Ye Olde Screed Resistor Thread

Discussion in 'Amp Tech Center' started by Luthier Vandros, Aug 27, 2015.

  1. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Fellow Ampites:

    I’m looking at adding some screen resistors to my current 5E7 build. Have any of you figured out a neat way to implement these without a major rewire of the octal sockets? I like to use pin 6 to mount grid resistors. I figure worst case scenario involves me installing a little terminal strip to hang the cinder blocks from.

    Thanks!
     
  2. Wally

    Wally Telefied Ad Free Member

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    LV, I was thinking about this while I had a '57 5E5A on the bench. If you are building this circuit, then things get simple since imho one does not have to do what FendeR did....and I can't think of any reason to do things the way Fender did back then. We had a little thread about it.
    For one thing, I would think you are running a twisted pair for the heater filaments, so there is no reason to run that ground from pin 8 through the blank pin 1 to pin2 and then to ground..(..not that there was any reason for them to do it that way back then, right? They may have done that as a continuation of the process of taking the metal shell of the metal power tubes to ground. This reason was proposed by a tech who has a long experience.) Take the cathode to ground. Use pin 1 for the bias grid stopper contact going to pin 5 and use pin 6 for the connections for the screen grid to pin 4.... Just like the later Fender approach to those sockets.
    Side note...I was looking at that original way of wiring that socket because I noticed how high the screen grid voltage was in that circuit.....3 volts above the plates in that '57.....385 on the screen, 382 on the plates. A fellow member...Fred Mertz iirc...suggested that one could set up things to run that screen with 3 different voltages for 3 different types of operation. Interesting way to get just a bit of variety out of what I think is one of the better Fender circuits from the tweed era. If that '57 had been mine or if the owner had wanted to afford it, that would have been done because the amp was not a pristine all-original...and the change is not of an intrusive nature, anyway.
     
  3. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Ah so a blackface style socket.

    I'm building into a little weber chassis, so I've been punching cement to prepare my knuckles for the beating they are going to take.

    Like occasional binge drinking, you have to build into a tweed chassis from time to time to remind you how good you had it with Hammond.

    Thanks for the tips and interesting commentary, my man. I've enjoyed your contributions as I've run across them.
     
    Last edited: Aug 28, 2015
  4. Wally

    Wally Telefied Ad Free Member

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    Ah, to have some of those smaller hands that Emily or Lily had, right? Those tweed chassi can pose an access problem, no doubt. Good luck with the project....and let us know how it things go.
     
  5. BobbyZ

    BobbyZ Doctor of Teleocity

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    Get yourself an early 1960s 6v6 Revererocket.
    When you're done changing out the electrolytics you'll think "dang there's a lot of room in a Fender tweed chassis."
    Or just get some hemostates at the local cheap tool store.

    As far as the E series tweed wiring my 5E7 is wired like an original. I've been side tracked with other things since I built it so I haven't really messed with it. Sounds good though.
     
  6. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Indeed! Will do.

    My hemostats are one of the tools I use for everything. Can't do without em!
     
  7. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Alright, just to make sure I'm on the same page - is this what you mean?


    [​IMG]
     
  8. mRtINY

    mRtINY Tele-Afflicted

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    I'm adding a screen-stopper to my 5E8A build. My plan is to do a j-bend splice from the wire to the resistor, put heat-shrink tube over the splice and attach the other end to the tube socket.


    -tINY
     
  9. Wally

    Wally Telefied Ad Free Member

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    Yessir, that is the basics of what I was saying. IF you would be so kind to let us know what that screen grid voltage is and what your plate voltage is when you get it wired up. The last part of my sugggestion was that there could be options as to what voltage is applied to the screen.
     
  10. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Will do, Wally. Provided she passes the broom stick test*, I'll provide measurements.

    Quick sidebar: Heater wiring - I've notice that one could simply wire the preamp heaters low and over the sockets instead of having the untwisted leads curving under the other component leads. You could literally keep the twists right up to the pins themselves. Where am I going wrong here?

    *Back in 99 I was repairing a home theater amp. At power up, a cap blew right off of the board, hitting me in the chest and sending fibers all over the place. From that moment onward, I turn on all of my builds for the first time at distance using a broomstick while cowering behind large furniture.
     
  11. jhundt

    jhundt Doctor of Teleocity

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    isn't that how Fender did it? seems to me my Fender amps are just like that.

    But I guess it's 6 of one, half-dozen of the other. You can lay the heater wires on the chassis and fly the leads over them; or you can fly the heater wires and lay the leads under them, along the chassis.
     
  12. Wally

    Wally Telefied Ad Free Member

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    The twisted pair can be lain low and toward the back of the chassis in order to allow sone access to the circuit board, right? This is how Fender when they went to the twisted pair filament wiring later in the tweed era.
    Here is a picture of a build that gives us just a peek at what should be done, imho. 5th picture down shows the best aspect of the filament wiring.

    https://sites.google.com/site/stringsandfrets/Home/bassman
     
  13. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Here's what I mean:

    [​IMG]

    VS

    [​IMG]

    The first example looks perfect. Routed against the chassis and away from other leads, the pair stays twisted all the way up to the lug. In my 5E7 build, the board leads will literally have to come straight down onto the sockets due to space constraints and this layout will be perfect.

    The second example is the typical fender styled routing, which leaves between 1" - 2" of lead right up against the socket, forcing one to route board leads over them.

    Roger that. I'm familiar with this style. Just curious if the example above is "better". I know the old adage, "If it ain't broke...".


    Cheers!
     
  14. Wally

    Wally Telefied Ad Free Member

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    your example and the one I showed are done exactly the same way.
     
  15. Luthier Vandros

    Luthier Vandros Tele-Holic

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  16. Wally

    Wally Telefied Ad Free Member

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    I found that pic just looking for an photo example of how the twisted pair was laid out. I haven't studied anything else they did there.
    I prefer the heater filament layout on the pic in that link I gave you to the black and white wiring above. I prefer not to have those heater filaments in the way on top of the sockets in these cramped tweed chassi. That looping around the socket is not the problem I have with the way Fender did their filament wiring on those earlier tweeds. I prefer to see twisted pairs for noise reduction. If you want, you can maintain the original approach by grounding one side of the heater filament--pin 2 on the power tubes--- by itself, then ground the cathode by itself..thereby freeing up pin 1 for the bias grid stopper resistor. That frees up pin 6 for the screen grid resistor...that Fender did not use in these amps, either. But...as I understand it....the twisted pair eliminates some noise.
     
  17. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Yah I intend to wire it up as depicted and not the old way. Gonna ground the cathodes through (2) 1 Ohm resistors to help with biasing.

    Best~
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I like to tie pin 1 and 8 together to be EL34 compatible so this is how I installed screen resistors on my 5E3:

    [​IMG]
     
  19. Luthier Vandros

    Luthier Vandros Tele-Holic

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    How dare you upstage my MSPaint rendition.
     
  20. Wally

    Wally Telefied Ad Free Member

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    When I was looking at that '57 5E5A; I decided that if I wanted to do anything there, I woudl want a small terminal strip to support those screen grid resistors. I like to use 5W sandcast wirewounds for the screens, and they have some weight.

    Side note: I had an EBS T90 bass head open this afternoon...non-operative condition.
    The interior fuse was burn. I looked around and told the owner: "I don't like this." while pointing to a bunch of huge 15W sandcast resistors that were standing tall off of the chassiss...supported only by their solder contacts and a dab of glue holding them in duets. You guessed it.....two of those huge resistors were torn loose...one wire broken at the resistor....two legs loose on another...one of them never had a chance....you could see the bad solder joint on the solder 'blob'. That was a really poor way to install those resistors...but I don't know if there would be much of an alternative. They don't build amps any more. side side note: the recent buyer thought he had bought a 'boutique' amp. ON the back....blacked out with a marks-a-lot..."Made in China".
    HE is getting his money back on the buy.
     
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