LTP PI - Is this normal?

[Daedalus]

I was scope probing around in a very generic LTP PI (schematic here).
Sine wave input and fairly low levels and see hard clipping on the negative cycle of the signal with the probe on the PI cathode or below the 470R.
Is this just the normal behavior of this circuit or is something wrong? I've read alot about this circuit but I don't understand it.
Thanks

[jbrew73]

is this on a operational amp? if so try pulling the power tubes and retesting as your power tubes may be drawing grid current.

[Daedalus]

Yes, it is. I just tried your suggestion but it didn't make any difference. I also have a 1M MV pot across the two output lines after the the .1uf caps. Turning that up and down doesn't change the signal on the PI.
Thanks

[Chris / CMW amps]

Seems like a Fender AA763 PI (very early blackface) and that circuit did use a 12AT7/ECC81 tube.

[cbarrow7625]

What does the signal look like at the plates? Off the top of my head, that does not sound normal but I could be incorrect. However, regardless of what the cathode sine wave looks like, it is the output at the plates that really matters.

Does the amp sound bad? Or is this more an academic curiosity?

[MWJB]

"Seems like a Fender AA763 PI (very early blackface) and that circuit did use a 12AT7/ECC81 tube."...and both PI plate resistors were 100K in that, short-lived, design.

27K tail in this circuit, especially with a 12AX7, may not give acceptable headroom?

Wouldn't it make more sense to scope the plates of the PI to see if it's doing it's job? At the point you are scoping the amp is still effectively single-ended.

LTPI is a differential amp - Triode A is modulated by the grid (fed by tonestack & PI coupling cap), triode B is modulated by it's cathode.

Your very large tail (for an LTPI with NFB) will improve balance over 22K or 10K, but may impair dynamic response & headroom, making the amp a little loose?

[ETR]

I was scope probing around in a very generic LTP PI (schematic here).
Sine wave input and fairly low levels and see hard clipping on the negative cycle of the signal with the probe on the PI cathode or below the 470R.
Is this just the normal behavior of this circuit or is something wrong? I've read alot about this circuit but I don't understand it.
Thanks

Try disconnecting the feedback. How much signal are we talking? It may be normal. A bigger tail resistor may cure it.

[Daedalus]

Thanks for the replies. I'll try to answer all in this post:
1) Currently the NFB is not connected to the OT output. I built this amp with a cathode biased output stage and wanted to experiment without NFB in and out.
2) I started with a 12at7 (4024) and saw the same symptom. The at7 drew the plate voltages down about 30v lower than the ax7, so I switched to ax7 thinking the higher plate voltage would help.
3) I originally had a 22K tail resistor. I disconnected one end put a resistor sub box in there. I didn't see any real difference in the clipping of the cathode signal with values between 10K and 47K. I put 27K back in kinda on a whim.
4) The first scope picture shows the signal on the plates (top trace is pin 1). The second picture shows the signal at pin2 (top trace) and at pin3 (bottom). Both pictures were taken with the volume control at about 3.
5) The amp actually sounds pretty good to me (tin ear though), so it might be more of an academic question. I want to figure out if there is something out of whack or if that clipping is just normal because of the "current source" design of the LTP.
Thanks for your help so far.

[MWJB]

"2) I started with a 12at7 (4024) and saw the same symptom. The at7 drew the plate voltages down about 30v lower than the ax7, so I switched to ax7 thinking the higher plate voltage would help." probably more than 30v drop at the plates I'd think, once you've factored in rise in voltage at the cathode?

The 12AT will give more headroom in that circuit.

What kind of signal are you putting in at the input (mV)?

"5) The amp actually sounds pretty good to me (tin ear though), so it might be more of an academic question." Well, if it doesn't sound broken it probably isn't ("ideal" and to your "taste" is another matter).

"I want to figure out if there is something out of whack or if that clipping is just normal because of the "current source" design of the LTP." Put the 12AT back in & go to Fender.com and check out the schem for the 65 Super reverb...see if your voltages are severely out of whack.

If both PI plates are doing the same thing, at the same voltage, then the PI is doing it's job, we just need to establish a datum/reasonsble expectation.

[Daedalus]

What kind of signal are you putting in at the input (mV)?
If I'm reading the scope correctly, it looks like 15divisions at 5v/div or 75V p-p(?) Kinda high?

Put the 12AT back in & go to Fender.com and check out the schem for the 65 Super reverb...see if your voltages are severely out of whack.
Will do.

If both PI plates are doing the same thing, at the same voltage, then the PI is doing it's job, we just need to establish a datum/reasonsble expectation.
Makes sense. I'll report back tonight.
Thanks

[Chuck H]

Your scope traces for the plates look fine.

The scope for the cathode traces always look like that. Not sure why exactly but there's a lot I don't know.

What I do know is that your plate traces show the output reaching cutoff and saturation at about the same time. If anything it looks like the bias is a little too hot. You might try increasing the 420 ohm bias resistor to 470 ohms or 680 ohms and then re-check the plates on the scope to see if the top and bottom of the wave form for each plate start to clip at the same time.

Chuck

[Daedalus]

"2) Put the 12AT back in & go to Fender.com and check out the schem for the 65 Super reverb...see if your voltages are severely out of whack.

My voltages are quite a bit lower than that. My PT has a higher voltage output available that could get me up there, however, I have a pair of 6V6GTs in this, don't know if they could take the 460v plate voltage.
Here's the whole thing...

[ETR]

Thanks for the replies. I'll try to answer all in this post:
1) Currently the NFB is not connected to the OT output. I built this amp with a cathode biased output stage and wanted to experiment without NFB in and out.
2) I started with a 12at7 (4024) and saw the same symptom. The at7 drew the plate voltages down about 30v lower than the ax7, so I switched to ax7 thinking the higher plate voltage would help.
3) I originally had a 22K tail resistor. I disconnected one end put a resistor sub box in there. I didn't see any real difference in the clipping of the cathode signal with values between 10K and 47K. I put 27K back in kinda on a whim.
4) The first scope picture shows the signal on the plates (top trace is pin 1). The second picture shows the signal at pin2 (top trace) and at pin3 (bottom). Both pictures were taken with the volume control at about 3.
5) The amp actually sounds pretty good to me (tin ear though), so it might be more of an academic question. I want to figure out if there is something out of whack or if that clipping is just normal because of the "current source" design of the LTP.
Thanks for your help so far.

You may want to try that again but set it where the plates are just starting to clip. It's impossible to tell anything from the hard clipping you have there. If your tail is big enough the non grid driven side and the cathode of the PI should just start clipping about the same time as the grid driven side. Don't worry about the assymetrical clipping on the cathode waveform.

[Daedalus]

I'm sorry, but I don't understand: try what again? Thanks

[ETR]

I'm sorry, but I don't understand: try what again? Thanks

I was referring to the scope screen shots of the PI fully clipped. You may want to try the tail resistor swap and note the differences just on the verge of clipping.