1973 Silverface Champ Modified with questions

Discussion in 'Shock Brother's DIY Amps' started by mjcyates, Apr 1, 2020.

  1. mjcyates

    mjcyates Tele-Meister

    Age:
    56
    Posts:
    308
    Joined:
    May 4, 2010
    Location:
    louisville ky
    So I picked up a 1973 Fender Silverface Champ. It had already been removed from original cabinet and housed in a Mojotone cabinet with 12" baffle. The speaker in it was a 12" WGS G12C/S. The cap can had already been replaced as well as the bypass caps and 1 coupling cap. I wasn't fully enamored with the tone and plus I just wanted to tweak it a bit. What I did:
    • Replaced Cathode Resistor with 10 watt 470 ohm Ohmite
    • Replaced Coupling caps with Sozo NextGen Blue Molded
    • Replaced bypass caps with Spragues 25uf/50v, except for the 2uf
    • Replaced Output Transformer with Allen T011S
    • Replaced Speaker with WGS 12" Reaper HP
    • Replaced the voltage dropping resistors with Ohmite 2 watt resistors
    • Replaced treble cap with Silver Mica 250pf/500v
    • Tubes are Zenith 5Y3, JJ 6V6S, JJ ECC83
    Sounds way better now. I know these amps are notorious for running hot so my questions are related to that. Here are some current readings:

    Current Amp Readings

    6V6 Cathode Voltage: 25.23 vdc
    Cathode Resistor Value: 470 ohms
    Cathode Resistor Current: 25.23/470 = .0537
    Plate Voltage: 370 vdc
    Power Reading: .0537*370=19.87

    5Y3
    Pin 2 - 402vdc
    Pin 4 - 370.2 vac
    Pin 6 - 371.4 vac
    Pin 8 - 402 vdc

    6V6
    Pin 3 - 394 vdc
    Pin 4 - 396 vdc
    Pin 8 - 25.23 vdc
    B+1 - 402 vdc
    B+2 (screen/grid voltage) - 396 vdc
    B+3 (preamp voltage) - 371 vdc

    So is this way too hot? Should I make any changes or just let it roll? Here are some pictures:

    IMG_0455.JPG IMG_0456.JPG IMG_0457.JPG IMG_0458.JPG IMG_0460.JPG IMG_0461.JPG IMG_0462.JPG IMG_0463.JPG
     
  2. Rockinvet

    Rockinvet Tele-Holic

    Posts:
    589
    Joined:
    Oct 17, 2019
    Location:
    Virginia
    Nice rig!
     
  3. Whatizitman

    Whatizitman Friend of Leo's

    Posts:
    3,441
    Joined:
    Feb 18, 2018
    Location:
    WV
    Can't say for certain. But I think those plate voltages are pretty typical for SF Champs. High, but typical.
     
  4. Lowerleftcoast

    Lowerleftcoast Tele-Holic

    Posts:
    954
    Joined:
    Dec 30, 2019
    Location:
    california
    The JJ is said to be able to handle 19w, so maybe.
    The tube may have a shorter life. More importantly how hot does the transformer get? If you can put your hand on the transformers, I would say give it a try.:cool:
     
  5. mjcyates

    mjcyates Tele-Meister

    Age:
    56
    Posts:
    308
    Joined:
    May 4, 2010
    Location:
    louisville ky
    Transformers do not get hot at all. Forgot to mention that the previous owner had also disconnected the NFB, I reconnected that.
     
  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    3,990
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    With Champs, we throw the numbers out the window. Easily can be 140% max dissipation. Main issue is does the tube redplate? It not, and it sounds good, play on!
     
  7. mjcyates

    mjcyates Tele-Meister

    Age:
    56
    Posts:
    308
    Joined:
    May 4, 2010
    Location:
    louisville ky
    That's pretty much what I figured. It does sound good and there is no red plating so I guess I will leave well enough alone. I do have one other question. I know single ended amps typically have more hum as compared to push pull circuits and this one does. It is probably more noticeable since I put in a larger OT and Speaker. Are there any tricks to reducing the hum in this circuit. I know the heater circuit is not twisted pair and the ground scheme is not what I would do if I were building from scratch but I don't want to waste my time if there is not going to be a noticeable improvement.
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    3,990
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Another common issue is "I replaced/upgraded a speaker in my Champ and now it hums more! What gives!?!?!"

    The answer, is your more full-range 12" speaker probably accents the 60Hz range more than the previous speaker a bit, so it might be more noticeable than the original 8". The only way to see if things would make a difference is to try it out and see.
     
  9. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,010
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    @corliss1 is of course right, and anyway revising the grounds would be tedious. But I'd be sorely tempted to try twisted-pair heaters. A trick that makes that easier is to use 20 or better yet 22ga wire -- Marshall did. Ampacity is not an issue on these little guys, and it's a lot easier to fit and fasten lighter wire around the old socket lugs without breaking 'em...
     
    tubegeek likes this.
  10. mjcyates

    mjcyates Tele-Meister

    Age:
    56
    Posts:
    308
    Joined:
    May 4, 2010
    Location:
    louisville ky
    Thanks, the next time I get into it I think I will rewire the heater circuit.
     
  11. tubegeek

    tubegeek Tele-Afflicted

    Age:
    59
    Posts:
    1,006
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    Once you do that, the next thing to try on the heaters is:

    1) Create a "virtual center tap" for the heaters. Two 100 ohm (I think 1/2 watt is ok) in series, outer ends go to each side of the heater winding, center junction is the virtual center tap.

    2) connect VCT to the cathode of the 6V6.

    3) make sure heater circuit is not referenced to ground anywhere else.

    This will give you less of an interaction between the heaters and the cathodes of any of the tubes, and may improve hum behavior. Depends on the tubes whether this will matter much.

    You'll want to plan your heater rewire to account for 3) as you do it if you want to make it easier to try this.
     
    King Fan likes this.
  12. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,010
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Oh, and here's a possible step. We were just talking elsewhere about Rob's implementation of the neglected idea of running your HV center tap straight to the first filter cap negative pole instead of the power amp ground bolt. Can definitely quiet hum/buzz due to ripple.

    That said, the Champ raises a question. With a modern-style cap can that has a ground lug, you run it there. Here's his PR.

    upload_2020-4-2_17-7-1.jpeg

    But to be fair I'm not sure what you'd do if you had the old-style cap can that grounded through its shell. I can't tell from your pics how your can is grounded.

    Hmm, now I wonder, how *is* it grounded?
     
  13. Lowerleftcoast

    Lowerleftcoast Tele-Holic

    Posts:
    954
    Joined:
    Dec 30, 2019
    Location:
    california
    It looks like there is a heater CT from the transformer. If there is, route the heater CT to the cathode of the V6 and use a heater twisted pair. No need for the virtual CT.
     
    tubegeek likes this.
  14. tubegeek

    tubegeek Tele-Afflicted

    Age:
    59
    Posts:
    1,006
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    To quote Emily Litella, "Never mind."
     
  15. mjcyates

    mjcyates Tele-Meister

    Age:
    56
    Posts:
    308
    Joined:
    May 4, 2010
    Location:
    louisville ky
    Cap can is grounded to the chassis. You guys have definitely given me some ideas.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.