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

New (and first) build: 5f2a Tweed Princeton

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

  1. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    You're doing great here, Prof. But working on the floor sounds harsh. Can you tack a couple chunks of 2x4 on edge on some scrap and put the chassis on a table? Even having it in a cab isn't quite as accessible.

    'Live' measurements on that 6V6 socket aren't the easiest for sure. You may, as you suspect, be measuring pin 3 with or instead of pin 4. But you can check the wiring on a cold amp with drained caps using just the continuity setting on your MM, pin 3 to the left side of your 10K dropping resistor, pin 4 to the right. Also make sure pin 3 and pin 4 don't show continuity -- solder drops and wire tails love to snuggle up in those tight quarters.

    As long as your dropping resistor measures ~10K, you shouldn't worry too much about the actual B+1 to 2 drop, but if they're wired right you'll see a similar drop between 6V6 pin 3 and 4. (OK, dead horse, take a break). :)

    If you have a JJ 6V6S sitting around, I use those for initial startup -- you get 14w and if you kill it somehow it isn't $$ to replace. Likewise if you have more than one 5Y3, see which one gives you the lowest B+. And if you have a cheap but reliable modern 12ax7 sitting around, same idea.

    With all the tubes in, did you re-measure rectifier pins 3 and 5 (equivalent to 4 and 6 due to diodes, as you note)? I may have missed it, but that'd be helpful.

    Finally, I doubt this has anything to do with your B+, but for safety and sanity, let's double check your AC power wiring. There's a white wire where I expected a black one, but overlapping stuff always makes it hard to trace in photos, and maybe that's a white jumper and not the household neutral? Anyhow, AFAIK, it should go like this, noting that the two PT primaries are interchangeable, but the order of the wiring connections and which one is hot v. neutral is a big deal.

    Untitled.001.jpeg
     

  2. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    To the other stuff in your post later. First, for the wiring of the power cord "hot" and the "neutral." I think mine are not wired as you say they should be. The layout is little help here, since it can't show white wires on a black background, and it assumes that you're not as ignorant as I am about electricity. :) [see Mojotone layout below]

    I have the black PT Primaries going to the fuse and the switch respectively, both hooked up on the left side of each one. The black (hot) from the power cord goes to the end of the fuse, and I sent the white (neutral) from the power cord straight to the switch. Maybe this explains why I didn't have household voltage at the switch? Maybe it explains why my lightbulb didn't light up at all or even glow dimly? Yikes!
    IMG_9524.JPG
    Mojotone 5f2A Layout.jpg
     

  3. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Yes, I'm gonna do something like that this afternoon. I have only been working on the floor when I was doing my initial startup process. With the tubes in, I can't as easily use the work bench I've been using for everything else, since I have glass attached to the chassis. So I'll build something makeshift this afternoon. I'll also go back and work on pins 3 and 4 on the 6V6 and maybe clean up the soldering. I definitely over-soldered in a few places.
     

  4. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    LOL, hide the women and children. You might wanna revise the AC power -- in any thread I can say it, I say Mojo is wonderful but their power wiring layout is crazy.
     

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I'm really glad you caught this. And I'm also glad I didn't blow up or get shocked. What you say comports with @robrob 's 5f1 layout [see below]: one primary to the switch, one primary to the neutral of the power cord, and the fuse connected to the switch.

    Yeah, the Mojotone layout is not great. As we've noticed already, it even includes the death cap, and you get one in the package with your small parts! Also, they jumper pins 8 and 1, as we noted last night. And they have crazy power wiring! Note, too, that they don't have a dedicated earth ground. Rather, they ground the power cord earth and the center tap of the PT together. That's why I was confused about that earlier in the thread. And you rightly corrected me. And again, @robrob 's 5f1 layout is right on that score.

    5F1_Layout_Robrob.png
     

  6. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Let me take a commercial break here for a minute to say: Holy Cow, I'm learning a lot! I'm still ignorant and taking baby steps, but goodness, this has been an education. The fields I know anything about couldn't be more different!

    Another commercial: I would recommend those who are completely green like I am not to order a Mojotone kit. My suspicion is that the Boothill layout isn't crazy like this one, and he sources good parts. Also, he's available on TDPRI and watches the builds of his customers that way. Mojotone vs. Boothill for a 5f1 or 5f2a for a first-timer? Boothill.

    It's not that I'm upset about the quality of the parts from Mojotone. It's that stuff like leaving the death cap on the layout (and sending you one) and giving you old-fashioned power wiring is inexcusable.

    Break over. Back to our regular scheduled programming...
     

  7. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    The voltage you want is 12 or 15 -- not 100. Refer back to tubeswell's post #237. Zeners are voltage regulating diodes, and the specified voltage is the regulated voltage.
     

  8. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Shoot! I wondered about that, but thought that 100V might be the rating. Well, it does make sense that the number would indicate the amount of drop. They are the only ones that Tube Depot sells. I'll have to look elsewhere for the correct ones. Thank you for pointing my mistake out to me, RLee77.
     

  9. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    OK. I just ordered some Vishay zener diodes at 15V from Mouser.
     

  10. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Hey, Boothill is great, and I *always* suggest folks look at Dave as a supplier. But my intention here is *most definitely* not to dump on Mojotone. We've had long TDPRI threads that discuss their good and bad points. Most seasoned builders end up defending them. They're a small shop, but hey, there aren't any big shops in this game. I've seen Weber and Triode and other small firms print layouts that had serious errors or out of date wiring or various ambiguities. I've read of many kits from many suppliers here and in Europe that had any number of problems. Any small supplier can send out a wrong part or leave some missing -- I have a ton more sympathy since I assembled my own BOM for my last build and made way more mistakes than they do. :D

    On my non-Mojo 5F2a I had a big folder full of various 5F1 and 5F2a layouts and drawings -- like you, I had to do a lot of back-and-forthing. A lot more than I would have with Mojo.

    To be fair, I think a few small suppliers may sweat the details more -- I very seldom get a wrong part on long lists I order from Doug Hoffman, and I've heard Dave Allen is very thorough.

    Finally, though, as a clue not to follow Mojo slavishly, note their layout calls this a 5F2. IIRC the 5F2 had a choke, less filtering, and no bypass cap on V1...

    But don't let me even seem to criticize Mojo -- I would and will build with Mojo in the future.
     

  11. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I understand, King Fan. But perhaps you will let me criticize them? :) I really do think it's not kosher to send a layout with the power issues it has--including the stupid death cap. I have very little patience for a lack of thoroughness, especially in places where it really matters.
     

  12. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    These are proportionately more in line with what is expected, except the voltages are too high overall for the 6V6. 50V drop across the 10k supply resistor indicates 5mA through that 10k supply resistor (which isn't that much different from 4mA), and is in line with what is expected.

    The main thing is your screen is now hooked up to the B+2 supply node.


    The period from the late 1940s to the early 1960s was the height of tube production around the world. Tubes were the primary current/voltage control devices in electronic circuits, and everybody was competing to manufacture them, so quality control was higher etc as reputations depended on reliable product. The materials used in construction were good quality and quality control became increasingly more careful with precise machined assembly. This adds up to surviving NOS tubes made in that period generally being more robust (although increasingly hard to find real NOS these days).

    However, that doesn't mean all current production tubes are bad. JJ6V6S are rated at 14W maximum plate dissipation, but IMO can handle 16W. You could try running one of those.



    You need a string of 2-3 5W zeners rated at 12V to 15V each. Each 12V zener drops 12V, so three in line will drop 36V. 2 x 15V will drop 30V. Either way, 30-40V is about how much you need to lose.

    Axial mount zeners are easy to mount on a tag strip - that's why I suggested that.

    100V 5W zeners will be no good I'm afraid. Too much voltage drop for not enough power dissipation. The power rating is important because you want sufficient power rating to comfortably dissipate the heat that you get. To estimate the power consumption of each zener, we need to know 2 things, namely: the voltage drop and the current. The current each zener will see won't exceed the current draw on the PT's High Tension winding, so you can use the HT winding rating as a conservatively safe guide for estimating the current. In this case, the HT winding is rated for 75mA max. So each 12V zener will never dissipate more than 12V x 0.075A = 0.9W. Each 15V zener would never dissipate more than 1.13W. Ideally you want a component that is rated for at least twice the maximum power seen by the component. So a 5W rating is good.

    But a 100V zener should anticipate a need to dissipate 7.5W, and if it did draw that amount, a 5W rated zener would fry.

    As for the physical layout, mount the zeners in a string in series on a tag terminal strip between the HT winding's centre tap and the ground return buss, so that banded end (-ve terminal) of each zener points toward the ground return (as per the schematic I posted above). You need to unhook the HT winding centre tap from its present ground attachment point and hook it to the available zener +ve terminal. Then the -ve terminal of the zener at the other end of the string hooks up to the ground return (buss) at the same point as the reservoir cap's negative terminal.
     

  13. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    You can criticize them, for sure. Or just write 'em an email and tell 'em you almost built an electric chair because of their unclear power wiring. :)For me, I'll continue to suggest people use their kits, but I'll continue to caution folks to use their parts a lot and their layouts (and instructions) a lot less. :D

    No doubt I'm slightly sensitive on this, since I think I endorsed your choice of Mojo when you were planning this build. And then I told you how to wire the tone 'defeat' switch wrong. But to salve my feelings a little, I also see I said at that time, "Mojo's layouts are OK unless they contain non-standard ideas (very often the power wiring, sometimes the grounds or other areas)." And you're nice to recall how I suggested Rob's power wiring instead.

    Hey, in the words of the immortal Tupac, "I ain't mad at cha…" Builds are stressful, and it'd be nice if there was a good road map. FWIW, I'm working on an idea to fix my own build and incorporate Rob and Tubeswell's ideas to see if I can make a better SPST approach to that tone pot.
     

  14. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    BTW, as an alternative to zenering down the voltage at the HT winding centre tap, you could use wire wound limiting resistors in series with each end of the HT winding and the protection diodes on your rectifier socket. Using the HT max current rating as a guide, a 25V drop at 75mA needs 20V/0.075A = 266R (and the closest available size would be 270R).

    But most of the time, we'd expect the actual current draw in a 5F2A would be around 45mA at idle, peaking at around 54mA under heavy signal conditions, (i.e. by a factor of 1.2 above idle current), which implies needing 444R (for 45mA) to 370R (for 54mA) to get a 20V drop. 390R is the practically available 'in-between' resistance value.

    The power rating would need to comfortably exceed:

    P = (1.1 × Idc)^2 × R

    So if there was 45mA drawn(1.1 x 0.054A)^2 x 390R = 1.4W, so 3W resistors should be adequate. 5W or 10W would be better in the hot confines of a tube amp chassis.

    However (noting from the above observation that we could expect that the forward current from the power supply will vary between idle and heavy load by a factor of about 1.2), using limiting resistors in this way will tend to make the power supply sag a bit under load (which you may or may not like). With 390R we'd expect to see between about 17 and 22V drop depending on the playing conditions. And the hotter the resistors get, the more the resistances would change. And the higher the limiting resistance, the more the voltage variation would be. So using limiting resistors doesn't provide the stable voltages that using zener diodes on the CT would give. Zeners give a guaranteed absolute voltage drop, depends what you want.
     

  15. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hey, it's no problem. I'm not upset at you in the least. All you've done is be patient with me asking tons of silly questions and also helping point out mistakes I've made. You've done nothing but lead me on the straight and narrow. I only mean that a company who mass-produces these things for anybody should care enough to revise their layout so that Joe Blow or Jane Doe don't do stupid things. It's not enough, in my opinion, to assume that people have to know enough already to build an amp. The layout just has to be right. It has to. So carry on, King Fan. You've been nothing but good to me!
     

  16. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ

    Attached Files:


  17. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, tubeswell. So these wouldn't work? https://www.mouser.com/ProductDetail/Vishay/1N5245B-TAP/?qs=aho4taYYNuoVEafX/9ir8A==&gclid=EAIaIQobChMIxZq5t7281wIVTSOBCh26bwELEAQYASABEgLum_D_BwE

    They're 1N5245B
     

  18. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Also, how should I connect the power cord neutral to one of the OT primaries? Does one solder the two leads together and cover it with heat shrink? Does one use a wire nut? What's the best way?
     

  19. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Their layout exactly mirrors the Fender schematic, so I'm not sure what all the fuss is about. Some builders prefer to build exactly from the vintage schematics, and cry foul if it's different, lol.
    The cap shown is not a "death" cap -- those were called as such because they were used with two wire power cords, so the chassis had no definitive ground, and you could use the ground switch to put hot ac (through the cap, not directly) on the chassis, which would give you a bit of a bite if you switched it incorrectly. In the configuration in your kit, this is not the case -- your chassis is always hard ground due to the 3-wire cord, and the cap is just providing a bit of noise filtering of the ac line. However, I'd likely omit it since it's not necessary, and is just one more component to fail.
    As far as fusing one side of the cord and putting the switch on the other, functionally it really doesn't make much difference. Ideally, however, you would fuse and switch the hot AC side, to cover oddball rare things like PT internal primary short to chassis ground, so the fuse blows in that case. It's not clear though from the layout which is hot and which neutral, which seems an omission (unless it's shown elsewhere).

    I have to add that I think you're doing an amazing job on your first build, prof... everything looks really nicely constructed and neat, and you obviously have an eye for getting the details just right.
     
    Last edited: Nov 13, 2017 at 4:33 PM
    King Fan likes this.

  20. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, RLee77. Maybe I'm just in a bad mood about it. It just seems to me that, as a first-timer, you'd like to have as much clear and up-to-date information as possible. Perhaps the fault is my own, and I'm looking for somewhere to lay the blame. I'll cease and desist and try to have a better attitude about a few bumps in the road, rather than blaming Mojotone. Sorry, everyone, for letting some frustrations get the better of me.
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.