291AEX power transformer UK

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Aug 22, 2020.

  1. Linkjr

    Linkjr Tele-Meister

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    Guys I got a 291AEX PT for a vibrochamp in the U.K. So need to wire it up for 240v. getting a bit confused with the wiring of this. I'm about to wire the brown and black to the switch is that correct?

    And is there a trick to wiring with this pt to get closer to vintage spec in a Vibrochamp

    Here is the spec sheet

    https://www.hammfg.com/files/parts/pdf/291AEX.pdf

    IMG_1507.JPG
     
  2. Paul-T

    Paul-T Tele-Meister

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    291AEX.jpg

    Blue goes to switch/live. White to neutral. Brown and black are connected together.

    That allows 240V across the primary winding.

    I am not sure about the Vibrochamp, but I know the 275V tap secondary is good for 5F2as, Harvards etc. I"m sure others who've built the Vibrochamp will have a more informed opinion,
     
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  3. BigDaddy23

    BigDaddy23 Tele-Holic

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    Paul-T is spot on with the 240v primary connections. I recently built a VC with this tranny and used the 275vdc secondaries - voltages were perfect.
     
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  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The 275v option on my last couple 5f2a's hit right around 340v with a 500R bias resistor, using JJ tubes. I think it comes in around 375v-380v on the 325v option, but again that was with a 500R bias resistor, while the VC schematic (and the 5f2a) calls for a 470R.
     
  5. King Fan

    King Fan Poster Extraordinaire

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    Yeah, by all means start with the 275V tap; leave the 325 taps long enough you could swap 'em in if you ever wanted to.

    The VibroChamp is an especially confusing case of 'which Fender do you believe'? The first schematic showed a B+ of 355V, but later versions showed 420V, and the actual amps, even early ones, often came in with B+ over 400. Is that a good thing? Not necessarily. This TGP thread discusses some pros and cons, but many restorers and most modern builders seem to prefer the cooler voltage for reasons of tube life and overall function. In any case you're looking for plate voltage around 340V and plate-to-cathode around 320...
     
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  6. Linkjr

    Linkjr Tele-Meister

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    Thanks For all the replies guys its a great help
     
  7. Linkjr

    Linkjr Tele-Meister

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    1 more thing according to the spec this transformer has a centre tap the red/yellow.
    The layout I'm working from mojotone has 2 100ohm resistors to ground from the lamp, I thought I could lose them if I put the PT CT to ground.

    Please excuse my basic pic to try and explain:
    IMG_1515.JPG
     
  8. Edvin

    Edvin TDPRI Member

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    Green/yellow is center tap for the 6,3v.
    If u dont have a green/yellow wire you need those 2x100ohms.
     
  9. Paul-T

    Paul-T Tele-Meister

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    OP does have a centre tap.

    It should be quite easy to elevate the heater supply I believe, I think you would tie the centre tap to the 6V6 cathode, pin 8.

    I have one done this on a 5E3, hopefully someone with more experience will comment re the vibro champ
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Your drawing is not correct...
    Inkeduk power_LI.jpg

    The power cord assuming UK color code:
    Yellow/green = Ground to a dedicated chassis bolt. (No other connections.)
    Brown = Hot to fuse.
    Blue = Neutral to switch.

    The PT:
    Blue to switch. (From fuse. Hot)
    White to switch. (Neutral)
    Black and Brown connected together.
    2 Green to 6.3v pilot lamp/heaters
    Green/yellow is the heater CT. Connect to 6V6 pin8 junction with bias resistor/cap.
    Red/Yellow is the HT (high tension) CT. Connect to negative side of first filter cap.

    The CT locations may be different than you have seen in other diagrams.
    The heater CT in this suggestion elevates the CT to the approx 20v of the bias voltage. It usually makes for less heater hum and is easy to do on a cathode biased amp.
    The HT CT is terminated at the neg side of the first filter cap to help reduce hum/buzz from rectifier ripple current. It keeps the ripple current from running through the chassis.
     
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  11. Linkjr

    Linkjr Tele-Meister

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    @King Fan
    Wired up using the 275 secondaries but getting a bit lower than the 340v plate and 320v plate to cathode.
    I got 320v plate and 300v plate to cathode.

    It's sounds good but tremolo intensity is a bit weak sounding. There is a 500ohm bias resistor have clipped in some greater values bit couldn't hear any difference to the trem

    @Lowerleftcoast thanks for correcting my drawing can confirm its nice and quiet at idle.
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Please provide a complete voltage chart for the amp.

    You have a few options. With a voltage chart maybe a best option will become more clear.

    What output transformer are you using? What is the reflected primary impedance?
     
  13. Linkjr

    Linkjr Tele-Meister

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    Thanks for looking @Lowerleftcoast I'm using a hammond 1760c OT, I'm using the 8000ohm primary tap and 8ohm secondary with an 8ohm speaker
    Working on a complete voltage chart
     
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  14. Linkjr

    Linkjr Tele-Meister

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    Ok guys here's the voltage chart for this amp with the tremolo off as in switched off with the footswitch.

    Hoping will be able to get the tremolo a bit stronger.

    V1 12ax7

    1 153

    2 0

    3 001

    4+5 20 9

    6 253

    7 0

    8 003

    9 20


    V2 12ax7 middle

    1 135

    2 0

    3 001

    4 20

    5 20

    6 329

    7 135

    8 158

    9 0


    V3 6v6

    1 0

    2 20

    3 323

    4 328

    5 0

    6 0

    7 0

    8 20


    V4 5y3

    1 0

    2 337

    3 0

    4 AC 293

    5 AC 0

    6 AC 292

    7 0

    8 337
     
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