NAD 1976 (1977) Vibro Champ

Discussion in 'Amp Central Station' started by HD28, Sep 16, 2021.

  1. GAS Giant

    GAS Giant Tele-Meister

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    A shop here in town has a 76 VC..also with a Weber...just sounds great, sweet, more bass response than you might expect and breaks up well...they are getting pricey! Nice catch!
     
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  2. HD28

    HD28 Tele-Meister

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    The schematic I was using and printed off was AA764, not AB764. I may have made an error. I will disassemble and recheck.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Well, alright…those two schematics, which run the same PTs, do show two different voltage levels. Interesting…how can that be?
    Curiouser and curiouser…. Hey, this is the same PT that runs in the AA964 Princeton and AA1164Princeton Reverb amps that call for and run 420vdc+ on the B+. ???????
    Life goes on…..and confusion grows…. At least I’ll be prepared if I ever see a 125P1B PT that runs low voltages.
    If I had that amp in front of me, I would have to measure the 5V supply on P2 of the 5Y3 as well as the 6.3v heater filament supply…because as I said, I have never seen such low voltages in any BF/SF amp….but I have not seen everything..as I learn from day to day….and today!
    If you have not buttoned that back up or if you did make a full voltage chart, what are those two numbers, please, sir?
     
    Last edited: Sep 25, 2021
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  4. HD28

    HD28 Tele-Meister

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    Wall 121.9VAC
    All knobs 1
    Tremolo RCA shorted
    Dummy load of 4Ohms resistance
    0.8Ohms resistance across thermal fuse
    P2 on the 5Y3 354.8VDC
    P4 365.5VAC
    P6 363.1VAC
    P8 357.7VDC
    Heater 6.56VAC
    Cathode 6V6 21.0VDC
     
  5. Wally

    Wally Telefied Ad Free Member

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    The rectifier P2 to ground should read 5v..unless my brain fog is occluded my processes. I am thinking all is fine, and that this PT is a learning experience for me. Thanks.
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Okay…6V6 plate voltage read 324vdc. 6V6 cathode reads 21vdc. 470 ohm bias resistor. 21/470 = .044 amps.
    0.044 x (324-21)= .044 x 303 =13.5 watts of plate dissipation
    13.5/14 = 96.7% of max plate dissipation.
     
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  7. HD28

    HD28 Tele-Meister

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    Today the plate voltage is 324VDC and current is 39mA
     
  8. HD28

    HD28 Tele-Meister

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    Pins 4 and 6 on the 5y3 read as per ab764 (365VDC) but the P8 voltage is considerable less than 420VDC (367.7VDC).
    I don’t have another 5y3 to swap out and try. I have a 5y4 an some 5u4g’s
     
  9. Wally

    Wally Telefied Ad Free Member

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    HD28, IF the 6V6 were drawing a lot of current..excessively…one might expect to see the B+ drawn down…but not this much. However, the 6V6 is not drawing much current, ime. With the HT at 365vac, I would expect the 6V6’s Pv to be closer to the schematic. It seems odd to me. But…your heater filament voltage is strong. Once again and I don’t like to belabor a point….what is the filament voltage on pin 2 of the rectifier. It should be 5vac….measured from the pin to ground.
     
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  10. HD28

    HD28 Tele-Meister

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    I have the Band-Master in my jig now, but will measure and report asap.
     
  11. HD28

    HD28 Tele-Meister

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    I swapped the JJ 5Y3S for an old Westinghouse 5Y3GT. Plate voltage went up, heater voltage the same.
    P2 on the 5Y3 to chassis is 0.13VDC and oscillating OL - ~400VAC. P2 to neutral is the same. Oscillating OL - ~400VAC
     
  12. HD28

    HD28 Tele-Meister

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    Wall 121.5vac
    All resistors within spec limits
    Heater 6.59vac
    5y3gt P2 and P8 402vdc, P4 365vac P6 366vac
    6v6 P3 391vdc P4 391vdc P8 24vdc
    Plate voltage 361vdc. Plate current 45mA
     
  13. Wally

    Wally Telefied Ad Free Member

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    HD28, P2 of the 5Y3 to ground should read somewhere around 5VAC. I just measured a 1964 Champ….to confirm my old mind’s understanding. P8 of that rectifier to ground should read the B+ voltage in DC. I do not understand your measurements.

    Also, P3 of the 6V6 is the plate. Plate voltage to ground is the plate voltage. There is a positive cathode…P8..voltage there. One needs to measure that. That 361VDV that you are calling plate voltage is more than likely a voltage reading taken between the plat..P3…and the cathode..P8…, correct? That is the plate to cathode voltage, which should equal the plate voltage minus the cathode voltage…voltages measured from those pins to ground.
    IfnI understand what you/are doing there, that 6V6 is dissipating something around 116%….a bit over 16 watts. Hat is common and imho desirable in these amps.
    As I noted, I do not understand your voltage measurements on the rectifier. ?????
     
  14. Wally

    Wally Telefied Ad Free Member

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    I also do not understand your statement….

    What is ‘neutral’???? Ground is ground. Neutral is one wire of the AC that connects to one end of the primary winding?????
     
  15. HD28

    HD28 Tele-Meister

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    Measuring DC on DMM, P2 and p8 on the 5y3 are both 402vdc to ground.
    Measuring AC on DMM, P4 is 365vac, P6 is 366vac to ground.
    Measuring AC on DMM, P2 to ground oscillates from overload to ~400vac. Once up and down about every second. It’s like measuring the voltage on a tremolo circuit but the range is greater.
     
  16. HD28

    HD28 Tele-Meister

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    When you said to measure the filament voltage on the 5Y3 by measuring the potential from P2 to ground is ground, what you meant to say was to measure the potential from P2 to P8.
    5.31VAC.
     
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  17. HD28

    HD28 Tele-Meister

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    Plate dissipation is ~16.5W which is 137.5% of max PD (12W according to Tung-Sol tech sheet) for a 6V6GT.
    At the risk of having my head bitten off again, there are conflicting opinions regarding the bias on Champ and Vibro Champ circuits. Some people enjoy the higher ~16W PD while others recommend decreasing the PD to a value closer to spec.
    Why?
     
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  18. studio1087

    studio1087 Telefied Silver Supporter

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    Fantastic amp!
     
  19. Wally

    Wally Telefied Ad Free Member

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    @HD28, with regard to the rectifier filament; I have always measured pin 2 of a 5Y3, GZ34, 5U4 rectifier from the pin 2 to ground. It will measure 5VAC…or a bit more if it is a vintage amp using higher wall voltage. I have been tracking this 1964 Champ on the bench…5.35VAC from pin 2 to ground. In AC mode from pin 2 to pin 8, it also measures that same voltage. In DC mode across pins 2 & 8, it measures 425VDC. From pin 8 to ground, it measures 425VDC.
    How that filament AC voltage measures from pin 2 to ground, I cannot say….but it does…and iirc has on every amp I have worked on for 25 years despite there being no obvious relationship for pin 2 to ground???? I do admit that the advice I find says to measure across pins 2-8 in AC mode to read that filament. I am wondering why your voltage does not read that way when using pin 2 to ground….but we’ll agree to let me wonder about that.
     
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