| Author | Message | ||
     Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
While following the signal on a scope I noticed that the signal begins clipping on the positive waveform first and doesnt clip the negative at the plate of the 1st stage of the PI tube, pin 6. This positive only clipping carries on through to the plate of the 2nd stage, pin 1. The signal at the grid of the 1st stage, pin 7, is clean with no clipping. Caps look fine with no DC leakage, resistors on the driverboard check fine (the cathode R48, 1.5 k measures a little high at 1.6k) and I have reflowed all solder joints. Is this normal? any thoughts ? Thanks | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Well, I replaced the cathode resistor and no change. I also raised and lowered the cathode resistor with no change. I wonder if a cap is leaking DC. here are links to a couple of pics the display. One is the input of the PI, from the preamp. the other is at the plate of the PI. http://home.earthlink.net/~awilson40/pin _7_grid.jpg http://home.earthlink.net/~awilson40/pin _6_plate.jpg | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
some more thoughts, if its not a leaky cap, would altering the 100k plate resistor better center the signal ?? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Where are your controls set when checking the wave form? I'm just trying to orient this in my head(NOT and easy task!). | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
The signal is from a sig gen program on the laptop set at 1khz. I run it to a clean boost pedal to set the volume about the same as my guitar was. I tried different combinations of settings on tone, channel 1 and 2, they all looks the same. If I turn up the volume on the clean boost or the volume on the amp channel, the signal will begin clipping on the positive direction first. If I switch to high power, then I have more headroom befor clipping, but when it does clip, it clips the positive first. | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Someone on another board suggested I look at the grid voltage on the 1st stage of the PI. What DC voltage (if any) should I see at the grid with no input ??? | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Here are some further checks and observations. NOW...the 1st stage is clipping the neg going signal...It was clipping the positive. I cleaned , wiggled,and bumped the PI tube to see if it would change the signal. It didnt. Power supply voltage is not changing when a signal is applied. Its holding steady. here are the voltages with no signal/ High power : Voltage at point F 343v at pin 6 (plate) 219v pin 7 (grid) 0v pin 8 (cathode) 1.6v Low Power point F 233v pin 6 160v pin 7 0v pin 8 .95v here are some test settings and observations. test signal at pin 7(grid) is a 750hz wave, 5v peak to peak output signal at pin 6 (cathode) is 2 v p2p with no clipping grid DC level at this point is -1vdc If I increase the signal at the grid to 10v p2p, the output signal has bad negative clipping and is at 3 v p2p. At this time the grid is at -.8v. As I increase the signal strength to the grid, clipping at the output gets worse and the grid DC goes further negative. This neg voltage at the grid is not coming from the preamp as its dc level remains zero. If I throw the 'Deep switch" removing the coupling cap and directly connecting the preamp to the PI, then the grid DC voltage stays at zero but the output still clips. Any other ideas, opinions ?? Thanks | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
scratch that whole post above (wish we could edit a post ) here is a revised adition, I was at the wrong pins. Here are some further checks and observations. I cleaned , wiggled,and bumped the PI tube to see if it would change the signal. It didnt. Power supply voltage is not changing when a signal is applied. Its holding steady. here are the voltages with no signal/ High power : Voltage at point F 343v at pin 6 (plate) 219v pin 7 (grid) 0v pin 8 (cathode) 1.6v Low Power point F 233v pin 6 160v pin 7 0v pin 8 .95v here are some test settings and observations. test signal at pin 7(grid) is a 750hz wave, 5v peak to peak output signal at pin 6 (cathode) is 120 v p2p with no clipping signal at cathode is 2v p2p, no clipping grid DC level at this point is -1vdc If I increase the signal at the grid to 10v p2p, the output signal has bad positive clipping and is at 140v p2p. At this time the grid is at -.8v. signal at cathode is at 3v p2p with neg clipping As I increase the signal strength to the grid, clipping at the output gets worse and the grid DC goes further negative. This neg voltage at the grid is not coming from the preamp as its dc level remains zero. If I throw the 'Deep switch" removing the coupling cap and directly connecting the preamp to the PI, then the grid DC voltage stays at zero but the output still clips. Any other ideas, opinions ?? Or is my input signal just driving too hard. Thanks | ||
| Chris Metcalfe (chris_m) Username: chris_m Registered: 10-2006 |
Just a very very quick post with no research - but if the PI is a cathodyne PI, which I seem to remember it is, then generally speaking the cathode voltage should be approximately half that of the total voltage drop across the PI. I'd look ar the resistor/ joint integrity across the PI circuit ... in haste, Chris. If not cathodyne, ignore this!! | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Thanks, I have pulled , measured and resoldered all resistors in this circuit. BTW, the above results were in Low power mode, High mode yeilds similar results, just at a higher level. Other comments from other boards : Quoting now : "ACK! 5V p-p on a 12AX7 grid??? The only saving grace here is that the cathode is unbypassed, so the cathode is coming up by a volt to add to its normal volt or so of reverse bias and the grid is ... marginally... below the cathode. Any more signal than that and you go into grid conduction." and : "That's because the grid starts conducting on positive peaks, and starts Hoovering in electrons from the stream passing through it on the way to the grid. Grid conduction is building up electrons on the coupling cap. You're into grid blocking. That's what it will do. Again, normal." so perhaps there is nothing wrong , I just have much hotter pickups and have to keep the preamp vol below 3 with this guitar. I do know that I am able to turn up the vol more with a single coil guitar | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Looks like its probably OK, I'm just pushing the PI too hard. I did some more checking with signals and a scope and as long as I keep the input to the grid of the PI below 6v p2p, its fine. One thing I did notice, the original 12ax7 breaks up later and smoother than the JJ Tubes. Sounds a lot better also. |