Need some technical FX loop help...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Kenny D, Mar 10, 2008.

  1. Kenny D

    Kenny D Member

    Messages:
    1,668
    Joined:
    Jan 21, 2008
    Location:
    The middle of Nowhere
    I recently purchased a 50 Watt Plexi clone. I ordered the FX loop modification.

    When I plug a TC G-Major into the FX loop and get a bit of a gain drain. Shouldn't the FX loop just send the signal into the FX as-is without draining the signal? I have the output on the G-Major up all the way.

    My forst thought is to simply boost the signal coming out of the G-Major by pumping it through an Alesis Compressor with no compression set -just the gain cranked up until the signal balances against tone of the amp with no FX in the loop.

    Aside from a minor FX loop problem, this amp absolutely smokes. I love it.

    I guess what I really need to know is what do I have to do to get this FX loop to act like FX loop on other amps I own. I have a Peavey 5150 II that passes the signal perfectly with no gain drain.

    Are there some electronics I am missing? What is up with this? I am not an electrical engineer.

    What is a "buffered" FX loop?

    Any help appreciated.
     
  2. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Nigh-on impossible to say what's going on in your amp without knowing a bit more about what's in the "FX Loop modification".

    As for the buffer thing... where to start?
    In a typical guitar amp most of the stages in the preamp behave poorly when you use them to drive a low impedance load. That is, the gain for a particular stage is often drastically reduced if the thing it is driving (the next amplification stage, a tone stack, an FX loop) has a low input impedance. The reason is that these stages are designed to amplify a voltage, but cannot deliver very much current. When you try to push a lot of current, the gain drops off. So if E is voltage, I is current, and Z is impedance, then I = E/Z. So as Z goes down, I goes up and, as explained, gain in this kind of stage drops like a stone. An example of this stage design is any place you've seen a 12AX7 where there's a resistor connected to the plate and the output signal comes from between the plate and resistor (often through a coupling capacitor). The design has names like "common cathode" or "anode follower".

    An alternative is something like a "cathode follower", like you find just before the tone stack in most Marshalls and many old Fender tweed designs. In these, the load resistor is on the cathode and the output is also taken off of the cathode. By definition, this design has a voltage gain of less than 1. However, it will amplify current all day long. It's also easy to get input impedance of 1 Megohm or more.

    So back to buffers... if we want to connect something with low input impedance (a pedal, a tone stack) to a voltage gain (anode follower) stage and not suck the life out of it, we can chuck a cathode follower between the anode follower and the low impedance load. The cathode follower has very high input impedance so we don't kill the anode follower, and it can deliver current to the low impedance load. In this application, we'd call that cathode follower a buffer -- because it buffers the low Z load from the sensitive anode follower.

    So, drum roll please, a buffered FX loop has a buffer amp (like the cathode follower or a solid state equivalent) with high input impedance and the ability to source current on the "send" side. The buffer is between the amp and the effect you're driving. The "return" side is usually buffered as well, but the rules for a good return buffer are a bit different (e.g. now we're concerned about what the pedal is driving into and how the whole mess mixes back into the amp's main signal path).
     
  3. Groovey Records

    Groovey Records Member

    Messages:
    3,146
    Joined:
    Nov 19, 2007
    Location:
    Jackson Heights Birthplace of Johnny Thunders
    Thanks Todd,

    I knew if I was patient someone would explain this, Excellent !

    EnJoY ThE MuSiC
    Groovey Records
     
  4. Randy

    Randy Member

    Messages:
    3,885
    Joined:
    Sep 9, 2002
    Location:
    CT
    Have you tried changing the input range on the G-Major from 'Pro' to 'Consumer'? I've had loops before that did not give me enough signal in the pro setting, but switching the input level to consumer solved the problem.
     
  5. ScottR

    ScottR Silver Supporting Member

    Messages:
    2,104
    Joined:
    Dec 30, 2005
    Location:
    Mrlnd
    could be the well documented G-Major Tone Suck
     
  6. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Glad to help :)

    The other half of the story is "why can't an anode follower source current?" and the related "why a cathode follower can".

    Midband gain in an anode follower is found as follows:
    Ri = input resistance of the following stage
    Rp = plate load resistor
    Rl = total plate load = (Ri*Rp)/(Ri+Rp)
    mu = amplification factor
    Ra = plate resistance
    Av = midband gain = (mu*Rl)/(Ra+Rl)

    So poking through the equations we see that midband gain is a function of plate load and that's just the plate resistor in parallel with the input resistance of the following stage. For all of these terms you can substitute "impedance" for "resistance". So, trivially, Rl is always less than either Ri or Ra (cuz they're in parallel) and so as Ri gets smaller, gain decreases.

    For the effects of increasing (or trying to increase) the current draw through an anode follower keep in mind that the plate voltage (Ep) seen by the tube is always B+ less any voltage dropped by the plate load. By Ohm's Law, as current increases, the voltage drop across the plate load increases, so Ep has to decrease. And what happens to a gain stage when you reduce plate voltage? :)

    The problem is that the load current passes through the plate resistor in the anode follower. In a cathode follower, the load current does not pass through the cathode resistor and there's nothing between B+ and the plate to mess with Ep, so it'll source current (up to the limits of the tube and power supply) all day and hold essentially constant.
     
  7. Kenny D

    Kenny D Member

    Messages:
    1,668
    Joined:
    Jan 21, 2008
    Location:
    The middle of Nowhere
    Thanks to everyone who answered.

    I will have to try this. I have a 5150 II that the G-Major workes well with.

    If that fails, then I am going to try putting a spare compressor (Alesis 3630) after the G-Major and use the gain control to boost the signal back to the amp.

    I was not aware of the "well published" G-Major tone suck. I'll google that.
     
  8. Randy

    Randy Member

    Messages:
    3,885
    Joined:
    Sep 9, 2002
    Location:
    CT
    I've found the G-Major to be pretty transparent with every amp I've tried it with, as long as the levels were set correctly. Where is the tone suckage documented?
     
  9. scottl

    scottl Member

    Messages:
    17,041
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    Are you saying that this loop is just two jacks in the back, no controls?? If so, it is most likely similar to what Dumble had on his amps. I would not call it an effect loop. It is more of an insertion point for a "buffered" active loop such as the Dumbleator. It is misleading to call these jacks, designed IMO for inserting an external loop, an effects loop.

    You may want to search "Dumbleator" in here and find who is making external loops. There are several companies and individuals who can help you.

    Here is a link to the Iron Sounds loop. Inexpensive and effective.

    http://mysite.verizon.net/resqaaya/products.html

     
  10. Kenny D

    Kenny D Member

    Messages:
    1,668
    Joined:
    Jan 21, 2008
    Location:
    The middle of Nowhere
    Scott, you answered my next question befre I asked it. Thanks for the link! This is exactyly what I was looking for.
     

Share This Page