I've been reading through the ManZamp threads on the Dumble section. It is a Dumble derivative of a 4x10 tweed bassman-ish amp, perhaps only one was built. Some years ago, ic-racer built one based on photos of this amp, some very long threads on his adventures. A couple of years back, another user on this forum, dorrisant, built one too.
The circuit is pretty straightforward. The only thing that really is different is the use of an inductor in series with the first pre-amp cathode bypass capacitor.
The discussion goes on a bit on the various threads, with the value on the schematic noted as " 1 to 22 H". As I can discern from the discussions, ic-racer settled on 3H of inductance.
It is kind of puzzling what function it might have, since the circuit functions more or less like an unbypassed cathode for most of the audio range.
The resonant frequency is about 18hz and the total reactance is about 700 ohms. At 82hz, the inductor dominates with about 1.5k of reactance and at 1khz it is about 19k reactance. So really, little influence at all. The circuit behaves mostly like a straight unbypassed cathode resistor.
Since the circuit was derived from photographs of the original amp, and I can't find any clear ones at this point, I would tend to think a different cathode bypass circuit might be built from the same or similar value components. One could arrange inductors and capacitors with resistors to achieve various EQ curves in the first preamp stage. I would tend to think something to create a mid-range bump rolling off the bass and the treble ends of the response might be quite useful in that stage.
Ponderings, comments?
Cathode inductor in this Dumble circuit?
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Cathode inductor in this Dumble circuit?
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martin manning
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Re: Cathode inductor in this Dumble circuit?
Useful if you fancy a 3db bass boost or treble cut. Green trace is 1nH (no inductor), blue is 100m, then 200m, 400m, 800m, and 1.6H. 20H produces a sharp peak around 8Hz. Anything over 1H would seem to be of little value, besides being physically huge. Power inductors in the 100-600 mH range might be useful.
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Re: Cathode inductor in this Dumble circuit?
I got interested in ic-racer's Manzamp build because I have a re-issue tweed bassman carcass in the garage I need to do something with. Some 20 years ago, it was popular to replace the re-issue cabinet with a solid pine reproduction as part of the "tone secret". I got the carcass for nothing basically.
On the ic-racer project circuit, I graphed out some options as well.
That raises some thoughts too:
1. If want an EQ curve in the first preamp stage, what should it be?
2. The straight-up LC in series pair gives a pretty sharp peak. It might be better to lower the Q of the reactive element to widen the curve a bit, less peaky.
3. Contemplating it, whatever LCR one picks to shape the frequency response, the impedance of that has to scale against the resistor that sets the operating point of the preamp tube.
The only example of a Dumble Manzamp seems to belong to Bonnie Raitt.
I applaud TAG user "ic-racer" project, it's an amazing story and build. I don't know if there are any better photos of the subject Dumble Manzamp 4x10 tweed. But it is amazing that he did so much work based on gut-shot photos. It looks like a Bassman, but as ic-racer discovered, it was all scratch built, every bit of it.
The photos also appear to show a 12AX7 first preamp, and the next two preamps appear to be taller tubes, perhaps 6CG7?
Followed up with EL34's and a GZ34 rectifier. But the project build had a lot of capacitance on the rectifier, which would really burn the GZ34 hard. Dumble generally knew what he was doing and I suspect he may have employed a choke-input filter rather than the common capacitor input filter design.
On the ic-racer project circuit, I graphed out some options as well.
That raises some thoughts too:
1. If want an EQ curve in the first preamp stage, what should it be?
2. The straight-up LC in series pair gives a pretty sharp peak. It might be better to lower the Q of the reactive element to widen the curve a bit, less peaky.
3. Contemplating it, whatever LCR one picks to shape the frequency response, the impedance of that has to scale against the resistor that sets the operating point of the preamp tube.
The only example of a Dumble Manzamp seems to belong to Bonnie Raitt.
I applaud TAG user "ic-racer" project, it's an amazing story and build. I don't know if there are any better photos of the subject Dumble Manzamp 4x10 tweed. But it is amazing that he did so much work based on gut-shot photos. It looks like a Bassman, but as ic-racer discovered, it was all scratch built, every bit of it.
The photos also appear to show a 12AX7 first preamp, and the next two preamps appear to be taller tubes, perhaps 6CG7?
Followed up with EL34's and a GZ34 rectifier. But the project build had a lot of capacitance on the rectifier, which would really burn the GZ34 hard. Dumble generally knew what he was doing and I suspect he may have employed a choke-input filter rather than the common capacitor input filter design.
Re: Cathode inductor in this Dumble circuit?
dbharris wrote: ↑Mon Sep 01, 2025 8:40 pm There is also the Manzamp preamp built for Eric Johnson:
https://reverb.com/item/73170202
-Dan
Yeah, except other than the name, has absolutely no relationship to the tweed 4x10 Manzamp for Bonnie Raitt.
The Dumble 4x10 Manzamp has the same controls as a tweed bassman and looks like one. Circuit is definitely, not at all the same.
The Eric Johnson pre-amp, is just completely different.