Fat PAB mods - LTSPice simulations

[markusw]

While planning my second build, a 50W reverb Non-HRM with switchable Skyliner/80s stack I also did some simulations on "fat PAB" mods.
I also included the mod described e.g. on the second page of this thread
https://tubeamparchive.com/viewtopic.ph ... sc&start=0
and shown in this image (the additional 500p between the plate of V1a and the unused pin of the upper PAB relay).

[img:883:758]http://img.photobucket.com/albums/v21/m ... Fatmod.gif[/img]


The schemo I used for the sims is more or less like the above schem. Only difference 22M PAB resistors and negative feedback loop at V1b.

Here's an image of the simulations with the frequency response at V1b plate.

[img:999:632]http://img.photobucket.com/albums/v21/m ... rd01-1.jpg[/img]

Starting at the bottom of the left Y-axis:
green: "normal" PAB

Blue, red and green (more or less overlapping): without PAB, treb, bass, mid all at 50%

Light blue: mid boost

Red: fat PAB mod as shown in the schem above, just with 220p; increases bottom end but also slightly overall PAB level (compared to normal PAB)

Blue: fat PAB by replacing the 500p (or actually 220p) with a 2M2 resistor as shown in the schem below. Bottom end increases but high end remains the same as with "normal" PAB
If the resistor is decreased to 1M bottom end further increases but highs remain unchanged. Increasing the 2M2 shifts bottom end down towards "normal" PAB. A 5M trim pot I believe should give a reasonable range.


Top light blue: normal PAB plus mid boost (just for completeness)


[img:883:758]http://img.photobucket.com/albums/v21/m ... atmod2.gif[/img]


BTW, I checked removing the NFB at V1b. Didn't make a significant difference on the relative levels of the various curves.

The difference between the two "fat PAB" mods isn't huge. Nevertheless it might be worth a try to test the "resistor" variant, because only the bottom end is increased and the mod reqires shorter wires for the connections (from mid boost switch or mid boost relay to PAB relay)...FWIW

Let me know what you think

Peace,

Markus

[Tonegeek]

While planning my second build, a 50W reverb Non-HRM with switchable Skyliner/80s stack I also did some simulations on "fat PAB" mods.
I also included the mod described e.g. on the second page of this thread
https://tubeamparchive.com/viewtopic.ph ... sc&start=0
and shown in this image (the additional 500p between the plate of V1a and the unused pin of the upper PAB relay).

[img:883:758]http://img.photobucket.com/albums/v21/m ... Fatmod.gif[/img]


The schemo I used for the sims is more or less like the above schem. Only difference 22M PAB resistors and negative feedback loop at V1b.

Here's an image of the simulations with the frequency response at V1b plate.

[img:999:632]http://img.photobucket.com/albums/v21/m ... rd01-1.jpg[/img]

Starting at the bottom of the left Y-axis:
green: "normal" PAB

Blue, red and green (more or less overlapping): without PAB, treb, bass, mid all at 50%

Light blue: mid boost

Red: fat PAB mod as shown in the schem above, just with 220p; increases bottom end but also slightly overall PAB level (compared to normal PAB)

Blue: fat PAB by replacing the 500p (or actually 220p) with a 2M2 resistor as shown in the schem below. Bottom end increases but high end remains the same as with "normal" PAB
If the resistor is decreased to 1M bottom end further increases but highs remain unchanged. Increasing the 2M2 shifts bottom end down towards "normal" PAB. A 5M trim pot I believe should give a reasonable range.


Top light blue: normal PAB plus mid boost (just for completeness)


[img:883:758]http://img.photobucket.com/albums/v21/m ... atmod2.gif[/img]


BTW, I checked removing the NFB at V1b. Didn't make a significant difference on the relative levels of the various curves.

The difference between the two "fat PAB" mods isn't huge. Nevertheless it might be worth a try to test the "resistor" variant, because only the bottom end is increased and the mod reqires shorter wires for the connections (from mid boost switch or mid boost relay to PAB relay)...FWIW

Let me know what you think

Peace,

Markus

Great to see this in a visual format.
Until I converted my amp to the BM spec, i was doing this same thing. I first saw this in a post by 'yeayea'. I tried 500p, thne upped it to 910p. If you find yourself using mid boost AND PAB most of the time, then this mod will give you that sound with only one relay. At the time I was going for the fatest boost possible and this was it.

[markusw]

Great to see this in a visual format.
Until I converted my amp to the BM spec, i was doing this same thing. I first saw this in a post by 'yeayea'. I tried 500p, thne upped it to 910p. If you find yourself using mid boost AND PAB most of the time, then this mod will give you that sound with only one relay. At the time I was going for the fatest boost possible and this was it.

Just did the simulation with 910p. Exactly like you said, mid boost + PAB with one relay

[Bob-I]

I really appreciate the work going into this, but I have a request.

Could someone please tell me which line is which by position on the graph, I.E starting point of the upper line on the Y axis. I'm almost completley colorblind and I can't make this out.

Thx again.

[talbany]

Markus

Don't know what LT Spice is or does.. I assume it's similar to the tone stack calculator.. If so do you have a plugin or one that does the sweep for the skyliner.. I think someone posted one way back and remember it working well..This was several hard drive crashes ago..Also if anyone here still has it and wouldnt mind posting it again..Woul'd be greatly appreciated!! Thanks..
Tony VVT

[markusw]

I really appreciate the work going into this, but I have a request.

Could someone please tell me which line is which by position on the graph, I.E starting point of the upper line on the Y axis. I'm almost completley colorblind and I can't make this out.

Thx again.

Sure
Imagine a vertical line starting at the first tick on the X-axis (the 160 Hz tick).
Along this line starting at the bottom:
* three more or less identical curves: without boost
* normal PAB
* mid boost
* fat PAB, "resistor version" as in the 2nd schem
* fat PAB, "cap version" as in the 1st schem
* PAB plus mid boost

Hope this helps.
Let me know if this works for you.

Peace,

Markus

Edit: I added the legends to the image.
fatPAB1 is the "cap version", fatPAB2 the "resistor version"
[img:999:632]http://img.photobucket.com/albums/v21/m ... legend.jpg[/img]

[markusw]

Markus

Don't know what LT Spice is or does.. I assume it's similar to the tone stack calculator.. If so do you have a plugin or one that does the sweep for the skyliner.. I think someone posted one way back and remember it working well..This was several hard drive crashes ago..Also if anyone here still has it and wouldnt mind posting it again..Woul'd be greatly appreciated!! Thanks..
Tony VVT

It can be done with LTSpice easily. Will do it after coffee

Edit: here they are.
Skyliner stack, bass, treb and mid all at 50%. Log pots are Piher pots (i.e. about 10% taper). Frequency response is again shown at plate V1b this time without NFB loop.

[img1033]http://img.photobucket.com/albums/v21/m ... ssweep.jpg[/img]

[img1033]http://img.photobucket.com/albums/v21/m ... dsweep.jpg[/img]

[img1033]http://img.photobucket.com/albums/v21/m ... bsweep.jpg[/img]


Peace,

Markus

[markusw]

Again did a sweep for the 2200k resistor in the fat PAB "resistor version".
5M gives a slight increase of about 2.5 dB at 80Hz compared to normal PAB. Suppose a 5M trim pot really should be OK for testing....

[martin manning]

Nice work!

A couple of questions, if you don't mind:

How much of the circuit is modeled? Sounds like you have at least the whole clean section since you say no NFB is included. It would be nice to establish a reference case for the standard pre-amp circuit.

What are the setting of the switches (Bright, Mid, and R/J), Off, Off, Rock?

When you say all controls at 50%, I assume you mean each is swept full range while the others are kept at 50%... There is no legend, but I'm guessing the dark green line is the response with all controls at 50%?

And one nit, in a set of plots like these, it is a good idea to set all the scales the same.

Thanks,

MPM

[markusw]

Nice work!

A couple of questions, if you don't mind:

How much of the circuit is modeled? Sounds like you have at least the whole clean section since you say no NFB is included. It would be nice to establish a reference case for the standard pre-amp circuit.

What are the setting of the switches (Bright, Mid, and R/J), Off, Off, Rock?

When you say all controls at 50%, I assume you mean each is swept full range while the others are kept at 50%... There is no legend, but I'm guessing the dark green line is the response with all controls at 50%?

And one nit, in a set of plots like these, it is a good idea to set all the scales the same.

Thanks,

MPM

Thanks for your comments and questions
I knew that I forgot to add some details


Frequency response at V1b plate is shown, rock mode, bright off, mid boost off, deep off.
"I assume you mean each is swept full range while the others are kept at 50%..." exactly
And again yes, the dark green line is with all controls at 50%
Pots are swept from 0 to 100% in 10% increments.
You're totally right, it would be better to have the scales all the same.
Will try to motivate myself to do the plots again
Edit: the above three sweep plots should have the same scale now. Firefox shows the updated versions, Opera not. Probably some cache issue

[talbany]

It can be done with LTSpice easily. Will do it after coffee

Edit: here they are.
Skyliner stack, bass, treb and mid all at 50%. Log pots are Piher pots (i.e. about 10% taper). Frequency response is again shown at plate V1b this time without NFB loop.

Markus
Thanks for running those for me..Very Cool!!!


Tony VVT

[markusw]

Markus
Thanks for running those for me..Very Cool!!!


Tony VVT

You're most welcome

Peace,

Markus

[mdroberts1243]

It can be done with LTSpice easily. Will do it after coffee

Edit: here they are.
Skyliner stack, bass, treb and mid all at 50%. Log pots are Piher pots (i.e. about 10% taper). Frequency response is again shown at plate V1b this time without NFB loop.

Markus

Hi Markus,
I've been using ltspice quite a bit but haven't figured out how to do multiple plots like those very efficiently. How do you produce your sweeps?

Thanks,

[markusw]

Hi Markus,
I've been using ltspice quite a bit but haven't figured out how to do multiple plots like those very efficiently. How do you produce your sweeps?

Thanks,

You can use a step command.
e.g. you want to sweep the value of a cap.
You give the cap the value e.g. "{c1}" (only add the brackets)
Then you add a command line
".step param c1 list 100n 200n 500n" (this command just steps through these three values). Alternatively, you can use:
".step param c1 100n 500n 50n" (this command goes from 100n to 500n in 50n increments)

Same is basically true for pots. If you download the pot models from the LTSpice yahoo group these pots go from 0 to 1. Pot value is defined separately.
To make a pot sweep you would assign the pot travel e.g. the value "{treble}" and then again add a command line like
.step param treble 0 1 0.1 (would sweep from 0 to 100% in 10% increments).


One trick: if you want to keep one param fixed you previously used a step command for just change it to a comment. This way you can use it later without having to type it again. You will need another command then which defines the fixed value for the respective parameter.
e.g.
".param treble=0.5" or ".param c1= 350n".
If you have one "fixed parameter command" and one ".step param" command for the same parameter active the "fixed parameter command" is ignored.

Hope this helps

Peace,

Markus

[mdroberts1243]

Thanks Markus!

That's gonna help a lot.