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5e8a chassis question

Discussion in 'Shock Brother's DIY Amps' started by mgrossm, Nov 20, 2020.

  1. mgrossm

    mgrossm TDPRI Member

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    does anyone know if webers chassis for the 5e8a will accept a hp pt (3× 3 & 3/4 mount) without having to cut it? Im assuming the one sold by mojo is also used for other amps and so would not have a pt cut out big enough for the hp pt.

    Also one other question...if i decided after buildingbto try ss rectifiers...wouldni just pull BOTH tube rectifiers and replace with 1 SS? Anyone have any thoughts on using the mojo752 for a pt vs their 753 or MM's PTs?
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The 752 is higher voltage than the 753. With SS rec the 753 would probably be as much voltage as you would need or want. Yes one ss rectifier will be fine.
    Sorry I can't help with the size cutout of the chassis.
     
  3. NTC

    NTC Tele-Meister

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    Have you asked the Weber people?
     
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  4. mgrossm

    mgrossm TDPRI Member

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    I havent yet
     
  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    You would need to file out the mounting holes, if nothing else. The low-power Twin PT has a hole spacing of 2.75"x3.5".

    Whatcha got in mind? A hopped-up low-power Twin with KT88s or some such? :)
     
  6. mgrossm

    mgrossm TDPRI Member

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    Its less the mouNting holes im worried about than the actual body of the pt...if that rectangular hole needs to be enlarged that could be a real PITA. And its just that im a bit worried, despite all reassurances (even from MM who i trust), about the capabilities of the lp pt to handle the dual rectifiers plus the fact that quite a number of original 5e8as actually shipped with hp pts. Seems like the 752 has enough current to drive the 2 recs and on the surface mm's does not (4v?) But apparently theirs is rated very conservatively and are based on measurememts of pts taken directly from original amps and i do not think they would be selling a pt for a 5e8a that is destined to fail for people, am i right?

    I ended up putting a ss rec in the 5f8a i built this summer and really like it and i have a feeling i might end up doing that...but anyways....Im just not sure what to do. Either go all MM because ive had good luck with them or MM OT and heyboer pt. But according to pat at MM the chassis mojo makes the 5e8a chassis dimensions so it will also fit several other models .
     
  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Man, every time I look at the Mojotone specs, I feel like I'm losing my mind. The 40-watt PT (MOJO752) is rated at 355VAC and 300mA, and the 80-watt PT (MOJO753) is rated at 325VAC and 250mA. Huh??
     
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  8. Ess Eff

    Ess Eff Tele-Afflicted

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    Mine.....
    :)
    .
    IMG_20191024_164957.jpg
    IMG_20200216_131610.jpg
     
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  9. mgrossm

    mgrossm TDPRI Member

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    Yeah no doubt....mm's hp is 348v and lp is 380v...so seems right that the lp pt have higher voltages. Its just the 4 amps on the 5v on mm's that makes me nervous.

    Has anyone used the mojo752?

    Or the MM lp pt?
     
  10. Wally

    Wally Telefied Ad Free Member

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    Fwiw, the 5F8A ran almost 20 volts less on the B+ than did the 5E8A...according to the Fender schematics, at least.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    As for a SS rectifier vs. 2 x GZ34??? Ime with the Fender Pro Sonic, there is a great difference between a single GZ34 and a solid state r3ctifier. Basically, one is moving from the first generation Marshall to the second generation...harder, punchier sound with later break up. I l8e the 5E8A as it is, and pulling...or switching out....one of the rectifiers gets one back to the Lo-po Twins contemporaries...the Pro, Super, and Bandmaster amps. Versatility. The difference between SS and 2xEL34s might be less of a difference?
     
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  12. mgrossm

    mgrossm TDPRI Member

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    i pulled the gz in my 5f84 build and put in a ss and it really increased the volume/headroom. Really like it. May try the tube again.

    Still hoping someone chimes in about their experience also with PTs in 5e8a builds. And if i can beg while im at it, i really need some advice, pics on an adjustable bias scheme. Ive been using those white 50k trim pots from hoffman with 2 lugs in the front and one on the back and im not sure which to wire what to...thinking of using one of these..a 50k trim pot with a 22k (?) resistor to ground. If anyonr had any experkence or could post a pic on how to wire up this it would help. Ive seen ceriatones but im not sure if the the three prongs are the same...attached file of pot i used on 5f8a
     

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  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I always meter the pot. The center is usually the wiper. When a pot is used in a bias circuit generally it is used as a variable resistor. One side and the wiper will make a variable resistor. It can be helpful to attach the wiper to the unused lug so should the wiper contact fail the full resistance of the pot will be in the circuit cooling the bias to protect the tubes.

    It is easy and typical to replace the 56k resistor with a 22k resistor in series with the 50k pot.

    EDIT: Rob has an illustration of using a 27k resistor with a 50k pot on his 5F6A mods site.
     
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  14. mgrossm

    mgrossm TDPRI Member

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    Thats whats in the photo above
     
  15. mgrossm

    mgrossm TDPRI Member

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    So in a 50k pot like the one attached the middle is usually the wiper? How about on the white one i used on the 5f8a (in the attachment above this post)...i ran a wire from the back/middle to the front right (yellow wire) which is connected to the 27k resistor. The front left lug is connected to the 8uf cap and 15k resistor.

    Im still confused on a pot like this which lug would be wired to what in an 5e8a i.e., which would go to the 22k resistor, which would go to the diode amd also we're at it....should the 220k resistors go to the neg of 100uf cap which then goes to diode or should they go to middle lug of bias pot...attaching 2 more photos to show what im talking about.
     

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  16. JRapp

    JRapp Tele-Meister

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    I had a '56 5E8A for about 15 years and I replaced the 5U4s with one 5AR4. A SS plug was too tight sounding and a 5AR4 helped that 5V winding survive after 60 years.
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    This is connected exactly how Rob has it on the 5F6a site. This design uses the pot as a variable resistor.
    The hand drawn design is using the pot as a voltage divider. It is a different method to arrive at the same place. (Adjustable bias.) The middle lug is the same as the back/middle on your white 50k pot.
    This design does not use the pot as a variable resistor.

    Either design can be used.
    Where you connect the left or right lugs just determines which way the pot rotates to make the bias hotter or colder. Clockwise/counterclockwise.


     
  18. mgrossm

    mgrossm TDPRI Member

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    If i wanted to use the variable resister method then for a 5E8A circuit could i just do this? See attchament. What do you do in a pot like that with unused lug? If i used my white pot could i simply connect wire from back/middle to the front and then connect it to the 22k resister....the other front lug going to the diode as pictured....heres someone elses example...and a drawing i just did (forgive! Please)
     

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  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The drawing with the blue ink will work fine.
     
  20. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Yep, it's the current rating that is confusing to me. 250mA seems kind of light for a 4x6L6 amp -- their Twin Reverb PT is rated for 450mA, which seems more like it.

    And the 40-watt PT being rated for 300mA seems like massive overkill since a 2x6L6 amp only needs about half, maybe two-thirds of that. It's almost like they mixed up the spec sheets...
     
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