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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. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    You're kind, Keithb7. You and King Fan are good at that. Yes, I am thorough, but that is both good and bad. Sometimes I get ticked when I don't understand something or am tired of being out of my comfort zone of things I know a lot about and can control. Anyway, I don't have an ETA on the cabinet. I think he'll be able to get a good bit done this week. I'm hoping that maybe I'll have it sometime before (U.S.) Thanksgiving. But I don't have a speaker yet, either. I got the best price on Amazon, but they're currently back-ordered. I must say that I'm itching to have those things, but it's probably good that I don't. It will give me time to work through these bugs in the chassis for a bit and get things right.
     

  2. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Y'all don't be lighting a match under me! Haha. The weather looks pretty good here for the coming week, so I'm hoping to get most of it knocked out soon.

    On a side note I thought I'd share, I posted a long time ago about the finger joints always looking so loose on old Tweed cabs. Some people suggested the wood was shrinking, but if that was so, the outside would draw in as much as the inside. I saw recently that Fender built the cabs around steel frames. As long as the joints where deep enough, any variation in the individual boards could be played with to match the frame. So, those old cabs that could hold a 16 penny nail in the gaps between the joints weren't glued and clamped like everyone does today. I'm guessing they were cut quickly, jointed deep, and when they got to the assembly frame the extra length on the fingers was knocked down and then rounded over. The hot hide glue set in a few minutes on that frame, and it didn't matter if the joints met up.
     
    King Fan likes this.

  3. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Wow, Prof, you've brought in a ringer -- I've seen your cabinetmaker's work and it's top drawer (unintentional pun). :)

    I may have missed it (this train is highballin') but have you had a chance to review the wiring around 6V6 pins 3 and 4? How about the voltages there?

    Your Mojo760 PT puts out a nominal 330v HT. I was surprised you B+ was so high, and I'd be curious to see if another 5Y3 might drop it some. But then I see where that PT has been noted to run high B+ in Champs (Mojo again -- the PTs they choose for their kits are another place I'm not a huge fan).

    BF/SF (factory) Champs / VibroChamps ran notoriously high B+, but tweed Champs weren't supposed to, so maybe we'll just say you have the more 'modern' style in your amp. Zeners should be a good fix, and I'll watch with interest as you do this -- I used a dropping resistor in my VibroChamp, but zeners are, I believe, a more sophisticated solution.

    The Mojo thing: Keith is right, I shouldn't come across so much as defending them. I'm just trying to say *nobody's perfect and we need all the small shops out there to stay in business.* They sure ain't perfect. I recall, Keith, you had to deal with a too-big PT cutout (for their too-big PT) and misplaced fuse/power holes on their 6G2 chassis. Their 'brown' faceplate and backplate are well-known to be the wrong brown -- like weak cocoa instead of strong coffee. My PR kit was really nice (and is what *many* specialty amp shops start with), but it was weak that they supplied only white wire for all non-heater use, and worse that they showed Stokes' mod wiring without saying so -- it's pretty dang subtle to catch on the layout. For sure, @RLee77 is right that swapping hot for neutral in power wiring won't kill you in ordinary use. And he's right: that isn't a functioning death cap -- it can't possibly kill you. And he's right that plenty of builders get upset if their wiring isn't more or less 'old style.' We should all complain to them when we find these issues -- but we do really need them to stay in business, so I try to temper my critique with realism. A small company mostly doing their best is not like Fender or Gibson screwing up, or, say, Takata airbags. :D
     
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  4. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hey King Fan. Thanks a bunch for your continued interest. It must be like a marathon! :) I did not yet go back and measure pins 3 and 4 on the 6V6. Right now, I'm fixing the power wiring. I also went to the electronics store around here this morning and got a few 13V and 14V zener diodes (5W rating). I just came up from my crawl space, where I'm moving the earth (safety) ground on the chassis to create more space for those diodes to go in. I just have to figure out how to mount them (i.e., what material I'll use to mount them).

    I'll post a pic in a few minutes of how I re-did the power wiring. Then I'll go back and check on pins 3 and 4 of the 6V6 and post the numbers here. I'll try it with the GE 5Y3 I was using, as well as an RCA 5Y3 I have to see which one puts out the lowest voltage.

    Meanwhile, I found a very helpful discussion and layout of the zener diodes in @robrob's treatment of 5f6a mods. I was having a hard time picturing how they'd be arranged. Here is the link:
    https://robrobinette.com/5F6A_Modifications.htm#B+1_Voltage
     

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I'm re-wiring the PT and power cord now. I was trying to figure out, though, which part of the Fender-style fuse holder gets the hot coming from the power cord. I found a post discussing the differences between the Mojotone wiring of the 5f1 (and thus also the 5f2a) and Rob's wiring of the 5f1, which we noted above. Here is the link:http://www.tdpri.com/threads/5f1-champ-ac-cord-wiring-differences.711653/

    Rob says there in post #10: "Yes, the central connector in Fender style fuse holders should get the hot wire so a blown fuse will cut the connection and not shock you when you pull the fuse out with your bare fingers."

    I assume this means the one on the tip?
     

  6. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I created some space now where the zener diodes can go between the high-tension center tap of the PT (red-yellow-red wire) and the B+1 node on the ground (-) side. Here are some pics of what I did. I hope it's right!

    You'll see that I moved the safety ground up to create more space in the middle. I re-wired the PT so that one of the black PT primaries is now joined in that plastic block terminal to the white (neutral) of the power cord. The power cord black (hot) goes to the tip of the fuse (is that right?). The side of the fuse is jumpered over to the switch. The other black PT primary is on the other side of the switch.
    IMG_9541.JPG IMG_9542.JPG IMG_9543.JPG
     
    Last edited: Nov 14, 2017
    King Fan likes this.

  7. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes, the fuse tip terminal is where the active/phase mains wire should go to for safety. If you unscrew the fuse cap with the amp plugged into the power, you don't want to get an electric shock from accidentally touching the ring terminal on the fuse holder.

    The fuse holder ring terminal then goes to the mains switch, and the mains switch goes to the appropriate black lead from the PT primary.

    The neutral mains wire should go to the same screw terminal as the appropriate black lead from the PT primary.
     

  8. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks very much, tubeswell. I think we simul-posted. See my pics above of the re-wire. Now I think I'll go through all the startup process again, just to be careful. Then I'll work on putting these zener diodes in, if everything still looks about the same voltage-wise. I got three morning that are 13V and rated at 5W from a local electronics store.
     

  9. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Great stuff, Prof. You really do extremely tidy work! The photos are super too.

    I might hold the zeners until I was certain about your 'best-case' B+. I'll wait for the pros to advise, but if I'm right, you can test all your voltages and make sure your amp is working, then decide on how many to mount, re-measure, and listen.
     
    Last edited: Nov 14, 2017

  10. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, King Fan. Yes, I think I'll try not to get in a hurry with them. I'll turn my attention back to the measurements and pins 3 and 4 of the 6V6 and play a bit with different rectifiers, etc. I'll also take bias measurements the way @tubeswell suggested above. And I think I read something about the import of heat-sink when mounting the zener diodes, too.
     

  11. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Mount the zener string on the terminal tag strip before installing the whole shebang in the amp. The diodes go in series (anode to cathode). Leave a bit of lead length on each one so that the diodes are not too closely bunched up. This helps with heat dissipation. The HT centre tap (yellow-red lead) goes to the anode (non-banded) end of the string. The banded end goes to the ground return where the reservoir cap ground lead is grounded. When you get the amp going and measure the voltages again, you can decide whether to hitch the HT Centre tap to the 1st, 2nd or 3rd diode in the string, so make sure there is room to unscrew the terminal from the chassis so you can get at the soldering easily.
     
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  12. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Cool. Literally. I recall a (?) Weber page where they described the special thermal goo you were supposed to embed them in. Air sounds like a much easier heat dispersant. :)
     

  13. D'tar

    D'tar Tele-Holic

    949
    Jan 11, 2013
    WNY
    From Mr. Robinette

    [​IMG]
     

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    It also just occurred to me that I have a NOS RCA 5R4GYB sitting around that may gain me a few volts at least. I think it should be able to be substituted for the 5Y3GB, since:
    • A 5Y3GB and a 5R4GYB both pull 2 amps of filament current
    • The 5R4GYB has twice the plate current rating (250 mA) as a 5Y3GB (125 mA).
     

  15. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes you can sub in any 5V octal rectifier tube that draws up to the maximum current rating of the PT's 5V winding. Different types of tubes will have different (internal) anode resistance and forward current supply characteristics, which will affect the B+ voltage.

    One thing to watch out for is the maximum reservoir capacitance rating of the tube. Some rectifier tubes are 'rated' to handle more reservoir capacitance than others*. The ratings were put there by manufacturers who wanted conservative operational ratings on their tubes so that they would last a long time, or so that they could avoid warranty claims for 'improper use'. But tubes are pretty robust, and in general, most types will handle 16-20uF. 5Y3G and 5R4GYB are both rated for 20uF but can routinely handle 32uF (even though they are 'rated' for less). The tube will wear out faster, the greater the actual reservoir capacitance. Where the capacitance greatly exceeds the tube's recommended rating, the tube will have quite a short life. If in doubt, look at the relevant tube datasheet and use it as a rough-order guide.

    https://frank.pocnet.net/sheets/049/5/5R4GYB.pdf

    * the reservoir capacitance determines how hard the tube plates have to work to pull electrons out of the reservoir cap's +ve pole. The bigger the capacitance, the harder the tube has to work per unit of time, in order to charge up the cap.
     
    Last edited: Nov 14, 2017

  16. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN

  17. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Here are some more progress pics on the cabinet:
    IMG_20171114_183239422_HDR.jpg IMG_20171114_183253077_HDR.jpg IMG_20171114_184412.jpg
     
    Outlaws likes this.

  18. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Looking good. Is he going to rout the edges with a 1/2" roundover bit?
     
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  19. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Yep.
     
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  20. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    That white stuff is thermal grease - a non conducting grease than is used for improving heat transfer between surfaces. Typically when used this way the heat producing component gets bolted to a heat sink that is vented either to outside air, or to a forced air circulation system (fan). A tube amp chassis heats up in operation anyway, so running a 25W bolt on zener to a tube amp chassis will attract more heat to the zener than the zener actually produces in this instance.

    However, thermal grease has no practical application for axial-mount zeners on a tag terminal strip. The three 13V 5W zeners you have will be able to dissipate a maximum of 15W between them with no need for thermal grease. All you need to do is ensure the zeners are mounted so as not to be all bunched up together. The best thing is lead length plus the flat surface of the terminal tags, to help dissipate heat. The zeners can each dissipate 5W, but they won't actually be dissipating anywhere near this much in this application. 13V x 0.054A = 0.7W each (tops). The air inside the chassis won't get hot enough to transfer much heat into the zeners in normal operation. Nowhere near as hot as the chassis itself.
     
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