amsdenj Posted February 23, 2015 Share Posted February 23, 2015 In a post Power User said "the main things you need to tweak are the 2 sources of tube distortion: drive and amp master volume (not to be confused with the master volume physical knob or the channel volume).." regarding the Brit 800, but this would likely apply to may of the high-gain amplifiers. I wonder what your experiences are on the differences between preamp and power amp distortion? On a typical Fender amp, you don't get much preamp distortion unless you add some gain effect in front of the amp to drive it harder. When these amps do distort, they do so symmetrically, the top and bottom half of the wave form are clipped the same since these are push-pull power amplifiers. Preamp distortion, from a typical triode tube, is not symmetric. The top half of the wave clips while the bottom half goes into saturation which is not as sharp. How do you balance preamp distortion and drive vs. master amp distortion? What is the difference in tone to you? How does it effect how the guitar feels when playing? Quote Link to comment Share on other sites More sharing options...
baron55 Posted February 24, 2015 Share Posted February 24, 2015 On typical guitar amps that are AB1 types with a typical long tail Phase inverter. The phase inverter always clips or distorts before the power amp does. The type of tube (12AX7, 12AT7, 12AU7, etc) plate resistors, tail resistor and supply voltage will aso determine how much headroom and when and how the PI will distort. Aso a huge factor is the negative feedback supply fed back to the phase inverter. Negative feedback ( low speaker signal fed back) delays on comming clipping in the by atenuating bandwidth. This is a pass filter effect that limits lows and highs. Most amps have this feature. BTW Mesa Rectifiers do not use negative feed back when the amp is switched on the high gain channels. This is what leads to the flubby booming bottom end and fuzzy highs people seem to complain about. The Rectos are the only high gain amps I know of that don't utilize negative feed back. Amps with master volumes fall in to mostly two types. Pre-phase inverter master volumes and post-phase inverter master volumes. Most modern high gain amps use a pre- phase inverter master volume. This controls how much signal is fed to the phase inverter. Thi is typical when low phase inbverter distortion is desired since the preamp is doing all the clipping. The way the taper is on most of these potentiometers (audio) usually the amp is at full wattage at the 10:00 position, turning up the master just clips the PI harder and then drives the power tubes into overdrive. Post phase inverter moster volumes, a popular mod for vinatge low gain amps does two things. One it allows the preamp to slam the PI with full volatge casuing it to clip heavily, second when the master is turned down, it loads down the phase inverter making the headroom even less and causes it to clip heavily. So unfortuneatley it is not that simple. Most so called power amp distortion is actually phase inverter distortion. Quote Link to comment Share on other sites More sharing options...
dbagchee Posted February 24, 2015 Share Posted February 24, 2015 How do we know which levels the master volume was modeled at? Is there a reference somewhere? I didn't know that about the Recto. Quote Link to comment Share on other sites More sharing options...
amsdenj Posted February 24, 2015 Author Share Posted February 24, 2015 Most so called power amp distortion is actually phase inverter distortion. That may be true of modern high gain amps with a lot of preamp stages. But distortion in the old Fender Twin, Showman, Deluxe, Baseman was definitely caused by the power tubes. Those amps didn't have enough gain to clip the phase inverter. I recall my friend Doug and I attempting to duplicate what those power tubes were doing with biased diode clipping circuits back in the late 60's, early 70's. I still have the old Showman with an external Fender reverb unit in which I added an extra tube, low voltage DC power supply (off the tube heater supply) and high-speed switching diodes biased at 1.5 volts. It was remarkably close, missing the class B notch. We spent many hours in the Physics Lab at UMO with a scope and signal generator exploring and building. Doug's gone now, died young at 54. I still have some of the old notes we kept. He's missed. Quote Link to comment Share on other sites More sharing options...
spaceatl Posted February 24, 2015 Share Posted February 24, 2015 I don't know but it would seem that the 800 might have been shot a little on the hot side...And I think that model probably differs from how most people run those... My own opinion is that the DEPs they chose the middle settings to offer as much of the dynamics of the power amp as they could...and they are all different... The DEP amp parameters are really the trick to finding yourself in a FULL model...Sure, turning the master DEP can help the 800 sound better...Another way to smooth out some of the xover distortion in those class AB models and tighten it up is to increase the bias DEP and decrease the biasX...I find these settings to be subtle and a little deceiving in headphones...I generally have to check with some decent air moving as these settings are more about dynamic response than the tone...they are all very interactive... Here's a little refresher from the Advanced Guide... Page 2 of the Amp:Edit Screen (not displayed for Preamp Models) • Knob 1: Use this Master Volume to adjust the amount of power amp distortion. This parameter is highly interactive with all other power amp parameters - the lower the Master is set, the less effect the other controls will have. • Knob 2: Setting the Sag to minimum offers a “tighter†responsiveness, and turning clockwise provides more “touch†dynamics & sustain. • Knob 3: Controls how much heater hum & AC ripple interacts with your tone. At the maximum setting things get really freaky. Page 3 of the Amp:Edit Screen (not displayed for Preamp Models) • Knob 1: Changes the Bias of the power tubes. Set to minimum to achieve a very “cold†Class AB biasing. At maximum the amp is operating in Class A. • Knob 2: The Bias Excursion determines how the power amp tubes’ voicing reacts when they are pushed hard. Set low for a tighter feel. Set high for more tube compression. This parameter is highly reactive with the DRIVE & MASTER settings. Quote Link to comment Share on other sites More sharing options...
baron55 Posted February 24, 2015 Share Posted February 24, 2015 That may be true of modern high gain amps with a lot of preamp stages. But distortion in the old Fender Twin, Showman, Deluxe, Baseman was definitely caused by the power tubes. Those amps didn't have enough gain to clip the phase inverter. I recall my friend Doug and I attempting to duplicate what those power tubes were doing with biased diode clipping circuits back in the late 60's, early 70's. I still have the old Showman with an external Fender reverb unit in which I added an extra tube, low voltage DC power supply (off the tube heater supply) and high-speed switching diodes biased at 1.5 volts. It was remarkably close, missing the class B notch. We spent many hours in the Physics Lab at UMO with a scope and signal generator exploring and building. Doug's gone now, died young at 54. I still have some of the old notes we kept. He's missed. The PI distorts in the Fenders as well. I have build a showman, and seen it first hand. It only takes about 4 volts AC input to a PI to clip it if it is a 12AX7. Long Tail phase inverters will always clip before the power stages do. Just not as heavily as a high gain amp Quote Link to comment Share on other sites More sharing options...
spaceatl Posted February 24, 2015 Share Posted February 24, 2015 I thought a stock Fender used a 12at7 for PI...higher current rating... Quote Link to comment Share on other sites More sharing options...
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