Rusty Box/Traynor TS50B - are these caps backwards? + any tips

[MrStab]

hi,

a friend has asked me to make them a Rusty Box, which i understand is a rip-off honourable tribute to the Traynor TS-50B's preamp section. i've done the first draft of a vero layout (cool tip: forget to label all the resistors, then you'll HAVE to double-check it!).

just wondering: there are a few polarised caps in this schematic which seem backwards (for example, the 4u7 near "Line out"). i understand the voltage system on this isn't the same as your average stompbox, so apologies if i'm asking something dumb here.


(full-size here: http://i.imgur.com/zVRVdWZ.png)

i've a long way to go - need to read up more on charge pumps and the whole bipolar thing etc. - but does anyone have any tips on this circuit for a kinda-but-not-quite n00b? would be much appreciated. the Rusty Box seems to only have one input as opposed to the high & low - how would i go about that?

cheers!

[psychedelicfish]

I would stick with the original cap orientation, if you look at the 4u7 cap you mentioned there is an 8k2 resistor to -15V on its negative end. Just looking at the schematic, it would be reasonably simple to convert to using a single supply by connect almost all the preamp GNDs to a 1/2 V+. I'll draw up a schematic some time.

[psychedelicfish]

Here's a quick rough schematic:

If anyone sees any errors, please let me know and I'll fix them. I wonder how this would sound with "normal" guitar?

[slacker]

That looks good to me, you've drawn it as though it's using the LO input if you want HI remove the resistor to ground after the input resistor.

[MrStab]

it would be reasonably simple to convert to using a single supply by connect almost all the preamp GNDs to a 1/2 V+. I'll draw up a schematic some time.

wow, thanks, that makes things a lot simpler! i'm a bit confused - some (well, most) -15V have become VBias, where as others (ie. the bottom of the diodes just before the opamp) have become ground. could you give me a rough explanation of the pattern there? sorry to be annoying! oh - and the FET has disappeared. was that an unnecessary component?

my plan is to go over my vero layout part by part and convert to your schematic. i'm guessing as it's all just a case of reference, a single supply would still have all the same tonal characteristics of the bipolar, so long as it's 15V, right?

cheers!

[psychedelicfish]

All points that were grounded and necessary for biasing have been connected to VB.

[psychedelicfish]

The FET was used as an electronic switch so I replaced it with a mechanical one to simplify things.

[MrStab]

All points that were grounded and necessary for biasing have been connected to VB.

so what you're saying is that some -15V points become vbias, while others become ground? my minuscule brain would've just expected one similar rule of change across the board, which is why i think i got confused.

a mechanical switch would make for a lot less hassle, good call.

[psychedelicfish]

-15V has become ground, ground has become VB, and +15V has become +9V

[psychedelicfish]

It should sound the same, but as we are running it on 1/3 of the supply it will have 3x the distortion

[MrStab]

-15V has become ground, ground has become VB, and +15V has become +9V

ahhh okay, i get you. i could still set up a charge pump (or 15V supply) to get 15V, right? would that even be necessary? (i'm aware 15V isn't the same as to 15 to -15, but still). there's a user on another forum somewhere (can't remember the link right now) who went down that route - not sure if he used a bipolar supply or not. too late for that now, though - i've just redrawn the whole layout lol.

just a few questions, if that's cool: in your schematic, vbias connects to the middle PNP on the furthest-right, whereas that connection was originally on the bottom of that column. doesn't matter either way, right? also, at the very end, where there's a volume pot followed by a cap and resistor to ground, would there be any harm in putting that pot after the cap? and could i just use one 18k resistor from V+ with jumpers branching off of it to the collectors of the PNPs, or would i need to use one for each? my guess is one would be fine

thanks a lot for your help, it's really appreciated!

[MrStab]

progress so far, if anyone wants to point out any glaring errors (and i'm sure there are a few)
Updated (but probably still wrong! despite the update, i just noticed 6n8, 1M and jumper just before NPN were a bit out of place, fixed). yellow indicates vbias, black is ground, blue is general jumpers.

[psychedelicfish]

That looks good to me, you've drawn it as though it's using the LO input if you want HI remove the resistor to ground after the input resistor.

I added the resistor to ground after the input resistor to prevent switch popping. I probably should have put it before the other resistor so I don't get a voltage divider happening.

I'll fix up the schematic a bit and add some component values.

[psychedelicfish]

Here's an updated schematic:

R6, R8, C8 and C14 can be connected to either VB or ground depending on what's most convenient for your layout.

[MrStab]

thanks a lot Edward, i'll cross-reference that later. i didn't even notice the pulldown resistor in your first schem! there's fairly easy access to ground from there.

in the original schematic, there's the 10k, the 68n, then another 10k (all in series) - is that 2nd 10k before the 68n still meant to be there? 

besides that, i'm up to the NPN in terms of double-checking my layout.

[psychedelicfish]

in the original schematic, there's the 10k, the 68n, then another 10k (all in series) - is that 2nd 10k before the 68n still meant to be there? 

Good catch! You could probably get away with using one 20k (22k) instead of the two 10k resistors. Before or after the capacitor is your choice, whatever's best for your layout

[MrStab]

i have room for two 10ks, guess i'll just go with that because i'm lazy. i only have 22ks, and that extra 2k could drastically affect the cork-like odour! lol

you seem to have reversed the very last (4u7) cap, but not the one right after the NPN, is that right? i'm used to using transistors as nubish switches , forgive the stupidity.

i noticed a mistake in my layout: vbias instead of V+ was going to the NPN collector, so i took care of that. i'm not looking forward to double-checking the jumper hell that is the gain stage (if that's what that is).

why'd you think they might've put 3x18k from V+ to each PNP pair instead of just one? could it have been a layout thing, or would there be some other advantage to that?

cheers

[duck_arse]

I was just looking at the original ciruit out of interest. what on earth is that stack of 6 transistors doing, top right? band-splitting tone shaping w/ individual buffers?

[MrStab]

^ that makes the most sense, now i think about it - maybe that's why the circuit is so sought-after by bassists.

[MrStab]

i know it's unrealistic to expect anyone to go through entire vero layouts, but reckon anyone could check the part with the transistors at least? no worries if it's too much hassle tho. still unsure about the orientation of those 4.7uFs...