Weak sounding Super Reverb

Discussion in 'Amp Tech Center' started by ShawnRocker, Feb 17, 2016.

  1. ShawnRocker

    ShawnRocker TDPRI Member

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    I brought home a buddies 1967 Super that he hasn't played in a few years because it didn't sound good at all. It was thin and weak sounding with a lot of noise/bad distortion against the notes. I didn't know a lot about his history with it.

    Got it home, poked around a little and noticed V3 felt loose. I tightened up the sockets and the noise / distortion went away but it's still quiet. I sit right next to it turned up to 6. It sounds OK actually and the reverb and vibrato sound good, it's just quiet. Replaced all the tubes w known good ones. Nada

    I stick a bias probe in with #1 power tube and read 2ma. 2.

    So I pulled the chassis the other night to do a visual. It's got new electrolytics in 2008, whoever did it wrote down dates, including filter and bias caps. The V3 tube socket looks like it's a replacement. The bias pot is about in the center of its range. That's all the time I had and I've been too busy the last couple days to go back into it, but it's been on my mind.

    Called buddy and got the story. He bought it sounding terribly in 2008 and took it to a random tech. Tech put in some tubes he had laying around and did the cap job. Amp sounded great for a handful of outdoor gigs, then sounded like I 1st heard it when he showed up for a rehearsal. Out of frustration it's been sitting in his garage since.

    My thought is to check the plate voltage, look for cold solder joints, and work my way back down the bias circuit. Experience tells me it's usually something simple you should've checked at the outset, so I'm hear asking what I've overlooked.

    Anybody have any experience with this they're willing to share?
     
  2. clintj

    clintj Friend of Leo's

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    That tube's heaters light up properly? What are all that socket's voltages? Tried retensioning that socket, and a shot of contact cleaner? There's a few other possibilities, but those answers can get us moving in the right direction.
     
  3. uriah1

    uriah1 Telefied Ad Free Member

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    Rectifier tube swap if it is on way out..might be another, other than the 6L6s
     
  4. Wally

    Wally Telefied Ad Free Member

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    +1 on needing to know the voltages. What is the B+? Then, , one might go to the bias circuit since you read such a low current draw. Was the bias cap replaced when the filters were done? What is the feed voltage for that bias circuit...what is the voltage on the other side of the bias cap? What is the bias voltage on the control grids of the power tubes? Once correct voltage is found or perhaps re-established there, one would want to go through the entire circuit checking voltages.
    It sounds as if your friend needs a tech.
     
  5. ShawnRocker

    ShawnRocker TDPRI Member

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    I had a little time this morning and took the following measurements.

    Plate voltage V7 - 340V
    Plate voltage V8 - 340V
    Bias cap at the (-) lead -60V - Replaced in 2008 Sprague Atom
    Bias at Center lug of trim pot -52VDC

    V1a - 215V
    V1b - 209V
    V2a - 221V
    V2b - 217V
    V3a - 337V
    V3b - 334V
    V4a - 215V
    V4b - 213V
    V5a - 230V
    V5b - 216V
    V6a - 230V
    V6b - 211V

    B+ at Rectifier Pin 2 344VDC

    I swapped out the Rectifier Tube with a known good one and the voltages remained pretty close to identical.
     
  6. Wally

    Wally Telefied Ad Free Member

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    that B+ is low, ime, for a SR. One would want to know the cathode voltages on those 12A-&'s, too. Heater voltages would be of interest imho...especially with the B+ that low.
    Your bias voltage where you took it would probably be to the cool side of things IF the B+ were where it should be. What is the voltage on pin 5 of the power tubes? What is the current draw for the plates in the output? What is the AC feed voltage to the rectifier?
    IF you had another known GZ34, it would be of interest to sub it in...especially if the bias voltage on pins 5 of the 6L6's is 'good'.
     
  7. Wally

    Wally Telefied Ad Free Member

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    Sorry...missed the thing about you had already subbed in. What is the bias voltage on the tubes and what is the current draw? And...what is the PT giving to the rectifier on pins 4 and 6...in AC. One might pull the rectifier to read those voltages. IF they are good, reinstall the rectifier and pull the power tubes....what is the B+. IF the AC out of the PT is low, you have a problem there.
    Danger.....tech work and all.....
     
  8. ShawnRocker

    ShawnRocker TDPRI Member

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    First, thank you for your time & wisdom Wally.

    I think I found the issue getting these readings you requested, there is 0 voltage on the Cathode, pin 3 of V5; I tried 3 different tubes in there and cleaned & tightened the socket. Otherwise my readings are as follows.

    AC across Pins 4 & 6 of V9 710VAC (355VAC)

    Pin 4 V8 - 40VDC

    Heaters are at 6.4V


    Cathode's are
    V1a - 16mV
    V1b - 8.4mV
    V2a - 17.2mV
    V2b - 8.4mV
    V3a - 3.5mV
    V3b - 3.5mV
    V4a - 4.6mV
    V4b - 4.6mV
    V5a - 0 mV
    V5b - Ranged from 20mv to 20mv as I swapped 3 different tubes through.
    V6a - 3 mV
    V6b - 3 mV
     
  9. ShawnRocker

    ShawnRocker TDPRI Member

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    I'd actually found 0V on both cathodes of V5. After cleaning and tightening it I was able to get the 20-30ma on V5b but nothing on V5a. I consulted the schematic and started over. Now I'm getting more like 455V at B+ and Plate. All the other cathodes are now in Volts (Ranging 2 to 20) instead of mV, but still 0 on V5a.

    I think I have a bad tube socket in V5
     
  10. Wally

    Wally Telefied Ad Free Member

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    Well....you are making progress. What to do now? First...???20-30 ma??? Are you reading current draw?
    SEcond: Why is that voltage on the first triode in that Vibrato tube at 0vdc? there should be 2.5VDC or so there. The schematic calls for 6.5vdc on the second triode.
    And that tube has nothing to do with the circuit unless you are using the vibrato function.
    What does the amp sound like now that the B+ is where it should be and V1(Normal ch preamp) and V2(Vib ch preamp) and V4 are presumably looking like they should work. V4 has one triode that is a gain stage in the reverb circuit. You can pull V3 and V5 and still play through that amp, right? The phase inverted cathodes should be seeing 106VDC....V6 pins 3 and 8. What cathode is showing 20 volts? That would be odd, ime. Are you looking at a schematic? Schematic Heaven has it if you don't.
     
  11. jazzguitar

    jazzguitar Tele-Afflicted

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    I do not think so. V5 is the vibrato tube which is shut down by the power tubes' bias voltage to switch the vibrato off, unless a pedal (or shorting plug) is in the pedal jack on the back. So zero volts on the cathode is OK at least for one half of it.

    I cannot make sense of your cathode voltages, they should all be about 2 volts except for V3 and one half of V5 that should have about 5 to 8 volts.

    Your preamp plate voltages look OK to me except for V3, that's too low. And you should have more than 400 volts on the power tube plates. What is the DC supply with the power tubes pulled?
     
  12. ShawnRocker

    ShawnRocker TDPRI Member

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    I don't know what's going on. I went to see what the bias is on V8 with the plate voltage suddenly being higher. It was 16mA, the furthest I could adjust it too was around 41mA.

    Turned it off, took the probe out. Switched my meter back to DC and thought I'd recheck every point.

    B+ was back down to 345V
    Plate is at 340V
    Bias at the (+) lead is -61V
    Bias center lug pot is -49V

    Heaters are running at 5.47VAC

    The following numbers are in order Pin 1, 6, 3 & 8 in VDC; except V5 which had its cathodes go back near 0 again.

    V1 222 212 1.78 1.71
    V2 228 226 1.75 1.71
    V3 348 348 7.95 7.95
    V4 221 222 1.82 1.82
    V5 343 289 9mV 41.4mV
    V6 238 219 50.9 50.9

    It seems like there's something loose or something that causes it to power up around 480V B+ one time, then 340V the next. Could it be the power transformer itself?
     
  13. clintj

    clintj Friend of Leo's

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    Check AC line voltage at the PT, both when it's acting normal and weird. Might just be a dirty power switch or fuse contacts. Oh, and you did verify the fuse is the correct rating and not wrapped in foil, right?
     
  14. ShawnRocker

    ShawnRocker TDPRI Member

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    I think I fixed it. I noticed that when it was only getting 340V it would still be 485V at the rectifier B+ pin in Standby. So with standby off I checked the voltage a couple places between the Rectifier and power tubes. I touch the probe back to the rectifier B+, and as the probe touches it makes a pop and the hum increases a bit. I measure B+ and Plate and both are at 485.

    That's when I notice shiny solder on the 4 used rectifier contact points and the eyelet on the board where the B+ joins the filter caps. I powered off, discharged the caps and heated up those 5 points to get the solder to better adhere.

    I've messed with it a touch since, pulling & reinstalling tubes, shaking it, turning it over, etc., and it's holding steady at 485 and all the preamp tube voltages are at or near what's on the schematic.

    I learned a ton through this process. Thank you all for your guidance. I'm gonna keep this out and keep testing it a few days to be sure, but I'm 95% convinced it was just a no w repaired cold solder joint at the rectifier socket.
     
  15. Wally

    Wally Telefied Ad Free Member

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    Kudos where did you leave the bias at with that 485vdc...which is high. You did have 455vdc at one time when the voltage wasn't unnaturally low.
     
  16. ShawnRocker

    ShawnRocker TDPRI Member

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    It's actually about 461V at the plate. I set the bias around .038. I'm not gonna be able to mess with it over the next couple days so I put it back in the cab and fired it up. Seems I'm halfway there. The clean side sounds like thunder. Really rich. Vibrato side still sounds thin and weak.
     
  17. ShawnRocker

    ShawnRocker TDPRI Member

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    2nd problem solved.

    Suspecting another cold solder joint, I set out this morning to heat up every joint where it looked like the last "tech" did work. Didn't take long to solve the issue. As I touched the joint where the .1mfd Cap joins the 100k resisters connected to pin 1 V2, I saw the cap move. On close inspection I found the lead was snapped right at the contact point. I piled a little solder to make the connection strong. Slid the chassis back in and now she just sings. Amazing big tone. I'm gonna have to find one of these beasts for myself, though have no idea where I'd ever need such a loud amp.

    Here's the cap where the lead was snapped. Is it recommended that I replace it?

    [​IMG]
     
  18. clintj

    clintj Friend of Leo's

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    Solder is not a great replacement for the component lead. It's not forgiving of vibration and the abuse gigging amps get, and there's a chance that joint may fail again someday.

    Option A: lift the lead, and extend it with a scrap piece taking care to splice them properly like this. It's a little bit of work, but highly reliable if doneright and soldered.

    [​IMG]

    Option B: replace the cap. Some people are very touchy about replacing those blue caps, so make sure the owner is 100% ok with that before taking that step. I'd go with A if the cap still tests fine.

    Glad you found the problem, and get to hear what that amp can sound like. They're awesome!
     
  19. ShawnRocker

    ShawnRocker TDPRI Member

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    Thank you, I will follow option A before I bolt it all back together.

    And thanks again everybody for your help and advise.
     
  20. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    FWIW your voltages should be a tad higher than the ones printed on the schematic. Normal wall power was +/-115VAC when those schematics were drawn, and if you check your power you are probably getting 120VAC+.

    Close to/higher than the schematic is fine...lower usually isn't.

    As mentioned solder isn't wire, and it sounds like (from your description of intermittent problems) some pretty extensive parts replacement and desoldering is necessary.
     
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