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AA764 Tremolo Depth

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Sep 14, 2020.

  1. Linkjr

    Linkjr Tele-Meister

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    Hi Guys,

    I want to get some more tremolo depth out of my vibro champ, it's using 275v secondaries on the PT I have tried increasing the bias resistor and couldn't hear any improvement, currently there's a 500ohm 5w in place there and a cap of 25uf 50v.
    the sound is great but the tremolo is a bit weak. There is a difference moving the intensity pot but even on 10 it's only getting there.

    Please see below voltage chart with tremolo disengaged

    V1 12ax7

    1 153
    2 0
    3 001
    4+5 20
    6 253
    7 0
    8 003
    9 20

    V2 12ax7 middle

    1 135
    2 0
    3 001
    4 20
    5 20
    6 329
    7 135
    8 158
    9 0

    V3 6v6

    1 0
    2 20
    3 323
    4 328
    5 0
    6 0
    7 0
    8 20

    V4 5y3

    1 0
    2 337
    3 0
    4 AC 293
    5 AC 0
    6 AC 292
    7 0
    8 337
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I see you have started a new thread specifying the weak trem. First thread was 291AEX PT.

    Some of the numbers are troublesome. On the 12a_7 tubes, the heater voltage is measured with your meter on VAC. The original heater wiring only had one heater wire to each tube. The more modern approach has a twisted heater pair to each tube.
    Which method are you using? The voltage should be around 6.3 VAC.

    The cathode voltages - V1 pin3 and pin8 should be about 1.5 VDC to 2 VDC.
    Same for V2 pin3.

    You are running the 275 taps from your PT. This gives lower B+ throughout the amp. Most people like how the amp sounds with lower B+, and you say the amp sounds good. I think you should leave it on the 275 taps. The lower B+ to the preamp tubes is also lowered which may be part of the low volume trem. The low cathode voltages could also be a reason for the low volume trem so let's figure that out first.

    So the game plan is to find out what is going on with the heater voltages and the cathode voltages.
    After that, you may want to get more voltage to the preamp tubes. You can get the B+ up a little by using a different rectifier tube (5U4) or changing some of the dropping resistors.

    Some pictures of the board and tube pin wiring would be helpful.
     
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  3. Guy Guyatone

    Guy Guyatone TDPRI Member Silver Supporter

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    Check the value of the 470k resistor feeding Pin 1 and 7 on V2, it may have drifted upward. The voltage should be 170, not 135. Why are the secondary voltages on your PT only 275v, should be more like 315v?
     
  4. Linkjr

    Linkjr Tele-Meister

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    Thanks for looking again @Lowerleftcoast @Guy Guyatone
    Yes I decided to start a new thread as the pt problem was sorted out successfully @Guy Guyatone in that thread was discussed using the 275v secondaries of a 291aex pt which others had used in previous builds and worked out fine for them.

    I'm using modern twisted pair one side to pins 4+5 the other to pin 9. Will take some more measurements of these in VAC.

    I have measured the 470k resistor on v2 it's reading at 437k
     
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  5. Guy Guyatone

    Guy Guyatone TDPRI Member Silver Supporter

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    Have you tried a different tube in V2? Tremolo circuits are noted for needing a "strong" tube, what ever that means.
     
  6. Linkjr

    Linkjr Tele-Meister

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    @Lowerleftcoast

    Getting a little worried - set up for VAC I got 43.5 vac on the heaters
    v1 pins 4, 5 and 9
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Where do you have the heater center tap grounded?

    Pictures...
     
  8. Linkjr

    Linkjr Tele-Meister

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    Pin 8 of the 6v6, rechecking it now
     
  9. Linkjr

    Linkjr Tele-Meister

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    IMG_1526.PNG
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Pull the tubes out. Even the rectifier. Check the voltage of the heaters. Careful the rectifier socket will have full voltage.

    Pictures of everything please.
     
  11. Linkjr

    Linkjr Tele-Meister

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    Ok I think there is a misunderstanding possibly on my part! I should be getting 6.3vac between the 2 pins not 6.3vac at each pin, my measurements of high 43vac are from heater pins on v1 to ground. I'm using twisted pair wiring scheme so between them I'm getting the correct vac 6.3
     
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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So with tubes in, speaker in, are you still getting small voltages for V1 pins3 & 8, V2 pin3?

    VDC. Meter pin to ground.
     
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  13. Linkjr

    Linkjr Tele-Meister

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    Ok now onto the next part of the game plan :)
    Had meter set too high so here are the more accurate readings for v1 v2

    V1
    3 1.4vdc
    8 3.1vdc


    V2
    3 1.5vdc
     
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  14. Wally

    Wally Telefied Ad Free Member

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    Edited due to simultaneous posting by OP that answered my query.
     
    Last edited: Sep 14, 2020
  15. Linkjr

    Linkjr Tele-Meister

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    @Wally thanks for taking a look, is plate voltage on v1 p6 possibly because of the lower 275v secondaries I'm using?
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You probably want to recheck the voltages on V1 and V2.

    Do you have a spare 10k resistor? How about a spare 1M resistor? Maybe we can temporarily piggyback them to raise some voltages. for the preamp.
     
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  17. Linkjr

    Linkjr Tele-Meister

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    I have a few 100k carbon comp 1/2 watt
    @Lowerleftcoast But i think we are going to use Higher rated additional voltage dropping resistors? i have a spare 10k and a 22k 2w resistors
     
    Last edited: Sep 14, 2020
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am still interested in whether the voltages are true.

    You should have a 10K dropping resistor between two of the filter caps. Clip in the spare 10K parallel to the existing 10K for a resistance of 5K. See how much that changes your V1, plate voltages.
    To increase voltage of V2 pin1 plate. Parallel around 1M to the existing 470K which is connected between V2 pin1 and pin6. If you don't have a 1M resistor, maybe you have a spare 1M pot? This is the plate that may help the trem volume the most.

    As I said earlier perhaps a 5U4 rectifier will raise all of the voltages and get to a sweet spot.

    I think the higher tap on the power transformer might take you past the sweet spot.
     
  19. Linkjr

    Linkjr Tele-Meister

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    Ok With the spare 10k across existing 10k between filter caps
    Plate v1 12ax7

    1 155 vdc

    6 233 vdc
     
  20. D'tar

    D'tar Friend of Leo's

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    photos please
     
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