1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

My 5F11 Build (aka The amp who wouldn't be)

Discussion in 'Shock Brother's DIY Amps' started by brucerbc, Mar 5, 2021.

  1. brucerbc

    brucerbc TDPRI Member

    Posts:
    91
    Joined:
    Oct 11, 2013
    Location:
    A little below the middle, BC, Canada
    An obscure Canadian literary reference in the build thread title seems the only thing straightforward about my latest build: 5F11 Vibrolux in a Mojotone chassis

    First efforts were stymied by a FedEx parcel thief, but Mojotone came through with a major assist and replaced the package.

    Now I've managed to get most of the board and chassis components and wiring done to find that my Hammond 290CAX transformer lacks the 5F11's 50V bias tap.

    5F11 - Layout w Bias Pot - Mojotone1024_1.jpg


    290CAX-page-001.jpg

    Urgh. Can't blame anyone else for that mistake.

    Still ... , perhaps a question only the truly ignorant seldom-poster might ask the learned minds of this forum ... is there anywhere to steal the 50V bias tap elsewhere in the circuit?


    Or do I just need to suck it up and buy the Hammond 291MZ?

    291MZ-page-001.jpg
     
    Paul-T likes this.
  2. NTC

    NTC Tele-Holic

    Posts:
    542
    Joined:
    Mar 17, 2003
    Look at a AA964 princeton schematic. See how the bias circuit is done on that amp. Also note how similar the circuit is other than the tone stack and some of the voltages. You will need to adjust resistor values to get the bias voltage in the ballpark (rink?), but the basic idea is there. I recommend putting a potentiometer in so you can adjust the bias easily.
     
  3. JuneauMike

    JuneauMike Poster Extraordinaire

    Posts:
    6,719
    Joined:
    May 5, 2015
    Location:
    Alaska
    Last edited: Mar 6, 2021
    NTC and Junior Little like this.
  4. Emiel

    Emiel Tele-Meister

    Posts:
    184
    Joined:
    Jan 14, 2009
    Location:
    The Netherlands
    I've noticed that the tone cap in Mojotone's 5F11 schematic (and 5F10, for that matter) is connected to the third terminal of the volume pot, as opposed to the tone pot's housing as in the original Fender schematic. Is this simply a different way of grounding, or does it give a tonal difference too?
     
  5. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    36
    Posts:
    3,532
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    Yeah, don't replace the transformer. Where Mojo has the brown bias tap going to the board
    • take your bias voltage from pin 4 or 6 of the rectifier (EDITED from pin 8 earlier - my mistake)
    • change the first resistor before the diode to 100k
    • use a bias pot and set the bias change the bias resistor from 56k to somewhere around 18k-22k

    As stated above, reference the Princeton Reverb layout for all you bias info for this. http://www.prowessamplifiers.com/schematics/fender/princeton_reverb_aa1164_layout.pdf
     
    Last edited: Mar 6, 2021
    brucerbc, lathoto and NTC like this.
  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,320
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Jason, you know more about the 5F11 than anyone, but do you mean pin 8 or pin 4, like the PR you mention? Maybe either? I’ve definitely heard of tapping B+ for bias, but I’ve always just used the HT feed. (Note tho if I fit recto protect diodes, I tap pin 3 before the diode.)

    BrucerBC, another excellent source for this 'PR style' bias is Doug Hoffman’s classic page, where he discusses the PR specifically.
     
    brucerbc and jsnwhite619 like this.
  7. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    36
    Posts:
    3,532
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    Thanks for catching me on that! Yes, I definitely named the wrong pin for some reason. I edited my response above.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,205
    Joined:
    Dec 30, 2019
    Location:
    california
    It is just a different (better) ground scheme. You will notice the cap attaches to the volume terminal that is attached to the input jack ground.
     
  9. brucerbc

    brucerbc TDPRI Member

    Posts:
    91
    Joined:
    Oct 11, 2013
    Location:
    A little below the middle, BC, Canada
    Thanks all for quick and super-helpful replies! Really appreciated.

    I reviewed the linked schematics, did some reading on the linked sites, then made the recommended resistor changes:

    Had to go back to Ohm's Law to figure out which of my on-hand resistor combinations would get me in the 18k-22k range. Ended up with 33k+56k for 20.8k in parallel.

    Thinking I'm once again ready to install the board and complete connections, but posting pictures here in meantime for interest. Constructive criticism also appreciated.

    Again, thanks for sharing knowledge and guidance!


    20210306_141107.jpg

    20210306_141122.jpg

    20210306_141144.jpg

    20210306_141220.jpg

    20210306_141226.jpg

    20210306_141230.jpg

    20210306_141306.jpg

    20210306_141307.jpg
     
    jsnwhite619 likes this.
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    36
    Posts:
    3,532
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    @brucerbc Did you catch my correction - and @King Fan good eye - about how to get the rectifier voltage for bias? Just follow the Princeton Reverb layout, and I use pin 6 on a Tweed amp because it's closer to the board.
     
    brucerbc likes this.
  11. brucerbc

    brucerbc TDPRI Member

    Posts:
    91
    Joined:
    Oct 11, 2013
    Location:
    A little below the middle, BC, Canada
    Thanks @jsnwhite619. Yes, did catch correction to take bias voltage from rectifier pin 4 or 6.

    Hoping to get to the board install this evening or tomorrow.
     
    jsnwhite619 likes this.
  12. JuneauMike

    JuneauMike Poster Extraordinaire

    Posts:
    6,719
    Joined:
    May 5, 2015
    Location:
    Alaska
    You're doing great. Looks like a nice clean build.
     
  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,320
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Nice workmanship. Here’s something I hope is constructive. In this pic we see neutral to switch and hot to fuse side.

    AAB5702A-F4BB-4588-AD87-65342CDB65F8.jpeg

    For technical reasons, like what gets hot if fuse fails or rat eats insulation, most experts prefer hot to fuse tip, fuse side on to switch and then to primary A, and neutral spliced to primary B, with A and B being arbitrary. From Rob's 5E3:

    84831D58-B707-4E39-AC5D-2D590CEB00A7.jpeg

    That brings up a question, just to check. The pic parallax and the Hammond overspray make it hard to see; which black wires are your primaries? Where do they go right now?
     
    Paul-T likes this.
  14. 2L man

    2L man Tele-Meister

    Age:
    61
    Posts:
    292
    Joined:
    Nov 23, 2020
    Location:
    Finland
    If potentiometers are installed without insulators there is possibility for ground loops when you also have a ground loop soldered on pot cases and obviously all those circuit return currents are made to run thru chassis when you don't have a solid ground bus.

    DC Current follows kirchoff law but when there are also AC on amps because on impedances things get complicated but it is not good to use parallel ground loops which there comes when circuit has many grounds. And then if return current is made run thru chassis it can become what ever...

    Tube amplifiers are often very simple and single ground bus for all stages is enough! Star ground is very good method but it does not use chassis for return current either!

    Think chassis as a shield which protects circuit from electromagnetic stray fields.

    Chassis is huge and it gets over circuits so it does not make sense to run current in it because then it can act as antenna and spread electromagnetic stray field to circuit.

    There is need for only two electrical connection to chassis. First is mains cable earth wire very near mains input and it makes amp electrically safer. Second=last is the end of ground bus very near the input jack which connects shield and ground where ground bus current is lowest.

    Screw mounted lug makes good contact but non insulated input steel jack can be used because there does not run any current and if more than one jack is used I think all can be mounted without insulation because current between them is very small.

    I think loudspeaker jacks should be insulated although there is AC and no return current runs there but when they are easy to obtain and are cheap this removes noise possibility.
     
    Last edited: Mar 7, 2021
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,205
    Joined:
    Dec 30, 2019
    Location:
    california
    As @2L man pointed out, you definitely have ground loops now. A better (more likely quieter) ground scheme can be chosen. It would less messy to change the grounding from the back of the pots now than to change it later.;)
     
    King Fan likes this.
  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,320
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    ^^^ true. I was gonna mention this, but BrucerBC, since you’ve already done the soldering (another suboptimal item Mojo shows) I also figure you might be able to leave it. Yes, Merlin mentions it creates (small) ground loops, but tons of homebuilds as well as factory builds do it; Marshall etc. did it after all, and even Merlin basically admits those little loops must not create much noise. A practical problem is soldering to pots, but you've done that already. The bigger problem, as LLC says say, is *unsoldering* pots if they need replacement. If you want to upgrade now, a standard separate ground bus is shown in many builds and in many of Rob's diagrams. If you decide to stay with the current plan, and do it right, your amp will still work.
     
    Last edited: Mar 7, 2021
  17. brucerbc

    brucerbc TDPRI Member

    Posts:
    91
    Joined:
    Oct 11, 2013
    Location:
    A little below the middle, BC, Canada
    Thanks all! Great spotting and suggestions.

    I don't mind the de-soldering so much. The iron will be hot when I get to installing the board in chassis anyway.

    I will re-jig the primary lead wiring and remove the pot-mounted ground bus as recommended. That won't get it to the Rob-Rob standard, but will at least be in line with my Two Stroke, 5E3 and Vibro Champ builds which are all pretty quiet.

    I've learned some things here, for sure, that I can better apply in future builds.

    Thanks again everyone.
     
  18. 2L man

    2L man Tele-Meister

    Age:
    61
    Posts:
    292
    Joined:
    Nov 23, 2020
    Location:
    Finland
    Possibly you can use that wire you have already soldered without problem when you continue it to power supply Negative IF you connect ground bus to chassis ONLY at input jack. Then no return current runs thru chassis and all thru ground bus.

    If you connect power supply negative to chassis and that partial ground bus to chassis at input jack then some current flow first towards input jack but some flow thru potentiometers to chassis and it is called ground loop.

    Practical ground bus is installed to turrett board and it begins at power supply negative and ends to input jack and every ground points in between "feeds" amp stages.

    Power supply negative and chassis should not be connected together because negative is where all the electrons flow to the system and chassis is very grong place to let them escape!
     
  19. brucerbc

    brucerbc TDPRI Member

    Posts:
    91
    Joined:
    Oct 11, 2013
    Location:
    A little below the middle, BC, Canada
    Thanks @2L man for that suggestion.

    If I understand correctly, that would require insulating washers on all pots to break the return current runs thru chassis? Or are insulating washers also needed for 2 of 3 input jacks?

    With luck, insulating washers may be a part I can source locally and void waiting on shipping.

    If only change, that would mean 3 ground points in this build: power cord at transformer bolt tab, pre-amp bus at input jack and power amp bus at another transformer bolt tab.

    Would that be considered a significant improvement (at least design-wise, if not necessarily noise-wise)?
     
  20. 2L man

    2L man Tele-Meister

    Age:
    61
    Posts:
    292
    Joined:
    Nov 23, 2020
    Location:
    Finland
    When potentiometer are mounted without insulation like most of us do and pot case is not used as active circuit no current flows there and then pot cases act as a shield around pot resistors almost like chassis makes a shield over amplifier.

    Do not connect power supply to chassis!!! If you do and you also connect this partial ground bus to input jack negative then all this current returns to power transformer thru chassis and it passes input jack proximity and it is not good because there instrument signal is weak and amo input impedance is high.

    Yes mains cord earth is connected to chassis and I believe it is written to law.

    Ground bus is connected to chassis to prevent potential difference only at input jack because it is best place and when it is only place there does not flow any current. I think all input jacks can be without insulation IF it is only place ground meets chassis and no current can flow thru and between them.

    If you use the wire between potentiometers as ground bus finish it at input jack. But do not connect any other active circuit part to chassis. Connect beginning of this ground bus to power supply negative.

    If you want you can make own grounds for power stage and phase inverter which you also connect to PS Negative and it becomes Star Grounding but they are not connected to chassis, only to PS negative.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.