More on 6V6 bias: Fender Champ

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Mar 14, 2006.


  1. mbratch

    mbratch Member

    Messages:
    2,390
    Joined:
    Nov 3, 2003
    Location:
    Webster, NY
    Sorry... everyone's probably sick of my biasing questions by now. But I cannot help myself. ;)

    I was checking voltages in my Fender Vibro Champ and looking at the schematic. I know the Champ is "cathode biased" right? But nonetheless, a 6V6 is still max 12 watts (or possibly 14 watts depending upon the tube), so the same rules should apply with respect to currents, shouldn't they?

    So when I measure plate voltage, I get about the same as stated in the schematic: nominally 340V. That would mean my absolute maximum plate current should be 12/340 or we'll say about 35mA (if this is class A, then I don't need to take 70%). The cathode voltage at the 470 ohm resistor also roughly matches the schematic at about 24V. That makes the current through the cathod 24/470 or about 50mA, which is much larger than 35mA. That would mean, to remain under the maximum power dissipation of the plate, that the current through the 220K screen resistor must be at least 15mA (and, unfortunately, I didn't measure that last time I had the thing open--duh).

    Does that make sense?
     
  2. JJman

    JJman Member

    Messages:
    991
    Joined:
    Mar 12, 2006
    Location:
    New Jersey
    Expect to get a “high” result. The AB764 scheme implies 18.8watts! :dude

    Mine was running 18.5. I changed the 449ohm cathode resistor to a 516ohm and it came down to 17. Some go to a 740 or 1k. I probably will go 740 next time I'm in there. Already added a 2k screen resistor to get it below the voltage on the plate. Use good tubes in them.
     
  3. phsyconoodler

    phsyconoodler Member

    Messages:
    4,351
    Joined:
    Jul 11, 2005
    Location:
    calgary canada
    the thing you need to shoot for here is the TONE.If it sounds good with less current,then by all means do it.But if the amp sounds thin and shrill if it's biased colder,then you've gone the wrong way.Get a robust 6V6 tube like a JJ and let 'er rip.Those tubes have been living in a champ for 40 + years with a 470 cathode resistor.
    Go for the tone and stop worrying so much about the numbers.
     
  4. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    Here tis again.

    Remove the 12AX7,(s), to remove any plate current being pulled.
    (Thus only the screen current will be indicated for the 6V6...)

    Measure the actual value of the 1K resistor coming from pin 8 on the rectifier. Write it down as Rs.

    Measure the 6V6 cathode resistor, write it down as Rk.

    Measure the voltage across the 1K coming from the rectifier pin 8.
    Divide this Voltage by the resistors actual Resistance. Write it down as this is the 6V6's Screen current. Let's call it Is.

    Turn amp on and let it warm up. Measure voltage across Rk. Write down as Vrk.

    Measure plate voltage and write it down as Vp.

    Get yer calculator and figure these:

    Vrk / Rk = Ik or cathode current.

    Ik - Is = Ip or plate current.

    Ip X Vp = IPD or Idle Plate Dissipation in Watts. This should not exceed 14W.
     
  5. mbratch

    mbratch Member

    Messages:
    2,390
    Joined:
    Nov 3, 2003
    Location:
    Webster, NY
    Thanks Jon. I must have missed the first time you posted that stuff.

    psychonoodler, you told me not to worry about the numbers. I wasn't really worried about them. But per my other post regarding bias, I have read so many conflicting accounts of what to be examining and what not to examine w.r.t. setting bias in an amp, I was very curious about how the Champ was set up in this regard.
     
  6. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    It's a look,listen,run the numbers process here. (I don't worry about numbers, I just change stuff to get them right. Then there's no need to worry...)

    May The Tone Be With You,
    Jon
     
  7. mbratch

    mbratch Member

    Messages:
    2,390
    Joined:
    Nov 3, 2003
    Location:
    Webster, NY
    Thanks Jon. My quandry has been finding so many definitions of "right" described in different places. ;)
     
  8. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Your measurements are absolutely correct... it's Fender's design which isn't :).

    No kidding.

    The stock 470-ohm cathode resistor is too low for that circuit, and results in the 6V6 running far too hot - not only over the maximum rating, but reduces the power available (clean) since the tube reaches forward clipping a long way before cutoff, which produces asymetrical clipping, and a rattier, fartier overdriven tone as well as less clean headroom.

    (IMO the most likely reason is that the earlier 4W Champs - Tweeds and the earliest BFs - had lower plate voltages, which makes the 470-ohm value correct. In about 1965 CBS/Fender decided to upgrade the power of the Champ, which they did simply by fitting a Princeton PT. This raises the B+ voltage far enough to give 6W output, but someone apparently wasn't paying enough attention, and they didn't change the value of the cathode resistor to keep the same operating point.)

    Biasing a single-ended amp correctly is about getting the operating point centered between cutoff and clipping so the waveform ends up symetrical (not to a particular power dissipation). You need a much higher cathode resistor value to achieve this with the high B+ in later Champs - typically 1K, but you could experiment with other values near that if you want.

    This will also take the cathode voltage up well above the 25V rating of the stock cap - usually into the low 30s - so you MUST replace this as well. I like to use a 50uF/50V cap since the bigger value compensates for the greater resistance.

    It's also a good idea to fit a screen resistor while you're in there - you may need larger than the 'traditional' 470-ohm here too, since the voltage drop across the OT primary is greater than in the higher-powered amps. (I would use 1K 2.5W or 3W wirewounds for both this and the cathode resistor).

    You should now find your Champ is louder, sweeter and more balanced sounding.
     
  9. mbratch

    mbratch Member

    Messages:
    2,390
    Joined:
    Nov 3, 2003
    Location:
    Webster, NY
    Thanks John. When I have the chance I will certainly like to try those changes out. Fender also added a 1.5k control grid resistor in some amps (e.g., DR). Do you think it's necessary in this case?

    I would like to know the function of the screen grid resistor and (if present) the control grid resistor.
     
  10. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    The function of the screen resistor is to limit the current drawn by the screen (which has a dissipation rating just like the plate, but much lower), by making sure the screen is operated at a lower voltage than the plate. If the plate voltage becomes lower than the screen, the screen then appears more attractive to the electrons in the tube than the plate and it starts to draw more and more current as the tube is pushed harder, and will heat up - you'll actually see this happening often, if you watch the tubes while you play at high power - and can lead to screen failure. Having a resistor in line to the screen both lowers the initial screen voltage slightly and also makes it drop further as the current increases, so it tends to limit this problem.

    On the lower-power amps that Fender didn't fit with screen resistors, there is a resistor in the power supply chain instead of a choke, so the voltage is already reduced somewhat... but fitting screen resistors as well is still a better idea really.

    The control-grid resistor ('grid stopper') is also to limit the current to the control grid, which can otherwise make the amp unstable - I forget exactly why! :) - but it does. Fender must have found it necessary in the Deluxe, since they changed the circuit almost immediately, but it probably isn't in a Champ.
     
  11. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    I know, but why? :)

    Why are they needed in some amps but not others with the same tube type? Why do some amps even have them on some tubes but not others?! (eg some 100W Marshalls). I'm sure I knew, once, along with a lot of other theory, but it's lost in the mist of wherever else my university electronics education went... ;)


    I've never needed this knowledge for any practical purpose I don't think - unlike with screen resistors, where you sometimes have to add them, or change their value or power rating (especially with a lot of modern-production tubes), and you need to work out what the new ones should be. Grid stoppers are either there anyway or not, and the only times I've ever had to change them is when the old ones have cracked with heat from the tube or been fried by a burning screen resistor.
     
  12. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    The math behind the madness is to calculate the Q for the grid-cathode circuit with and without the stopper resistor. Dropping resistance into the circuit quenches the high frequency oscillation. It's also why grid stoppers are most effective when installed directly on the tube socket -- to eliminate any lead capacitance.

    A secondary purpose is to mess with the RC time constant for the combination of grid-cathode capacitance and the capacitance of the output coupling caps from the PI to improve recovery time from grid clamp (blocking distortion).

    As to why they appear in some amps and not others, I'll fall back on my usual "Leo didn't say and it's too late to ask". I'm guessing cost control (not just the nickel for the resistor, but the labor to install it and the layout considerations of where to install it -- you see 'em done flying lead from the main board to the tube socket in old Ampegs). I use them as a matter of course.
     
  13. donnyjaguar

    donnyjaguar Member

    Messages:
    4,232
    Joined:
    Nov 4, 2005
    Location:
    Canada
    Yes, they limit the gain at ultrahigh frequencies. Why you always see 1k or 1k5 values in the power tubes I'm not sure. There's another benefit to putting them in your first voltage amplifier. If the tube melts down and the plate shorts to the grid it will limit the current through your pickup(s) and lessen the change of smoking them. :)
     
  14. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    You probably won't anyway, since the plate resistor is still in the way.

    This does remind me of the idiot that didn't realise that an 'electric guitar' was not an appliance that you plug into the wall - and needed an amplifier - though. He cut off one end of a guitar cord, fitted a mains plug (you can still get these separately in the UK) and connected it to the wall. 240VAC right across the pickups :eek:. They melted... as did the volume pot and the pickguard. Luckily it was just a cheap Strat copy, and even more amazingly he didn't kill himself.

    I'm really not kidding, BTW.
     
  15. mbratch

    mbratch Member

    Messages:
    2,390
    Joined:
    Nov 3, 2003
    Location:
    Webster, NY
    Sounds like something our Guitar Center employees would suggest or do. ;)
     
  16. JJman

    JJman Member

    Messages:
    991
    Joined:
    Mar 12, 2006
    Location:
    New Jersey
    When I was young my friend and I would test tubes on his father’s tester to pass time. One day we took a ~6” speaker and connected a wall plug to it. 110v in the USA. We pluged it in on the bench. Out came a shower of white sparks and fire from the center of the cone. Along with that was the loudest scariest noise I ever heard.
     
  17. Shea

    Shea Member

    Messages:
    540
    Joined:
    Sep 7, 2004
    That explains how grid stoppers reduce gain at very high frequencies.

    This explains how limiting gain at very high frequencies prevents oscillations:

    http://www.ken-gilbert.com/techstuff/compensation.html

    Put 'em both together, and there's the answer to John's question.

    Shea
     
  18. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Excellent, thanks! I should have known Randall would have the answer on his website... :)

    Now I'm reminded of exactly why, I did of course 'know' that all along ;).

    Funny how the detail of stuff you don't actually use disappears as you get older. I'm sure I remember studying and using Differential and Integral Calculus at some point too :rolleyes:.
     
  19. Shea

    Shea Member

    Messages:
    540
    Joined:
    Sep 7, 2004
    Just another thing that Marshall nicked from tweed Fenders.

    Look at this layout diagram:

    http://www.ampwares.com/ffg/schem/twin_5f8-a_layout.gif

    See, when you have pin 1 tied to pin 8, there isn't any handy mounting point for the input side of the grid resistor. So rather than leave that end of the resistor sticking up in the air unsupported, like some people do, Fender mounted the grid resistors in between the pin 5 terminals of each pair of tube sockets.

    So, it looks to me like it was just a cheap and easy way to mount some grid stoppers without having to install supporting terminals next to the tube sockets, which would have been more expensive.

    While it's not as effective as having a grid stopper on every tube, perhaps they figured it was just good enough. The aim is to reduce gain at very high frequencies to prevent oscillations, so it looks like someone decided that putting grid stoppers on two sockets provided just enough reduction in gain to get the job done.

    In my homemade tweed twin, I originally had grid stoppers standing up in the air on pin 5 of each tube, with the lead on the free end wrapped around the end of the grid lead wire and covered with blue shrink tubing. Eventually I decided that it looked too dang sloppy, so I changed over to the old Fender/Marshall way that has only two grid stoppers.

    Here's what I noticed afterwards: if I were to hit a really loud chord, all the 5881s would glow blue a bit more brightly, then if I suddently stopped the chord ringing, the two outer tubes would immediately go dim but the blue glow in the two inner tubes would flicker for another second or two. So I presume an oscillation was occurring in the two middle tubes, which are the the ones that lack grid stoppers, but would die out on its own. So, you might say that those grid stoppers are actually "needed" on all four tubes after all. But I still have it set up that way.

    Shea
     
  20. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    The whole pin 1 tied to pin 8 thing...

    In the olden days when folks still used metal can 6L6, pin 1 was tied to the can and typically grounded. In a fixed bias amp, the convenient ground is pin 8.

    But with the advent of 6L6G and later variants, pin 1 is now either not connected or not present. A convenient tie point -- and used this way in the blackface Fenders and other products.

    There's also another tie point on the socket at pin 6, but Fender (and others) tend to use this as a mount point for screen resistors.

    So...
    The easy thing to do is to break the 8/1 connection, run your grid wires to pin 1 and put the stopper from pin 1 to pin 5.

    For those who want a socket that can be used with 6L6 type and EL34 type (where pin 1 is connected to the suppressor grid), I'd argue it's more important to have the stopper on the socket than the screen resistor, so the screen R goes to the board, and the grid stoppers get pins 5/6 with grid leads to pin 6. Now you can tie pin 1 to ground (and/or pin 8), or, for the EL34 enthusiast, actually tie the suppressor to a negative voltage independent of pin 8/cathode.

    Or, of course, you can always put a terminal strip next to the socket for tie points (or.. yecchh.. use a PCB mounted socket).

    Gawd I love this stuff :AOK
     

Share This Page