Jet City Picovalve mod thread

Discussion in 'The Squawk Box' started by Dr.Distortion, Aug 27, 2013.

  1. kfowler8

    kfowler8 Senior Member

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    Great. Thanks. I started reading your other link.
     
  2. wagdog

    wagdog Member

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    Cool. One of the things I love about modding amps is it's such an excellent place to learn about electronics - you not only see the difference in the circuits with the relationship between voltage, resistance and current (and gain), but actually get to hear them. If you have access to an oscilloscope, you can follow the AC (audio) signal through the amp and see the changes as well. Fun stuff.
     
  3. rhythmtech

    rhythmtech Junior Member

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    Here's a clip from my latest round of mods. I swapped C12 for a 22uF and changed the gain on V2:1 to ~15. I also switched over to a different speaker cabinet, which helps the distorted tone quite a bit.


    The clip is increments in gain, guitar is left without changing anything, I change music according to the amount of gain.


    [SC]https://soundcloud.com/jared-corgan/picovalv-mod-set-3[/SC]
     
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  4. kfowler8

    kfowler8 Senior Member

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    Went to rack shack tonight and bought a variety of resistors and caps. I did a some short recordings of different options. Take a listen and tell me what you think. I'm using 2 12AX7s and a 6L6GC.

    First: 3.3K Resistor Only
    Second: 3.3K + .22uf Cap
    Third: 3.3K + .47uf Cap
    Forth: 3.3K + 22uf Cap

    Cap Comparison by kfowler8 on SoundCloud - Hear the world

    Here's my thoughts. I think I hear a difference between resistor only and adding any of the caps. I think adding the cap tightens it up. I really can't say I hear a difference between any of the caps. Perhaps you do.

    What do you think.
     
  5. wagdog

    wagdog Member

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    That sounds great man. Nice riffage/playing too, very nice. Like the cleaner tones better (but then again, I generally like cleaner tones, lol).

    I'm not hearing any difference at all man, to be honest. Can you post a pic of your wiring and the switch wiring? There should be a difference between the .47 and 22uf. There probably won't be much of one between the .22 and .47.
     
  6. rhythmtech

    rhythmtech Junior Member

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    Thanks, I don't really play with a lot of gain normally either, honestly I am not really even sure I've ever heard that guitar with a lot of distortion through any other amp, maybe I'll try a different guitar at some point too. I still don't think that unless I could figure out a way to get the power tube to distort it will ever sound distorted to my satisfaction.

    Here's the local nfb hack I did:
    [​IMG]


    Here's a schematic of basically what I've done to the circuit, although in looking at the pic and trying to make it make sense with that cruddy schematic I don't think it reflects what I actually did. I think I'll have to trace it out again and double check.
    SCHEMATIC
     
  7. kfowler8

    kfowler8 Senior Member

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    I was determined to make the cathode bypass work so I decided to take a different route. I took a spare DTDP toggle switch with 6 lugs and hooked up the caps and wires. Finally I could hear the difference. I even had my wife verify it just to make I wasn't going insane.

    I ended up going with a three way mini switch and drilled a hole in the front. I went with the 3.3K resistor which is the middle position. Up and down add either a .22uf or 22uf cap. This was a huge relief to get this to work. It was driving me crazy!!!!

    So I need to label the switch now and was trying to think of what the cap does to the sound. I've heard tighter but to me it sound like you took a pillow out front in front of the speaker. Almost like a presence knob without it becoming harsh.

    Any ideas?

    Also, I'd like to add something else to the old 5w/2w switch. Perhaps something that impacts gain. Any ideas?
     
  8. duhvoodooman

    duhvoodooman Member

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    Assuming you have the two caps soldered across the two pairs of end lugs, then it should be straightforward. With virtually all toggle style switches, the toggle bat points the opposite direction of the selected end pair of lugs.
     
  9. wagdog

    wagdog Member

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    I have a champ project with a similar configuration (on-off-on). Mine is labeled tight-off-full on the schematic... never got around to labeling it on the amp, lol. A 3.3k cathode resistor is going to sound muffled in v1a. Suggest you go with a more conventional 1.5k, or change the plate load to 150k and go with a 2.7k cathode. FWIW, I rarely (never) run my champ project without a bypass cap, just sounds dead.

    Just for grins, I would check that 2w/5w with a continuity tester to make sure it's working as expected.
     
  10. kfowler8

    kfowler8 Senior Member

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    I ran some tests using a 1.5K, 2.7K, and 3.3K. Honestly I couldn't hear a difference at all. I actually recorded them in hopes of posting but my PC crashed when I was trying to save. I've got rehearsal Thursday night so we'll see how it goes. It's a pretty easy switch to go back to the 1.5K.

    I checked the 2w/5w switch thinking it might be bad. I get correct resistance switching between positions. Not sure what the issue is.
     
  11. kfowler8

    kfowler8 Senior Member

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    Just to satisfy my own curiosity I could take a 2.7k resistor and jump it off the 3.3k. That should change the resistance down to 1.5k. Hmmm
     
  12. rhythmtech

    rhythmtech Junior Member

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    What kind of change are you looking for?
     
  13. kfowler8

    kfowler8 Senior Member

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    Well to the extreme I accidentally put a 1.5M resistor in there. The amp had no gain or significant volume. It was anemic.

    So when I was comparing the 1.5K to the 3.3K, I was listening for volume drop, lack of gain, "mushyness". I just didn't hear a difference.

    I believe duhvoodooman is using a 3.3K in his.
     
  14. rhythmtech

    rhythmtech Junior Member

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    If you have a bypass cap in the circuit also, then what you're changing mostly is frequency response. At the 1.5M you probably have a lot of boost at the high end with mids and bottom dropping to dropped out (anemic in my book).

    I like to play with this calculator - Bypass Capacitor Calculator to get a visual idea of what happens when you change bypass cap values and cathode resistor values for a rough guideline of what I wanna try. I like to try to avoid any long response slope lines. I like the way my guitar sounds so I try to not remove anything via the amp, just amplify and add what the tubes can. However like all things guitar it's your ear that will tell you what's best.
     
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  15. kfowler8

    kfowler8 Senior Member

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    This is really cool.

    When it says "gain at 82Hz (low E):" Is this how much it's boosting the 82hz from the base cathode resistor or is it the absolute value. For example using a 22uf cap, is it boosting the signal across the entire spectrum by ~35dBs? Compared to a .22uf cap where you'd get a spread of ~ 9dBs for low to high.

    I assume this is what duhvoodooman meant by "gain spread".
     
  16. rhythmtech

    rhythmtech Junior Member

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    I tend to think of most of this sort of circuit as high pass or low pass circuit. In this case it's a high pass and you're shaping tone by adjusting where the bass response is cutoff. You heard one extreme when you put the 1.5M in for Rk.
     
  17. duhvoodooman

    duhvoodooman Member

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    Clearly, the 1.5M resistor was a mistake and screwed things up royally. But let's look at more reasonable values....

    The function of that cathode resistor is to determine the stage gain--lower resistance will equal more gain, other things being equal. As the Amp Books calculator shows, if there is no cap across the cathode resistor (this can be approximated by picking the lowest available cap value in the calculator, 100pf), the stage will have a gain of +29.5 dB across the entire audible range of a guitar signal for a typical 12AX7 tube type and a 1.5K cathode resistor. Raising the resistor value to 3.3K will drop this value to about +25.4 dB, which should be a slight but audible decrease. However, the decrease is across the board on the audible frequency range, so I really would not expect to hear any dullness/loss of tone for this modest gain decrease. I know I couldn't hear any significant change in tone on my own Picovalve for this resistor change--it just lowered the V1A gain a bit.

    At the other extreme, if you install a bypass cap of sufficiently large capacitance, it will increase the gain across all audible frequencies. A 22uf cap will certainly do this, raising the stage gain to +35.4 dB. This value is the same whether a 1.5K or a 3.3K cathode resistor is used. There is a slight low frequency rolloff below ~100Hz, but the magnitude is so small that the change would be imperceptible.

    The really interesting behavior is when a cap value is used that creates a high pass filter with its transition in the audible frequency range, as rhythmtech referred to. By judicious selection of the cap value, you can adjust the gain boost to be in the upper frequencies while leaving the lower frequencies at the "baseline" gain of the stage. Whether you refer to it as a "bright switch" or a "tight switch", the bottom line is that the high frequencies are emphasized relative to the lows, and the magnitude of that difference is the "gain spread" that I referred to earlier in this thread. The position of the frequency transition is determined by the combination of resistor and cap values, so changing either one will shift the transition range up or down in frequency. Play around with the Amp Books Cathode Bypass Capacitor Calculator and see how the two values interact. Set the tube type for 12AX7, the grid resistor = 1M, the plate resistor = 100K, the cathode resistor = 1.5K and the bypass cap = 0.47uf. Then change the cap value, first to 0.22uf and then to 1uf and see how the transition range shifts up or down in frequency. Do the same thing with the cathode resistor value, increasing it to 3.3K and then decreasing it to 820 ohms.
     
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  18. kfowler8

    kfowler8 Senior Member

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    Man this is really helpful. Starting to make sense now. Thanks!!!!
     
  19. wagdog

    wagdog Member

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    The calculators on that amp book site are awesome! Thanks for link!
     
  20. kfowler8

    kfowler8 Senior Member

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    I started reading a book on tube amps and it's been really helpful undestanding the role of the components. I'm still on the input to V1 stage.

    Here's some updates on the mods I've completed:

    FX Loop: seems to work really well
    Mute Switch in place of Standby: Had to add the shielded wire but really works well and no popping
    Cathode Bypass Switch: Added three way mini switch to go between no caps, .22uf, and 22uf with a 3.3K resistor. Nice options here. I may still play with the resistor and cap values.
    Replaced R9: Replaced R9 with a 10K and added a 3.3K resistor via the old 5w/2w switch. I really like this. It does add a nice boost option.
    Remove C1: Not exactly sure what this did. Can someone explain?

    Before I move further I'd like to understand impact some of the other mods wagdog listed with do. I noticed many are similar to the mods from Ultimate Guitar forum. I don't want to reduce the gain anymore than I have.
     

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