Dr Boogey question

[bancika]

Hi,
I found "other" boogey emulator, made by Pablo De Luca (aka Gringo) and there are few components with different values
R15 39k / 3k9*
R21 100k / 10k*
R28 25k L / 100k L* Pres
which ones are better?
Tnx

[Gringo]

There are no "better" ones. You can use the layout to build either version of the mesa premp emulation. The *'s show which values are different between versions. The rest of the parts share the same values.

[Bucksears]

I tried the 'other' values and they sound awful IMHO. Even with a 250K pot, there is way too much gain with all J201's. There is no mention of it anywhere, but I'm assuming on the 'other' schem that you are supposed to adjust the pots to 1/2 of the voltage (to 4.5v). I had a problem with Q3 and couldn't get it below 7v no matter how many J201s I tried. It's a pain to swap out these parts without taking the whole thing apart, but I'm going to go back to the original Dr. Boogey values.
- Buck

[bancika]

Thanks, I'll go with Dr Boogey values then

[petemoore]

  No one knows what the characteristics of the transistors used are, even if all are of the exact same brand/type, there can be differences in gain, bias, sound.
  Trimpots for Drains certainly does help to get a circuit up and biased reasonably.
  That and sockets, chances are if you have enough transistors to interview for positions in the Jfets circuit, the chances for bias and 'better' everything increases to a 'satisfactory' amount or something.
  To try your Jfet circuit through your airears'n ampn guitar's'n *whatever, to try it out fully, or some very similar topology of circuit, breadboard or even messing around with values other than just drains and Q gains may prove to increase mileage...I mean, since you're going to all that trouble, and since these circuits, judging from the feedback [of the typed, 'why does/why doesn't my' kind] shows you'll plain, flat out benefit from either breadboarding, or building some type of moddery into your board so that the stages containing the active components can be tweeked to work well for you.
  Depending on the Jfet used, combined with the shooting match set...Long Story...the target moves...to look at you and yours from the circuits point of view, depending on guitar and amp and what you want...what transistors are you putting in me and what are they doing there...
  another approach would be to do a report on each individual Jfet, comprehensively creating a Data Sheet with much more precise data than is available, then use these paramaters in *tube amp designs morphed to Jfet @9V technology and hit targets all day that way. The Target being '*', I'm not thouroughly convinced which transistor works exactly like which tube...ie no one can know that...neither am I convinced that what the tube likes, exactly, is what your Jfet will like, exactly..all the topologies being the same, I would guess your Jfet might 'like' a bit of individualized treatment, considering it is being applied in a circuit designed around tubes.
  ...tips
  Use Q sockets when building Jfets circuits onto Tube amp topologies, trimpot each *drain.
  Allow room or lead access [above board or...] so that bypass caps or even *source resistors values [*bases too?] can fairly easily be temporarily or permanently altered...
  testclips on a pot
  hookended lead of <resistor> flexwire to testclip
  resistor hooked right onto above board access lead [tricky]
  Drain Resistor socket...use pots wires into socket temporarily [pot used as Variable Resistor], to find drain resistance value @ decent bias, then stick fixed resistor into socket...when you run out of trimpots...
  These are not tubes, so there will be a departure from exactly what a tube 'does, "there". Stringing Jfets gain stages'll get distortion, more stages or higher gain per stage [especially toward the input of a multigain stage circuit] renders...Gain.
  These tube amp schematics are a great place to start with the voicing and structure of the gain...course...I've got tube amps...and, well, I've tried messing with the exact voicing, and every time I did I found the re-voiced circuit to be more to my liking, fairly hot bias on them helps also.
  Thing is with the Jfets @ a safe ~9vdc, is you can do anything you want...they do sound bad misbiased, but other than that you have a pallete from which to work the magic into a more figured out manner than just slappin' er together and that's that.
  Whether or not the values on the schematic are truly 'optimal' for you is a question only answered by considered, evaluation.
  Time consuming? Yes and No.
  Yes, it will take more time to mess around with the values [and swap actives and re-bias] on your circuit after populating it.
  No, you might not have to build another [>enter number here between 1-39] Jfet circuits, looking for the one that is 'right', possible passing very near to the right, but right by it because 'the target moved'.
  By target, I mean what the soundclip sounds like by the person who schematicized and build the Jfet circuit and tweeked it, hopefully trying out transistors or getting lucky.
  Pretty much everything counts. There is alot of tunability in multigain Jfet topologies, from the input cap to the drainpots to the source bypass caps, and the Jfets themselves, some of them kind of fly off the map compared to others, refusing to bias well under 'normal' conditions.
  It's easy to go overboard also, with this one there looks to be lots of tonal voicing variability, gain structurings to be had, just getting decent bias and trying out different gain Jfets in it should find you in a very nice ballpark.