And away we go!... Another 5f2-A first amp build

Discussion in 'Shock Brother's DIY Amps' started by Robear, Jul 21, 2019.

  1. Robear

    Robear TDPRI Member Ad Free Member

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    My plan was to shoot for period voltage as well. I guess I’ll see where I land on the voltage when it’s fired up?
     
  2. Robear

    Robear TDPRI Member Ad Free Member

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    Since I wanted to cut the ‘tone stack’ out of the circuit to make the amp a pseudo-5F1 I used Tubeswell’s suggestions in theProfessor’s old thread New (and first) build: 5f2a Tweed Princetonand came up with this wiring diagram. If anyone wants to give it a look to see if it looks like it will work, it’d be great. Thanks!
     

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  3. willholt92

    willholt92 TDPRI Member

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    Can't wait to see how this turns out. I'll be watching very closely, as I have on all the other threads you listed above.

    I have my own 5F2-A kit from Mojo which I am modifying from orignial in a similar way to you (I.E putting a tone cut to give a Champ circuit)
    However I am doing mine differnetly and building it into an old Fender Frontman as a black face champ/princeton.

    I just need to finish a tweed cabinet project first.

    Best of look with it all
     
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  4. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Speaking of period voltages, @tubedood and his good thoughts remind me that Fender's voltages for the 5f2a don't appear on its schematic. But the (very rare, almost mythical) 6G10 Harvard was the *exact* same amp, with a 10" speaker in fact, and its schematic very helpfully does list voltages. You don't have to go *too* low.

    Harvard schematic with voltages.jpg
     
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  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    I'm in a rush and didn't check the diagram, but if you want to keep it simple, @RLee added a helpful note in the excellent build thread by @theprofessor noting that an on-off switched tone pot will *in practice* achieve much the same thing more simply and with less space and wire. That's what I'd done in ignorance of EE theory when I built mine and it sounds great...
     
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  6. Robear

    Robear TDPRI Member Ad Free Member

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    King - I have that “Harvard” schematic with the voltages in my build folder ready to go. Dang, that’s a heck of a thread, ain’t it? BTW, since this is a Princeton with a 10” speaker I was playing around having a nameplate made calling it the “Hahvahd”

    I thought about doing it with a on/off pot switch, but I never got it ordered. I had plenty of DPST switches around and didn’t mind the hole in the chassis front. It doesn’t look bad where it is, and if it wants to become a NFB switch someday, or a cap switch or whatever I can moved the tone cut switch to a new pot. That was my rationalization, anyway.

    Will - I’m going to go and look for your builds. Look forward to seeing how the Princeton/Frontman turns out. Thanks for the interest!



    So... couldn’t sleep this morning so I got up and worked on the back of the boards and getting the leads ready to go. Here’s some pics.

    BA0FF0CD-26E8-4C10-BAFD-0F357AA8777A.jpeg

    I don’t have the ground leads there yet and everything is hanging on for dear life until I pull out the soldering iron. I didn’t forget the jumper between the large filter cap cathodes. It’s on the front:


    Although I have a few 5W zener diodes on hand to handle any high B+ voltage issues I might have, I thought I’d leave it easy to get to in case I want to put a resistor there in the future and don’t want to take out the whole board.

    I have to run myself this morning. Heading into the big city for the day. Later all!
     

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    Last edited: Jul 25, 2019
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  7. Robear

    Robear TDPRI Member Ad Free Member

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    Hi all. This project is still alive and so am I. No life threatening shock to me yet. But my poor wife has been through a lot this last few days, though. After taking care of me for a year while I underwent treatment and surgery for a major cancer, last Thursday after we returned home from running some errands we got got a call letting us know her father had just had a stroke while traveling through Montana with his wife. We were still dealing with the shock from that when we get a call her sister had just broken her forearm and nose in a freak sports accident (she was doing sports and shouldn’t have been...).

    I wish I could say what the prognosis is for her father, it’s too early and her brother is still on his way to Montana where he is going to help him back to his home in South Dakota and stay awhile to help he and his wife. I got back earlier today from dropping my wife off to her sister’s to help her through the next few days since it looks like her break is bad enough to warrant surgery.

    Needless to say, the last few days have been too crazy to even think about this little Princeton project. But after I got home today I really needed something to occupy my brain, so I cleaned off the work bench where everything had been tossed for the last few days, reviewed where I was and made sure everything looked like it was where it was supposed to be on the board, and I started to solder it together.

    Here’s a couple pics of the front and back if anyone cares to review it for me. It’s been a crazy few days and it would probably be a good idea. HA!

    Copper bus in place, not soldered yet:
    71C443E8-BB16-46FE-8DE6-2340D87DA2E1.jpeg

    AD8553B6-12B6-4868-87EC-9928C31629D9.jpeg

    Thanks all.
     
  8. tubedood

    tubedood Tele-Meister

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    Looking very good my friend. Good to keep a positive outlook and try to occupy your thoughts a bit when things get difficult. Hoping things go well.
     
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  9. Robear

    Robear TDPRI Member Ad Free Member

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    Thanks, tubedood. It’s good having a project or two to turn to. Luckily the news today is my father-in-law isn’t as bad as we feared and is going to be able to travel home Friday. He has a lot of physical and other therapy lined up to assist is his recovery. My BIL is up there now helping him out.

    Speaking of distracting projects, I kept myself occupied with my Princeton this afternoon. Lots of progress. Here’s the evidence.

    Into the chassis we go today. Here are the standoffs I used to keep the board off the bolt holding the OT in place:

    1991A48F-6E1E-49C5-81CF-140EAB1F288E.jpeg



    In and ready for lead soldering:

    83DEE10B-AD03-45D8-B5CE-3549C8C84881.jpeg



    Started with the inputs. I rather like the looks of the romex ground bus:

    02C186CA-2ABD-421A-AF4F-75713EE282DB.jpeg


    Volume and tone pots with the tone pot kill switch:



    Pre-amp socket sans heaters:

    C6A96217-DAC2-46AF-978B-633D2DBB64F3.jpeg


    Speaker jack along with 4/8 ohm selector switch with NFB soldered in for both selections. Also the power tube socket sans heaters:
    0265FD2C-6534-48E6-835F-27ECAEA0BE1B.jpeg

    If anyone cares to review and sees anything I did wrong or has any suggestions, please let me know. I’m pretty sure about my ground bus, but I’d like some confirmation I have the vol and tone pot grounded correctly. Yeah, I know my wire colors got a bit confusing in there. Darn kit just didn’t have quite enough yellow wire. lol

    Thanks all!

    Robear
     

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  10. Robear

    Robear TDPRI Member Ad Free Member

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    Weird.. I don’t know why the tone/vol pot pic is out of order. I hope you can figure it out...
     
  11. D'tar

    D'tar Friend of Leo's

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    Hi Robear, looking good. FWIW if you want your NFB level to be consistent while changing speaker load 4/8ohm, connect the NFB feed to the correct tap on the switch itself instead of the speaker jack. Even if you are not using the tap it will still provide the NFB signal. IIRC this would be the 4ohm tap and a 22k resistor maybe someone else can confirm that if you are not sure. 4ohm 22k would be moderate feedback, 8ohm 22k would be heavy feedback. This is a good spot for versatility by using a minimal resistor and a linear pot say.... 10kr and a 50kl pot.
     
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  12. Robear

    Robear TDPRI Member Ad Free Member

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    @D’tar, so if I’m clear, if I hook the NFB to the 4ohm tap, switch to 8ohm and plug in an external speaker, I still get the NFB modified signal? I didn’t realize that and thought I was being smrt. Luckily after I soldered the NFB in I thought might not want to if I want to follow Steve Luckey’s startup procedure. So I was going to unsolder it anyway. I’ll hook it back up as you suggested.

    And I already have a hole drilled and a 50K pot on hand for that adjustable NFB circuit. I wanted to get the thing up and running before I did that mod. :)

    Thanks for the input!
     
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  13. Robear

    Robear TDPRI Member Ad Free Member

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    Well, nearly ready for first power up. I’m going to be following Steve Luckey’s startup procedure as recommended by KingFan. So my power has been brought in, hooked into the PT. No secondaries are wired in yet, other than the 6.3v leads are soldered to the pilot light. I have the heater wires attached there, but the other ends are covered with heat shrink and not soldered to the sockets yet.

    Here’s a pic of the power section:



    I’m going to let my daughter get home, then hopefully I’ll be back with some initial voltages.

    Robear
     

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  14. D'tar

    D'tar Friend of Leo's

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    Correct
     
  15. tubedood

    tubedood Tele-Meister

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    I had researched the NFB resistor value during my build as some schematics showed the 22k NFB resistor with an 8 ohm speaker but the originals were as D'tar pointed out 22k to a 4 ohm tap . I ended up using a 47k resistor to the 8 ohm speaker to reduce the level of negative feedback. It is a bit bright sounding as is tho I might try the stock value to hear the difference.
     
  16. Robear

    Robear TDPRI Member Ad Free Member

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    Thanks, D’tar and tubedood. As I have a 4 ohm speaker I’ll voice the amp for that and if I swap out for 8 ohms in the future I’ll deal then.

    And Look! The pilot light is on!

    And my 300 watt lightbulb didn’t make a squeak.

    After I settled down I took some measurements. I think they’re what I expected:

    Wall: 121.7 VAC
    Fuse: 121.8
    Switch: 121.8

    6.3v pilot light is on (see visual evidence)
    6.3v leads to 6.3v center tap: 3.46v/3.51v

    Rectifier heaters (yellow leads): 5.62v

    650v AC leads: 342.8/343.1

    Unless anyone sees anything I should be concerned about I’m going to move on to step two (connecting the heaters) in the morning when I’m rested. It’s been a long day.

    G’night everyone!

    Robear
     

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  17. Robear

    Robear TDPRI Member Ad Free Member

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    Quick update. Moving on to step 2 in Luckey’s startup procedure I wired in my heaters. After another uneventful startup with the limiter I plugged tested my 6.3v pins.

    6V6 6.3v to 6.3v ground (pin 2 to 7): 6.95vAC
    12AX7 6.3v to 6.3v ground (pin 5 to 9): 6.97vAC

    And here is something I don’t think I’ve seen in my home for over 35 years. The soft glow of a vacuum tube.


    E

    Off to the next step.
     

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  18. Robear

    Robear TDPRI Member Ad Free Member

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    Frustrations!

    I’ve been working on this today and I’m beginning to wonder if I may have a fried OT.

    I kept following Luckey’s startup procedure to great success until the final stage. When I plugged in my guitar hoping to hear something all I got was dead air. Frustration when everything seemed to go according to plan during the startup. Before I came whining here I started working through Rob’s troubleshooting page. While doing so I saw a problem with the 6V6 plate (pin 3). With my MM attached when I started up the amp I would watch the voltage DC rise on the plate up past 350V (what I was hoping to see) to 450-460V when my MM suddenly went blank. Just kinda wigged out. When I turned off the amp I watched the readings come back as the voltage dropped.

    Here are my latest voltages during the startup stages:

    Heater Test:
    12AX7: 6.97vAC (pins 5 to 9, no jumper between pins 4&5)
    6V6: 6.95 (pins 2 to 7)

    Rectifier Test
    HT voltages:

    Pin 4: 345.5
    Pin 6: 345.2
    B+ voltage (with tube): 382.4vDC

    At this point I plugged in a speaker and my guitar. To no noise. Following Rob’s troubleshooting suggestions I double checked my speaker, input wiring, etc. before moving inside the chassis where I found the issue with the voltage at the 6V6 plate. Here’s the other voltages I pulled when I was testing:

    Startup
    B+: 378vDC
    B+1: 339.8 vDC
    B+2: 314.7 vDC (looks high?)
    6V6 pin 3 (plate): ??? <— the problem area?
    6V6 pin 4 (grid): 340v DC
    12AX7 pin1 (v1a plate): 311.6 vDC
    12AX7 pin 6 (v1b plate): 203 vDC

    My brain is tired so I’m giving it a break. I’d appreciate it if anyone has any ideas or thoughts to share.
     
  19. D'tar

    D'tar Friend of Leo's

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    Can you clarify here. Pins 4 and 5 should be connected here for both triodes to conduct.

    Its best to check voltage at every pin of each tube. If voltage is somewhere its not suppose to be or not where it should, it makes troubleshooting a whole lot easier.

    Looks reaaly tidy, wont be long
     
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  20. Robear

    Robear TDPRI Member Ad Free Member

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    Well D’tar, you were right. On both counts. It didn’t take long, and the idea I got in my head that the jumper was unnecessary was totally wrong. I need to go back and research where I thought I learned that.

    Jumper in, sound on. :D

    One happy camper here tonight. I need to hook in the NFB loop then I’m going to take a little time and get familiar with my new friend.
     
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