Recipe for a moderate and FAT PAB circuit

[dogears]

Bingo...

Another thing i thought of would be to add a small cap that switches in parallel to the treble cap. smaller than the usual mid boost cap to keep it from getting too extreme...of course that is getting you mostly more low mids too.

[yeahyeah]

But how would you do that without adding another relay??? I guess a 3PDT or 4PDT would work but there has to be an easier way.

I'd say 500pf would do it, but that is just a guess, luckily ceramics have allot of values to choose from.

[dogears]

You could just change the midboost value. Then make it relay and footswitchable. You could make a switch that latches the boosts....

[stelligan]

https://tubeamparchive.com/viewtopic.php?t=3067

Something like these switches to gang PAB along with Mid boost is going to get me where I need to be I think. Or just always ganging them together at the amp. I am still playing with mid boost cap values. I'd like to add that the clip is actually much darker than I am hearing in the room. It's a 57 mic on a Delta Pro 12A in a 2 x 12 cab. The other speaker is a much brighter warehouse CL80 clone that has sounded good with nearly every amp I've tried it in. It's a good pair. The Delta Pro by itself does not have any high end sparkle - good for the meat, but no hot sauce!

[yeahyeah]

You could just change the midboost value. Then make it relay and footswitchable. You could make a switch that latches the boosts....

OR you could keep the midboost as it is and do this without adding any other parts besides the extra cap.

This slightly different wiring of the switch allows the same connections to be made AND makes use of one of the unused lugs on the PAB relay and switches in a cap.

Now before you go all crazy about the cap being on the opposite side of the treble pot, I've simulated it and there is no audible tonal change when the treble pot is turned. It does change a little but only half a dB difference across the ENTIRE sweep.

The only problem I can think of with this is there may be a slight pop caused by switching the cap in and out. easy to fix though.

[yeahyeah]

~Frequency curves using above schem. PAB on.

[yeahyeah]

here is the total change in tone throughout the range of the treble pot.

As you can see its very minimal, less than 1/2 of a dB

[odourboy]

That looks pretty interesting too Mr. yeahyeah. Have you tried this or just modelled it? There's something to be said for maintaining the sanctity of the mid boost.

Do the 4M7 resistors affect the response curve with that extra cap (versus 22Ms which I'm running)?

[dogears]

Good stuff YeahYeah, although I might add that Glaswerks modeled the PAB in SPICE and it looks nothing like yours. Maybe Gary will chime in, but your knee is way too high on all counts. Assuming Gary is correct.

[yeahyeah]

Dogears, would you mind posting the glaswerks frequency curve you speak of?

Which knee are you talking about? Is it too high in regards to dB levels or frequency? Where do you think it "should" be? Keep in mind that the X and Y axis on my diagrams are probably scaled differently as well. On my diagram the input signal is 1v peak-to-peak with 38k source impedance.

The pink line was modeled of off the exact skyline schematics in the files section, The only difference is that I used 4.7M resistors instead of 22M on the switch. This lowers the amount of boost and fattens up the bass while still keeping the overall character of the normal PAB...

odourboy,
I haven't tried this arrangement with the extra cap on the PAB yet, but I'll be putting it in my next build.

I'll post another diagram of the PAB with 22M, 10M and 4.7M resistors across the switch so you can see the difference.

[odourboy]

Good stuff YeahYeah, although I might add that Glaswerks modeled the PAB in SPICE and it looks nothing like yours. Maybe Gary will chime in, but your knee is way too high on all counts. Assuming Gary is correct.

Maybe we should clarify exactly what's being modeled by yeahyeah versus Gary. My curves don't look like these either - but I model the whole preamp with tone controls set to 'typical' settings. I assmed yeahyeah is just modeling some portion of the tone stack.

That last set of curves indicates that the response don't deviate significantly above 60 Hz.... which is encouraging for me. But it does make me curious about what's being modeled - shouldn't we be seeing differing boost across the board from the different resistor values? (Of course, this is an assumption on my part based on discussions on the boar - I've never played with or modeled different PAB resistors myself.)

[yeahyeah]

I'm only modeling the tone stack, not the whole preamp. I just run the signal straight into the tonestack and take the frequency curve from the wiper of the treble pot.