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 joy was short lived. I soldered in the NFB, fired it back up to no sound again. This time I can see the 12AX7 tube is no longer lighting up.

    :confused:

    Went through and took voltages again. Here’s what I have now:

    B+: 381.8v DC
    B+1: 358.1v DC
    B+2: 357v DC

    5Y3:
    Pin 4: 333.8v AC
    Pin 6: 334.4v AC
    Pin 8: 23.1v dc
    Pins 2->8: 5.35v AC

    6V6:
    Pin 3: 368v DC
    Pin 4: 352v DC
    Pin 5: 54mV DC
    Pin 8: 23.14v DC
    Pin 2->7: 6.74v AC

    12AX7:
    Pin 1: 348.2v DC
    Pin 2: 15mV DC
    Pin 3: 1mV DC
    Pin 6: 348.3v DC
    Pin 7: 20mV DC
    Pin 8: .5mV DC
    Pin 5->9: 6.73v AC
    Pin 4->9: 6.73v AC

    I’m sure you noticed the lack of drop in voltage between B+1 and B+2 and the jump in voltage at the plates on the 12AX7. Those were not the voltages I measured earlier today. They were a little higher than I expected, pin 1 was 308v DC, pin 6 was 311v DC, and B+2 was also 311v DC. But NOT 348v. WEIRD.

    I double checked the 22k voltage dropping resistor, but it still checks out with 21.7k ohm resistance. Sigh.

    Could I have shorted something when I soldered in the OT? I don’t relish pulling the board out, but i may have to. Unless someone out there has a better idea.

    This time I am dropping it for the evening and coming back with fresh eyes in the am.
     
    Last edited: Aug 3, 2019
  2. D'tar

    D'tar Friend of Leo's

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    Looks like V1 is not conducting. Either lack of heater voltage to the heater filament, no cathode to ground connection, bad tube, tube to socket connection. Can you see the heater glowing?

    Can check v1 p3 to ground should be 1.5k

    Check v1 p 8 to ground, with nfb disconnected should be 1.5k... with nfb connectd you would see a very low resistance to ground.
     
  3. Robear

    Robear TDPRI Member Ad Free Member

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    D’tar, no, the heater isn’t glowing anymore. It was earlier. I have photographic proof. LOL.

    V1 p3 to ground is 1.478k. And v1 p8 to ground is 1.505k. Right in there. Would the high voltages on p1 and p6 blow the tube?
     
  4. Robear

    Robear TDPRI Member Ad Free Member

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    Obvs I’m still thinking about this tonight. My brain wheels are just grinding over it, actually. Typical.
     
  5. D'tar

    D'tar Friend of Leo's

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    Doubtful. Likely not making good heater circuit connection somewhere. If you have another 12a#7 give it a try. Ive gotten rid of preamp tubes for microphinics more than I can count. Not sure Ive ever actually had one stop working all together:)YMMV
     
  6. Robear

    Robear TDPRI Member Ad Free Member

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    Well I’ll be hornswaggled. I’m pretty sure i found my issue. While checking the voltage across the v1 heaters for the umpteenth time I pushed the heater wire to get clearer access to the pin and the wire popped out of the insulation. It appears while wrestling that 18 gauge wire to add the jumper I broke it. So you’re right, it was the heater circuit. Hopefully when I get new heater wires in all my issues will go away.

    Again D’tar, thank you SO much. If I’m ever in WNY I owe you lunch.

    I will return and report. But probably in the morning. My brain has had its “A-HA!” and I think I can relax now.
     
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  7. Robear

    Robear TDPRI Member Ad Free Member

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    Thanks to D’tar keeping me at troubleshooting the connection on v1 I found the broken wire causing the issue. I think when I was taking my heater voltages on v1 how I was attaching my MM leads kept closing the break and giving me a normal looking 6.75vAC. But finally I hassled those pins so much taking voltages I found the wire break. After a little food and relaxation I went back downstairs, fixed that little wire and...

    YES!!!

    Wow. These Princeton’s really do bring it, don’t they?

    I hope the neighbors didn’t mind an early Sunday wake up call. JK

    There are a few things still left to do. I need to get a read on final voltages and decide if I’m going to reduce the B+ voltage. I took out the 4/8 ohm switch chasing this down and I need to put it back in. Also, I have everything together to install an adjustable NFB. And then there is the question of the nameplate and a tube chart. I hopefully will finish all that up this week and I can put this project to bed. At least until I want to mod something. :) I’m sure I’ll be calling for advice before I’m done.
     
    Last edited: Aug 4, 2019
    King Fan likes this.
  8. D'tar

    D'tar Friend of Leo's

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    @Robear, good job, I can only lead you to water... It is up to you to take a drink:)

    GRab your princeton, get here by 4(est) and dont eat before you get here!!!
     
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  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Wow, great build... and great guidance from @D'tar.

    Six Sigma Signs of a good build:

    1. Looks tidy -- check
    2. Goes through intelligent startup sequence and correct testing -- check
    3. Builder knows circuit well enough to describe problems so experts can decode them -- check
    4. Builder follows suggestions of experts (not as common as you'd think) -- check
    5. Builder himself can also decode and diagnose problems -- check
    6. Not many problems, and the few that exist are found and fixed in short order -- check
     
  10. Robear

    Robear TDPRI Member Ad Free Member

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    Back after spending a few days in South Dakota visiting family. Time to get back to this!

    @D’tar - I hope the smoking went well! Sorry I didn’t make it for dinner. LOL
    @King Fan - THANKS! High praise indeed coming from you. Thanks for not calling it out when I got my B+ confused with my B+1. I did know what I was referring to. :lol:

    So... after a week playing it (and not enough playing with the travel) I am loving it. It took me until today to take the back off and finally get some baseline voltages. This is what I came up with (with vintage voltages in parenthesis from Fender’s 6G10 schematic).


    Power Supply
    Wall AC: 121.3V
    Fuse: 121.1
    Power Switch: 121.2
    Pilot Lamp On? Yes

    5Y3
    Pin 2 -> 8 (Heater): 5.28Vac
    Pin 3 -> GND: 331.2
    Pin 5 -> GND: 332.2

    B+1: 380V (6G10: 360V)
    B+2: 334.8V (6G10: 320V)
    B+3: 291.8V (6G10: 280V)

    6V6GT
    Pin 3 (plate): 368V: (6G10: 350V)
    Pin 4 (screen grid): 334.5V (6G10: 320V)
    Pin 8 (cathode): 21.5V (6G10: 20V)
    Pin 2 -> 7 (heaters): 6.69Vac

    12AX7
    Pin 1 (v1 plate): 196V (6G10: 190V)
    Pin 2 (v1 grid): 0.2mV
    Pin 3 (v1 cathode): 1.49V (6G10: 1.4V)
    Pin 6 (v2 plate): 196.8V (6G10: 190V)
    Pin 7 (v2 grid): 2mV
    Pin 8 (v2 cathode): 1.38V (6G10: 1.4V)
    Pin 5 -> 9 (heaters): 6.69Vac
    Pin 4 -> 9 (heaters): 6.68Vac

    It looks like my B+1 is over vintage by 20 volts. I debated whether to use a dropping resistor or diodes to reduce the voltage and decided to go with the diode method since since the Princeton is a cathode-biased amp. If my reading is correct to bring my voltage down 20V I would want to use 2 10V or 1 20V 5W Zener Diodes. 1N5347B’s or 1N5357B if I figure right. I went ahead and ordered a few 10V 5347’s and a tag strip from Tube Depot. Does this plan sound right?

    I think that leaves 2 more items for me to do before I think I can call this build complete. One, I need to finally wire in my adjustable NFB. I have everything on hand to complete, so I’ll probably have that done before the diodes get here.

    Second, I need a nameplate for it. And being me, I couldn’t just order one online. I have to etch it out of brass myself. The one item I managed to do this week was pull out Adobe Illustrator to work on my design.

    6AE896EE-60ED-45D3-ACEF-CA4856597310.jpeg

    I first designed the one on the right, but after time to reflect I decided I wanted a more 1959-era spaghetti font design and came up with the one on the left. Fender logo included for comparison.

    Well... I’m off to either work on the NFB loop or make a nameplate. Have a great Saturday evening, everyone!
     
    Last edited: Aug 10, 2019
    tubedood, FenderLover and King Fan like this.
  11. King Fan

    King Fan Friend of Leo's Ad Free Member

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    That's a tidy and informative voltage chart. I'd say those voltages are fine. Remember Fender had 110 at the wall when they measured theirs. :)

    I know you already ordered the parts, but personally I wouldn't add zeners to drop just 20V B+. OTOH one thing I have considered is Rob's simple bucking transformer, which could drop your whole amp to vintage voltages. You can read that running your heaters at 6.3V is great for tube longevity and function, and zeners won't do that.

    BTW, I like your logos.
     
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  12. Robear

    Robear TDPRI Member Ad Free Member

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    Thanks King Fan. I’m still futzing with the logo a bit. Darn perfectionism...

    I’ve seen bucking transformers mentioned, but I haven’t looked closely. Thanks for the link, I’m off to read more.
     
    Last edited: Aug 10, 2019
  13. Robear

    Robear TDPRI Member Ad Free Member

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    I think I was hung up on keeping the amp a self contained unit, but I see the advantage to a bucking transformer. Besides, it would give me something else to build.

    :D
     
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  14. willholt92

    willholt92 TDPRI Member

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    Loving the name plate designs. Very easy to spend HOURS on Illustrator perfecting stuff. I usually print stuff out and mock it up before I make my mind up.

    Can't wait to see them on there
     
  15. Robear

    Robear TDPRI Member Ad Free Member

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    @willholt92 - I'm glad you like the nameplate designs. I finally got started on producing the nameplate yesterday. It's baking in the toaster I keep in the garage right now. When it's done I'll post pics of the process and final result. Probably tomorrow or Monday. Then I'll call this thread complete. :D

    I was distracted when I installed an adjustable feedback loop this week and I've been playing it a lot. I think it could be a very useful mod and I'm considering how to bring it to the front panel as a 3-way switch. I have a design, just need to decide on final values and order some mini shielded cable. But that'll be another thread.
     
  16. Robear

    Robear TDPRI Member Ad Free Member

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    I finally finished the nameplate last night so one more post to finish up this build thread.

    Shout out to any who followed this thread. I appreciate it. I started planning this build 2 years ago. Last July I didn’t think I would have the chance to do it. Just as I was ready to start gathering parts I was diagnosed with a severe, life-threatening form of cancer and didn’t know if I was going to make it 6 months. I spent the last year in chemo, radiation, and undergoing major abdominal surgery to treat the cancer. A year later I’m still here and with no evidence of disease. Because of this my simple little amp build really came to mean a lot to me.

    Special thanks to @D’tar, @tubedood, @King Fan for their excellent advice and help during this process.

    Here’s how I made my nameplate. The process will be familiar to anyone who has etched a PCB, or took an etching course while pursuing a BFA when they were young...

    First I had to decide on and create a design. I really wanted the Fender spaghetti/50’s era logo look while not being aggressively retro. Here’s the final design with the brass plate ready to go. I printed the reverse image on coated magazine stock.


    Here’s the transfer applied to the brass stock. I used an simple household iron. And I got into the process so I forgot to take pictures of that part. Oops.

    57290CC9-026F-4D0C-BFBB-94A4C46C3F6F.jpeg

    I suspended the brass in ferric chloride by taping it underneath a piece of foam. The aquarium pump provide a little vibration to keep the solution from pooling in a single spot. I’m not sure how well that worked...

    FEDD6CC1-4CEE-4985-BCE6-FBBF5814B8D7.jpeg

    Fresh out of the acid bath

    100D0221-BD28-4FE6-81F5-AA072FEF9461.jpeg


    I hit it with a little brown spray paint and baked it in a toaster oven. 400 degrees for 90 minutes. Perfect every time!

    Beginning to sand paint away from the high parts.

    1EBE8A49-D5FE-436B-AE36-DB0F2F16F586.jpeg

    Working up the polish.

    D234F96A-6AC8-4693-8F0B-BA519DD6BA26.jpeg


    And in place. I am very pleased with how it turned out. Acid etching gives it a little extra vintage flair. I know the originals were simple silk screened badges (cost-conscious Leo...), but I like this. And using an old process to produce it gives it a bit of an imperfect ‘analog’ feeling that befits the amp.

    57318AD2-B6F2-4972-B550-17F5D6BD93F4.jpeg

    I’m off to have a nice Sunday with family. In the words of the late, great Warren Zevon and to paraphrase @keithb7 ’s excellent advice to me at the beginning of my build, “Enjoy every sandwich and savor every solder”! I’ll be seeing you around because apparently building amplifiers is addictive and there’s a 5E3 folder beginning to gather info on my iPad...
     

    Attached Files:

    Last edited: Aug 18, 2019
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  17. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    Bless you for being here to finish the wonderful build.

    That tag is awesome!
     
  18. Robear

    Robear TDPRI Member Ad Free Member

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    Thank you, @RollingBender . And I have to say, your builds are inspirational! In fact it was your latest chassis with the engraved control panel that inspired me to have a crack at doing the nameplate myself. Just used the old-school techniques I was familiar with as opposed to state-of-the-art methods.
     
  19. tubedood

    tubedood Tele-Meister

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    Great amp you made yourself there Robear!
    I am sure by now you are already a fan of the huge sounds this little circuit can belt out.
    I haven't found myself a nameplate for my lil amp yet but after seeing what you came up with I am inspired to try to make one! I dig it.

    I also have a few folders on the 'puter with some researched info for the next build (s). This hobby is addictive. You are leaning towards the 5E3? I might try something that can gimme some of the hair band keeeranggg sound of my youth.

    Relieved to hear the health scares are looking brighter, and eager to see more of your builds and contributions to this fine forum.
     
  20. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Your name badge just shot to the top of my 'best ever' list. Perfect finishing touch on a great build.
     
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