplify it again. With my version, all levels stay within the 5V and things stay drastically simpler.

    The volume pot is very high impedance, so I used a JFET as a buffer. I actually wanted to avoid this, I would have preferred the dry signal to take the old route through the tubes and only the reverb signal to see transistors. I console myself with the fact that a JFET is basically a tube too, just without the built-in pilot light and with less volts (quote from Dave's EEV blog on Youtube).

    The resistor 4k7 at the tap of the potentiometer was necessary because otherwise the amplifier would oscillate massively at Vol=0. There was still the problem to solve that the voltage between the heater and the cathodes of the tubes should stay below 100V, which was solved in the original by centring the heater voltage and raising it to the potential of the cathode of the third tube. In my version, I simply connected the 10V to the negative rail of the reverb board, which also achieves the goal.

    Finally, I had to deal with an unpleasant 100Hz buzz in the speaker. The small 100nF capacitor at the rectifier took care of the spikes, which are caused by the diodes in the rectifier not blocking immediately when the current direction is reversed. The main cause of the noise, however, was a too small smoothing capacitor behind my rectifier. The reverb component draws a whopping 100mA and drained the originally installed 470µ capacitor in every half-wave. With the 1000µ, peace returned.

    So, now the thing works and gives me a lot of fun. Now my excuse is gone and I'll have to go back to practising guitar ...

  • Hi Veit,
    thanks for sharing!
    I am interested in building/using small cabs.
    How many litres has your cab as volume?
    I am surprised, that you get good sound from such little cab without a notch filter or a negative feedback.
    Is the cab back completely closed? Is there some kind of a tube of a bass reflex device?
    Perhaps this is due to the relatively low efficiency of this visaton speaker? This is a visaton BG20 "Breitbandlautsprecher"?
    Best regards Christof

  • Hi Christof!

    You are perfectly right, the cabinet and the volume of the speaker cabinet normally matter a lot. In this case I got away with a simple 8" speaker because I use the amp only for practicing, so I do not need high volumes. My cabinet has open slots at the back because I need to cool the tubes of the Madamp. I selected the BG20 by looking at the frequency response and it covers the range from 100Hz to 5kHz quite evenly (see https://www.visaton.de/de/produkte/ch…eme/bg-20-8-ohm). In fact it is advertised as

    "Breitbandlautsprecher mit Hochtonkegel. Hoher Wirkungsgrad.
    Geeignet als Breitbänder für kleine Gesangs- und Gitarrenmonitore
    oder als Mitteltöner für PA-Anlagen und Musikerboxen."

    I have built a few other practice amps with even smaller speakers. I very much like the Visaton FR10, because it produces a very nice warm sound, has only 4 Ohms and the frequency response reaches down to almost 100 Hz (https://www.visaton.de/de/produkte/ch…eme/fr-10-4-ohm). However, in this case you really need to help the low end by using a vented box. I got very nice results with designs that are based on the dimensions of the cheap trick 247 (https://www.lautsprechershop.de/hifi/ct247.htm).

    Veit

  • Thanks again for your response!
    This is interesting, because you have chosen a very different approach as the usual guitar speaker with its frequency response in its usual very big cabinet.

  • In fact I was not quite certain, if this approach might lead anywhere, but it seems to work ...

    My goal was to design the sound, rather than building something and then find out what the amp sounds like when I finally switch it on. I think that electronic circuitry gives you a lot more control of your frequency response curves, distortion, reverb etc., while the sound properties of the cabinet are much more difficult to predict. Because I want to determine the sound electronically I cannot allow the speaker to add any unforeseen effects to it.

    What helps is that I am shooting for a warm Jazz sound, which may be a bit easier because you stay largely in the linear range. In Rock it is all about generating good sounding overtones and a lot of the nonlinear effects are produced by the deformation of the speaker cone.

    I have started to look into a new project which involves using a Raspberry Pi to run the open source Rakarack-software. It includes a cabinet emulator, which I want to play with. In this case I will really need a flat frequency response of the speaker, because otherwise the speaker will spoil the emulation.

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