Pharaoh Coupling Caps

[Hexjibber]

Hi guys,

Just wondering if anyone could clarify, I built a Pharaoh on the King Tut PCB and I have to admit it didn't quite have as much, well balls, as I was expecting! Just came across another thread which mentions that the coupling caps are 470n, on the King Tut build doc they are stated as 47n (C4, C6, C7, C9 & C12), it looks as though the King Tut values are from the Kit Rae schem whereas every other version I've found has them as 470n, can anyone shed any light on this? And I suppose most of all, would this account for the slightly less beefy sound I'm getting when compared to vids of the real thing?

Cheers!
Graham

[BaklavaMetal]

looking at |\/|ark's vero layout there are no 47n caps, only  470n, and substituting 470 with 47 would result in less bass.
explanation: coupling caps effectivly form a high pass filter. lower the cap - higher the cutoff frequency, thus less bass

[cooder]

AFAIK the coupling caps in original Pharaoh are indeed 470n and that should make a real difference in oummph and bass content.

[Hexjibber]

^^ Nice one guys! Have ordered some 470n caps so hopefully will give me that doomy growl I'm looking for! 

[selfdestroyer]

Looking forward to your compare. If you get a chance can you do a quick demo of a before and after? Mine has 47n's in it and wondered the same for a more chunky low end.

Thanks
Cody

[midwayfair]

Don't change the 47nF caps in the feedback loops of Q2 and Q3 -- increasing those will clip more bass frequencies. Just the ones between each stage.

[selfdestroyer]

Don't change the 47nF caps in the feedback loops of Q2 and Q3 -- increasing those will clip more bass frequencies. Just the ones between each stage.

That makes sense. Thanks Jon

Cody

[lincolnic]

Don't change the 47nF caps in the feedback loops of Q2 and Q3 -- increasing those will clip more bass frequencies. Just the ones between each stage.

For those of us playing along at home, would that be C6 and C9?

(Build doc with schematic here for reference: http://www.rullywow.com/build_docs/King%20Tut%20Build%20Doc%20v1.1.pdf)

[selfdestroyer]

Don't change the 47nF caps in the feedback loops of Q2 and Q3 -- increasing those will clip more bass frequencies. Just the ones between each stage.

For those of us playing along at home, would that be C6 and C9?

(Build doc with schematic here for reference: http://www.rullywow.com/build_docs/King%20Tut%20Build%20Doc%20v1.1.pdf)

Correct.

Cody

[cooder]

That's correct I think.
Edit: Cody beat me to it...

[Hexjibber]

Ok cool will leave the Q2 & Q3 caps as is, thanks Jon.

Can try and do a before and after demo if you want Cody, only got an iPhone to record with and a 5watt solid state combo amp (my testing rig) though, not sure the full difference will be apparent until I get it through my stack at which point the caps will already be changed unless I socket them, would still only be through an iPhone though! Any good to you?

Could really do with sorting my recording set up (or lack of) out!

[bordonbert]

Don't know if this has been touched on elsewhere but if I were you I would change these one at a time and listen after each individual change.

If you are changing from 47nF to 470nF you are effectively changing the frequency of the cutoff of the filter by a factor of 10x.  i.e. as an example, a filter originally set to break at 400Hz will break at 40Hz.  For this filter the frequencies from 400Hz down to 40Hz will be lifted by an increasing amount up to 20dB, that's 10x !  Each filter you change will have that effect separately and they all add up.  If you change 3 filters by that factor of 10 you have lifted the response of overlapping frequencies in their ranges by an extremely large amount.  That may be too much in one step.

In real life it won't be quite as vicious as that but there is another factor here.  With filters which go down very low you are going to allow a lot more hum and low frequency noise through.  This is not a hifi application where we want extended bass, the best strategy is to actually set the filters to act a little below the open bottom string on your guitar, which is just above 82Hz.  For 82Hz, a 47nF resistor would need to see 43k.  This is certainly within the right ball park of the stage design used so not much change to capacitors is required to lift your bass response noticeably.

The schematic I have of the Pharoah shows exactly the same gain stage for each of three stages.  Hence the filter effect of each of the coupling caps is set at almost exactly the same point.  That's a big potential lift!  If you have capacitors available you could try just increasing one of the caps by doubling it at each change and gauge the difference in sound each time.  You can make sure to only go as far as you prefer then.  Each doubling of the cap value halves the frequency of that filter.  You will tailor the sound exactly to your own requirements this way.

It is entirely possible to change the filters to set them at different break points to achieve different degrees of lift across that frequency range.  In fact this is often a good thing as it improves transient response and stability in some circuits, (though probably not of interest here).

Also, do you have the unit input resistor 39k/390k High/Low option in your clone?  If not what is the value of your input resistor and first coupling cap?

[Hexjibber]

Thanks for the detailed reply man! I think to be honest as I'm just going for a straight up clone of the Pharaoh I will just stick to the stock values. Also I tune in Bb standard so am looking for a lower frequency response by approx. 30Hz to that of standard tuning.

During my trawl of the internet I found this shot of the Pharaoh board, you can clearly see 6 larger caps which I think is logical to assume are all 470n, if you count up the remaining caps as well then it all matches up with the rest of the King Tut bom.



Will let you know my results when I get chance to swap them out!

Cheers!
Graham

[midwayfair]

If you are changing from 47nF to 470nF you are effectively changing the frequency of the cutoff of the filter by a factor of 10x.  i.e. as an example, a filter originally set to break at 400Hz will break at 40Hz.  For this filter the frequencies from 400Hz down to 40Hz will be lifted by an increasing amount up to 20dB, that's 10x !

Your numbers are really exaggerating the effect this change will have, and you're also up an entire decade from where the cutoffs actually are, which really makes a big difference in this case. The 3dB cutoff of each stage with the 47nF caps is only 41Hz (~82K [470K/100K] and 47nF). In the original BMP, the 3dB cutoff was 19Hz -- "no" cut for anyone except hi-fi folks, and that was achieved with a 100nF cap. The signal is only down a couple decibels on the low E of the guitar, so even a full decade cut is not that remarkable.

I agree with you that increasing the coupling caps is going to increase the hum in the circuit, though. It's possible that a better mod would be to increase the treble cut(s) at various places in the circuit.

A full decade cut is still only 6dB per octave.

[rullywowr]

Not enough balls?  Never heard that complaint before.... 

Just curious how yours compares to this demo here of the King Tut.

PCB and Schematic Link:
http://www.rullywow.com/product/king-tut-fuzz-pharoh-clone-pcb/