Need some help understanding this circuit diagram

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bgood, Apr 3, 2008.

  1. bgood

    bgood Member

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    Hi,

    Just starting to learn about all this and I came across this current flow diagram. I'm working through it and have run into something that I don't understand. Can someone take a look. My question is related to the El34 heaters. This shows the pins 2 and 7 as the filament pins but it also shows the heater wires from the PT going to pins 1&8 on the EL34s. All the circuits I have looked at have been cathode bias and this seems to be something new. Is this right and can anyone point me to some good resources on learning about this bias configuration (is it called fixed bias?)

    Also, regardless of bias - shouldn't the heaters be wired directly to the fils on the EL34's - I'm wondering if there is some mistake or is my lack of understanding?

    Thanks

    http://www.el34world.com/charts/currentflow.htm
     
  2. VacuumVoodoo

    VacuumVoodoo Member

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    There's no heater current flowing into the cathodes. Heaters and filaments are just two names for the same thing.
    Heater supply voltage is referenced symmetrically around ground by the two 100 Ohm resistors. That common practice where heater winding on power transformer has no center tap which would be grounded instead.

    Pin 1 on EL34 is the supressor grid and needs to be connected to the cathode on pin 8 (in most cases, there are circuits with supressor grid connected differently). The 1 Ohm resistors between cathode and ground are used as idle current monitors: measure voltage across them to get value of idle current, this in turn is set/adjusted by varying the negative bias voltage on control grids pin 5.
     
  3. bgood

    bgood Member

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    Thanks for your answer and you've probably answered my basic question but I don't quite understand due to lack of foundation. Sorry.

    Getting off bias for a second.....

    How are the EL34's getting power to their filaments if there is nothing connected to the pins that feed the filaments? That's one thing I'm definitely not getting from this diagram
     
  4. Blue Strat

    Blue Strat Member

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    In the interest of making the diagram readable, all wires aren't explicitly shown. Note the green "Heater Circuit" designation at the bottom and then note the green pins on all of the tubes. These are the heater connections.
     
  5. bgood

    bgood Member

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    AHHHHHHH. Thanks. Got it - now I feel stupid. What confused me is how the heater wires "appear" to connect into the yellow wire and then pin1 and 8 and my DEEP lack of understanding on how amps work prevents me from understanding the representation of what those 100ohm resisters mean - but now that I see it I recall putting two 100 ohm resisters in my 5E3 in a similar way. Off the heaters on the 6v6GTs to ground. I assumed that it was to reduce noise. But I definitely don't know. Just painted by the numbers on a Mission kit - came out great too.

    More reading..........what mountain to climb

    Appreciate the help.
     
  6. WaltC

    WaltC Member

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    Keep in mind that the diagram you're looking at is a stylized layout diagram, NOT a schematic. A schematic is a formal representation in symbols of the way a circuit is designed, a layout diagram is an attempt to portray the physical arrangement of the parts (from the schematic) in the actual amp build.

    A good example of the difference can be found on the Fender Amp Field Guide (http://www.ampwares.com/ffg) where you can see the schematics for most popular Fender amps, that contain all the parts and their place in the circuit, and the layout diagram for the same amp that shows (mostly) how the parts are placed in the chassis. In the layout diagram not all the parts (Filter caps for example) are shown and not all the connections necessary to make the amp work are shown either. You need to use (and understand) the schematic together with the layout diagram to put together a "finished" functional amp.

    Hope this helps ...
     
  7. Blue Strat

    Blue Strat Member

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    Yes, the 2 100 ohm resistors form a "faux centertap" (since the transformer doesn't have a centertap on it's heater winding) and the object is to reduce hum.
     

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