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New (and first) build: 5f2a Tweed Princeton

Discussion in 'Shock Brother's DIY Amps' started by theprofessor, Nov 2, 2017.

  1. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hi all -

    If it were not for TDPRI, I wouldn't be where I am now. I have been wanting a smaller amp that is portable and sounds good and that won't blow my family out of the house on 4 like my SF Deluxe Reverb will. With the encouragement of some here--especially @King Fan --I decided to build my own.

    I thought initially I'd build a 5f1, but it's a tight work space. The 5f2a is almost identical, and it has more work space (and a tone control). I'm going to build the tweed Princeton with a 10" speaker at 8 ohms. That's another reason I decided not to go with a 5f1. The Mojotone 5f1 kit comes only with a 4 ohm transformer, and they don't allow substitutions. The 5f2 kit comes with a transformer tapped for 4 and 8 ohms.

    I've been planning this for a couple of weeks now, and I so knew there were some parts I was going to want to replace. I ordered some F&T filter capacitors to use instead of the Mojotone ones, and I got two Mallory 150 coupling capacitors to use instead of the Orange Drops. I also ordered two Belton octal sockets and one Belton noval socket. Finally, I ordered a 1Meg Alpha audio taper pot with a SPST switch to replace the regular one that controls the tone on the 5f2a. The plan is to be able to defeat the tone control completely, which will essentially turn this 5f2a back into a 5f1 (with a 10" speaker). Those are my upgrades as of now. I also took the time while I was waiting to build a light-bulb current limiter, following Uncle Doug's video on YouTube.

    Last night I determined which end of the Mallory 150s was the foil-end, since it isn't marked on the outside of the capacitor. The folks over on this thread were very patient with me as I figured out how to apply a small amount of electricity to the body of the capacitor, first through my fingers, and then against the power cord of an amp in the "on" position, while I listened through the amp's speaker to determine where the foil was (more explanation on the thread). I marked the foil-end with a black permanent marker (it occurred to me later that somebody down the road might think they're burned or something. Oh, well).

    The chassis and small parts came in from Mojotone today. I opened them up, and I checked the inventory against the list. Everything looks good. You'll see in this picture that I wasn't able to resist getting the vise that @Romartin and @Outlaws mention on this 5E3 build thread.
    IMG_9438.JPG

    Then I got out my multimeter and tested the actual DCR of the resistors. I put some blue tape on the end of one of the leads of each of them, and I wrote the actual number there. I went through the parts box and replaced the Mojotone parts I decided not to use with the others I ordered (F&T, Belton, Mallory). Now the parts box won't close, because those F&Ts are honkin' big!
    IMG_9439.JPG

    I've been reading @robrob 's literature on electricity, amps, and amp-building and skimming build threads in the past week. Now to go to reading again before I start putting some things together in the next few days.

    But: Are there any remaining parts that seem to be a "must replace," either due to the nature of the material (e.g., carbon composition resistors, which tend to drift in value) or rating (e.g., 1 watt instead of 1/2 watt or 2 watt instead of 1 watt, etc.)? I can get some of them at a local electronics store, or I can simply order from Tube Depot, which is not far from me here in Chattanooga. Things arrive from Memphis rather quickly.

    More soon!
     
    Last edited: Nov 3, 2017

  2. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    My 1st build was a 5F2A. That tone control is awesome.

    They look like good quality parts (and that's very fancy packaging). The transformers and speaker are the key components that affect sound quality, especially the OT and speaker
     

  3. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Good luck. Sounds like you're up for it.
     

  4. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    You go, sir. Like Tubeswell, my first build was a 5f2a. They're well known to be a gateway drug to hardcore amp building. :D

    I'll be interested in what people suggest you swap. Don't swap much. An amp-wise friend suggested for my 5f2a to use metal film resistors on the input stage (i.e, all 3 resistors connected to the input jacks and the 1500 on V1 pin 3) -- all they can add here is hiss, and that hiss will get amplified at every step in the chain. In a small tweed like this, I would stop there. Just me.

    And don't mod until it works and you've played it a month. I agree a 10" speaker, a defeatable tone 'stack', and slightly better parts aren't mods, though. :)
     
    Last edited: Nov 2, 2017

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    So I started installing the chassis-mounted hardware, and I already have a few observations and questions.

    (1) As most of you know, the Mojotone kits don't come with any directions. There is a "manual" for the 5f2a that one can get on the website, though. That said, it's quite minimal. I've attached it here, in case that's helpful for others.
    (2) In looking through the inventory of the kit, I can't figure out why they include a Mojotone mono right angle plug 1/4" plug (pancake-style) when they also include a Switchcraft 1/4" 2-conductor mono phone plug.
    IMG_4404.JPG
    (3) The bill of materials lists (6) 4-40 Keps nuts. Those are the ones to use for the tube sockets. The octal sockets mount inside the chassis, with the keps nuts inside the chassis. The noval socket mounts outside the chassis, with the keps nuts inside the chassis. The orientation of the pins, looking from the bottom, is as follows. On the octal sockets, the guide pin is up and to the right (2 o'clock).
    IMG_4399.JPG
    On the nine-pin socket, the gap in the pins is down at about 4 o'clock.
    IMG_4400.JPG
    (4) I learned that you've got to be extra-careful with the outside of the chassis when installing the hardware. Maybe it's a good idea to tape it up. I seem to have scratched the area around the tone pot shaft a bit:
    IMG_4401.JPG
    (5) For now, I installed the 1 meg audio pot for the tone control that comes with the kit. I can't figure out how the Alpha 1 meg audio pot with a SPST switch that I bought would mount into the chassis, due to the tab shown here on the left. Do I need another switch to accomplish what I'm hoping for (to use a tone pot that allows me to defeat the tone stack if I want to, making it more like a 5f1)?
    IMG_4403.JPG
    (6) Finally, when I attempted to install the output transformer, I could not for the life of me figure out how it mounts into the chassis. I knew that the two black grommets go into the holes where the output transformer wires go through. But why did the output transformer only have one hole in the chassis it would attach through? It turns out that Mojotone sent me the Champ transformer (771) instead of the BF Princeton transformer (772) that's supposed to come with the 5f2a kit. So I've got to call them tomorrow to get them to send me the right parts. At least I think I do. The Mojotone site is confusing. Under the 5f2a kit, it lists the BF Princeton output transformer, but when you click on the link, it takes you to the BF Champ output transformer. Are those the same? Because when you do a search of "output transformer" on the Mojotone site, there are two different parts: a BF Champ output transformer (771) and a BF Princeton output transformer (772).
    http://www.mojotone.com/Pickups_x?search=output+transformer
    IMG_4402.JPG
     

    Attached Files:

    Last edited: Nov 2, 2017

  6. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Those tabs are meant to be broken off if your mounting situation does not include the matching small hole for it. They snap off pretty easily.
     
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  7. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, RLee77! I wondered about that. Good to know!
     

  8. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    The BF ('Blackface') Princeton OT is different from the type of OT that's required for a Tweed Princeton. The BF Princeton has a push-pull output stage, and has a push-pull OT. The Tweed Princeton OT is for a SE (single-ended) output stage, (and its exactly the same OT as the Tweed Champ OT). You have the correct OT already.

    The red and blue wires are the primary winding wires. Usually, red goes to the B+ supply node and the blue goes to the 6V6 plate, but if they've accidentally got the winding phasing wrong at the factory, then you swap those colours around, but you'll only find that out when you power it up and it squeals with positive feedback (but don't worry about that for now, just wire it up normally).

    The black and green wires are for the secondary winding (for the speaker) the green attached to the speaker jack tip terminal, and the black grounds at the speaker jack sleeve terminal.
     
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  9. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    After more investigation, it looks like the Mojotone 5f2a kit does come with the BF Champ transformer. That is what it says under the BF Champ transformer description: used in tweed Princeton kit. So perhaps 771 is correct. On the other hand, the BF Princeton output transformer (772) says that it comes in their BF Princeton Reverb kit.

    I must be really dumb, though, because I can't figure out where to mount it. The one hole between the two grommets makes sense, but where does the other tab mount?
    IMG_4405.JPG
     

  10. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Perfect! Thanks so much, tubeswell!
     

  11. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Your OT looks like it has a couple of secondary winding taps (i.e. green and yellow) and my guess is that the black wire is the common (ground return) wire that goes to the seeker jack sleeve lug. Test these winding ends for DC continuity with your ohmmeter (just to be sure), and read the instructions.

    If these are different secondary winding taps, this means that you have a choice about what speaker impedance to use. and if you like, you could wire up two speaker sockets, for convenience, or have an extension speaker arrangement -e.g. where you can run an second speaker that has a matching impedance to the onboard speaker (but have them switched so the both speaker jacks can be switched between the different secondary taps). Read the instructions that came with the OT
     
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  12. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    It also says on the mojo site that the primary winding colours are phased the other way around to most normal SE Tweed champ/princeton OTs, so you need to connect the primary wires the opposite way to what I suggested two posts ago
     
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  13. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Most excellent. Many of us are in the same boat. We'd not be here building a 5F2A except for great assistance of this forum.

    My 5F2A. Transformer location seen here. If one hole lines up, use it and drill another new hole at the other end.

    [​IMG]
     
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  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you very much, tubeswell! I'll come back to this and read it carefully (along with the diagram for the OT) when I'm wiring it up.

    Thanks, keithb7! I was looking around for awhile on the web for pictures of a 5f2a chassis that showed the transformers. I didn't find one. Everyone (understandably) wants to show the inside of the chassis. I figured that maybe I'd have to drill another hole. I just find it odd that a kit that comes with a chassis and an OT made for the amp doesn't have both mounting holes provided.
     

  15. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    If the chassis mounting holes don't line up with the OT bolt mounting holes, you may need to drill another hole in the chassis so that these holes line up. If you decide to do this, plan the hole location(s) carefully, so that you don't end up with the nuts behind the main eyelet board if you can. Makes swapping OTs easier later if you decide you want to.

    Edit - Oops I see keithb7 covered this just now
     
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  16. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Mojotone probably drilled the 5F2A chassis exactly as the original Fender. It was the second half of the 50's when Fender was using a certain (Triad?) transformer that had a longer mounting foot. Then the BF Champs came out. Also a single ended amp, a whole new chassis with front mount controls. Fender by now was using a different output transformer with narrower mounting holes. They cared not about the earlier 5F2A OT they used to use.

    Fast forward to today. Mojotone, in their wisdom only need 1 single ended 5W OT for Tweed, Champ & Princeton and BF Champ amps. They choose the smaller of the two. Which is what you have there in your hand. It can be used in the amps mentioned as well as the Vibro Champ, and Bronco too.

    So your holes don't line up. Mojotone probably expects amp builders to do what is needed to make it work.

    I can't say for sure why you have a straight and a 90 degree plug. A 90 is generally used for speaker jacks on the rear of a chassis. A straight plug for a tweed style with the jack on the bottom of the chassis.
     
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  17. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    You have the Penske team in the pits here, Professor. Perfect info.

    As a detail, are there two holes in a vertical line between the grommets? I think so but the lower one isn't clear.

    If so, then it's exactly what the guys say -- drill one new hole. (Either way, actually, drill one new hole). How do I know? I had exactly that situation with my Mojo PR kit. Keith's reason for it is genius, and Tubeswell's caution about double checking the hole(s) location is excellent too -- in the PR the (vintage) OT location prevented you from reaching the board mounting bolt.
     

  18. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Oh, FWIW, I think given the 'vintage' OT hole apacing and location, and with Tubeswell's note about the OT secondaries (there's always even odds you'll have to reverse them, and with a multi-tap OT you can't universally say which colors to wire up) you're starting to see why Mojo (and many other kits) aren't really suited to instructions. Luckily, you're a quick study, and you're seeking out a ton of information, so you're learning a ton more than if there *were* some 1-2-3 instructions. (Rose-colored glass now officially half full.). :)
     

  19. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, @tubeswell, @keithb7, and @King Fan . You guys are the best! As I said before, I'd have no confidence in going forward with this if I didn't have you fellows looking over my shoulder, so to speak. Thanks!
     

  20. Outlaws

    Outlaws Tele-Meister

    157
    Jan 16, 2007
    None
    It was suggested to me in my first 5e3 build to use washers on the ears of the OT for those bolts. The extra surface area made a huge difference in the mounting strength that’s visible just by trying to wiggle it.
     
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