1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

AA764 Tremolo Depth

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

  1. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    @King Fan good thinking, I just checked and shows continuity fine on those 3 wires
    I rolled another tube in v1 also and got slightly different voltages but still not in spec
    V1
    1 159
    6 266
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,619
    Joined:
    Dec 30, 2019
    Location:
    california
    I just saw a post from robrob saying sometimes the three disc caps can result in low trem.
    He also mentions taming the cathode voltages of V1.
    SF Vibro Champ - Weak Vibrato
    Discussion in 'Amp Tech Center' started by jondanger, Mar 11, 2017.

    The amp in that thread had high voltages and was resolved with a V1 tube change, so I still think you should systematically be addressing the odd voltages of your amp. If rob is correct, perhaps the disc caps could be an influence as well.

    Looking forward to the answer to Wally's suggestion.
     
    Wally likes this.
  3. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    @Lowerleftcoast thanks have read that thread and swapped in another known working 12ax7 of a different brand in post 61. I have not tried changing the 3 disc caps yet I know will slow it if I put in a .022
    Was wondering if putting higher bias resistor would sort out the v1 rogue voltages it's got a 500ohm currently.

    @Wally please see voltages below in vdc With tubes out except rectifier 5y3

    V1
    1 349
    2 0
    3 0.003
    6 351
    7 0
    8 0

    V2
    1 fluctuating
    2 0
    3 0.03
    6 411
    7 280
    8 .005

    V3
    3 409
    4 408
    5 0.33
    8 0

    V4
    2 404
    4 0
    6 0
    8 403
     
    Wally likes this.
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,619
    Joined:
    Dec 30, 2019
    Location:
    california
    I don't think changing the 500ohm bias resistor would boost the B+ voltages enough to effect this problem. This is more of a preamp problem. The preamp gain and how aggressively the trem varies the V1 pin8 cathode IMO is where attention is needed.
     
  5. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    @Wally please see below for voltages in vdc with all tubes back in and with v1 p8 disconnected which I think you mentioned takes the trem out.

    V1
    1 155
    2 0
    3 1.41
    6 268
    7 0
    8 4.53


    V2
    1 138
    2 0
    3 1.5
    6 325
    7 138
    8 fluctuating


    V3
    3 316
    4 324
    5 0
    8 20


    V4
    2 331
    4 fluctuating -03.5
    6 fluctuating -03
    8 331
     
  6. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    Ok UPDATE I swapped out 2 100k resistors from p1 and p6 of v1 the ones I had in there I took out and measured 162k and 168k so may have drifted up since they are silver band 10% tolerance.
    I measured the new ones before putting them in and they are 111k on left and 110k on the right.
    I have swapped in a sovtek 5y3 in V4 and I am now much closer to the correct fender voltages for this circuit.

    The Trem depth is still just average and amp seems to be sounding a bit more solid state like than before but since these are closer to fender spec voltages perhaps this is closer to how it should sound, please see below new voltages in vdc excepted where noted

    V1 12ax7
    1 194
    2 .001
    3 1.715
    6 193
    7 .001
    8 1.7



    V2 12ax7
    1 142
    2 0.45 fluct
    3 1.5
    6 354
    7 141
    8 151



    V3 6v6
    2 Heater
    3 351
    4 354
    5 .07
    7 Heater
    8 22


    V4 5y3
    2 362
    4 288ac
    6 288ac
    8 363
     
  7. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,855
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Post removed to eliminate confusion....mostly my own!
     
    Last edited: Sep 30, 2020
  8. Ten Over

    Ten Over Tele-Holic

    Posts:
    681
    Joined:
    May 13, 2015
    Location:
    Central California
    What in wonky world are you talking about? I can't find any of those resistors in an AA764.
     
  9. Ten Over

    Ten Over Tele-Holic

    Posts:
    681
    Joined:
    May 13, 2015
    Location:
    Central California
    V2b is a cathode follower that is triggered by the voltage on its grid. In order to get the biggest voltage swing out of the CF, you need the biggest voltage swing at its grid. A 470K plate resistor on V2a gives a bigger voltage swing than a 220K or a 100K. 220K swings more current than 470K and 100K swings even more. The larger voltage swing and smaller current swing is undoubtedly why Fender went that route.
     
    King Fan likes this.
  10. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    @Wally thanks for your recommendations and i appreciate your continued help, could some of the voltage discrepancies you mentioned be attributed to the 275v PT secondaries im using?
    There are 2 1m resistors in the aa764 do you mean the 1M which the Right side of it goes to ground left side is connected to the top of the .01 disc cap?
     
  11. Ten Over

    Ten Over Tele-Holic

    Posts:
    681
    Joined:
    May 13, 2015
    Location:
    Central California
    Your last voltage readings aren't that bad and the wonkiness could easily be attributed to a cheap voltmeter with a 1M input impedance.

    Turn the intensity clear down and turn the tremolo off with the footswitch when taking voltage readings.

    I also noticed that you used a 100K plate resistor on V1b instead of the 220K that Mojo shows on their schematic. But then again, they show 100K on their layout.
     
    Wally and Linkjr like this.
  12. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,855
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Well, my bad. I should have tripped to the fact that this is an AA764 Vibrochamp and not an AA764 Princeton. The single 6V6 listed above was enough for that right? Apologies from wonky world.
     
    Linkjr likes this.
  13. Ten Over

    Ten Over Tele-Holic

    Posts:
    681
    Joined:
    May 13, 2015
    Location:
    Central California
    That's pretty funny.

    I can calculate to within a few volts of the posted voltages by putting a 1M resistor in parallel with the tube in question. Kind of coincidental, isn't it?
     
    Linkjr and Wally like this.
  14. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    @Ten Over Thanks for taking a look, and steering Wally clear from a aa764 Princeton that's a relief. I will take a pic of my Voltmeter and - it is a cheap one

    will check if i got a 220k and sub a 220k in place of the 100k and see what happens.....
    there is another discrepancy between the mojotone and fender layout of the aa764 Vibrochamp in that mojotone have 50k RA pot in place of the fender 25k RA for the intensity control - not sure if this will help any.
     
    Wally likes this.
  15. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,855
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Not to minimize my error, Link, but this is an instance when a complete revelation of the amp in question is important. The BF schematic designations indicate nothing more than the date and version of the schematic. There numerous amps that use the AA763 and AB763 designations the Bassman and the Pro Reverb both use AA165 designations. (Edit note: I did have the Pro Reverb as sharing the AB165 designation with the AB165 Bassman, but in a clearer mind it occurs to me that the BF Pro Reverb Circuit had only one version...AA165.)

    In the tweed era the schematic gave us the model, date and version. In the brown/blonde era, the schematic designation gave us the amp model and version.
    Still...I apologize for taking a ‘wrong turn’
     
    Last edited: Oct 1, 2020
    Linkjr and D'tar like this.
  16. D'tar

    D'tar Friend of Leo's

    Posts:
    3,159
    Joined:
    Jan 11, 2013
    Location:
    WNY
    I think this is all a ploy to make us think @Wally is an actual human being!!! ;)
     
    King Fan and Wally like this.
  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    5,956
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Instead of supercomputer with a Lubbock sense of humor??? :)

    This whole AA764 thing has been humorous and ironic. First, we'd often *welcome* a specific 'model' number -- how often does somebody just post they're having trouble with their "Champ" or "Princeton"? Then, I took it for a PR cuz I have PR on the brain, and apparently stopped reading after "AA...". Then, Wally started to think it was a PR I imagine because Fender (weirdly, even by their standards) used the famous "typo" PR tube chart that said AA764.

    upload_2020-10-1_11-52-17.png

    And that tube chart, in turn, led to a famous internet misunderstanding that even fooled Uncle Doug into thinking *this* widely-seen PR AA764 schematic was real -- before the author (an honest soul) realized he'd created a monster and inserted the text block explaining how he'd created it in Photoshop.

    upload_2020-10-1_11-53-40.png

    We oughta create a forum here for the AA764 PR, and an owner's club. They'd probably be well-attended and popular...
     
    Wally and D'tar like this.
  18. Wally

    Wally Telefied Ad Free Member

    Posts:
    37,855
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    actually, I thought it was a Princeton(non verb) AA764. Lol......

    and super computer...no way. My memory banks are in failure mode very often....obviously. At one point in this tortuous thread I knew what amp was being discussed. When Link brought it back up, I looked at the thread header...and got lost.

    it looks as if Ten Over and others are going to get the help to Link that is needed to get this amp on track. I am watching from the sidelines at this point...having taken myself out of the game.
     
    King Fan likes this.
  19. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    Thanks Wally learning a lot here, i thought the designations were all that was needed to identify the amp. Already tried to change the thread title to reflect this new knowledge and for future reference without success.
    Anyway here is my cheap multimeter IMG_20201001_191143.jpg
     
    Wally likes this.
  20. Linkjr

    Linkjr Tele-Meister

    Age:
    50
    Posts:
    178
    Joined:
    Aug 16, 2017
    Location:
    Uk
    @Ten Over
    Last values are with trem intensity on 1 and trem off via footswitch I think rest of my voltages are also the same settings.
    Im still working my way around schematics are we talking about this resistor:
    Screenshot_20201001-204105~2.png
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.