Proco Rat Bass

[rocket88]

so i've been thinking about building a rat to use with my bass, as i've played a normal one with a bass and found it rather thin, and lacking the umph needed for bass. i know that johnk had made one, he called the bassface, and here's what he said he did

"i built this one last night( using the enclosure from my disappointing Retro Fuzz build). i didn't have any LM308N's yet (they'll be here later today) but it works perfectly with LM741's, LM301N's, and my favorite so far is a TL071CP. to me, it sounds a little more 'tubey with the TL071 (and it has less noise). i have absolutely NO oscillation issues at any setting . i made it stock as per the vero, but socketed the input, output and tone caps, the 560R & 47R resistors in the drive circuit, and i also socketed the clipping diodes. and since i'm using it for bass, i raised the values of the input and output caps to allow more low end thru the circuit, slightly changed the values of the drive circuit reistors, and am running 1N270 Germanium diodes instead of the 1N4148's. (i tried red leds, green leds, blue leds, yellow leds, and the 1N270's sound the best to me, even though it loses some level with them.


to make it even better suited for bass, i also added a blend control (split'n'blend) and am currently adding a Tillman FET (clean)booster so when the split'n'blend's balance control is in the center positon, the pedal will be at unity gain."

no i found the schematic, and looked at his board but i don't know what changing those resistors would do. i'm not looking at copying what he did, but since he tweaked it to work better with a bass i figure it would be a good starting point. i noticed though that he changed the 3.3uF cap on the bottom right, is that part of the tonestack? also, on his site he said he changed the coupling caps, but the only one i see is the 4.7uF in the middle, are there any others? any other suggestions as to modifications to make the rat better for a bass?

[sclemmer]

If someone wouldn't have ran JohnK off he would probably have already replied with his modifications...

[rocket88]

sclemmer. i don't want to get into the politics of what happened, it has nothing to do with me and i don't want/need to get involved, nor do i think anyone else should. i'm just looking for some suggestions on approaching something similar to what he did.

[IvIark]

If I was building this for bass it's the two high pass filters that I'd look at altering.  The 47R + 2u2 will attenuate frequencies below around 1.54khz at 6dB/octave, and the 560R + 4u7 will attenuate frequencies below 60hz even more aggressively at 12dB/octave.  Increasing those resistors will reduce the cutoff frequencies when the attenuation starts and so there'll be less meat of the usual bass frequencies taken out.

Increasing the input and output cap can alway help because more bass frequencies will get into the circuit in the first place, but it's those high pass filters that will be doing the most damage I suspect.

I'd socket those resistors and keep increasing the value until you get the response you want.

[rocket88]

awesome. as always thanks mark for the guidance. i didn't realize those resistors were part of the filters since it was mentioned that they were part of the drive circuit. i've finally got a handle on reading and understanding transistor based circuits, IC circuits still throw me for a loop. i'll give this one a shot and see how it goes.

BTW, i'm looking at the schematic, i see the tone pot comes off the clipping diodes after the 4.7uF cap, then connects to 1.5k resistor 3.3nF to ground, and 22nF cap into the JFET. i'm i right in that the 1.5k/3.3nF to ground is the tonestack, and could be tailored? and the 22nF is the coupling cap, and the only one in the circuit?

[IvIark]

It is part of the drive circuit, but also a high pass filter.  If you don't want to affect the gain at all then increase the capacitor values rather than the resistor.

The 3n3 and the 1K5 plus the tone pot value make up the tone stack.  So when fully counterclockwise you have the full pot value + 1K5 making 101.5K and the 3n3 giving you a cut off frequency of 475hz which will gradually cut high frequencies above that level.  When fully clockwise the pot is effectively bypassed so you have just the 1K5 resistor in circuit giving a cut off frequency of 32khz and so essentially letting all frequencies through.

[rocket88]

 thanks again mark. while i've finally learned enough to help others, i still have a lot to learn, so i can't thank you enough. so if i understand what you're saying about the 2 resistors in the drive circuit, they along with the two cap act as filters, and raising the value of the resistors will increase the amount of bass, so conversely lowering them will lower the amount of bass in the circuit. but, that can affect the amount of gain too, so if i want to keep the gain the same then i will need to also raise the 2 cap values along with the resistors?

[IvIark]

Yes they're simply RC high pass filters and so let all high frequencies through, but attenuate low frequencies gradually starting at frequency the resistor and capacitor combination calculate to.  You can use this to calculate the frequency:

http://www.muzique.com/schem/filter.htm

So increasing the cap or resistor will lower the cutoff frequency so that more low end gets through before attenuation starts.  This graph should make it clear how it works

[rocket88]

Awesome. That's exactly what I thought. I'm going to socket the resistors so I can get it to where I want.