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AB763 Build- Help!

Discussion in 'Shock Brother's DIY Amps' started by vtuna, Jun 19, 2016.

  1. vtuna

    vtuna TDPRI Member

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    Hey all!

    I'm building an AB763 Fender Deluxe Reverb Clone in an electrical engineering class at my University. Im only building one channel (the reverb/ tremolo channel) but I'm having some problems right now getting it to work.
    [​IMG]

    As I had a lot of problems getting this to work I basically imitated the normal channel (removing v4 and the whole reverb circuit) and I was able to get this to work. I haven't actually tried it with a speaker but I have a function generator putting a 440hz tone in the input and using an oscilloscope I was monitoring the signal across a 8ohm 5 watt resistor I have connected at the output. It seems to me that without the reverb and v4 the circuit works more or less as expected thus I think the problem has something to do with v4/ reverb in the preamp.

    Going back to the preamp now. I put the same 440hz tone in at the input, disconnected the preamp from the phase inverter/ power amp, and used an oscilloscope to measure what was happening at the output. Whenever I put in the reverb tube I get all kinds of weird noise in this circuit, but I figured it should still amplify the normal dry signal if I pull the verb tube. So I removed the reverb tube and monitored the output of the preamp- success- I had a perfectly amplified version of the input!! Now I connected this back to the power amp and monitoring the output signal of the entire amp (across my 5w 8ohm resistor) I get pure noise after about 2 o'clock on the volume knob!!!

    Next I logically thought to check the power amp. I disconnected the pre and power amp, moved my function generator to the input of the phase inverter and magically I get a perfectly amplified signal at the output of the power amp! Also after simplifying the circuit, removing v4 (reverb tube still not in) and taking the signal after the coupling cap (.02) that comes after v2 straight to the phase inverter (essentially the normal channel of the ab763) I was able to get a more or less working amp. So it seems to be not a phase inverter/ power amp issue

    This leads me to believe that first of all something with v4 is causing an issue? Also something in the reverb circuit is funky too but the amp should work fine (no reverb of course) without the reverb tube in? All the voltages on the tubes are more or less what they should be and I've tried swapping 12ax7's around in the preamp as well as using a 12at7 for v4 in case the tubes were the cause of the problem but that didnt help at all. If anybody has any thoughts they would be much appreciated!
     
  2. dan40

    dan40 Friend of Leo's

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    Welcome to the forum! We will most likely need some good clear pics of everything to help determine the problem.
     
  3. Wally

    Wally Telefied Ad Free Member

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    One triode in V4 is the reverb recovery while the other triode is the third gain stage for that circuit. So, no, the circuit will not process signal without V4 in the socket.
    You have isolated the problem to the area of V4. Figure out what is wrong....you have misfired or forgotten something.
     
  4. FenderLover

    FenderLover Poster Extraordinaire

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    ...and you said you have a sig gen and a scope - just follow the signal through the circuit.
    Maybe you need to start with an accurate schematic of what you actually built (if you don't have one already).
     
  5. RLee77

    RLee77 Friend of Leo's

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    Nice opportunity to learn more about the circuit by troubleshooting it, since you have a 'scope to see what's happening.
    I'd recommend a much higher wattage load resistor; you'll smoke that 5w pretty easily. A couple 4.7 10w resistors in series would work, they'll still get hot at full output though.

    I'd check the phasing of the output trans, if it's reversed your negative fb loop becomes positive, which can oscillate.
     
  6. vtuna

    vtuna TDPRI Member

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    thanks!! Pics coming soon

    sorry yea when I said pull the verb tube I meant the 12at7 reverb driver. Pulling this tube makes the preamp function by itself it seems.

    With the whole amp wired up and the reverb 12at7 pulled out I tried following the signal with the scope and it starts to distort right after the volume pot. Not quite sure what to make of this?

    When I wire the preamp like the normal channel (no v4 or verb circuit) there is no distortion.

    Have checked the output trans secondaries. When I reversed them there was crazy oscillation in the circuit, so I left them as is (blue to v7 and brwn to v8).
     
  7. Wally

    Wally Telefied Ad Free Member

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    Without the Normal channel, the tubes would be ..if I understand what you have built...
    V1....input preamp
    V2....reverb driver
    V3....reverb recovery and third gain stage
    V4....tremolo
    V5....phase inverter
    V6, V7...power tubes
    V8...rectifier


    As I understand it, without the reverb driver tube in circuit, the amplifier is 'working', correct? If so, then it seems that the problem is in the area of that tube....that reverb driver circuit is miswired somehow is my guess.
     
    boredguy6060 likes this.
  8. vtuna

    vtuna TDPRI Member

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    Yup!
    V1- input pre
    V2- verb driver
    v3- verb recovery/ 3rd gain
    v4- trem
    v5- phase inv
    v6/v7- power
    v8-rect

    Sorry i guess my original post was a bit confusing.

    I know I have some problem with the reverb driver as the signal gets super noisy with tube V2 in. The amp does not function though properly if I pull the reverb driver tube either though. I only get noise on the output of the amp.

    If I also remove tube v3 (3rd gain stage) and just wire the amp as if it only had a normal channel then it does function. So I think I have a problem with the reverb circuit and the 3rd gain stage. I'm trying to fix the problem with the 3rd gain stage first though as this seems like a more logical place to start to me.
     
  9. Wally

    Wally Telefied Ad Free Member

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    I would be concerned with the wiring from the second gain stage to the reverb circuit and all of the wiring in and from that reverb circuit. check that .02mfd coupling cap??
     
  10. vtuna

    vtuna TDPRI Member

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    Cool thank- Dont have access to tools at home but when I get back to the lab tomorrow will do! the circuit should work as normal (with no reverb of course) If I remove the reverb section (as shown below) entirely right? cut-verb.png
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That red line is cutting out the 3rd gain stage grid leak resistor--the 220k resistor to the right of the reverb pot, it is physically located on the reverb foot switch jack. Did you forget that one? Check the resistor's ground because weird crap will happen without a grid leak.

    Annotated AB763 Schematic:

    [​IMG]
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Also check the reverb driver (normally V4A) coupling cap (.003) and verify there is no DC voltage on the output side. If it leaks DC onto the 3rd gain stage grid, again, weird stuff will happen because the tube's bias will be shifted by the grid voltage.
     
  13. vtuna

    vtuna TDPRI Member

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    Awesome thanks for the info! So if I wanted to temporarily remove the reverb effect (for debugging purposes) and keep the 3rd gain stage I would cut everything where the red line is, but keep the 220k resistor to ground?
     
  14. vtuna

    vtuna TDPRI Member

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    Hey guys. So I have to turn in this project as a working guitar amplifier by the end of this week. Given the frustration I've had for the past few weeks trying to fix the reverb channel I decided to revert to the normal channel and have the amp at least working for my professor.

    I took out the reverb driver tube, reverb transformer, reverb circuit, reverb recovery, and 3rd gain stage. I also changed the .02 uf cap to a .047 one.

    Now when I put a 440hz tone (around 200mv amplitude) in at the input with the volume on full and measuring with a scope across the 7.5 Ohm resistor I have on the output I get the 440hz tone at about 2V amplitude.

    This is way too weak is it not??? The dr is a ~20 watt amp. If my calculations are right (P=V^2/2R, V=root(20*15)) I should be getting somewhere around 17ish Volts with volume maxed out??? Any thoughts on where to start looking here?
     
  15. RLee77

    RLee77 Friend of Leo's

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    Hmm, well you've modified the circuit quite a bit, removing the 3rd stage, which drove the phase splitter, so I'd look at the gains going through the stages and see where you're losing it. By removing that stage, depending on how you've bypassed it, you could have increased the negative feedback and lowered the overall gain excessively. (You could try keeping that 3rd stage in, just remove the components for the reverb part... )
    Otherwise check grid swing vs plate swing on each tube, and calc voltage gain, see where it's being lost. Check output of the phase splitter, see if both sides are being driven equally. If you're driving with a sinewave, does it look symmetrical?
     
  16. vtuna

    vtuna TDPRI Member

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    What i've got is basically this now: deluxe-lite.jpg A few differences- I dont have an R14 270k resistor to gnd (if anything that should boost the signal though I think right?). Also R15 I have kept the 47Ohm Resistor from the original diagram. Finally I dont have the 1 Ohm R27/R28 resistors (I have pin 8 on both 6v6 directly to gnd). I was having crazy noise problems with the 3rd gain stage in so I removed it. Will check gains on everything when I'm back in the lab tomorrow.

    By bypass I mean I actually physically removed it from the circuit so there are no connections to it anymore.
     
  17. RLee77

    RLee77 Friend of Leo's

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    I had read the original schematic wrong (bad eyes, or brain, or both), thought the normal channel was using v4b, but it's not, so no issues with you pulling it, disregard my comments about 3rd stage issues.
     
  18. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I have two suggestions.
    1. Remove the 220K/270K voltage divider and connect the 12AX7 plate directly to the .001 coupling capacitor feeding the PI. The voltage divider is cutting half of the AC input to the PI.
    2. Replace the 100 ohm feedback resistor with a 47 ohm resistor as shown in the Fender schematic. The 100 ohm resistor combined with the 820 ohm resistor gives a little over 6 db of negative feedback. The stock circuit has about 3 db of negative feedback.
     
  19. vtuna

    vtuna TDPRI Member

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    Thanks! I actually don't have either of those in :p

    Sorry if my earlier post was a bit unclear but form the output of the preamp 12ax7 I have a .047 coupling cap, then a 220k resistor, then straight to PI

    Also I have the 47ohm resistor on the nfb circuit not the 100 ohm one shown in the diagram.

    I guess i'll hit the lab tomorrow and see where the signal starts to die with a scope to see if i can diagnose this any better
     
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