ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Trainwrecked Expresso question

9/17/1999 12:53 PM
Mook	Trainwrecked Expresso question This question is in regard to the schematic at Steve A's site. After the tone stack and vol controls, the signal goes to 1/2 of a 12ax7. After that it "dumps" a bunch of signal (through a 56K resistor) and goes into another 1/2 of a 12ax7. This stage has a cathode resistor of 10K (if I remember correctly). What does this stage do? I remember Ken Gilbert saying something about cascading stages using very high cathode resistor values. For instance, I remember Ken saying he cascased 3 stages using cathode values of 10K, 22K, and 50K. He said this didn't increase gain, but increased harmonics. Do I remember this correctly?? Thanks, Mook

9/17/1999 12:53 PM

Mook

Trainwrecked Expresso question

This question is in regard to the schematic at Steve A's site.

After the tone stack and vol controls, the signal goes to 1/2 of a 12ax7. After that it "dumps" a bunch of signal (through a 56K resistor) and goes into another 1/2 of a 12ax7. This stage has a cathode resistor of 10K (if I remember correctly).

What does this stage do? I remember Ken Gilbert saying something about cascading stages using very high cathode resistor values. For instance, I remember Ken saying he cascased 3 stages using cathode values of 10K, 22K, and 50K.

He said this didn't increase gain, but increased harmonics. Do I remember this correctly??

Thanks,

Mook

9/19/1999 12:48 PM
Mook	Any Ideas about this, Ken?? Thanks, Mook

9/19/1999 3:38 PM

Gil Ayan

Re: Trainwrecked Expresso question

quote:
"This question is in regard to the schematic at Steve A's site.

After the tone stack and vol controls, the signal goes to 1/2 of a 12ax7. After that it "dumps" a bunch of signal (through a 56K resistor) and goes into another 1/2 of a 12ax7. This stage has a cathode resistor of 10K (if I remember correctly).

What does this stage do? "

For starters one would think it kills the gain, since the gain approaches Rp/Rc (p = plate, c = cathode). However, you will eventually get some distortion that way because you are starving the tube of some current.

Many modern amps like Soldano, The Mesa Boogie Recto, Broove Tuves (all of which can be found at Steve Ahola's schematics page, BTW), use a larger that usual cathode resistor at some distortion stage.

So, I "had" to try that a while ago and found that the distortion you get is "harsh," really... not what I would consider too musical. Of course, if you change the resot of the circuitry to accommodate that, that's one thing, but I am not sure why some companies are using that method. What probably happened is maybe Soldano started it and the others copied the approach...

Easy enough to try, Mook. In a distortion stage, put a 50K pot and hear the difference the various settings will make.

Gil

9/19/1999 4:17 PM
Ken Gilbert	Hey Mook... Just took a look at the schem from Steve's site, and I think I have some idea of what you're talking about. First off, while the 56K does create a rather low impedance to ground, it's not really about dumping signal level (or voltage). If it were simply about that, then a voltage divider is much more effective. What the 56K does is a couple of things. A) it makes for a pretty low grid circuit impedance on the following stage. Not incredibly low, but low nonetheless. This helps in avoiding grid blocking, where the coupling cap has a charge impressed across it because of the grid current flow clamping the AC voltage below ground. During these conditions, the grid circuit acts as if it had a single diode across it. Also note the small value of coupling cap here, only 1 nF. This, along with the low value of grid resistor, makes for a quick time constant, which makes the amp much quicker to recover from overloads and transients. B) the other thing the 56K does is create a pretty low composite plate load for the previous tube. The reactance of the coupling cap comes into play here, making the load heavier at higher frequencies. At DC, obviously, there is only the 100K load, and as the frequency rises the impedance of the cap falls, and te 56K comes more into play in parallel with the 100K. A tube heavily loaded on its plate tends to produce a heavy 2nd harmonic distortion component. So that following stage with the 10K Rk is operating at a low current. This lends a crispy upper harmonic laden tone. Because the stage is operating at a low plate current, very little voltage is dropped across that 100K plate load resistor. Therefore, positive signal excursions are limited to the voltage drop across said resistor. Negative excursions, OTOH, are not really limited per se, but they ARE decreased to some extent by the heavy degeneration that's going on in the cathode circuit. As the grid goes up, so does the cathode, and the peak plate currents are reduced. This will limit the negative-going swings of the plate. So, as a result of the quiescent plate voltage, and the large Rk, output voltage from this stage is less than optimal. I'd expect someting like 30 Vpp. The output will have a very heavy 2nd harmonic distortion component, since the positive excursion is brick wall limited at B+. This relatively small output swing capability is actually good, though, because the PI does not need (or want, really) a very huge signal pumped into it. This keeps the stage from farting out too much, and screwing up the NFB loop in the process. Judging by the circuit values, you would expect a stage gain of about 10 or so, but gain is not really what this stage is about. It's really there to add some higher harmonics and complexity to the tone. The lack of cathode bypass cap, the higher value of Rk, the low value of grid resistor all make this stage harder to "fart out" when hit with a heavy signal. The relatively small output swing capability will also make the PI harder to "fart out" as well. Hope it's clear enough to follow. Ken

9/19/1999 4:17 PM

Ken Gilbert

Hey Mook...

Just took a look at the schem from Steve's site, and I think I have some idea of what you're talking about.

First off, while the 56K does create a rather low impedance to ground, it's not really about dumping signal level (or voltage). If it were simply about that, then a voltage divider is much more effective.

What the 56K does is a couple of things.

A) it makes for a pretty low grid circuit impedance on the following stage. Not incredibly low, but low nonetheless. This helps in avoiding grid blocking, where the coupling cap has a charge impressed across it because of the grid current flow clamping the AC voltage below ground. During these conditions, the grid circuit acts as if it had a single diode across it. Also note the small value of coupling cap here, only 1 nF. This, along with the low value of grid resistor, makes for a quick time constant, which makes the amp much quicker to recover from overloads and transients.

B) the other thing the 56K does is create a pretty low composite plate load for the previous tube. The reactance of the coupling cap comes into play here, making the load heavier at higher frequencies. At DC, obviously, there is only the 100K load, and as the frequency rises the impedance of the cap falls, and te 56K comes more into play in parallel with the 100K. A tube heavily loaded on its plate tends to produce a heavy 2nd harmonic distortion component.

So that following stage with the 10K Rk is operating at a low current. This lends a crispy upper harmonic laden tone. Because the stage is operating at a low plate current, very little voltage is dropped across that 100K plate load resistor. Therefore, positive signal excursions are limited to the voltage drop across said resistor.

Negative excursions, OTOH, are not really limited per se, but they ARE decreased to some extent by the heavy degeneration that's going on in the cathode circuit. As the grid goes up, so does the cathode, and the peak plate currents are reduced. This will limit the negative-going swings of the plate.

So, as a result of the quiescent plate voltage, and the large Rk, output voltage from this stage is less than optimal. I'd expect someting like 30 Vpp. The output will have a very heavy 2nd harmonic distortion component, since the positive excursion is brick wall limited at B+. This relatively small output swing capability is actually good, though, because the PI does not need (or want, really) a very huge signal pumped into it. This keeps the stage from farting out too much, and screwing up the NFB loop in the process.

Judging by the circuit values, you would expect a stage gain of about 10 or so, but gain is not really what this stage is about. It's really there to add some higher harmonics and complexity to the tone. The lack of cathode bypass cap, the higher value of Rk, the low value of grid resistor all make this stage harder to "fart out" when hit with a heavy signal. The relatively small output swing capability will also make the PI harder to "fart out" as well.

Hope it's clear enough to follow.

Ken

Page 1 of 1