Marshall question. Resistors on the control grid

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Mar 25, 2005.


  1. pfrischmann

    pfrischmann Member

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    I have a queston for the Marshall techs out there.

    My amp (78 super bass) has a 1.5K 1/2w resistor tied across each pair of output tubes. (2 resistors total). They are on pin 5 of each socket.

    I was reading in gerald Weber's book about puting a 5.6K (or a 1.5) 1/2w resistor on each output tubes control grid (pin 5).


    Is this the same thing?
    Is it an either or approach or should I do both?

    Thanks,
    Paul
     
  2. WailinGuy

    WailinGuy Member

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    I think what you're describing is that the drive signals go directly to the control grids (pin 5) for one of the push-pull pairs of output tubes (inside pair?), but those signals have to pass through 1.5K resistors before hitting the control grids of the other pair of output tubes. In other words, only two of the four output tubes are fitted with control grid resistors.

    It's really best if all four tubes have control grid resistors on them. 5.6K is the standard Marshall value. Chances are, you won't notice any significant difference in tone, but you may find the amp to be more stable at high volume levels, with smoother distortion and less of that "about to blow up" (I mean in a bad way) sound.
     
  3. pfrischmann

    pfrischmann Member

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    Hi Jim,
    thanks for the help. Actually I believe it's a bit different than you described. It's not the inside pair, it's the left and right pair. The left pair of el-34's are sharing a single 1.5k resistor. the resistor is tied to each individual tube socket's pin 5. The same thing on the right. Each "pin 5" represents a seperate EL-34.



    pin5------1.5k-------pin5 pin5----------1.5k-----pin5
     
  4. WailinGuy

    WailinGuy Member

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    Actually, the "picture" you drew is exactly what I thought you had described. But the 1.5K resistors are NOT being shared! For each resistor, there is a drive signal wire (which comes off of a .1uF phase inverter coupling cap connected to a 220K bias feed resistor on the circuit board) connected to one end. Whichever end it is, I don't know, because you didn't show that on your diagram. But whatever end that is, it is not affected by the resistor, because the drive wire is connected DIRECTLY to the control grid for that tube. Only the tube at the OTHER end of the resistor (the one that doesn't have the drive signal wire directly attached to pin 5) is getting the benefit of a 1.5k grid resistor.

    This is what you want:

    Each output tube socket has its own dedicated 5.6K control grid resistor, connected to pin 5. The other ends of the left pair of resistors are connected together and with the left drive signal wire. Similarly, the other ends of the right pair of resistors are connected together and with the right drive signal wire. (Terminal strips and/or 18 or 16 guage bus wire is typically used to support and connect the grid resistors.)
     
  5. pfrischmann

    pfrischmann Member

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    Thanks Jim,
    I'm afraid I'm losing you somewhere

    Is the idea to replace these 1.5k's with individual 5.6K's for each tube?





    I'm not sure if this pic helps...

    Can you direct me to somewhere I can get a better understanding of how to mount these

    //pic15.picturetrail.com/VOL625/3397225/6975410/90531390.jpg
     
  6. WailinGuy

    WailinGuy Member

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    Yes.

    The orange wire going to pin5 of the socket on the left side of the picture carries the drive signal for the pair of sockets shown.

    You want to replace this:

    pin5-----1.5k-----pin5 pin5-----1.5k-----pin5

    with this:

    pin5----5.6K--+--5.6K----pin5 pin5----5.6K--+--5.6K----pin5

    where the two drive signal wires connect to the junctions of the two pairs of 5.6K resistors (at the "+" signs in the above diagram). The drive signal wires should no longer connect directly to pin 5.
     
  7. pfrischmann

    pfrischmann Member

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    aha!

    That is both sides of the PI, right?

    I get it. Instead of a 1.5K in series we will now have two 5.6 in parallel.

    Now, can anybody explain how to mount the resistors...or better yet, show me?


    Thanks,
    This is fun!
     
  8. John Phillips

    John Phillips Member

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    The best way is to get four pieces of terminal strip, each with one ground tag and one free tag, and mount them under the nuts that hold the tube sockets, next to pin 5. Connect the resistors between pin 5 and the free tag, then link the free tags on each pair together and connect to the feed wire.

    Marshall did it without the tag strips, with the resistors soldered to the tube sockets and the other end connections floating in space with the wires soldered to them. This works, but it can cause the resistor leads to break at the socket pin since the resistor can vibrate around.

    FWIW, the nut on the right-hand socket in that pic looks loose anyway :). And probably the screw has been replaced (it looks nickel-plated)... these screws, and the ones holding the filter caps down, often break on old Marshalls. Brass was not really the best material to use...
     
  9. pfrischmann

    pfrischmann Member

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    Got it so I will still need some wire to connect each side together.

    Any suggestions on how to route it?


    wire--tag board----resistor---- middle tubes pin 5 (new piece of wire)----outside tubes tag board-----resistor, outside tubes pin 5...


    Like that for each side?
     
  10. WailinGuy

    WailinGuy Member

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    You should need only two new pieces of wire. One piece connects the free (not grounded) tags on the left pair of terminal strips and the other piece connects the free tags on the right pair of terminal strips. (As John explained, the free tags connect to the other ends of the resistors - the ends not connected directly to the pin 5 of each socket.)

    The two feed wires should get disconnected from pin 5 of the center two sockets. The left feed wire should connect to either one of the two free tags on the left, and the right feed wire should connect to either one of the two free tags on the right. Generally, it's a very good idea to keep the feed wires as short as possible, so that might dictate which free tag each one connects to.

    Paul, this is pretty basic stuff. I'm concerned that you are trying to figure out the WHAT, but maybe don't have quite enough background in the HOW and WHY. If you're still having trouble understanding John's and my instructions (which I think are fairly clear) , then I suggest you either:

    a) Have either tech or a possibly a friend who is experienced in working on tube guitar amps do the rewiring; or

    b) Step back for now. Read up on basic tube amp circuits, study some schematics, paying particular attention to how push-pull 4-tube output sections work. Then, armed with more knowledge and understanding, come back to this project.

    Before you pick up the soldering iron, I want you to be 100% confident that you know what needs to connect to where (and why) and how to implement that. The voltages are too high and there's too much opportunity for serious damage to result (this is a 100-watt Marshall!) if you are not in this state of mind.

    Sorry to sound like an ass.
     
  11. pfrischmann

    pfrischmann Member

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    Thanks for the help guys,

    I know enough about amps to do simple stuff like change out the electrolytics, drain the caps and biasthe amp. I've read several books but have not been able to apply the knoledge. There is no one to teach me but nice people on the net. I'm sure you know better than I. Application is far different than reading it.

    This is unusual in that is does not show on the schematic this way and I am essentially modding the amp.....especilly this schematic as I'm not sure one exists for this amp....maybe Park.

    I truly understand everything at this point but how to get the feed running parallel. I'll keep reading. I just learn better visually.

    A terminal strip has several points on it but they are not wired together. I have to figure out a way to get current from my feed to each resistor without it being affected by the other (running in parallel)

    It's funny, everytime i try to apply some knowledge on this stuff, someone says .....get a book or find someone to help. This is a bit of a dying art. There are huge holes in my education and bigger holes in the material out there if your not an EE or tech...It's like an old boys network.

    I'm open to suggestions on how to get to the next level.
    How did you guys learn and start applying knowledge?
     
  12. WailinGuy

    WailinGuy Member

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    Technically, the grid resistors are not "wired in parallel". The wiring can be described very simply: The left signal wire feeds the two resistors on the left and the right signal wire feeds the two resistors on the right. Each of the resistors, in turn, feeds its respective control grid (pin 5 of each socket).

    To see how this looks on a schematic, you don't even have to look at a Marshall schematic. Just look at a Fender Twin Reverb (circuit AB763) schematic. You can go to www.schematicheaven.com and look it up. There's probably a wiring diagram as well. (On a Twin Reverb, the control grid resistors are 1.5K and they connect between pin 1 and pin 5 - pin 1 is unused by 6L6GC tubes and is available as a "free tag" for connecting the feed signal, so no terminal strips are necessary).

    Since you work better visually, try to draw a picture as you review John's and my previous posts on this thread. All the information is there.

    When I started out doing amp repair, I learned a lot from Tom Mitchell's book and accompanying video, "How To Repair Your Own Tube Amp". You can Google it and see if it's still in print. It's starts out with basic electronics but has a lot of good information. There are other books out there for when you've gained some more knowledge. Keniv O'Conner's, "The Ultimate Tone" ("TUT") series and "Principles of Power" are very highly regarded.
     
  13. WailinGuy

    WailinGuy Member

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    The name of that Tom Mitchell book/video set is actually "How To Service Your Own Tube Amp".

    This page lists some good books for sale, including the Tom Mitchell one (book only; no video):

    http://www.vibroworld.com/parts/tech13.html

    Also highly recommended (and listed on this page) are "The Tube Amp Book 4.1" (from Groove Tubes - lots of schematics!) and "Dave Funk's Tube Amp Workbook". "A Desktop Reference of Hip Vintage Guitar Amps" is worth owning just for the section titled, "The Trainwreck Pages", written by Ken Fischer.
     

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