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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    Hi to all you amp builders.

    I’ve been a member of TDPRI since 2016 when I first started to contemplate building an amplifier. I was nearly ready to get moving when work and health issues intervened and I had to put it on the back burner.

    I continued to do a lot of reading here and around the web and finally decided to build myself a Princeton when I was ready. I have to give credit to @theprofessor ‘s wonderful build thread (here) for giving me the confidence to move forward with the project. Thanks for asking all the questions, Prof!

    I was in the hospital for major surgery at the end of March. 2 weeks later I ordered a kit from Mojotone along with a 10” 10A125 speaker from Weber. The team at Mojotone custom cut a baffle for the speaker. Nice.

    So here’s the obligatory ‘everything laid out and ready to go’ pic. There are a few upgrades coming from TubeDepot in the next couple of days such as Belton tube sockets and a pair of FT 16uf filter caps because I want to open up the amp and see those big bad boys.


    I’m going to drill the chassis today for a tone stack in/out switch and a 4ohm/8ohm speaker selector. I purchased a 4ohm speaker from Weber but I’m not sure that was the correct choice. Time and playing will tell I suppose. An NFB selector is the other mod I’m contemplating at this point, but I think I’ll slip that in after I’ve played it for awhile.

    I have to give thanks to a few more forum members whose posts have taught me a lot and clarified a lot of issues for me, @tubeswell, @King Fan, @keithb7, and of course @robrob. Hey King Fan, I just noticed you’re from Salt Lake City. I’m just a little west of you... so if I get stuck... LOL

    Anyway, off I go to drill some holes. I sure hope to have this thing up and running in a couple of weeks.
     

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  2. theprofessor

    theprofessor Friend of Leo's

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    I'm very happy that my thread was helpful to you. That's why I started it, and that's why I asked all the rookie questions. I'm looking forward to seeing your build!
     
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  3. keithb7

    keithb7 Friend of Leo's

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    Looks like a great kit with a quality cab. Enjoy the build. Pause and relish in the moment once in a while during the process.
     
  4. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Great amp choice. I agree with Keith. Slow down and savor the build process. You'll have months and years down the road to enjoy the amp's *sound* -- it'd be a shame to rush through the fun of the *build.*

    If you get stuck I'd be happy to pitch in -- but share questions and any problems with this gang first. There are dozens of people smarter and more experienced than me here.

    And a question for you: You said you're ordering Belton sockets as an upgrade. What sockets came in the kit?
     
  5. Robear

    Robear TDPRI Member Ad Free Member

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    Prof and Keith - I’m glad to see you here! I will take your advice and savor the build. I savored it a lot yesterday so today I may actually try to accomplish something. ;)

    King Fan - Hello from out in Tooele. To answer your question about the tube sockets, there is no indication on the sockets from Mojotone who made them. They look generic. I had come across a thread a few weeks where the builder had to troubleshoot an issue that came down to a problem lug on the power tube socket. I had already decided to upgrade the early signal chain resistors to metal oxide and to get those badass F&T capacitors so I decided to toss in the Belton tube sockets to see if there was a difference with the ones that came with the kit. They just got here and comparing the Beltons to the ones included in the kit there is a definite difference. The Beltons are more solidly put together. The materials appear to be of similar quality, but the lugs on the ones provided in the kit are looser and rattle about in their slots. I don’t know if that could be an issue down the line. The Mojotone sockets appear to hold the tube snugly just fine, but I’m comfortable that I made the purchase if just for the fact I now have a head start on parts for the next build. Someone somewhere said you can’t build just one...

    Oh, and I’m sure the team here will definitely have the answers to anything I come up against, but I have to noticed you’re plenty smart yourself with a lot of valuable input and suggestions. Very glad to see you here.
     
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  6. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The pins inside of the sockets are manufactured to be loose so that they have a bit of give when installing tubes. If they were made rock solid, the chance of cracking a tube would increase greatly unless the pins were perfectly straight. The main thing to look for is how tightly the socket grips the tube's pins. Beltons are top notch and should last you for many years!
     
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  7. Robear

    Robear TDPRI Member Ad Free Member

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    The Beltons do seem to grip more firmly so they’re going in. I like planning for many years. Thanks, Dan40.
     
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  8. tubedood

    tubedood Tele-Meister

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    Right on Robear!
    I just finished up a lil 5F2-A also. You are sooo going enjoy the build and the sounds you can get from it. You have came to the right place to ask a question or two (or ten!). These folks will steer you right for sure.
     
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  9. tubedood

    tubedood Tele-Meister

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    I have heard great things about those Mojo Tone kits... how are you feeling on the quality and contents? I think they used to have directions which were at times unclear?
     
  10. BobSmith

    BobSmith Tele-Meister

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    In my opinion the parts will make a solid, quality amp. But they aren’t premium. They use CTS and switch craft which is nice. I personally upgraded a number of the parts on mine including sockets (had some trouble fitting one of my tubes in their socket during some prefit tests), going to a turret board (felt much more solid than the eyelet board). The caps and resistors are probably fine but nothing special (I went with Sprague atoms for filters and Jupiter for coupling caps. I also had some NOS carbon comps that I put in where I could.)

    I’m not sure if my money on caps/resistors was well spent, but in the long run with all the time spent building the amp and the likelihood that well treated it could last 50 years (not crazy! plenty of vintage amps still out there, right) it seemed like no big deal.

    For the most part the assembly instructions are very good. The testing area could be a bit more complete and I intentionally changed the order of assembly in places to accommodate my upgrades which literally left no room to work.
     
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  11. Robear

    Robear TDPRI Member Ad Free Member

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    Hey Tubedood! I saw your build thread a while back. I need to go back a review how it went for you, but I remember your turret board is a thing of beauty.

    Dood and Bob - Yes, in my limited experience with amplifier parts but with more experience with pedals, I agree with Bob the kit seems like it would build a solid amp as is. Although I purchased the Belton sockets I really think the ones included in the kit would have worked fine. Whether or not the caps make a huge difference, I have no idea. I don’t have enough experience to render an opinion. I’ve read some people say the Mojotone caps are fine and of similar quality to the F&T’s. I think I admitted I bought them for the black, badass look they have when I open the amp up to show it off more than what I think or now they’ll sound like. Don’t @ me... :rolleyes:

    Oh... I think I forgot to mention the Mallory coupling caps. Which reminds me I need to figure out the foil end of those today.

    I didn’t consider a turret board for this build. But I’m seriously doubting this is my last amp so I am going to have to think about using one.

    PROGRESS REPORT:
    Slow, savory progress. We had some visitors drop by the last couple of nights so work was slow. So far I’ve drilled out the chassis for the extra switches and pots I’m considering as mods. Speaking of drilling the chassis, a lot of people have had issue drilling their Mojotone chassis due to electroplating issues. I had good result with my hand drill using a center punch to start, a 1/8” drill to start the hole, then opening it up to size slowly with a titanium step drill. I didn’t see any tear out of the electroplating. YMMV

    I’ve also mounted most of the components in the chassis, soldering N1007 diodes to the PT socket before it was mounted so I don’t try to do that inside the chassis.

    Yesterday also saw the construction of my capacitor bleeder. Or snuffer. Or whatever we call that life-saving little device. I have the parts for my lightbulb limiter, it still needs to be put together.

    Today I’m going to finish soldering the input jacks prior to mounting them inside the chassis. And then I’m going to finally start populating the board.

    Question to anyone following along - What is the opinion nowadays on using a variac for startup? I’ve been reading both sides of the issue and can’t decide if I’m going to use one at startup. I budgeted one into the project, but I have pulled the trigger on it yet. Should I? Does forming the caps really make a big difference? Anyway, curious here. Maybe there is a thread discussing this I’ve missed?

    I’ll try posting some pics later, after I’ve spent some time getting the board together. But first the dogs need walking.
     
  12. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Sounds like you're doing great. You're right, we're all jonesing for pics. But it's not just about us. I've learned to take pics as I go, every step, several angles, check the lighting, and enlarge them on the phone for clearly showing the details before I go back to work. You'll be amazed how often it helps you down the line.

    Re the variac. I thought 'forming' the caps was for bringing old amps back to life. There may be other advantages to a variac for startup, but lots of us just use a light bulb limiter. I'll await more information.

    Details of build sequence are tricky, especially on a narrow chassis like these tweeds. For example, those input jacks can go in now, but you may want or need to take 'em back out to test fit the board as you go, and then to squeeze the board into final location.

    A logical build sequence is something I only learned on amp 3. The way 90% of us do it is pointless: fit components to chassis, populate board, yada yada. If you're interested, a test-as-you-go sequence *way* simplifies startup (and makes it likely to succeed). I like the one described by guru Steve Luckey on EL34 (cited below). I adapted it to integrate some great ideas from Rob. I'll paste the whole thing here in case you might find it useful.

    *************************************
    Most of this article comes from master builder Steve Luckey:

    http://el34world.com/Forum/index.php?topic=2376.0

    Notes from other sources are in italics.


    There is never only one 'big power on' event. Instead, I power up in stages as the build progresses. LOOK, LISTEN, FEEL, AND SMELL during all phases!

    As per Rob Robinette, perform available mains and grounds safety testing before adding power in any phases -- see below. In Phase 1 you can test the AC mains power cord for chassis ground, hot (small prong), and neutral (big prong). Then add ground testing each time you complete a phase and before the next power on phase.

    If you have a light-bulb current limiter, it’s good to use it at every power-up step to make sure there are no shorts. (If the circuit is not shorted, the light bulb goes bright only briefly, or not at all.) But after this ‘limiter start-up’ you should then do a ‘no-limiter’ startup before measuring voltages or testing function.


    PHASE 1. The PT primary circuit is the first thing I wire up. As soon as the power cord is in place (but before the secondaries are connected) I plug in for the first time. I make sure the power switch, fuse, indicator light, etc. work and that the unloaded secondary voltages are as expected. No filter caps at this stage.

    PHASE 2. No filter caps yet. Next I wire the 6.3v filament circuit. (If you do Fender-style overhead filament wiring, this step would come later, in Phase 6.) Then power on again and measure voltage across the filament pins of each socket. All 6.3v tubes are then plugged in and visually checked for glowing filaments. REMOVE ALL TUBES.

    PHASE 3. Rectifier tube is wired (nothing else). Check for proper filament and HT voltages on the rectifier socket. Then plug in the rectifier tube and check for B+ (VDC) at the cathode (pin 8 – which is also the filament measured in VAC). There are no filter caps at this time, so readings will be low. At this point the entire PT circuit has been checked, except for a possible bias winding that's still flying loose.

    PHASE 4. Wire in the OT, filters, board(s), pots, input/output jacks, etc. IOW, complete all remaining wiring. (Don’t trim the OT plate leads to exact length until you see if they need to be swapped at the end of Phase 7.)

    Now that the jacks are wired, do Rob’s tests of the entire safety ground scheme:

    Before adding power to a new amp be sure to verify the safety ground connection. Plug a guitar cable into an input jack and check that you have continuity (meter "beep") between the guitar cable's sleeve (the part that isn't the tip) and the amp's power cord ground prong.
    Plug a cable into the main speaker jack and test again for continuity to the ground prong.
    You must not power up the amp until you have safety ground continuity between the power cord ground prong, guitar cable sleeve and speaker cable sleeve because the amp is a deadly shock hazard without it. If you don't have continuity use your meter's continuity function to verify your power cord wiring: Hot (small prong) to fuse, Neutral (large prong) to the power transformer primary and the ground prong wire bolted directly to the chassis.


    Now use the schematic and an ohmmeter to verify that grid and cathode resistances measure expected values to ground and that plates are correctly connected to the proper B+ nodes. Visually trace the schematic (not the layout) and compare to your actual build. Verify polarity of ALL electrolytic capacitors and bias diode.

    PHASE 5. Bias: No tubes in yet. (You can ignore this step if you are building a cathode-biased amp, eg, 5F1, 5F2a, 5e3, etc.) If you have a fixed bias amp, power up and verify proper negative bias voltage is present on the control grid pin of every output tube socket. If adjustable, be sure the range is adequate and then set it for max negative voltage. DON'T PROCEED UNTIL THIS IS RIGHT!

    Added note from user PRR in the same thread: CAREFULLY use clip-leads to connect a voltmeter, Black to ground and Red to the point where the 220K resistors come together.
    Plug-in, turn on. This bias voltage should go to NEGATIVE 40V or 50V, and it should be adjustable with the trimmer. Don't linger, the unloaded voltage may be hard on the filter caps. But you MUST be sure you have LOTS of NEGATIVE bias voltage, before you put tubes in.


    PHASE 6. (If doing Fender-style ‘overhead’ heaters, wire them now. Then power on again and measure voltage across the filament pins of each socket. Insert all tubes, and visually check for glowing filaments. Rob: “Also make sure you have heater voltage on the preamp tubes at pins 4 to 9 and 5 to 9.”)

    Plug in the rectifier tube ONLY. Power up and check for plate voltages on every empty tube socket. They will be high.

    Rob says: “You should check voltages with just the rectifier. The DC voltages will be much higher than normal with no current use. You should have rectifier voltage at all the tube plates because there will be no voltage drop through the B+2 and B+3 dropping resistors and across the plate load resistors with no current flow.”

    PHASE 7. Plug in all tubes and connect a speaker. (If you have an NFB lead, you can leave it lifted for this phase.) If OK, plug in a guitar and see how it sounds. If you've successfully gotten this far, there shouldn't be too many surprises. Watch for red plates, smoke, etc., and listen for howl, hum, buzzes, bacon frying, etc.

    If you have NFB, connect it now if you haven’t already. With NFB connected, there'll be a 50/50 chance you may need to reverse the OT plate leads. There may be an obvious howl, or you may just have a bad or weird sound, possible squeal/howl on certain notes, the kind of things you can't explain. Just reverse them. You may find out that it was right to begin with, but at least now you'll know for sure. (Don’t trim the OT plate leads to length until you see if they need to be swapped.)

    Play guitar for a short time. Then measure plate, grid, and cathode voltages for all tubes. Measure on the socket.

    Steve Luckey concludes: “That's basically what I do. You may have to adapt details for different amps, but the idea is to power up in stages. Divide and conquer. You'll have confidence built up for that final complete power up.”
     
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  13. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Consider adding a permanent 220k-2W resistor from the power supply to ground. It will consume a very small amount of your high voltage DC supply, it will generate a small amount of heat, but it will always drain the filter capacitors in the power supply within about a minute of shutdown, even when no power tubes are installed or when you flick the power on and off without the heater circuit having the chance to get installed tubes conducting. Eliminates the whole wand with a resistor and a lead and an alligator clip thing and the steps associated with it. Instead you just confirm the voltage drop with a single probe of your voltmeter; the other probe remains clipped to ground. You would have to do this after using the wand thing anyway to confirm that you drained the caps before proceeding.
     
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  14. Robear

    Robear TDPRI Member Ad Free Member

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    King - Yes, I’ll be taking a lot of pics as I do this build. I’ve found in pedal builds I’ve done that having that record is invaluable.

    Oh, and although I have the components in place they’re only in finger tight right now. I remember you telling theProf how you always seem to take them in and out more times than you expect.

    And thanks for posting Steve Luckey’s start up method here. I coincidentally just came across it myself a couple of days ago and bookmarked it to look at closely before I started up. And now it’s here. How about that?

    Snfoilhat - I’ve seen that mod over on Rob’s site. I’ll have to go back over and review it. Thanks for the suggestion.
     
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  15. Robear

    Robear TDPRI Member Ad Free Member

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    OK... first few pics.

    Here’s the rectifier socket with protector diodes soldered in:

    3E522280-F3B4-4DDF-A09F-958B05AAE7B4.jpeg

    And inside:

    26B4C851-831B-4310-B3FE-CCA4DFAF3514.jpeg

    And the input jacks wired and ready to be moved inside

    6F2F2E26-222B-4764-A6E2-2E0309706C06.jpeg

    I’m going to start looking at the board now.

    BTW Tinfoil Hat - I needed more solder so I ordered a resistor for the capacitor bleed circuit at the same time. Doing that made as much sense as the protector diodes. In other words a total non-brainer. Thanks for getting me to look at it again.
     
  16. Robear

    Robear TDPRI Member Ad Free Member

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    I put some time into the board this afternoon and here is the front. If anyone care to check my work, I’d appreciate it.


    If I understood the videos and discussions I reviewed this afternoon and my old ears heard the right things then the Mallory coupling caps foil ends are pointing where I can easily add leads to the 12AX7’s plates. That was fun.

    I still need to do the back, but it’s time for supper and wife. Later!
     

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  17. tubedood

    tubedood Tele-Meister

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    Looking great Robear! I ended up putting a 1k 5 watt resistor between those first two filter capacitors to lower my B+ by about 30 volts.
     
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  18. tubedood

    tubedood Tele-Meister

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    And thank you for the nice comment on how my lil 5F2-A turned out.. Very kind!
    I learned a LOT from the build pages by the folks on this fine site.
     
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  19. Robear

    Robear TDPRI Member Ad Free Member

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    I was curious if I might need to lower my B+. I’ve noticed that seems to be a trend with 5F2-A builds.

    And yes! I’ve learned a lot from just lurking around here for the last few years. I’m hoping I’ve gleaned enough info so my build goes smoothly and successfully.
     
  20. tubedood

    tubedood Tele-Meister

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    I am getting about 334 volts to the 6V6 plate by adding that little resistor. I was a bit too high (360vdc) without it, as I was shooting for a somewhat era correct voltage.
     
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