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Starting a 5F11 Vibrolux build - bias question

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Dec 12, 2016.

  1. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    I'm using a Mojotone parts kit. The layout diagram has a 10k bias pot with a 33k resistor that is grounded/soldered to the back of the pot, and the chassis is already drilled in the right place to add a bias pot. However, the parts kit only came with a 56k resistor to use instead, and all I have on hand is a 50k pot. Any thoughts? Should I go ahead and add a bias pot, and is the 10k pot/33k resistor the best combo?

    Second, I am using a Hammond 290CAX PT that has dual 630/550v primaries @ 115mA. Would there be any benefit in wiring the dual primary on a switch for a hi/lo power option, or would that just cause trouble with the bias? Since this PT didn't come with a 50v bias tap, I have thought about just doing it cathode biased, but I think it may be getting too close to my Deluxe territory if I do, and I'm not wanting 2 of the same amps.

    That's all for now, so thanks in advance.

    Just for further info., I'm using the same Hammond 1750E, 15 watt OT that I have in my 5E3. This will also be a tweed cabinet build as well, but I'm sure the whole process is going to take a few weeks.
     

  2. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Here is a shot of the diagram.

    [​IMG]
     

  3. King Fan

    King Fan Tele-Afflicted

    Jan 1, 2013
    Salt Lake City
    Can't answer either question, but I'm sure someone smart will be along soon!

    IIRC, the 5F11 originally had a selenium bias rectifier. You might also look at modern solutions for the 5G9, which did also.

    Hmm, I wonder if Robrob's several bias options for the 5e3 would have some parallel information.

    https://robrobinette.com/5e3_Modifications.htm#Switchable_Bias

    Sounds like a fun project -- I'll be cheering for ya.
     

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  5. aabbs20

    aabbs20 Tele-Meister

    132
    Mar 21, 2014
    Colorado
    I've built several 5f11s, I love them. I think you definitely want to add the bias pot to dial in the swampiest tremolo. The bias has a huge impact on the trem. I like the 10k pot. Check out the link below for more info. You'll have to scroll down a bit but this thread has all the info you need.

    http://166.63.127.229/Forum/index.php?topic=15153.0

    Check out the Princeton Reverb schematic/layout for your bias voltage supply.
     
    jimytheassassin and jsnwhite619 like this.

  6. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Thanks! That is a good thread to have on hand right now!
     

  7. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    What value resistor do you like with the 10k pot? The schematic has 56k without a pot. So, a 47k to make up the difference? The Mojo diagram has 33k, but it seems that would come up short from the original.
     

  8. aabbs20

    aabbs20 Tele-Meister

    132
    Mar 21, 2014
    Colorado
    I don't remember off the top of my head...if you use a variation on the Princeton Reverb bias circuit it will probably be different from your mojotone layout. 47k would be a good place to start. I think they discuss this all in that thread also. You'll probably have to try a few different values to get the right one.
     
    jsnwhite619 likes this.

  9. dankilling

    dankilling Tele-Afflicted

    Oct 28, 2014
    Lehigh Valley, PA
    Without the -50V bias tap, you will need to set the circuit up more like a Princeton - the values in the 5f11 kit are designed around having the tap. Check here for some methods to do this - http://el34world.com/charts/Biascircuits.htm
     

  10. dbl9

    dbl9 TDPRI Member

    93
    May 25, 2013
    USA
    My son and I built a Mojo 5f11 (actually got it through Marsh). We used a 33k with the 10k pot. Worked great for us. Tremolo sounds great.
     

  11. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Just in case it helps someone in the future, I found this thread, appropriately titled "PT w/no bias tap in 5F11". http://www.tdpri.com/threads/pt-w-no-bias-tap-in-5f11.573959/ along with this modified diagram. In the thread, the OP was using pretty low powered PT, so the 10k resistor was bumped up to 100k at first, but then dropped down to 47k for the low voltage transformer, just to clear that part up here.

    I assume this is what I need to do with a 100k resistor, then diode, then 10k pot + 33k-47k resistor?
    5f11NoBiasTap_v01.jpg

    Since I have to order that stuff anyway, does anyone have thoughts/suggestions on increasing the filter caps or anything like that so I could save on shipping and before I have it soldered?

    Also, the first dropping resistor -- the diagram says 4.7k, the schematic says 470, and they sent a 470 ohm resister. It looks like the old Fender diagrams I found online say 470 ohms, so I assume that's correct, but I just wanted to double check.
     

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    This is my "universal bias from the HT tap circuit:

    [​IMG]

    If you stick with one filter cap I would up that to 50uF for better filtering.
     

  13. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    So, have I been over complicating this whole thing? I feel like I've been worrying about this whole process, and all I need to do is change one resistor and use a different lead to get there. Am I right?
    I did some Paint editing on the layout here to show what I think is correct, but I have the high voltage (RED wire) coming from the recitfier, to a 100k resistor (compared to 10k original for a -50v tap), to the diode, with a 10k bias pot, and I am ordering 27k, 36k, and 47k resistors to have on hand to try. There is already a hole drilled in the chassis in the right spot for a pot, so I'm hoping to just keep things close to the original board layout as possible for organization & aesthetic purposes.
    5f11 Layout.jpg
     

  14. dankilling

    dankilling Tele-Afflicted

    Oct 28, 2014
    Lehigh Valley, PA
    Yes, that's the method I would use. You will just need to make sure you can get into the proper biasing range for the plate voltage you are seeing. I would start conservatively with more negative voltage at the bias point than needed and 'warm' it up from there. That way, you minimize the possibility of cooked parts.
     
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  15. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Thanks everyone! I'm going to get the parts ordered today.
    One more thing I forgot about, and since I'm making an order, I assume this is the switch I need to wire up a footswitch for the tremolo? I'm thinking about building a little box in matching tweed for it. :cool:
    footswitch.JPG
     

  16. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Thought I'd add a pic of the progress so far. I found some demo clips of the 5F11 on YouTube that sounded amazing. I'm getting kinda stoked for this. [​IMG]
     
    CapnCrunch likes this.

  17. CapnCrunch

    CapnCrunch Friend of Leo's

    Jan 7, 2011
    Washington, USA
    I've recently got the bug to build another amp myself and have been watching your thread and the 6g2 head build thread. I'm thinking of building a 6g2 in a 5f11 chassis. I always get excited when I get the board populated too. It means you're not far away from putting it all together. Nice build, keep up the good work!
     
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  18. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    That sounds cool too! A while back, Weber had a closeout on a Tweed Super style chassis for a Super Reverb that I wanted to get really bad for the novelty of it, but I have no use for an amp that big. Still, I like the idea of the later year models built in a Tweed style setup.
     

  19. dankilling

    dankilling Tele-Afflicted

    Oct 28, 2014
    Lehigh Valley, PA
    For the most part, you can use the 5f11 fiber board to do exactly this so you don't have to worry about the cap-can. The biggest difference will be your B+ so select an appropriate transformer set and you're there.
     
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  20. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Finally got through the holidays and got my extra parts in to get back into it. That Hammond 290 CAX PT has I think 275v & 315v secondaries. Would it be worth trying to put both on a switch? Or will it just be a hassle to deal with the bias? If I could build it for a quick "hi- lo" option, great. If I would need to reset it each time, I won't worry about it. Thanks
     

  21. dankilling

    dankilling Tele-Afflicted

    Oct 28, 2014
    Lehigh Valley, PA
    You would have to re-bias with each switch. I'd just go with the 315V taps and use different rectifiers to get the the voltages you like. The original came with 350-50-0-350 75ma taps and was rectified with a 5y3. B+ around 340-350B+ when it was all said and done. With today's voltages, the 315V and a 5y3 will likely be close. I build mine with a 330-50-0-330 120ma PT for around 360-370vdc B+ on a 5y3 to give a touch more headroom and overall wattage.
     
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