No input resistor?

Discussion in 'Shock Brother's DIY Amps' started by JohnnyCrash, Jun 8, 2021.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    So I’m about to wire up a modified version of the old Supro Coronado and I’ve got a few questions. I’m only building the Trem channel, without the actual Trem.

    First, is it safe to omit an input resistor? I thought I read something about problems if something failed and shorted.

    The Treble input does not use an input resistor, though it uses a capacitor.

    I’m not married to this schematic, I plan on making several changes, but I am interested in trying to keep the overall sound.



    E58668B4-9912-4C14-8D4D-8485FEE1EE9D.png
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The input preamp tube that is connected to the trem circuitry is cathode biased. It has a resistor from the cathode to ground. In this case the resistor is bypassed with a capacitor.

    Yes, it is safe to omit the control grid resistor. The reason you do not want to omit this resistor is due to RF radio frequencies that are all around us. Most of us have heard a radio station coming through a guitar amp. This resistor helps to eliminate that from occurring.

    The minimum resistance of this resistor should be around 15K. Fender typically used a resistance of 33K and Marshall used 68K. There is not much difference in the input signal when this is in the circuit.

    The Supro has a few caps which will let some high frequencies go to ground. Unless the amp is demonstrating high frequency problems, ime the caps should be eliminated. This is your project so your rules.;)
     
    Last edited: Jun 8, 2021
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  3. Big-ez

    Big-ez TDPRI Member

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    I have made a few amps without the first input (grid) resistor. Never had any issue with them. It depends on how close you are to radio frequencies that might get picked up (apparently I am not close). That resistor adds noise, and some builders omit it or keep it a small value to minimize the noise. On the amps where I do use one, I usually use a 10k.
     
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  4. tubeToaster

    tubeToaster Tele-Meister

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    I have used 10k without any RF problems and I’m about 2 miles from the WSM radio tower broadcasting at 100,000 watts.
     
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  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've done a little experimenting in this area lately. I used different values each with their own 1M resistor on the jack and independent of each other, but altered the value of the grid stopper resistors. The 1M on each helps retain the high end and punch that you are used to hearing with the #1 input on a Fender, but you can tune the gain of each channel. I've used 33k & 68k, and 48k and 68k before. I haven't gone higher than that, but after not liking the results of my voltage divider experiments in low gain Fender circuits in the end, I wonder if this might be a better area to work? There is a definite drop in gain that can't be attributed to the lower impedance because they are separated and both have the 1M. They both function just like a regular #1 & #2 input, but #2 has more bite than a stock arrangement. Anyway, maybe worth playing with it. My next build might of a 50k pot in the #2 slot and see if I can dial in my input level? A master volume that works at the source?
     
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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am not sure I am visualizing what you are describing. Can you show this on a schematic?
     
  7. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Not sure I follow the idea, but the volume pot on the guitar is basically "a master volume at the source".
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Wire both inputs exactly like a single Fender #1, except pick your own grid stopper value. Instead of 2x 68k that are parallel for #1, and split for #2, each is it's own and pick how hot/cold you want it.


    It is, but if you don't have 50's wiring the tone changes as you turn it down. You also have to pay attention to where you keep the volume at that point. If you swap from a vintage output Tele to a modern Les Paul, then you get to turn the LP way down, and if you haven't rewired it, you get to sound like there's a blanket over the amp with the humbuckers turned down.

    I regularly use 50's wiring in my guitars, and utilize the whole "wide open for lead, roll back for rhythm" idea. But, I don't like paying attention to knobs, and I don't own a single one that sounds the same turned down as it does on full. The guitar on 10 and amp on 3 does not sound the same with the guitar on 3 and amp on 10. I would love the idea of plugging in two different guitars and tuning how hard that first stage gets hit with a 25k-50k pot on the amp, but with a full level guitar signal going in.
     
  9. bebopbrain

    bebopbrain TDPRI Member

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    My first DIY amp received WWWW 106.7 FM Detroit country western loud and clear. So I ran the lead through a cardboard cylinder from a roll of paper towels. Then I stuffed the cylinder with steel wool. Problem solved. In those days we didn't have the internet, but we had the ARRL handbook.

    If you do add a resistor (and you should), solder it right to the tube socket grid so RF and positive feedback from later stages can't sneak in after the resistor.

    I love how 68K became some sort of de facto standard even though Fender used 34K.
     

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  10. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I’ve always used input resistors. I’ve been crazy busy as well as dealing with a new child and chronic physical and mental health issues, so I get mixed up easy these days...

    I was recently looking at building an old circuit that had an unusual grid leak bias in the preamp (from what my failing memory can recall)... and I read something about the dangers of plugging right into these types of amps (especially without a three prong AC cord), so I apologize for the dumb question... my brain started fogging and blurring together when I looked at this particular schematic.

    I really don’t understand my brain these days.
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Typically the "input" resistor is considered the grid leak resistor. On Fender amps the 1M grid leak is mounted on the input jacks so that's where it got its name. It sets the amp's input impedance to 1M.

    In the Supro the 6.8M is the grid leak. The upper channel first stage is using grid leak bias--notice no cathode resistor--that's why the grid leak is so large. It traps electrons captured by the grid on the grid.

    The Supro uses the cap paralleling the grid leak resistor to remove high freqs above human hearing--what a grid stopper resistor does on the Fender input circuit.

    The cap just upstream is for the grid leak bias to separate it from the input jack circuit.

    The .005 cap on the Treble input jack is a very small coupling cap that filters out low freqs to emphasize treble.

    I'm not a fan of the input circuits in this amp but the bottom line of course is how does it play and sound. Playing clean through a grid leak bias preamp stage can add some cool dynamics but the circuit does not handle humbuckers, hot pickups or drive pedals well.
     
    Last edited: Jun 10, 2021
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