1962 6G3 re-cap advice/affirmation please...

Discussion in 'Amp Tech Center' started by capohk, Jul 25, 2020.

  1. capohk

    capohk TDPRI Member

    Age:
    46
    Posts:
    65
    Joined:
    Apr 26, 2019
    Location:
    Hong Kong
    Thanks for all of your thoughts. The amp has replacement Tolex and speaker. I fabbed up a bracket and it would have worked fine, but somehow it looked less 'correct' than a proper fender-style pot. With a bias probe, I reckon I could do the bias adjustment without removing the chassis. Or at the very least, just take it out and flip it over. So I drilled the hole...

    Some pics below for interested folks:

    [​IMG]
    After all of your advice to keep the circuit stock, I went with F&T caps - the closest values were 10 and 22uF. Hopefully not too far out of spec.

    [​IMG]
    Cathode bypass filter caps replaced with Sprague Atom 25uF50V. I found another change to the circuit - the .0033uF cap in parallel to the plate resistor on ch1 has been removed. I don't have one to hand so I'll have to try it out to decide if I need to put it back in. Everything else is back to stock according to the schematic. I used carbon comp resistors throughout. They all measured within spec, but they might be a bit noisier than modern ones.

    [​IMG]

    Bias cap is replaced - Sprague Atom at 100uF100v. I've put a 15K resistor in series which gives the range 15 - 25K, but that may end up being too low. Bias pot installed. I wonder here the if resistor could be grounded to the back of the pot as in later circuits? Next job is biasing...

    [​IMG]
    Bias pot from the underside of the chassis.
    3-prong power cord installed. I used Doug's instructions for that. Time to do the start-up routine, I made a lightbulb limiter for my 5F6A build according to RobRob's instructions. I'll go through the same process for this.
     
    Last edited: Jul 31, 2020
  2. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    12,656
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    Something doesn't look right on the bias board. Two wires off the pot going to the board, but I could he looking at it wrong.
     
    Wally likes this.
  3. Wally

    Wally Telefied Ad Free Member

    Posts:
    36,019
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX

    I am in the middle of wishing I was in the middle of a Saturday afternoon nap, but I don’t like what I see either. If my concern is misplaced, maybe someone will wake me up and explain it to me. however, I would think that that pot needs to be in between the the bias cap/diode junction and the tremolo intensity potentiometer, which is being fed by that yellow wire that leaves the upper right corner of the board. Would one not have the bias voltage feeding the number 1 lug, the resistor as it is a going to ground from the #3 lug, and the bias voltage feed to the circuit going to the trem pot. That is....the wire from the junction of the cap and diode...black in the setup above...needs to move to the #1 lug. Then, a wire from the trem intensity pot would hit the middle lug of the bias pot. If the yellow wire could be unsoldered from that connection under the board, you simply run that to the pot? Am I seeing things correctly or am I asleep and unable to compute?
    Although it is not an error as far as I can see, my eye wants to have that resistor grounded to the bias control pot. Habitual I am.
     
  4. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,437
    Joined:
    Dec 30, 2019
    Location:
    california
    I don't think it would make a difference as long as the black wire is connected to the middle lug of the pot.

    The way it is hooked up now would be the same as the stock Princeton Rev. The 22k bias resistor has been replaced with an adjustable resistance.

    No problem with the resistor grounding directly with the cap + to ground.
     
    Wally likes this.
  5. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,437
    Joined:
    Dec 30, 2019
    Location:
    california
    I believe the way @BobbyZ and @Wally want it would be safer, should the pot fail. It should work either way.

    EDIT: Maybe it wouldn't be safer. I will have to think about it some more. Ha!
     
  6. capohk

    capohk TDPRI Member

    Age:
    46
    Posts:
    65
    Joined:
    Apr 26, 2019
    Location:
    Hong Kong
    [​IMG]

    This is the bias board section from the 6g3 layout. What I have done is to change the 22K resistor to 15K and put the 10K bias pot in series with it. The resistor attaches to the left lug of the pot as you look at the back of the pot and goes back to ground in the normal spot , and the centre lug goes back to the negative side of the bias cap (and the diode). As you adjust the pot from the top of the chassis in a clockwise direction, plate dissipation increases, which felt the correct way to me. The lead disappearing from the top right in the diagram goes to the tremolo intensity.
     
    BobbyZ, Wally and tubegeek like this.
  7. tubegeek

    tubegeek Friend of Leo's

    Age:
    59
    Posts:
    2,486
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    Short the center lug of the pot to the right (unused) lug in case the wiper fails open. That way you'll have a fixed 25K instead of an open between the diode [-] and ground.

    More safer.

    It's good practice when using a pot as a variable resistor ("rheostat" connection) instead of a 3 terminal variable voltage divider. For a bias circuit that can do some expensive damage I'd say it's worth the bother even though it doesn't happen every day.
     
    Last edited: Aug 1, 2020
    Lowerleftcoast and Wally like this.
  8. capohk

    capohk TDPRI Member

    Age:
    46
    Posts:
    65
    Joined:
    Apr 26, 2019
    Location:
    Hong Kong
    Here's a better picture:

    [​IMG]
     
    BobbyZ and _Steve like this.
  9. tubegeek

    tubegeek Friend of Leo's

    Age:
    59
    Posts:
    2,486
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    I'd clean out the wire strand under the lower phillips head screw too, it'd suck if it shorted the bias to ground.
     
    capohk likes this.
  10. capohk

    capohk TDPRI Member

    Age:
    46
    Posts:
    65
    Joined:
    Apr 26, 2019
    Location:
    Hong Kong
    Good spot. The whole thing needs a good brush and hoover out.
     
    tubegeek likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.