I've been seeking a better sounding overdrive for my amp and have looked at lots of other designs for suggested values etc. I've run across one design for a supposed Dumble amp that uses a 500pf cap between the plate and cathode on a 12AX7 preamp. I've not seen this in other designs, but I'm not real experienced at this, so, anyone know what this does and how it affects the tone? Here's a link to the schematic. Thanks.
http://www.schematicheaven.com/dumbleamps/distorto.pdf
http://www.schematicheaven.com/dumbleamps/distorto.pdf
I'm not real sure just what this circuit is for, but I'm assuming it's some sort of distortion/overload device for a guitar amp. Correct me if I'm wrong please. However, small capacitors used this way will slightly roll off the very highest frequencies by swamping them to ground and through a tiny amount of feedback. This is most likely done to prevent oscillation. Perhaps ultrasonic. Because of their small value, I doubt if you'll actually hear much high frequency loss.
Victor
Victor
It does two things in a guitar pre-amp.
1) As Victor says it rolls of the HF above the audio range (way above)
2) It also introduces these frequencies back into the valve via the cathode were they intermodulate with the signal in the valve and change the sound of the distortion it produces - not a lot but enough to help give the Dumble its unique character.
James
1) As Victor says it rolls of the HF above the audio range (way above)
2) It also introduces these frequencies back into the valve via the cathode were they intermodulate with the signal in the valve and change the sound of the distortion it produces - not a lot but enough to help give the Dumble its unique character.
James
It's a pretty typical guitar preamp mod that you see to prevent oscillation and roll off some highs. You see the same thing on input stages of some Marshalls.
With the Zo of the stage being around 40K or so, the 500p cap should form a 6dB/octave single pole filter with a knee just under 8kHz should it not? That would be audible.
With the Zo of the stage being around 40K or so, the 500p cap should form a 6dB/octave single pole filter with a knee just under 8kHz should it not? That would be audible.
Thanks for the replies, I can follow that, and I think I'm going to give it a try on my amp. Will adding this cap have any affect on gain? Also, I've been struggling to get tighter sounding lows, will this have any affect there as well, I suspect not?
And, yes Victor, the link to the schematic is an overdrive model for a Dumble preamp. I'm attempting to model my overdrive in my Music Man RD-50 after this design.
Thanks again
And, yes Victor, the link to the schematic is an overdrive model for a Dumble preamp. I'm attempting to model my overdrive in my Music Man RD-50 after this design.
Thanks again
Buckeye Sound said:
Looks like a common practice from SS RF practice to prevent parasitic oscillation. works pretty good in that capacity, however, returning the bypass capacitor to the cathode (emitter, source) seems to cause less distortion than simply grounding it. However, it's essential to have the cathode resistor well bypassed, otherwise, that capacitor will cause positive feedback, and either damped or continuous oscillation.
Plate to cathode cap
Fi you notice that the cathode is bypassed to ground by 5 uf, then the 500pf is effectively to ground but is easier to wire directly on the tube socket? No
The effective source impedance is about 20K since plate out is about 40K plus the parallel of the plate resistor and following volume control. Approx 16KHz -3dB times 2 because of cascadeing makes that -6dB or about 12KHz -3dB.
Fi you notice that the cathode is bypassed to ground by 5 uf, then the 500pf is effectively to ground but is easier to wire directly on the tube socket? No
The effective source impedance is about 20K since plate out is about 40K plus the parallel of the plate resistor and following volume control. Approx 16KHz -3dB times 2 because of cascadeing makes that -6dB or about 12KHz -3dB.
We now have two explanations for the 500pF. As a novice re guitar amps and effects, I accept the re-introduction of harmonics into the circuit - but hardly with the given one! As Brian mentioned, the magnitude of this here would only be round 0,7% of the input signal (with an ECC83). There is also audible Miller effect with the 180K+ grid resistance of the second triode. I furthermore have a problem to see which possible instability needs to be cured with this circuit - the 500pF will not cause meaningful positive feedback connected to the low impedance cathode with that degree of Miller feedback (not talking of r.f. effects). Is the tube ECC82 or ECC83?
But since we have gurus of the guitar genre in our midst and I have much to learn, I put the above by way of questions. The specific Dumble sound was mentioned. Is this particularly at very high (audible) frequencies?
If someone can kindly broaden my horizons here ........
Regards.
But since we have gurus of the guitar genre in our midst and I have much to learn, I put the above by way of questions. The specific Dumble sound was mentioned. Is this particularly at very high (audible) frequencies?
If someone can kindly broaden my horizons here ........
Regards.
Johan Potgieter said:
Good to see you on the board again Johan.
It's just a standard snubber. it prevents oscillation as well as rolling off some of the eekey upper harmnics. You see them ranging from 47p - 500p usually but sometimes higher in a few Fender amps. They are run from plate to cathode for easy mounting, but it could be sttrapped across the plate resistor, or run to ground as well. It does add a bit of feedback, but the amount is almost negligible tonewise.
If it is indeed there to snuff out rf oscillations or pickup, the socket-mounting grid-to-cathode (instead of grid to ground) is good practice and the value is appropriate. Working backward, the socket-mounting of the cap, its size, and its connection suggests that indeed it is there for the purpose of preventing oscillation or pickup at rf.
Two things again 
1) I missed the 5uF bypassing the cathode resistor when I scanned the circuit.... mea cupla! So the harmonic reinjection isn't working here to any degree...
2) I've used this harmonic reintroduction in guitar amps I have designed and it works quite nicely - but I didn't fully bypass the cathode resistor...
Dumble do some nice things in there circuits to get the sound they have and pick some very intersting operating points too - but this isn't one of them this time...
James
1) I missed the 5uF bypassing the cathode resistor when I scanned the circuit.... mea cupla! So the harmonic reinjection isn't working here to any degree...
2) I've used this harmonic reintroduction in guitar amps I have designed and it works quite nicely - but I didn't fully bypass the cathode resistor...
Dumble do some nice things in there circuits to get the sound they have and pick some very intersting operating points too - but this isn't one of them this time...
James
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