PAB logistics - can someone teach me?

[kleinm]

I posted this over at the Brown Note forum, and haven't had any takers yet. I thought I'd try over here. Note, on the BN D'Lite, PAB actually physically breaks the connection from the Treble pot to Bass pot, rather than the 22Mx2 I've seen in schematics here.

Okay, so PAB...we know what it is, what it does, and how it effects our sound.

..but, I don't understand how it works. Let's start here, with a quote from Normster taken from BN forum, again.

When you engage the PAB, you disconnect the ground connection for the tone stack so ALL of the signal from the volume control goes into the next stage.

Unfortunately, I'm seeing a permanent grounds on two controls. I'll go one at a time....

First, the connection on the Midrange control. Since the mid pot is set up as a variable resistor, its just controlling the frequency knee of the Midrange control. Right? PAB does not remove the permanent connection to ground (via lug 1) on Midrange control...yet Midrange control is useless in PAB mode, at least to my meager ears.

Second, the connection on the Bass control. Bass, again, is setup as a variable resistor with a .0012uF cap across it...which looks like a -3dB filter that vaguely reminds me of the classic 470pF/470k Marshall high-pass filter. But no matter what PAB does, Bass is always connected to ground. And yet the control is seemingly dead in PAB?

All PAB does, that I'm seeing, is break the connection (or add two series 22M r's) from Treble to Bass. This disconnects Treble from ground, via the Bass pot. Bass is still connected to ground, and Middle is still connected to ground.

So how does breaking (or creating 20m to that path, in the classical Dumble fashion) that singular connection render ALL controls dead? Is it seriously just a matter of "path of least resistance"?

And furthermore, signal STILL has to run from Treble lug 3 to lug 2, PAB or not. Correct? Even though it wouldn't necessarily effect frequency response per se, wouldn't the position of that Treble pot still effect the amount of signal running into the 1M-A Volume control?

I guess in other words, what does PAB-engaged look like to the signal path? Is it just the treble cap (BN D'Lite stock 270pF) and whatever resistance is between lug 3 and 2 on the Treble control? I've attached a drawing of what I'm thinking is going on, but surely I'm missing something here.

If anyone can explain this, I'd be very grateful!

Thanks.
mk

[ayan]

I posted this over at the Brown Note forum, and haven't had any takers yet. I thought I'd try over here. Note, on the BN D'Lite, PAB actually physically breaks the connection from the Treble pot to Bass pot, rather than the 22Mx2 I've seen in schematics here.

Okay, so PAB...we know what it is, what it does, and how it effects our sound.

..but, I don't understand how it works. Let's start here, with a quote from Normster taken from BN forum, again.

When you engage the PAB, you disconnect the ground connection for the tone stack so ALL of the signal from the volume control goes into the next stage.

Unfortunately, I'm seeing a permanent grounds on two controls. I'll go one at a time....

First, the connection on the Midrange control. Since the mid pot is set up as a variable resistor, its just controlling the frequency knee of the Midrange control. Right? PAB does not remove the permanent connection to ground (via lug 1) on Midrange control...yet Midrange control is useless in PAB mode, at least to my meager ears.

Second, the connection on the Bass control. Bass, again, is setup as a variable resistor with a .0012uF cap across it...which looks like a -3dB filter that vaguely reminds me of the classic 470pF/470k Marshall high-pass filter. But no matter what PAB does, Bass is always connected to ground. And yet the control is seemingly dead in PAB?

All PAB does, that I'm seeing, is break the connection (or add two series 22M r's) from Treble to Bass. This disconnects Treble from ground, via the Bass pot. Bass is still connected to ground, and Middle is still connected to ground.

So how does breaking (or creating 20m to that path, in the classical Dumble fashion) that singular connection render ALL controls dead? Is it seriously just a matter of "path of least resistance"?

And furthermore, signal STILL has to run from Treble lug 3 to lug 2, PAB or not. Correct? Even though it wouldn't necessarily effect frequency response per se, wouldn't the position of that Treble pot still effect the amount of signal running into the 1M-A Volume control?

I guess in other words, what does PAB-engaged look like to the signal path? Is it just the treble cap (BN D'Lite stock 270pF) and whatever resistance is between lug 3 and 2 on the Treble control? I've attached a drawing of what I'm thinking is going on, but surely I'm missing something here.

If anyone can explain this, I'd be very grateful!

Thanks.
mk

I think your diagram is not 100% correct. The slope resistor will feed, on the one hand, the midrange cap in series with the midrange pot; on the other hand, it will feed the bass cap in series with the bass pot at full value and with the tail resistor to ground -- and this is not what your diagram shows.

The way I look at it is: since the bass pot is disconnected from the "Feed ahead" network that would take some of the lower frequencies to the ground lug of the treble pot, in PAB mode we have the equivalent of the bass pot being 100% off in terms of feed ahead. Granted, since the path to ground from the .1uF cap is the full pot value + tail, not all the bass will be lost to ground. Turning the midrange pot in PAB mode really has no effect whatsoever in the tone, so the equivalent network is obviously not sensitive to the change in resistance from the middle pot.

And, on the top section you end up with a coupling cap equivalent to the treble cap working alone.

One thing you could do is lift the ground on the bass pot and see if there is any difference in sound in PAB mode. If the sound becomes thicker, then the bass and middle pots being grounded help to "drain" some lower frequencies before they hit the treble cap, which I suspect is the case. If it makes no difference whatsoever, then the whole circuit is simply equivalent to the treble cap all by itself acting as the coupling cap for V1A-V1B.

Cheers,

Gil

[dogears]

Agreed.

I'd add 4.7M resistors on the break points. All my amps have them, and others who have tried both ways claim better PAB tone with the resistors.

Also, I'd consider upping the stock treble cap size if you want a more effective PAB tone. 270pf in PAB mode may be thin sounding. Much more bass with a 330pf, IMHO.....

[greiswig]

Agreed.

I'd add 4.7M resistors on the break points. All my amps have them, and others who have tried both ways claim better PAB tone with the resistors.

Also, I'd consider upping the stock treble cap size if you want a more effective PAB tone. 270pf in PAB mode may be thin sounding. Much more bass with a 330pf, IMHO.....

Could you elaborate on this a bit, dogears? What are the break points, and what does the other end of the R connect to?

Thanks!

[kleinm]

I think your diagram is not 100% correct. The slope resistor will feed, on the one hand, the midrange cap in series with the midrange pot; on the other hand, it will feed the bass cap in series with the bass pot at full value and with the tail resistor to ground -- and this is not what your diagram shows.

The way I look at it is: since the bass pot is disconnected from the "Feed ahead" network that would take some of the lower frequencies to the ground lug of the treble pot, in PAB mode we have the equivalent of the bass pot being 100% off in terms of feed ahead. Granted, since the path to ground from the .1uF cap is the full pot value + tail, not all the bass will be lost to ground. Turning the midrange pot in PAB mode really has no effect whatsoever in the tone, so the equivalent network is obviously not sensitive to the change in resistance from the middle pot.

And, on the top section you end up with a coupling cap equivalent to the treble cap working alone.

One thing you could do is lift the ground on the bass pot and see if there is any difference in sound in PAB mode. If the sound becomes thicker, then the bass and middle pots being grounded help to "drain" some lower frequencies before they hit the treble cap, which I suspect is the case. If it makes no difference whatsoever, then the whole circuit is simply equivalent to the treble cap all by itself acting as the coupling cap for V1A-V1B.

Cheers,

Gil

Yeah, I didn't think that my diagram could be correct because of the various experiences noting differences between the full "break" PAB vs. the "padded" PAB. If there is a difference, which it certainly sounds like, then it goes beyond my little MS Paint diagram.

Your second paragraph was as good as an explanation I could have hoped for! Thank you so much, Gil! I'm going to marinate with that information for a while.

I hope to get some experimenting going on in this over the coming days. If there's interest, I can post my results.

I've attached a revised "PAB always" diagram. The Mid pot can be subbed for a fixed resistor, and the Bass pot can be turned into two fixed resistors, with the lower 4M7 PAB resistor connecting to their junction.

Thanks again, I really appreciate it.
- mk

[kleinm]

Agreed.

I'd add 4.7M resistors on the break points. All my amps have them, and others who have tried both ways claim better PAB tone with the resistors.

Also, I'd consider upping the stock treble cap size if you want a more effective PAB tone. 270pf in PAB mode may be thin sounding. Much more bass with a 330pf, IMHO.....

Cool, thanks for the information. I still have my PAB in the stock D'Lite fashion, but I'll experiment with the 4M7s on the break points. It sounds like its certainly worth it.

BTW - I've already increased my treble cap to 330pF. I liked where it placed the frequency knee, compared to the stock 270pF. Good stuff. Thanks again for that mod, btw.

[dogears]

Mike,

There are two points that get broken with the PAB switch. Your diagram is confusing me... Why did you put the resistors in series?

You only show one break....

[kleinm]

Mike,

There are two points that get broken with the PAB switch. Your diagram is confusing me... Why did you put the resistors in series?

You only show one break....

Dammit...I'm apparently still missing something.

From the diagram below, I thought it shows that when the switch is up in real-life (using the throws on the bottom) the signal has to pass through both resistors, from the treble pot to the bass wiper.

https://tubeamparchive.com/download/file.php?id=1179

Hope I'm not being a total dunce with this. Sorry in advance, if so.
- mk

[fabiomayo]

These 4M7 resistors are the ones shown as 22M in the ODS-101 schematitc? Does their value matter at that point?

[kleinm]

I've still been wrestling with what's going on to the signal in PAB. If the ODS-101-HRM schematic is correct, its most certainly not a tonestack lift. Its adds a significant amount of resistance to the tonestack, making most of the signal go through the treble cap, hence acting somewhat as a coupling cap.

I've attached two drawings of what I'm visualizing this as. What's in hard to read (intentionally so) green, is part of the regular tonestack that's not of much interest pertaining to PAB. What's in hard to read (intentionally so) blue is not active given where the PAB switch is. What's black and easy to read (intentionally so...see a theme? ) is active, given where the PAB switch is. I'm aware the PAB disengaged is somewhat misleading regarding the rest of the tonestack, but I kept the diagrams consistent to focus solely on PAB and the related areas.

NOTE - the images will be distorted unless opened in a new window, or downloaded.

Someone please correct me if I'm wrong.

I really appreciate all of your input. Thanks!