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AA764 Tremolo Depth

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

  1. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Yeah, that’s the biggest drawback of the Vibro Champ’s tremolo circuit. I’ve tried a handful of others in my Deluxe Reverb build, but they were all much subtler than the Vibro Champ’s.
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The V1 voltages seems odd. The plates are fed B+3 through 100k resistors. Pin1 155v, pin6 264v. Maybe the intensity was interacting a bit?
    Make sure those 100k plate resistors are close to spec.
    Do you have another 12a_7 to roll through there?

    If you have a 10uF (or higher) cap, try it parallel with the 25uF of V2 to up the uF to 35 (or more).
     
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  3. Linkjr

    Linkjr Tele-Meister

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    @Lowerleftcoast thanks again for some suggestions you seem to be very knowledgable about this. Just so I understand the plan 100% I'll put back the 2 components I replaced with a red LED and clip an additional cap parallel to existing 25uf?

    Was sure I switched off the trem when measuring will measure again with intensity 0 and again on intensity 10 trem off.

    @Dan_Pomykalski yeah that's what I keep hearing about the vibrochamp trem is fantastic , mine at the moment is OK I think some of the beauty of old amps and these circuits is you can get a variance in sound depending on tubes etc and still call it a vibrochamp Since I made this myself I'm hoping to get an idealised imo :) version of a Vibrochamp.
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Basically I am looking at the low voltages compared to the schematic.

    I suspect if you use the PT higher voltage tap the voltages will be too high affecting the character of the amp.

    I think a 5U4 rectifier will raise all of the voltages a little, keeping the character of the amp and providing the preamp and trem with enough voltage. I would definitely pursue this path if you can't nudge the preamp and trem enough for your taste.

    Like Wally, currently I am not sure what resistor/cap values you have installed but I am assuming it is back to stock values.

    So with comparing voltages for V1. The voltages can be raised by reducing the 10k filter resistor. That is why I wanted you to try paralleling another 10k to reduce that value to 5k.
    The Pin1 155v, pin6 264v is very uneven which I think needs investigating/correction. I am concerned it may be causing some trouble if the gain is less than what is needed by the trem.

    Comparing voltages with V2. The 470k plate resistor (pin1) can be reduced with at the same time reducing the 4k7 cathode resistor (pin3). Values as low as 100k plate and 1k cathode can be used. As mentioned before, the valvewizard suggests using a 100uF cathode cap so raising it more than the 25uF may be beneficial. (The higher value cap allows lower Hz through which is where the trem fluctuation starts.)
    The use of a 100k plate resistor(pin1) with a LED on the cathode (pin3) is just another way of doing the same thing.
    Changing the 68k resistor (pin8) can also help to get more trem.
     
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  5. King Fan

    King Fan Poster Extraordinaire

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  6. Wally

    Wally Telefied Ad Free Member

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    I don’t think we do, but we have the members to figure the power biasing and plate dissipation. However, this bias vary trem acts on that last gain stage’s cathode biasing in V2....so the power tube biasing is not 8n play with the trem...at least to my limited understanding. That is why I was wondering about that very low voltage on V2P8 since it drives the variation to the V2V3 cathode. In looking at V2P1 voltage, it is low. That voltage is part of the biasing of that tube and perhaps that is part of the remaining problem with the trem????
    As Lowerleftcoast has noted, there are some odd plate voltages in those 12AX7s.
     
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  7. King Fan

    King Fan Poster Extraordinaire

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    Oh, yeah, Vibro Champ. D'oh. Thanks, Wally. :oops: I shoulda known there was a reason you smart detectives were all sniffing around the preamp voltages.

    So I will just renew my question about pics. I may be wrong, but it seems like good pics are the most common way these questions get resolved when numbers and words aren't working.
     
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  8. Linkjr

    Linkjr Tele-Meister

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    @Lowerleftcoast
    I have taken some more measurements as I said I would as you kindly mentioned something maybe up with the low voltages in v1 in #42
    See if this below makes sense , (I think you were right about intensity level interacting)

    Trem off (foot-switched off)

    Trem intensity on 1
    V1
    P1 150
    P6 150

    Intensity on 10
    V1
    P1 152
    P6 267

    I'm working from the mojotone scheme for this circuit I believe this is stock component values but they have a 500ohm 5w bias resistor in place of a 470 and I have added a red LED from v2 p3 in place of a 4k7 and 25uf cap this was suggested by lowerleftcoast which has improved the trem somewhat. I am also using the 275v secondaries on my 291aex PT.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The mojotone scheme looks like the same values as the original.

    From the voltage charts you have provided I am assuming you replaced the 470k plate resistor as well? (V2 connects between pin1 and pin7)

    Considering the B+2 voltage... If the 470k plate resistor has been replaced with ~100k, that would be about as strong as that portion of the circuit could get. The only other resistor value to change would be the 68k V2 pin8 to try for more intensity.

    You may try to increase B+3 to eke out a little more gain from V1, but you are near the end of the rope. You can give it a shot but probably you'll have a better luck with the 5U4 rectifier.

    Some 12a_7 tubes are stronger than others so it is always a good idea to throw a few at it and see if they stick.
     
  10. Wally

    Wally Telefied Ad Free Member

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    The B+ is only 18 volts less that the schematic....a little over a 5% variance...understandable, imho. However, at the latest reading of 150vdc on the V1 plates, that voltage is a decrease of 25%. V2.....pin 1 at the latest 328vdc is almost at 200% above the schematic, also the cathode in that section is 154vdc when the scheme calls for 1.6vdc. ?????.
    Imho, something not wired correctly somewhere in this circuit, or perhaps some component is way out of spec or bad. We have not seen definitive pictures of the build. Some folks here have great eyes for picking up on a simple wiring error. pull all of the tubes except the rectifier and give us a fresh voltage chart of the unloaded voltages along that power rail, if you could please. Then, Lift that trem circuit from V1P8, reinstall the tubes and give us a fresh and complete voltage chart.
     
  11. Linkjr

    Linkjr Tele-Meister

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    @Lowerleftcoast I Tried 100k in place of the 470k across the pins of v2 but had an unwanted effect similar to lowering the 68k resistor in that it affected the useable volume if trem intensity on 10 and trem switched off

    voltage readings are with mojotone configuration so 470k across pins of v2 and the last of voltages readings of v1 also as stated were taken with the red led in place of 4k7 and 25uf cap. No voltage readings have been taken with the 100k across v2.
     
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  12. Linkjr

    Linkjr Tele-Meister

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    @Wally
    Thanks Wally that sounds like a great idea, will action this plan tomorrow AM - need to get some sleep, but theres a few things I'm not 100% certain of please forgive my limited knowledge in this subject you want the voltage readings across the power rail is this the pins of 6v6? And just to be certain - lift trem v1 p8 - disconnect the wire from this pin?
     
  13. D'tar

    D'tar Friend of Leo's

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  14. BigDaddy23

    BigDaddy23 Tele-Holic

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    Haha......'1000 words' and all that.
     
  15. Linkjr

    Linkjr Tele-Meister

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    @D'tar @Lowerleftcoast @King Fan
    Why didn't you ask! I'm sorry gentlemen i was pursuing lowerleftcoast component swap in fix to make all my troubles go away.
    IMG_20200920_093217.jpg IMG_20200920_094556.jpg IMG_20200920_094618.jpg
     

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  16. King Fan

    King Fan Poster Extraordinaire

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    Thanks, you're right, we shoulda asked. :)

    I flipped 'em to horizontal since for me at least it's easier to read, but now I'll let people who can really 'see' circuits and values read 'em.

    IMG_20200920_094556.jpg IMG_20200920_093217.jpg IMG_20200920_094637.jpg IMG_20200920_094618.jpg

    While they get out their surgical loupes, it might be good to add a bright low-angle shot of the pots and their wiring, esp. the trem pots. It's tricky on these amps with back-slanted faces, but it may be helpful here.
     
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  17. King Fan

    King Fan Poster Extraordinaire

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    Oh, and though the surgical loupe crowd have all these as wallpaper in their workshop, I'll post this for those keeping score at home...

    champ_vibro_aa764_layout clean.png

    vibro_champ_aa764_schematic.png
     
  18. Wally

    Wally Telefied Ad Free Member

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    Link, can you give us some good pics of the pots with emphasis on the trem circuit pots. One really good shot of all pots might do the job. Another one concentrating on the trem depth and intensity. We have come this far, right? Let’s see it all....
     
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  19. Linkjr

    Linkjr Tele-Meister

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    @Wally and rest of you guys please take a look, full disclosure ha ! - really appreciate the thorough investigation

    IMG_20200921_114654.jpg
     

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  20. King Fan

    King Fan Poster Extraordinaire

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    That helps. Let's cone in on this one (click to expand). The trem pot wire colors look right.

    upload_2020-9-21_8-53-19.jpeg

    But looking at Fender's layout, the *source* of the wires to the trem pots is about as hard to track as could possibly be... and it involves (of course) underboard leads. :D

    Just for grins, you might use your continuity tester to confirm those three wires originate at the right point -- and don't interconnect or short to something or each other at some eyelet/passthrough.

    If I'm right (!) the continuity map should show these connected -- and they shouldn't interconnect with their passthrough neighbors.

    upload_2020-9-21_9-9-5.jpeg
     
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