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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

5F1 Build Check

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Oct 10, 2017.

  1. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    Ok, unload on me, ha ha. I'm very close to installing the board on my marathon 5F1 build. Chipping away at it here and there when I get a chance. Could y'all take a quick look and give some comments.

    Overall, none of my leads are straight. I've added some stress relief to them all as per advice I've read elsewhere. (Also, anything that's in my possession for more than 48 hours ends up with some kind of abnormality, so they are all funky to start with.)

    I've left a pretty good tag end on each resistor in the dropping string with the idea that they are probably the most likely to have to be swapped out once I put power to the amp.

    board_front.gif

    The V1A Cathode was attached to the bypass cap by necessity. I like the way Hoffman wires his busses and jumpers so I sorta copied that idea, just moving all the turrets on the preamp circuit out away from the power side. The downstream effect of this was that the cathode resistor no longer reached the turrets. Poor planning on my part.

    I like to run my under board wires up the turret and crimp them over and then solder. Its a nice visual que to me that there's something else going on at that junction. Otherwise, I try to keep the turret holes empty until they are needed.

    board_back.gif

    This shot gives you a better view of the PT cathode resistor, it appears to have adequate space between the other components.

    pt_cathode.gif

    The PT bears some explaining because I'm not sure that I've got it right. This power chord black wire is to the bottom lug of the fuse. White neutral wire goes to an ungrounded lug on the side of the chassis where it meets one of the 120v primaries from the PT. Green ground wire loops around the PT and is attached to a grounded lug at the lower right PT mounting bolt. If the power chord is ripped out, the ground wire should be long enough that it's the last thing to tear out. The PT center tap is grounded to the same lug. The other 120v PT primary goes to the switch.

    pt.gif

    This is where the transformer meets the power chord. The center lug is a grounding lug, the outer lugs are not.

    pt_union.gif

    I ran two laps of 22 AWG through the holes here to tie all the lugs to ground. The CT and the mains ground is here. I will attach the power section ground to this as well. The signal ground will go to a similar lug on the other end of the chassis and all my inputs, outputs and volume/power pot will be isolated. I'm not isolating the lamp.

    power_ground.gif

    Anyway, that's kinda it. The grid stoppers will be flown from the input jacks. I'll use shielded cable from there to V1. Also considering shielded cable to replace the white signal wire from the volume pot to the preamp tube, but not sure of how effective that will be in reducing noise. It might be an unnecessary waste of RG174. Overall I'm using 22 gauge for signal and power and 20 gauge for grounds. Filaments are also 22 AWG. Based on what I've read here, those seem adequate.

    I'm still not quite sure how to ground the isolated speaker output jack. Would I run a ground wire all the way to R6 (V1B Cathode)?

    Every solder has been checked for continuity and they all surprisingly check out. I love my new soldering station, it actually makes soldering enjoyable.

    My goal is to build a Champ with an 8 inch speaker that's solid enough to record with, so I want to limit noise as much as possible.

    Thanks in advance for any help.
     
    Last edited: Oct 10, 2017

  2. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    Wow, crickets. I guess that's good news.

    But does anyone who has built a 5F1 know how to ground the isolated speaker jack. Would it have to be after the Negative Feedback, at the turret between R6 and R13?

    Why couldn't I attach it to the first cathode on the 12AX7, pin 8. That would accomplish the same thing and would be a shorter lead, right? If that's the case, then I could instead ground it to pin 3 for a shorter route to ground, right?
     

  3. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    You didn't link or post a schematic, so I don't know what R6 is. The speaker ground connection always goes back to the same stage that NFB goes to, when present (the ground connection in the second stage preamp). Otherwise it is referenced to the output stage. It should always be isolated, but if you already connected the circuit ground to the chassis in more than one location it doesn't matter. A ground loop has already been created so isolating the jack would not be necessary in that case. It's all or nothing. (Remember these things don't matter too much on a 5F1 but it's fun to do it right....)

    This is where "ground" may be confused with the chassis or the power supply. Output current flows through the OT and is connected to speaker terminals. The 'ground' connection is just a circuit reference and only require a light gauge wire to the the location noted above. There is no current to speak of.

    Here is a layout that does it right. Notice the output jack is isolated and the reference goes back to the preamp:
    5F1 layout.gif

    Here is also a schematic that shows the grounding a little more literally than most schematics.
    5F1_ground.GIF
     
    Last edited: Oct 11, 2017

  4. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Nevermind. I thought that coupling cap to ground was wrong, but that 220k resistor is a grid leak.
     

  5. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    So to get rid of the ground loop I would just attach the center tap to the negative side of B+1 filter cap, jumper B+2 to B+3 and attach the Input 2 ground to the buss at the V1 cathode bypass? That would eliminate the separate grounds for power and input?
     

  6. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska

  7. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Yes, the chassis is connected to the circuit ground only at the input jack(s) as shown in the schematic. One connection, no loop.
     

  8. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    So no preamp and power amp grounding buss. Just one main buss.
     

  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    The Fender 5F1 schematic grounding has no bearing on how the 5F1 was actually grounded. All the schematic's ground symbols mean is a connection, direct or indirect to the chassis.
     

  10. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Examine the layout in post 3, there is no bus. A physical bus is not necessary in any build. The physical connections match the schematic posted below it, to show the intention of the path the ground connection takes. There are no loops. Notice how each capacitor is the reference for the circuit it is connected to.

    Fender uses the ground symbol to show that all of those connections are in common, often using the chassis to make connection. In reality, the chassis is not ground, it is should not be an electrical connection, especially from the input jack to the power supply, or between any two circuits.

    Highlight the ground connection in the layout and schematic and see ONE path from the power supply center tap to the input jack and chassis. I like the schematic as a demonstrative tool for the deliberate use of the chassis symbol and the lack of use of ground symbols sprinkled about.

    Using a physical bus is a rudimentary method to replace the chassis as a connection because it is often done incorrectly. Even when it is done correctly, a neater layout is often achievable without one. Sometimes a bus wire is integrated onto the board layout, and that certainly looks better. A lot of 5E3 busses can look a little sloppy, or like an afterthought.
     

  11. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    Thanks for the input, I do appreciate it. I think I'm gonna plow ahead with my original split bus plan and keep the output and the power sections grounded on opposite ends of the chassis and see how that goes. Seems like a lot of folks here have had success with that. If it doesn't work, its easy enough to change it.

    You're just going from this:
    5f1-layout_split.gif

    To this:

    5f1-layout_star.gif

    The original 5F1 used a buss ground (brass plate) so we'll see what happens.
    Thanks
     

  12. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Both will work just fine. It's when you get into much larger circuits and/or with more gain that the small differences start to matter.

    Do you have a speaker and cabinet ready to load up? You'll be done with this part in no time.
     
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  13. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    This has been a pretty big mountain for such a small amp. I finally finished the cab about a week ago and after pulling my hair out over speaker choices (size, cone comp, magnet type, etc, etc) decided on a Weber 8A125. Not because its the best choice, but because it was past time to make a decision. Time to get off the toilet, so to speak. Got word on Monday that it's on the way. But most everything about this amp has been paralysis by analysis.

    Bought stuff for a current limiter at lunch today. I'm looking forward to not thinking about this project someday. Ha
     

  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    An electrical engineer would like your ground scheme. Your speaker jack will need a ground connection.

    [​IMG]
    On the paralysis by analysis, keep in mind you can easily modify the amp later.
     

  15. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    Ha, knowing me I'd probably just live with it. I am so looking forward to not ever working on this amp again.

    The speaker jack is grounded at either the turret just north of the NFB or all the way up at the jumper turret above the cathode. This is a question I posed earlier and didn't get a response. Why can't I just ground it at the cathode on pin 8 of the 12ax7? It's a shorter run and they are all going to the same place. That's the second stage of amplification, so hmm?
     

  16. Nickfl

    Nickfl Tele-Holic

    681
    May 24, 2016
    Florida
    If you attach the speaker jack ground to pin 8 of the preamp tube, it will still have the 1500ohm bias resistor between it and ground, which will then elevate the speaker ground by the cathode voltage... which seems like it wouldn't be something you want (off the top of my head I'm not sure what would happen, but I'm not going to try to figure it out right now). Just ground it at one of the other points you mentioned. I wouldn't spend too much time worrying about hum and grounding with one of these, they are never going to be dead quiet, its just the nature of a SE amp, especially one with small filter caps and no choke...

    I've never isolated the jacks on any of my builds and it doesn't seem to have much effect on noise, some of them have been dead quiet and others have some background noise inherent in the design, but I don't think the jack grounding is a big factor.
     
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  17. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Post 3.

    On the 5F1, all of the grounds are within inches of each other. My dissertation above was long winded for how it could make a difference, where in fact the emphasis should be that most of the time it does not make a difference. When troubleshooting a new build, looking at ground loops can help to solve problems. Read on...

    I mentioned in another thread that I fixed a friends 5E3 last weekend. It was a new build that didn't work. I took out 3 feet of wire. I could have taken out 2 more feet without trying too hard. Needless to say, it was "over-grounded" but not a single error in connections that were made. Wiring can make a difference, and we all know it.

    In your case, just use a star washer on the output jack, and you're done. Or run a wire to the preamp ground, like in post 3.
     
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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    "In your case, just use a star washer on the output jack, and you're done. Or run a wire to the preamp ground, like in post 3."

    ^I agree.

    The "north side" of the NFB resistor isn't a ground either.
     

  19. JuneauMike

    JuneauMike Tele-Holic

    908
    May 5, 2015
    Alaska
    Ok, build is almost done. It sounds amazing, but has a little buzz that starts at 3, increases to 10 then drops dramatically after that. I'd like to get rid of that eventually. I used shielded RG-174 from the grid stoppers to V1 pin 1. 22AWG from vol wiper to V1. Buzz seems to be in the input section, it disapears when I pull V1.

    Did the stock build and used the grounding scheme listed first in post #11.

    be patient if I make some mistakes here, but voltages:

    Wall 119.3
    Lamp 6.5 VAC
    PT 5V winding 5.02VAC
    PT HT 335.1 VAC - 335.3 VAC
    5Y3
    Pin 8 381.1 DC

    B+1 381.1
    B+2 334.2
    B+3 291.0 (all within 20% tolerances but I think I'd like to get them down)

    12AX7
    Plate (1-6) 199 DC
    Cathode (3-8) 1.39 DC
    Grid (2-7) 0 DC

    6V6
    Plate (3) 370.5 DC
    Screen (4) 334.4 DC
    Grid (5) 13.3 mV (is that right?. AC read .001v but that should be DC, right?)
    Cathode (8) 20.86 DC

    Voltages seem generally high, but in the ballpark. PT is a P-TF22772 replacement for fender. 6v6 is a JJ 6V6GT. 12AX7 is a Ruby.

    What do you guys think? If I lower B+ voltages will that do anything for the buzz?

    Although I'm confused, a transformer voltage calculator estimated my voltage at the first filter cap should be more lime 406 volts with this PT and tube compliment. Not complaining.

    As I said, tone is great.
     

  20. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Looks about right on voltage with a 5Y3 rectifier. Real world diode rectification is about 1.32x (1.41x is theoretical). 1.32x335 = 442V. 5Y3 drop from 40-60V let's say (depending on current draw etc.) 380 puts you right at 62V drop, so I wouldn't be alarmed at this.

    Plate voltage is 350V. 350-375 is where I like to run 6V6 so that's right in the ball park (IMHO). Looks like you are biased at 15W total dissipation (20/470 =.0425; .0425*350 = 15) . Assume 5mA of screen current, and that puts you closer to 12.9W (350*.037) on the plates. I'd say you are right where you want to be.

    Regarding your Hum/Buzz.
    1. Play the amp for at least 8 hours before making final judgments. Or just leave it on for 8 hours even if you aren't playing. Burn stuff in a little bit. You've got a punchers chance of that going away.
    2. Is the RG174 shield grounded at the input side?
    3. 60hz or 120hz?
    4. Can you get a good gut shot of the chassis innards all wired up?
     

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