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

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

  1. capohk

    capohk TDPRI Member

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    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

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    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.
     
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  3. Wally

    Wally Telefied Ad Free Member

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    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 Friend of Leo's

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    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.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    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

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    [​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.
     
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  7. tubegeek

    tubegeek Friend of Leo's

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    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
  8. capohk

    capohk TDPRI Member

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    Here's a better picture:

    [​IMG]
     
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  9. tubegeek

    tubegeek Friend of Leo's

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    I'd clean out the wire strand under the lower phillips head screw too, it'd suck if it shorted the bias to ground.
     
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  10. capohk

    capohk TDPRI Member

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    Good spot. The whole thing needs a good brush and hoover out.
     
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  11. capohk

    capohk TDPRI Member

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    Thanks - I have done this now. I had to put a 22K resistor in place of the 15K along with the 10K pot to get the bias down cold enough. Even at 25K, plate dissipation was ~103%. Whoops. I've left it at 32K for now - plate dissipation is around ~55% so there is a bit of room to adjust hotter. FWIW I'm using JJ tubes.

    My bias measurements are here:
    [​IMG]

    The B+ is quite high. The Fender layout gives 375V for B+. This thread was interesting reading for me, a very similar experience!

    I should note that I have ended up replacing the PT with a Classictone 40-18028 after hearing about a case where a local stage tech plugged someone's US voltage amp into the local 220V wall voltage with some interesting results! As I hope to use this amp for gigs when the Covid embuggerance is over, I will carefully wrap the original PT and keep her safe.

    I've used the 330V tap for the 6G3. This PT also has a lower 300V tap. The Classictone is a straight retrofit, so I figure there is no issue with later value as it is about 10 mins of work to put the old one back in.

    I've used the 220V input tap as the wall voltage measures around 223V (today).

    [​IMG]

    I've left the lead dress fairly long for the moment as I'm considering using the ground switch hole for a Fender export-style carling voltage selector switch. I think it would work without any irreversible mods. There is even a Fender/Mesa reproduction fascia plate available at TAD.

    My problem now is that the volume is really quite low on both normal and bright channels, even at full tilt. The tremolo effect works as it should. I'm going to go back and map all of the component values on to the layout, just for peace of mind, but any advice as to where the lack of volume might be caused by would be very welcome.

    In the meantime, I am giving it a polish...that always makes my bike go faster!

    [​IMG]

    Cheers,
    Matt
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    What a great amp! I like your project here, and you've had great advice.

    Did the volume drop, or never come back very much? If drop, was it after the bias pot or the PT replacement or some other point? In any case, it might be helpful now to do a complete voltage table, especially the B+ and bias figures.

    I like your solution of drilling the bias adjust hole in the floor of the chassis. Much more 'Fender' and less objectionable than the back or front panel on an all-original amp. And your bias tail/shunt resistor sounds about right, if I can reason from similar 6G2/5F11 experience. A helpful reference here is Doug Hoffman's bias page, tho his PR example doesn't seem to need the increased value in the shunt spot.
     
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  13. capohk

    capohk TDPRI Member

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    Thanks for the vote of confidence, King Fan. I'm going to do the voltage chart today. The volume issue is independent of the PT swap as far as I can determine. I'm going to check all of the resistor values again. The electrolytics are new so I ought to be able to trust them but I need to make sure none of the blue caps have gone bad. I think I have to lift a leg on those to test their capacitance?
     
    Last edited: Aug 11, 2020
  14. capohk

    capohk TDPRI Member

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    So before I unsoldered any caps to test, I fired up the amp to map the voltages. Let it warm up for a couple of minutes and the V5 6v6 is glowing red so immediately shut off. I switched the tubes over and the same tube socket red-plated, so according to Rob Robinette's advice, it is either a loose grid pin socket or a leaky coupling cap. I've re-tensioned the sockets and V5 still red-plates. I'm trying to get my head around the next step. Obviously frustrating because this is a new problem and I am going backwards!

    I removed the power tubes and lifted the end of the cap nearest the tube socket. Then measured to see if there is any DC voltage coming through to the now-unattached end. Read ~0.03VDC once it had settled.

    [​IMG]

    Apologies if this is turning into one of "those' threads. I've changed the cap for an orange drop which is almost undetectable in the photo below. Tube no longer red plates, so I'm going to assume that the cap was the issue.

    [​IMG]
     
    Last edited: Aug 11, 2020
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  15. capohk

    capohk TDPRI Member

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    Anyone see anything wrong here?

    [​IMG]
     
  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Excellent sleuthing on the cap, though the OD is setting off my OCD. "Almost undetectable." lolol

    Waiting for smart folks to comment on voltages, I'll just share the Fender scheme for reference.

    deluxe_6g3_schem copy.png

    And IIRC you have 470R screen resistors from pins 1-4 with 1.5K grid stoppers from 6-5. Common safe mods, I believe. But without more coffee, I'm struck by the small voltage drops across your filter caps. Did you say you measured all the resistors?
     
    Last edited: Aug 11, 2020
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  17. capohk

    capohk TDPRI Member

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    Plate voltages on V1 look way high...any ideas?
     
  18. Wally

    Wally Telefied Ad Free Member

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    Your power rail voltages are high everywhere it appears. Your bias voltage is way more negative tha; the schematic.one would wonder what your current draw and plate dissipation biasing numbers are. I am going to suspect that this amp is running quite cool...if not cold, and that if the bias were warmed up the voltages would come down some. I don’t know if that would ‘correct’ for instance the plate voltages on V1, though. Those are about 130 volts out of ‘spec’. if this amp has been played a lot in its past, the power supply resistors I; the rail and the to the plates may have changed their values. Have you checked?
     
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  19. capohk

    capohk TDPRI Member

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    Thanks Wally. I went through and tested all of the resistors in the circuit - my scratty notes are below but there are several resistors that have drifted up beyond the 10% range. I think specifically the bias resistors are required to be within 5% tolerance and they are quite a long way out.

    [​IMG]

    [​IMG]
     
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  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You're being methodical, which is good. I'm no expert, but right about now I'd like to know what VAC is coming out of the PT to the rectifier, the resistances of the dropping resistors in the doghouse, and what your calculated bias is.
     
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