Hello all,
I have a Peavey Butcher that has become my mod test bed. I have been trying to get the amp to have more bass response and higher gain. I had previously added a choke, bias adjust, and changed the tone stack to be more similar to a Fender silverface than a Marshall JCM 800.
Recently, in order to increase low end I have also removed the bright cap and the "treble peaking" circuit between the first and second gain stages. This also entailed removing the 470k voltage divider and replacing with a single 1M load resistor.
I'd like to replace all the load resistors between gain stages with 1M audio taper pots for individual stage gain control.
In order to further increase gain, I wanted to convert the second half of the second 12AX7A, currently being used as the cathode follower for the tone stack, into a fourth gain stage. Concurrently, I wanted to ensure the tone stack does not excessively load the third gain stage.
Looking at the circuit design of many op-amp based preamps and pedals, it occurred to me to place the tone stack in the negative feedback loop between the output transformer and the phase splitter. Essentially I want to replace the presence circuit with a more comprehensive tone stack.
Questions:
Is this phesable? If so, does anyone know of any tube amps that utilize this circuit so I can cross-reference some existing schematics?
Are there simpler ways to increase gain?
What are some other things to try to increase the bottom end?
Should I still keep some sort of fixed resistive load between each stage, or is it OK to have pots as the only load resistors?
Are there any slick mods to simulate a Clean and gain channel (preferably footswitch operated) in a single channel amp? Maybe use a relay to shunt all the stage gain pots to full load resistance?
Thanks!
I have a Peavey Butcher that has become my mod test bed. I have been trying to get the amp to have more bass response and higher gain. I had previously added a choke, bias adjust, and changed the tone stack to be more similar to a Fender silverface than a Marshall JCM 800.
Recently, in order to increase low end I have also removed the bright cap and the "treble peaking" circuit between the first and second gain stages. This also entailed removing the 470k voltage divider and replacing with a single 1M load resistor.
I'd like to replace all the load resistors between gain stages with 1M audio taper pots for individual stage gain control.
In order to further increase gain, I wanted to convert the second half of the second 12AX7A, currently being used as the cathode follower for the tone stack, into a fourth gain stage. Concurrently, I wanted to ensure the tone stack does not excessively load the third gain stage.
Looking at the circuit design of many op-amp based preamps and pedals, it occurred to me to place the tone stack in the negative feedback loop between the output transformer and the phase splitter. Essentially I want to replace the presence circuit with a more comprehensive tone stack.
Questions:
Is this phesable? If so, does anyone know of any tube amps that utilize this circuit so I can cross-reference some existing schematics?
Are there simpler ways to increase gain?
What are some other things to try to increase the bottom end?
Should I still keep some sort of fixed resistive load between each stage, or is it OK to have pots as the only load resistors?
Are there any slick mods to simulate a Clean and gain channel (preferably footswitch operated) in a single channel amp? Maybe use a relay to shunt all the stage gain pots to full load resistance?
Thanks!
Plenty. Relays are your friend.
check what's been done (with switches) in the Duncan Amps' Blues 112 and the Seymour Duncan Twin Tube range (you'll need to search - unless you're a member at freestomp boxes)
The Peavey 5150, a higher gain descendant of the Butcher, uses an extra 12AX7A for two additional gain stages in the "Lead" channel. This is the only amp with a relay channel selector switch that I have studied in any detail.
If I plan on using the same gain stage amount (and physically the same tubes) for both the clean and gain pseudo-channels, what is the most salient feature of the stages that the relay should switch? I was thinking maybe inter-stage voltage dividers, but if there is better approaches, please feel free to educate me in the subject.
If I plan on using the same gain stage amount (and physically the same tubes) for both the clean and gain pseudo-channels, what is the most salient feature of the stages that the relay should switch? I was thinking maybe inter-stage voltage dividers, but if there is better approaches, please feel free to educate me in the subject.
Also, the bias resistors on the 5150 are drastically different than the Butcher. 220k plate resistors and very high value cathode resistors, like 86K!
How does this bias resistor selection change the gain and distortion characteristics of the gain stage. In other words, Why did they redesign the amp in this fashion?
I am going to also find a Peavey Ultra 120 schematic to see what this intermediary model between the Butcher and the 5150 has as its general gain stage design.
I have the 5150 II schematic, but it exceeds the file size limits for PDFs
How does this bias resistor selection change the gain and distortion characteristics of the gain stage. In other words, Why did they redesign the amp in this fashion?
I am going to also find a Peavey Ultra 120 schematic to see what this intermediary model between the Butcher and the 5150 has as its general gain stage design.
I have the 5150 II schematic, but it exceeds the file size limits for PDFs
Absolutely - the "cold clipper" (see Rob Robinette's analysis here) is a key component of the 'Marshall' sound.
Messing with bias points and pre/post distortion frequency shaping allows for significant changes in "tone". (for fun, stick an EQ pedal before your favourite distortion pedal and mess around for a while.)
For more overall bass throughput, you can increase the value of the coupling caps between stages. Just a little more capacitance can make a huge difference, don't overdo it or your preamp tubes will voice their discontent!
If you look at the 5150 and 6505 prints, you will see what they do with the extra stages in the higher gain channels. There is a huge amount of signal shaping and cold clipping utilized to make those "signature" overdrive and distortion tones. With that many stages of preamp, you pretty much have to throw away voltage in between stages to keep signal levels in a useful range. It took a ton of engineering and play testing to come up with the preamp sections in those amps.
Like thoglette mentioned, look around Rob's site and get a good grip on the underlying reasons the circuits are built like they are, they all serve separate purposes. Conjuring the sound you desire is not too difficult, you just have to identify the method and places to modify what you have to achieve a certain effect. Messing around with too much stuff at once will make the process more difficult and confusing.
If you look at the 5150 and 6505 prints, you will see what they do with the extra stages in the higher gain channels. There is a huge amount of signal shaping and cold clipping utilized to make those "signature" overdrive and distortion tones. With that many stages of preamp, you pretty much have to throw away voltage in between stages to keep signal levels in a useful range. It took a ton of engineering and play testing to come up with the preamp sections in those amps.
Like thoglette mentioned, look around Rob's site and get a good grip on the underlying reasons the circuits are built like they are, they all serve separate purposes. Conjuring the sound you desire is not too difficult, you just have to identify the method and places to modify what you have to achieve a certain effect. Messing around with too much stuff at once will make the process more difficult and confusing.
Apehead,
I will try to answer some of your questions in post #6, about relay switching and value changes. Especially on the lead/crunch channel. It must be said first that the relay switching enables foot switching of functionality, but that's a side effect in this discussion. The relays are employed before and after certain stages to alter the way they behave and to match signal levels. Without them, you would be twiddling gain and eq knobs and pots to achieve desired effects, which is undesirable on stage.
You asked about plate resistor changes, and what that affects mainly is voltage gain and headroom. A smaller plate resistor will swing less voltage with the same input signal, so to get more gain a larger value of resistance is what you want. At a certain point, the resistor will run out of current to flow, due to the tube cutting off or saturating. With increased gain comes the reduction of headroom and vice versa.
This plate resistor sets the window of input signal operation and plays with the sensitivity/saturation/compression demeanor of the stage and everything downstream from it in the signal path. If the following stages cannot handle the signal level produced, a voltage divider must "throw away" some of the voltage gain to keep the signal inside the window of the rest of the amp. However, this voltage divider doesn't change the altered behavior of the modified stage. Thus, the (insert adjective) sound is retained while not wreaking havoc on the whole channel.
You also asked about cathode resistor changes. The 12AX7 valves in these amps are all cathode biased, which means the cathode resistor sets the quiescent (DC) current value flowing through the tube. A smaller value resistor allows more current to flow, and sets the static current more towards saturation, and the tube runs "hotter". A larger resistance flows less current, setting the static current more towards cutoff, and the tube runs "colder".
The cathode resistor does have a small effect on the gain of the stage, but this effect is much smaller than that of the plate resistor value. This gain reduction is overcome by the use of a bypass capacitor connected in parallel with the cathode resistor. The capacitor is invisible to DC current flow (during linear operation) but sets up a high pass filter for the AC signal. The rule here is the "colder" the bias, the more effect the capacitor has at passing signal, and the larger the capacitance, the lower the "boosted" frequency response. The careful selection of cathode resistor and bypass cap has a huge effect on the "growl" or "punch" of the stage, as well as the symmetry of the distortion produced if (and when) the stage goes into clipping territory.
Setting the bias or voltage gain too far out of the "normal" linear operating point by changing these few parts too much causes bad things to happen. There is such a thing as too much clipping or too little headroom. Cutoff clipping, grid clipping, and grid blocking are all effects produced by overdriving the input of the valve. At a certain level of overdrive, the clipping starts to change the quiescent operating conditions of the stage. In some cases, this effect is desirable, but in most it is not. This is gone through in detail on Rob's website.
I will try to answer some of your questions in post #6, about relay switching and value changes. Especially on the lead/crunch channel. It must be said first that the relay switching enables foot switching of functionality, but that's a side effect in this discussion. The relays are employed before and after certain stages to alter the way they behave and to match signal levels. Without them, you would be twiddling gain and eq knobs and pots to achieve desired effects, which is undesirable on stage.
You asked about plate resistor changes, and what that affects mainly is voltage gain and headroom. A smaller plate resistor will swing less voltage with the same input signal, so to get more gain a larger value of resistance is what you want. At a certain point, the resistor will run out of current to flow, due to the tube cutting off or saturating. With increased gain comes the reduction of headroom and vice versa.
This plate resistor sets the window of input signal operation and plays with the sensitivity/saturation/compression demeanor of the stage and everything downstream from it in the signal path. If the following stages cannot handle the signal level produced, a voltage divider must "throw away" some of the voltage gain to keep the signal inside the window of the rest of the amp. However, this voltage divider doesn't change the altered behavior of the modified stage. Thus, the (insert adjective) sound is retained while not wreaking havoc on the whole channel.
You also asked about cathode resistor changes. The 12AX7 valves in these amps are all cathode biased, which means the cathode resistor sets the quiescent (DC) current value flowing through the tube. A smaller value resistor allows more current to flow, and sets the static current more towards saturation, and the tube runs "hotter". A larger resistance flows less current, setting the static current more towards cutoff, and the tube runs "colder".
The cathode resistor does have a small effect on the gain of the stage, but this effect is much smaller than that of the plate resistor value. This gain reduction is overcome by the use of a bypass capacitor connected in parallel with the cathode resistor. The capacitor is invisible to DC current flow (during linear operation) but sets up a high pass filter for the AC signal. The rule here is the "colder" the bias, the more effect the capacitor has at passing signal, and the larger the capacitance, the lower the "boosted" frequency response. The careful selection of cathode resistor and bypass cap has a huge effect on the "growl" or "punch" of the stage, as well as the symmetry of the distortion produced if (and when) the stage goes into clipping territory.
Setting the bias or voltage gain too far out of the "normal" linear operating point by changing these few parts too much causes bad things to happen. There is such a thing as too much clipping or too little headroom. Cutoff clipping, grid clipping, and grid blocking are all effects produced by overdriving the input of the valve. At a certain level of overdrive, the clipping starts to change the quiescent operating conditions of the stage. In some cases, this effect is desirable, but in most it is not. This is gone through in detail on Rob's website.
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