Home Forums GuitarPCB Build Support Adjusting BIAS

This topic contains 5 replies, has 3 voices, and was last updated by  KeneK 2 weeks ago.

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  • #6812

    KeneK
    Participant

    Hello,  I am an intermediate builder and I want to build the ’69 “Stones Drive’ and the ‘Ratt Deluxe’.  Both have a trimmer to adjust the BIAS.

    Question 1) The ’69 Stones Drive’ instructions say ,  “…the drain voltage should always be about half of the supply voltage (or 4.5 volts)…”   Please , between which 2 points on the PCB do I measure this voltage?

    Question 2) Concerning TR1 in the ‘Ratt Deluxe’ ; if I install the stock R7 resistor (100R) , and the stock TR1 trimmer (100R) , then between which points on the PCB do I measure the voltage? – and is the trimmer then supposed to be adjusted to 4.5 volts between these points?

    Question 3) The ‘Ratt Deluxe’ kit comes with a LM308. Are the following 3 chips 100% interchangeable with the LM308?  >  OP07 , NE5534AP , TL081 ?

    Thank you very much,  KeneK

    #6815

    wilkie1
    Moderator
    1.  Looking at the schematic you can see that the lug 1 (identified by a dot) of TR1 connects directly to the Drain of Q5.  To measure the voltage, place your red probe on lug 1 and the black probe on any ground point.
    2. On the Ratt, TR1 is not setting bias of a transistor.  It is changing the frequency response of the IC.  There is no need to measure a voltage here.  The instructions tell you that this is part of the Ruetz Mod.
    3. Since you have a LM308 you should use that for best results.  While other opamps may function in this circuit, they all will have some values that vary from the LM308.  You can look up the data sheets for the others and compare the electrical characteristics, but that may just confuse you further.  Or, you can just plug them in and see if they wlll work.  If the pin pattern is the same, they will probably function.  “Your results may vary”.
    #6817

    Cybercow
    Moderator

    Kenner – Wilkie1 has you covered well on your questions. I just want to add some info on the opamp used in the Rat. The original circuit has always used the 308 (LM308, uA308, etc. – plastic DIP or metal can). The key to the unique tone the Rat delivers is the low slew rate of that device. Based on my personal extensive single opamp testing in a Rat circuit, the closest near equivalent to the 308 is the OP07. Next would be the 301, the 709 and then the 741. At least, that’s order in nearness to the results of using a 308 in the Rat. My personal observation in a Rat circuit is to select an opamp with a slew-rate close to that of the 308; which is 0.3V/µs. Most other single opamps will work in a Rat circuit, but the sound seems to noticeably change as the slew-rate of the chosen opamp rises. I suggest, agreeing with Wilkie1, that the 308 be used. But once you have the circuit built, (and hopefully you using a socket for the opamp), try the 308 and listen to it with various control settings. Then swap out the 308 for the TL081 and I’m pretty sure you will hear a difference. Ultimately, you can then decide which opamp you want to keep in your Rat build.

    Over the years I’ve collected info on single opamps and have created the following list of PIN-FOR-PIN replacements (swappable) single opamps . . . .

    Single Op Amp Equivalents:

    • LM201 – Slew = variable
    • LM301 – Slew = variable
    • LM308 – Slew = 0.3V/µs
    • LM709 – Slew = 0.25/µs
    • LM741 – Slew = 0.5/µs
    • LM748 -<span class=”Apple-converted-space”> </span>
    • NE5534 – Slew = 13V/µs
    • TL061 – Slew = 3.5V/µs
    • TL071 – Slew = 13V/µs
    • CA3140 – Slew = 9V/µs
    • LF351N – Slew = 12-16V/µs
    • LM4250 – Slew = variable
    • TL070 – Slew = 18V/µs
    • MC1439
    • MC33171N
    • MCP601
    • MCP603
    • MUSES03<span class=”Apple-converted-space”>  </span>- Slew = 35V/µs (very expensive)
    • OP07CP – Slew = 0.3V/µs
    • OPA134 – Slew = 20V/µs
    • OPA602 – Slew = 20V/µs
    • OPA1641 – Slew = 20V/µs
    • LME49710HA
    #6818

    wilkie1
    Moderator

    Fantastic Free Findings from The Fabulous Cybercow!  Many thanks for this valuable research.  I will print and save for future use.  Normally I always considered that faster slew rate meant better performance in audio circuits.  But, you point out that in some applications, slower may be better.  Thanks again!

    #6820

    Cybercow
    Moderator

    Wilkie1 – you are correct with regard to audio quality being higher with faster slew-rates in opamps. But in the Rat (and a few other circuits) the slower slew-rate is low enough to have bearing on the upper registers of the audio range, providing a unique sort of distortion. From what I could make of my studies when I had a real scope, sweep generator and such (a few decades ago), the type of distortion harmonics seemed neither odd nor even – more like “diffused” or just degraded. Glad to hear you found some value in the info.

    #6821

    KeneK
    Participant

    Wow! Thank you.

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