Amp techs! Help me tweak out the harshness!!! (Engl schematic encl.)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Axeplyr, Aug 22, 2006.

  1. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Hey dudes... Any of you superior amp techs want to take a crack at solving a dilemma?

    I've been reading Dave Hunter's new book "The Guiar Amp Handbook", and some inspiration hit me. So now I'm I'm trying to solve a riddle with respect to the design of my Engl Classic combo. The Engl Classic has a gorgeous clean channel, but the overdrive channel is an icepick. Harsh to no end. So, I located its schematic and started comparing it to other amps.

    I've come to the following conclusions regarding the amp's design. Apparently, things are fairly standard for the most part.

    Preamp gain stages are pretty close to Fender. Cathode and plate circuit configs are fairly standard, like a Twin or Bassman (100k plate resistor, 1.5k ohm cathode resistor & 22uF cathode bypass cap). I haven't dissected what's between these gain stages yet, however!

    The tone stack is very similar (almost identical) to 5F6-A Bassman.

    The PI circuit is virtually identical to that of a Mesa MkIIB.

    The power section appears pretty standard, with 6L6 tubes for 50 watts.

    One thing I've noticed is the absense of a "grid stopper" resistor on the input. I'm finding most amps have a 68k resistor on the input - not this Engl. I'm not claiming this is the problem, but it's just one thing that stands out.

    Another thing that's obvious is how busy it is. Being loaded with features, there is lots of extra stuff in there to account for bright/deep switches, switching, reverb, and the recording out circuit.

    Can someone shed a light on this? Any ideas as to where this harshness is coming from? If you want to take a look, I've included a link to the schematic below.

    http://www.tonecentral.net/gearpics/Engl_Classic_E350_schematic.pdf

    I'm trying to tame the gain channel, so I'm considering manipulating the values on V1B, which serves as a footswitch selectable gain stage. Particularly the capacitor/resistor off of the cathode. Perhaps I can revoice this stage to dial out or tame the hashness a bit.

    Thanks tons - any help is appreciated!

    EDIT: Am I trying to polish a turd here? Maybe it's a lost cause?
     
  2. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Hi,

    The scematic didn't come through. Thinking off the cuff here.
    Try using a lower gain tube in that stage, might help.
    This is the easy solution. (NOTE: for you old farts
    out that there...recall your early days of FORTRAN
    and stacks of cards):
    IF 12AX7
    THEN SUB
    12AT7
    12AY7
    12AU7
    ELSE
    DO
    GOSUB1
    END

    You have a couple of other choices...

    Depending on the diode clipper circuit change them
    or make that part of the circuit more compliant.

    Maybe some diode gurus will step in and help.
    Some examples are also given in The Ultimate Tone v1.



    Depending on which caps are in that part of the circuit,
    change/upgrade them.

    RETURN
    SUB1
    IF ceramic caps
    THEN
    silver mica - smoother
    poly carb - less treble
    mylar - less treble, detail

    SUB1 END oops, forgot what that was just
    screwed the rest of the group as now the program
    has timed out (if you set that parm).

    See what you like best.

    Let us know what you find out. : )
     
  3. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Thanks AmpNerd! Good thing I'm a software guy. :cool:

    The schematic takes a couple of minutes. I thought it didn't come through either, found out I had to wait.

    It sounds like it's going to be pretty involved to get rid of the icepicks in this amp. I was hoping for a resistor or a cap or two. :(

    I have a 12AT7, I'll give it a shot. What about the PI tube? This book says a lower gain PI tube will smooth out the tone as well.
     
  4. Rod

    Rod Tone is Paramount Silver Supporting Member

    Messages:
    16,619
    Joined:
    Nov 29, 2004
    Location:
    Santa Fe New Mexico/Kittery Maine
    It really sounds like a design issue..I'd sell the amp, and find another that you like the tone of...If you modify it a lot, it won't be worth anything
     
  5. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    I'm starting to lean that way myself. These Engls are a nightmare to mod. Disassembly is a real hassle. The more I think about it, the more I'd like to just cut my losses and free up the cash for a JTM-45 kit.

    - John
     
  6. Roe

    Roe Member

    Messages:
    6,939
    Joined:
    Jul 12, 2005
    Location:
    tromsoe, norway
    yes, go for a jtm45 kit - a metro kit with RS deluxe OT!
     
  7. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Do you think this RS deluxe OT sounds better than the Marstran?
     
  8. donnyjaguar

    donnyjaguar Member

    Messages:
    4,244
    Joined:
    Nov 4, 2005
    Location:
    Canada
    Before you throw the kinder out with the badewasser, define "harsh" for me.
    Is it harsh even when the gain is turned down and you are playing relatively clean? Does it become harsh as you add more drive? Does the harshness occur at all frequencies, the mids, highs? Any other artifacts showing up like unloading on low notes? Also comment on whether the bright mode is where this harshness shows up.

    It might be worth mentioning which guitar from your vast collection you normally play through this thing too.
     
  9. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Thank you! That's exactly the type information I need to know. I will definitely look into R37, and see what type of work is involved in changing it.

    Great link on grid stoppers. This is an easy place to play as well. Maybe I'll experiment with a resistor or two.

    - John
     
  10. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    donnyj,

    The harshness is not an issue until the amp starts clipping. The clean channel is gorgeous. I can even manage to get an acceptable very light clipping gain sound out of it. It becomes progressively worse as gain increases. And even at significant volumes the harshness remains. Even with the bright button out (off), and the treble to 0, there is harshness. As far as where it occurs in the spectrum, I'd have to say everywere. It's a pretty unpleasant raspy tone.

    I have been running the Les Paul through this thing as of late. '59 LP reissue, with burstbuckers 2 and 3 in the neck and bridge respectively.

    I've narrowed it to the preamp - if I slave my Marshall JCM-800 into the fx loop return, I can obtain a killer distortion tone.
     
  11. V846

    V846 Member

    Messages:
    322
    Joined:
    Dec 22, 2003
    Location:
    Enfield, CT




    Marstran also offers an RS Deluxe style OT with the same primary & secondary configuration as the original RS Deluxe output transformer found in the old JTM45's:cool: . Sorry for going off topic..

    David
     
  12. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    No problem! Thanks for the info. I am really gassin' for a JTM45, and I think this Marstran is the one I'll be getting when I start accumulating parts.
     
  13. V846

    V846 Member

    Messages:
    322
    Joined:
    Dec 22, 2003
    Location:
    Enfield, CT
  14. trdlasvegas

    trdlasvegas Member

    Messages:
    167
    Joined:
    Aug 30, 2005
    Location:
    Las Vegas
    John,
    If your saying that the Lo Gain mode sounds fine but the Hi Gain mode is bad, I'd take a look at V1b like you said earlier. For all practical purposes it's the ONLY thing being switched into the circuit that wasn't there in the Lo Gain mode.

    Tube wise, it will be tricky because changing V1 is also going to effect the Lo Gain sound. There is a dual triode with different gains on each half of the tube, but off hand I can't remember the number. And I'm not sure which side is hi and which side is lo, which would be critical in this case.

    I'd play around with C10 and/or C4 as they will effect the frequency response of the V1b gain stage. I wouldn't change R37 as someone suggested as it will effect the Lo Gain sound as well. Instead raising the value of R11 will acomplish what they said but ONLY to the Hi Gain stage.
    -Tony
     
  15. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Wow - that is a killer amp you've got there... :drool
     
  16. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    I think it's mainly the Hi Gain mode because of the extra gain though. If I crank up the Lo Gain to where it's really, clipping, I'll probably end up with the same harshness.

    I hadn't gotten to C10 and C4. How can I expect them to change the tone of the circuit? And should I go bigger or smaller wrt the cap values?

    Thanks tons,

    John
     
  17. donnyjaguar

    donnyjaguar Member

    Messages:
    4,244
    Joined:
    Nov 4, 2005
    Location:
    Canada
    Okay, that's good information - and this would be easy to nail down with an oscilloscope I might add. :) Have a look at the FET marked "T1". Although it should be good for a 30V differential betwixt the gate and Source or Drain, I can see that its getting 24Vdc which, in my opinion, is much higher than necessary. Add to that the potential for much more than 30V to appear on the Drain say when V2 is removed and it could cause problems. I couldn't find the DS max voltage spec, but we can deduce its less than 60Vdc. I guess to test the theory disconnect the Drain and see if that helps. If that's the culprit, I'd consider replacing with say an MPSA NPN transistor that's good for 300V just to be safe.

    DJ
     
  18. rockon1

    rockon1 Member

    Messages:
    11,454
    Joined:
    Aug 12, 2005
    Location:
    Stamford CT
    12DW7
     
  19. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Excellent - thanks! Maybe it's time to pick up an oscope. I see where T1 appears to be involved with the circuit - I'll try to get a meter on there and measure what it's seeing on the FET.
     
  20. Axeplyr

    Axeplyr Supporting Member

    Messages:
    1,701
    Joined:
    Mar 26, 2004
    Location:
    Nelson Bay, Australia
    Hmmm. Gain of 20 on the first triad (pins 1,2 & 3) and a gain of 100 on the second (pins 6, 7 & 8). Any tubes with a higher gain (say, 50) on the first triad?
     

Share This Page