5F1 6V6 Power Tube Tweaks

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Jul 14, 2020.

  1. James Knox

    James Knox Tele-Meister Ad Free Member

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    I see that Fender has adds 470Ω 3W screen resistor and a 1.5KΩ 1W grid stopper resistor to the power tube socket. What exactly does that do and is this a mod I should add to my 5F1 build?

    75EEE2A8-5886-4CC8-9F90-5F5E87930754.jpeg
     
  2. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    The 1.5KΩ 1W grid stopper resistor helps to keep the amp stable. It helps to prevent (RFI) very high oscillations before they start and it helps to prevent blocking distortion. IIRC the 5F1 usually will be fitted with a larger grid stopper than 1k5 to battle blocking distortion.

    The 470R screen resistor is added to protect the tube. It will smooth the distortion of the tube which may or may not be desired. Most of us have ears that can hear the difference but it is slight. I will go out on a limb and say most 5F1 owners would go without and most deluxe reverb owners would go with. YMMV.
     
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  3. James Knox

    James Knox Tele-Meister Ad Free Member

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    Thank you for your reply! I’m going to use them both on this build. I tend to prefer smoother distortion.
     
  4. vvelt

    vvelt TDPRI Member

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    What would be a better value grid stopper to use on the 5F1? And what does blocking distortion sound like?
     
  5. andrewRneumann

    andrewRneumann Tele-Meister

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    Curiosity: Were these resistors added on later factory Champ builds? @James Knox What amp is that layout diagram?
     
  6. James Knox

    James Knox Tele-Meister Ad Free Member

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  7. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    What value grid stopper for the 5F1 will elicit many responses from as many shock brothers. To a large extent, it is a choice. Other factors can help prevent blocking distortion. Steps can be taken to reduce bass in the circuit. Preamp cathode cap size, RC filters, and grid stoppers on previous stages can help. A well known *fix* for blocking distortion in the 5E3 circuit is to install a 470k to 1M grid resistor on V2 pin7.

    Blocking distortion usually will not show itself until very high volume. Not all amps sound the same when exhibiting blocking distortion. It is not a pleasing distortion. A sound clip of blocking distortion can be heard:
    TDPRI shock brothers
    AB763 Blocking Distortion - Troubleshooting (w/ pix)
    IIRC the resistance of a grid stopper is not audibly noticed up to around 47k. Higher resistances may reduce highs which may or may not be desired.

    If there was a problem with blocking distortion on the 5F1, I would start with a 6V6 grid stopper of 10k and reduce bass frequencies elsewhere in the circuit. I would also add a 33k-47k grid stopper to V1 pin7.
    If more was needed I would go higher. OMMV.
     
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  8. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    This is one of the areas of a circuit where it is easy to install a 1k screen resistor and bypass it with another 1k (close to 470R) to hear the difference. Then you can bypass it with a jumper for *the no screen resistor sound*. Easy innit?

    Same with the grid stopper.

    :cool::cool::cool:

    LLC
     
    Last edited: Jul 15, 2020
  9. andrewRneumann

    andrewRneumann Tele-Meister

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    Has anyone experienced power tube blocking distortion on a Champ? I just didn't think you could overdrive the 6V6 enough to get it to happen, especially with only 220k on the grid leak. Even on the V2, with a fully bypassed V1 cathode, can you drive it enough to come into play on Champ? Maybe with a boost pedal out front? I just assumed blocking distortion was more of a problem on high gain, gobs of clipping designs and then you see some huge grid stoppers.
     
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  10. D'tar

    D'tar Friend of Leo's

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    Perhaps Fender has been reading Robinette, Valve wizard or Aiken? Me personally, I don't see a need for these two components on a 5f1. I agree with the rectifier back up diodes though.

    This is relatively like your filter cap questions.... One can build a bone stock 5f1 and be a happy camper. I like the stock values for both 6v6 grid stop and screen grids;). I think we are more concerned with grid current limiting on this circuit, but hey its a champ!!! Start with large resistors and small caps right!

    By now you surely own an assorted resistor pack that will allow you to try all the values that will surely be suggested from 0 to1.5k to 1M.
     
  11. James Knox

    James Knox Tele-Meister Ad Free Member

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    Cool - I’ll have to try that!
     
  12. James Knox

    James Knox Tele-Meister Ad Free Member

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    Good points! I feel like I have so much to learn. Plus, reading about stuff doesn’t really click til I try it, see it or hear it.

    Yes, building a nice little stash of resistors. I through a few in on every order.
     
  13. elpico

    elpico Tele-Afflicted

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    A champ can overdrive the power tube with more signal than just about any other amp out there. At least double what a 5e3 can do. Most amps drive the power tubes directly from the phase inverter and those have less gain and headroom than the a regular gain stage. The 5e3 PI in particular has no gain at all (a slight loss even) and needs to fit two copies of the signal into the B+ voltage.
     
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