Tube driven effects loop and other questions

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Feb 16, 2020.

  1. NSB_Chris

    NSB_Chris Tele-Meister

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    I saw that post when I was researching but was not able to view the schematic. Looked really interesting.
     
  2. Ten Over

    Ten Over Tele-Holic

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    I was not aware that Sluckey had an active FX loop. After looking at the linked thread, I am still not aware that he had an active FX loop.
     
  3. Ten Over

    Ten Over Tele-Holic

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    Moving the Send level to after the cathode follower buffers the Send level pot from the Master Volumes so that the FX loop doesn't mess with your mixing circuit. Both positions on the Bypass switch present a very high impedance to your mixing resistors.
     
  4. Paul-T

    Paul-T Tele-Meister

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    tube effects loop &  tube reverb schem 12-10-1.jpg

    More info and schematic in this thread.

    It is an active FX loop but plugs into the passive loop of the amp.
     
  5. NSB_Chris

    NSB_Chris Tele-Meister

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    I did see that circuit somewhere, but I wasn't interested in the parallel option. Also, I wasn't able to get my head around what was happening with the parallel circuit and why. Seems the dry signal is mixed back in with the wet after the effects return trim pot, which makes sense, but then that signal is split and mixed back in with the inverted signal from the return gain stage.
     
  6. NSB_Chris

    NSB_Chris Tele-Meister

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    Just curious why the output of the return stage has a coupling cap and 1M resistor to ground when this goes directly to the PI which has its own coupling cap? Is that because the bypass switch would leave the output free and unconnected when the loop is bypassed?
     
  7. Ten Over

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    Well for starters, I just wouldn't send out a high voltage to anything. The input cap to the PI is referenced to ground by your mixing circuit. The output from the Return stage is also referenced to ground by the 1M resistor, so both sides of the bypass switch are referenced to ground and there will be no pop when you use the switch.

    The same is true for the other half of the switch where the switch side of the 10nF input capacitor is always referenced to ground.
     
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  8. NSB_Chris

    NSB_Chris Tele-Meister

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    If patience is not lost, I would benefit from more discussion on this topic. Rather than blindly implementing something, I would prefer to at least have a rudimentary understanding of the basics...

    I read through this article on cathode followers which echoes a lot of what Ten Over is saying: http://www.valvewizard.co.uk/accf.html
    Some things in this article don’t make total sense to me however.

    I also referenced the RR website for the mods of the Special 6. This section shows the schematic of the Special 12/20RT which was very interesting!
    https://robrobinette.com/Special6UltraMod.htm#Send_&_Return_Level
    They use a 470k resistor for the send stage grid leak instead of a traditional 1M like the valve wizard article discussed. On the recovery side, they seem to have the input to the recovery gain stage look like a typical amp input, which seems to agree with Ten Over’s advice … keep the total grid resistance less than 100k. They put the recovery volume control after the gain stage.

    @Ten Over: It would be great to understand your basis for the 820-47K-22K cathode string you put on the send stage your recommended to me. If I understand correctly you use a warm bias for the stage with the 820 and have a total of 69K for the load, but chose to position the send takeoff point between 47K and 22K on the load. For reference, based on the voltages that I got on my last build of a similar circuit, I expect to have a B+ voltage of about 280V for the effects loop drive and recovery.
     
  9. Ten Over

    Ten Over Tele-Holic

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    I made a bad assumption that the signal would get cut in half by the mixing circuit. When a signal of equal magnitude and phase is present at the wipers of both MV's, each 270K mixing resistor looks huge to the other 270K resistor. Therefore, the mixed signal will be nearly as large as the signal at the wiper under these circumstances. Accordingly, I have altered the effects loop to accommodate the larger possible signal.

    FX Loop NSB Chris 2.png
     
  10. Ten Over

    Ten Over Tele-Holic

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    FX Loop for NSB Chris  pg 1.png
    FX Loop for NSB Chris pg 2.png
     
  11. NSB_Chris

    NSB_Chris Tele-Meister

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    This is really Great!!! Thank you for taking the time to put all of that down. I have been reviewing it.
    I put together a quick spreadsheet to calculate some of the parameters you are specifying. I added values for when the wiper is at an intermediate setting to see how that changed things.

    For your circuit I get the following:
    Screen Shot 2020-02-23 at 1.34.03 PM.png
     
  12. NSB_Chris

    NSB_Chris Tele-Meister

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    If I arbitrarily put in a 25k pot, I get the following with the pot at 100%:
    25 0 25.png

    With the same setup, but the wiper in the center of the range (50%), I get the following:
    25 12-5 12-5.png
    So if I did that correctly, then the 25k pot just forces the pot to be set in the middle of the range to get the same output voltage. The length of cable that can be run without unwanted filtering is reduced, but still a pretty decent length.
     
  13. NSB_Chris

    NSB_Chris Tele-Meister

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    One more discussion point: Why would the assumption be that the input impedance of the effect in the effects loop is as low as 25K? Guitar amps have an input impedance of 500k to 1M, so wouldn't effect pedals have similar input impedance because they are designed to have guitars with passive pickups plugged directly into them? If I was worried about it, couldn't I just put a high input impedance pedal first in line in the effects loop and all is well?
     
  14. NSB_Chris

    NSB_Chris Tele-Meister

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    IMG_3697.jpg
     
  15. NSB_Chris

    NSB_Chris Tele-Meister

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    I think I am starting to see the method to the madness. If I assume a load impedance of 500k, it just means I need to move the pot position.
    Original 10k pot with 500k load and pot adjusted for an output of 0.45V with cathode input of 9.5V:
    Screen Shot 2020-02-23 at 7.25.51 PM.png
     
  16. Ten Over

    Ten Over Tele-Holic

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    The 25K is for line level gear. 100K is a common input impedance for these devices, but I must have had one with a 25K input impedance or something.

    The change in design is minimal for a 1M load because you still have the 22K in parallel with the 10K pot in order to get the 7K output impedance. You could change the 100K resistor to a 130K resistor and bias the cathode follower a little colder. Not really gaining much, so I would prefer to use the one that can drive a 25K load.
     
  17. Ten Over

    Ten Over Tele-Holic

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    The 2.2uF Send output capacitor was chosen using this formula: C = 1 / (2*pi*R*f)

    R is the parallel combination of the 10K pot and the input impedance of the effects device. For the 25K load case, the parallel combination is 7.14K. f is the low frequency cutoff that we desire and we are going to go with 10Hz. The capacitor we need is
    1 / (6.283*7.14K*10Hz) = 2.23uF. 2.2uF is close enough and readily available.

    With a 1M input impedance, the parallel combination is 9.9K and C = 1 / (6.283*9.9K*10Hz) = 1.61uF. The closest readily available capacitor is 2.2uF.
     
  18. NSB_Chris

    NSB_Chris Tele-Meister

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    Using a more common coupling capacitor size, say 0.47uF would give a cutoff of 34Hz to 47Hz with the input resistance between 1M and 25K. That would be ok as we don't much care about anything below 100Hz, correct?
     
  19. Ten Over

    Ten Over Tele-Holic

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    I care about frequencies below 100Hz. The E string is 82.4Hz and I habitually tune it lower than that.

    The low cutoff frequency is at -3dB and the signal drops off rapidly at frequencies below that. The signal rises much slower as the frequency increases from the cutoff frequency. Because of this, I always select cutoff frequencies that are well out of the range of the expected frequencies because I want a flat frequency response.
     
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  20. NSB_Chris

    NSB_Chris Tele-Meister

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    Brain fart. Don't know what I was thinking!

    One last question and then I am going to start preparing for my component orders.

    On the recovery side, what are the pros and cons for putting the recovery level adjustment before or after the tube? Below I have your loop design drawn up on the left (hopefully without errors). Then I have an alternate recovery option on the right with the input of the recovery stage like the input of an amp and the 1M-A level pot after the gain stage.

    Thoughts...

    Screen Shot 2020-02-25 at 6.14.37 PM.png
     
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