Building / modding a ProCo Rat for bass

[Widows]

Hi folks, a friend of mine has asked me to build him a bass Rat. Now I know that increasing the value of the i/p and o/p caps widens the low end freq response going into and out of the circuit.....but what I'm wondering is if I increased just the value of the o/p cap would that add more low end in the distorted output signal or would it add clean low end? Or alternatively, if I just increased the value of the i/p cap and left the o/p cap as standard, would the o/p cap just cut out the added low end created by the larger i/p cap?

Hope that makes some sort of sense. Any help, info, links etc would be most appreciated

Cheers

[midwayfair]

First, have you tried a RAT on bass? Many people like it just fine with no mods.

Second, please post a schematic when requesting help on mods. Everyone needs to know exactly what version of a circuit you're working from to give specific advice.

would that add more low end in the distorted output signal or would it add clean low end?

Well, think about it: how could it add "clean" low-end? There's only one path for the signal to follow, and that signal gets distorted.

if I just increased the value of the i/p cap and left the o/p cap as standard, would the o/p cap just cut out the added low end created by the larger i/p cap?

Not necessarily. You would need to calculate the output frequency cutoff and input frequency cutoff to determine whether there is a mismatch.
Here's a calculator for that: http://sim.okawa-denshi.jp/en/CRhikeisan.htm

Finally, here's a thread on Madbean about doing the same thing, though keep in mind my suggestions there are not verified and there should be plenty of people here able to help:
http://www.madbeanpedals.com/forum/index.php?topic=7643.0

[Mark Hammer]

The stock RAT has plenty of bottom.  The more pressing matter is whether there is anything that needs attending to at the top.

For instance, the Filter control is a single-pole lowpass filter, whose rolloff point ranges from 475hz to well over 30khz.  Clearly, a big chunk of that range is moot for a bass.  It might be more useful to exchange the 100k Tone pot for 10k (for a narrower range) and excahnge the .0033uf cap for .047uf.  That would yield a rolloff range from a little under 300hz to a little over 2.2khz.  Note that filter slope is fairly shallow, so some buzz will still come through, but it will be usable buzz.  That may be more suitable for bass.

The other aspect is the differential gain for the high end.  You will see that there are TWO resistor/capacitor networks in parallel from pin 3 of the LM308.  The 560R/4u7 network provides the "default" gain for the entire spectrum.  The 47R/2u2 network provides extra gain for content above 1.5khz.  Of course, the role that content plays for a guitar is different than the role is plays for bass.  Would simply increasing the 2u2 cap up to 3u3 (dropping the boost range down to 1khz and above) do anything useful?  I leave that up to your ears and your buddy's, but it is worth considering.

Apart from that, you should be good to go.

[Widows]

Thanks folks, I'll have a look into all of the above....and yeah I realise how daft that qeustion about clean low end sounds now i re-read it haha.

The schematic I'm working from is this one:
http://analogguru.an.ohost.de/193/schematics/Proco_TheRat-1986.gif

and the vero layout I'm using is this one:
http://tagboardeffects.blogspot.co.uk/2012/01/proco-rat.html

[ashcat_lt]

If you're looking for "clean low end" there's always the Gruntbox.  You can build the distorted side about exactly like a Rat.  That's kinda what I did for my bassist.

Kinda hoping Mark will post a link to the most recent Gruntbox.  Here's a link to a relatively coherent post re: my 1337Drive

[GGBB]

Just note that that schematic isn't 100% accurate.  R2 shouldn't be there and R10 should be 1k6, although neither will cause any problems.  Also, as it was based on a 1986 model, it's the later version of the original RAT not the 'early version'.

[Widows]

...excahnge the .0033uf cap for .047uf.  That would yield a rolloff range from a little under 300hz to a little over 2.2khz.  Note that filter slope is fairly shallow, so some buzz will still come through, but it will be usable buzz.  That may be more suitable for bass.

As it happens I didn't have any 3.3nF caps so I used a 4.7nF instead...although on re-reading your post it looks like you suggest changing it to a 47nF, I haven't yet replaced the 100k pot with a 10k though. Will give that a try today and see how it affects the sound.

The other aspect is the differential gain for the high end. The 560R/4u7 network provides the "default" gain for the entire spectrum.  The 47R/2u2 network provides extra gain for content above 1.5khz.  

I've tried the mod relating to the gain section (560R/4u7 + 47R/2u2 network) and removed the socketed 47R resistor so as to remove the added boost to freqs above 1.5kHz. I A/B'd this pedal with another Rat that i built exactly the same but without the 47R mod and it definitely removes some of the brightness from the sound without the 47R resistor, smoothing out the sound and giving a more balanced boost across the freq range.

Think I'll replace the 47R and socket the 2u2 cap and try different (higher) values to affect the freq range boosted by the 47R/2u2 network as you mentioned to see if that helps voice the pedal more effectively, and I may also drop the value of the 560R resistor to something like 470R, I'm assuming that would give a little more gain to the overall signal?

Having read some more about it on other forums/posts, I've noticed a few people say that the stock 22nF input/output caps already let in/out the entire audible freq range anyway so increasing their values would be a bit pointless as it appears that if you were to change their values, you would reduce them to knock out some low end. Is this the case?

Aside from the other ideas mentioned already, is there anything else I should be considering when tweaking the circuit to be more suitable for bass?

Much appreciated chaps

[GGBB]

Those two resistor/cap pairs are a big part of what defines the RAT sound, so when you play around with them and stray too much from the original design (such as removing the 47R/2u2 pair - the Ruetz mod) you end up with a very different sounding beast.  Note that changing the resistor will affect the gain whereas changing the cap doesn't.  If you just want to add a little more kick in the bottom end, try changing the other side but in tandem so as to not change the knee frequency add another 560R in parallel with the existing one and do the same with the 4u7 cap.  Or for more effect use 270R/10u in parallel.  These are easy mods to existing RATs and can be put on a switch: http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/RATBassBoost.gif.html.

[Widows]

Cool mod, thanks for the link  I'll have a go at that when I buy some more switches.

[GGBB]

If you want to use the bass boost mod permanently - replace the 560/4u7 pair with either 270/10u or 180/15u.

[Widows]

Righty, I'm back after a bit of a hiatus. Have been doing some more tinkering with this pedal and have made some mods after a bit of testing with the guy who I'm building it for.

Mark Hammer: I played around with the differential gain network (560R+4u7 / 47R+2u2) and upping the 2u2 to 3u3 was a bit much so I swapped it back to 2u2. However I swapped the 3n3 at C9 to 47n as you mentioned and that brought a nice amount of extra bass in there. I also upped the 560R in the differential gain network to 1K to reduce the 'default' gain a little and swapped the diodes for a pair of 3mm flat topped red LEDs. This combination seemed to nail the sound he was after.

The only other thing I'm having a problem with the 100K log pot used for the filter. I've wired it backwards (deliberately) so it works as a tone knob instead. Unfortunately with it being a log pot, it does barely anything until almost 3/4 rotation and then dials in a hell of a lot of high end very quickly which doesn't allow for any real control over the part of the sound I'd like to be able to have the most control over. Would switching this out for a linear pot of the same value solve the problem?

I'd also like to be able to have less high end in the signal with the tone control all the way up, as it gets a little overpowering in the last little bit before the pot is fully turned. Would a diffrent value pot solve the problem, or would I have to re-examine the circuit again and make a change somewhere?

Apologies for the possibly silly questions, I'm learning as I go with this one Cheers

[GGBB]

The only other thing I'm having a problem with the 100K log pot used for the filter. I've wired it backwards (deliberately) so it works as a tone knob instead. Unfortunately with it being a log pot, it does barely anything until almost 3/4 rotation and then dials in a hell of a lot of high end very quickly which doesn't allow for any real control over the part of the sound I'd like to be able to have the most control over. Would switching this out for a linear pot of the same value solve the problem?

Linear should work better.

I'd also like to be able to have less high end in the signal with the tone control all the way up, as it gets a little overpowering in the last little bit before the pot is fully turned. Would a diffrent value pot solve the problem, or would I have to re-examine the circuit again and make a change somewhere?

A different pot value won't make any difference because at full treble the pot is shorted.  Instead, try raising either the 1.6K resistor or the .0033 cap.

[Widows]

Thanks GGBB :-D

Yeah figured as much about the pot but wanted to be sure. I've switched the filter/tone pot for a 10K linear and it's much easier to dial in the right tone now.

I have the board hooked up to my breakout box now as it happens so I'll go n switch out that resistor now. I'm assuming you mean the resistor at R10 after the filter pot in the schematic, and that this should lower the level of the top end freqs if I increase the value of R10?

Will try values of 2K, 2.2K, and 2.7K as that's what I have in my drawer, I reckon anything over that might be a bit much. Let's see.......

[GGBB]

Thanks GGBB :-D

Yeah figured as much about the pot but wanted to be sure. I've switched the filter/tone pot for a 10K linear and it's much easier to dial in the right tone now.

I have the board hooked up to my breakout box now as it happens so I'll go n switch out that resistor now. I'm assuming you mean the resistor at R10 after the filter pot in the schematic, and that this should lower the level of the top end freqs if I increase the value of R10?

Will try values of 2K, 2.2K, and 2.7K as that's what I have in my drawer, I reckon anything over that might be a bit much. Let's see.......

Yes - R10.  Raising this is effectively the same as turning down the treble knob, but at the same time it gives you a little more range on the darker end of the pot as well.  Since you have it open, the best thing to do would be to jumper R10 and then dial in the max treble setting with the pot and measure this value on the pot and use it (or next step lower) for the value of R10.

[Widows]

ok cool, well I have a vero board of the Rat layout with all the components socketed so I can chop n change stuff at will. I was a bit pushed for time last night so I had the idea to simply put a linear pot across the sockets where R10 should be rather than incrementally increasing the value of R10 n trying to note the individual differences through a crappy practice amp. This allowed me to fine tune the high end roll-off after the tone/filter stage giving much more control over the 'bite' of the pedal's sound. The guy I'm building it for likes the idea of the 4th control so I think I'll keep it this way, unless anyone has any reasons why I really shouldn't.

Now I'm pretty much at the point of boxing it up, I could really use some recommendations for places to get cheap diecast aluminium enclosures like Hammond/Eddystone as the ones stocked by maplin are a bit on the expensive side.

Thank you all very much for your help, you've been an invaluable resource on this project 

[GGBB]

I can't help you with enclosure sources other than to say that the Asian online dealers and eBay tend to be the cheapest.

There's no problem with replacing R10 with a pot - the tone/filter pot and R10 are really just one resistor to the circuit so whether you replace them both with just one pot or two pots doesn't matter except that you typically want to have some resistance in there otherwise the pot doesn't change the sound much at the brighter settings. You might want to consider making R10 a trim pot that is adjusted internally so that he can set it and forget it.  Also, most people don't use a lot of the darker settings on the RAT so if that's the case here then you might want to consider a 50K pot and a different value C9 cap to get a more usable range.  See my post here for some ideas.  If it were mine I'd look at a combination something like pot=25K, C9=0.0082 or 0.0068, and R10 about 1.2K.

[Widows]

Hi everyone, I've finally got this bass rat finished. I had the circuit done months ago but I've been waiting on a friend to sort a design for the front.....well, I've got it back, art n all and it's now boxed, teseted and ready to go to the guy who ordered it. Sounds great on my little practice amp, and makes a pretty decent guitar pedal too!

Here's a pic of the finished product!


Uploaded with ImageShack.us

[GGBB]

That looks great!  "F" stands for

[Widows]

F is for "fine tune". I replaced the R10 resistor after the filter/tone knob in the schematic with a 250k lin pot to allow some fine tuning of the tone. It works pretty well and allows you to get some interesting sounds that I just couldn't get from the stock version.

I've put a basic vid on Youtube (before I bought the knobs for it sadly) but you can hear it/watch the vid here: http://youtu.be/IkhiNR-v3uY <-- give it a few mins as it's still uploading

Here's a shot with the knobs on it that came through the post today:





Cheers

[Gus]

After reading this thread and others I did a circuit design sim
http://www.diystompboxes.com/smfforum/index.php?topic=103110.0

Not built but the sim looks interesting