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SF Super Rreverb Oscillation?

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  • #1

    SF Super Rreverb Oscillation?

    Hi

    I'm having a frustrating time trying to troubleshoot a problem with a 72 Super Reverb and I was hoping some of you could help me with some ideas. I'm thinking I have an ultrasonic oscillation going on.

    SYMPTOMS:
    - With the treble knob at 9 or below, when the volume is pushed up to 10, the amp suddenly loses some volume. With both of those knobs at 10 it REALLY loses volume.
    - With the volume at anywhere from 3-10, flipping the Bright switch ON causes the amp to lose volume and get thin-sounding.
    - the amp generally has a 'peaky' sound, especially if I use a Strat and play up around the 9-12 fret on the D and G strings. In fact I have been using that as the test sound, as it seems to be most prominent in that frequency region. The distortion is on the unpleasant side when I hit those frequencies hard too.

    This occurs on both channels, and happens with any speakers I put it through.

    BACKGROUND:
    The amp has been modified in the following ways:
    - all new electrolytics
    - bias adjust added to existing bias balance
    - some coupling caps replaced
    - new 470 resistors at the output tube grids
    - a couple of 'blackface' resistor values were changed, specifically, the two 47Ks were replaced with 82K/100K, and the two 330Ks were replaced with 1M

    I believe the problem really started to show itself when I changed those resistors - the effect generally seems to be increased gain at the inverter, and I think that exacerbated an existing oscillation problem. I think...

    At one point I had removed the grid suppressor caps, but they are back on now, and they don't seem to help the issue. Note, I do not own a scope unfortunately.

    I have done quite a bit of chopsticking without much success. I have also tightened up some of the lead dress, especially the twisted wires on the heaters and pots, which were a little sloppy.

    I've measured voltages throughout the amp, and while they are a little higher, maybe 10-15V, than the schematic [originally an AA270, except for the 330K resistors I mentioned], they are consistently higher, and when I run the amp on a variac set to about 110 the voltages are largely identical to the schematic values. I've checked all the grounds against the chassis for continuity and haven't found any problems.

    I'm trying to think of anything else worth mentioning, but some of my memory seems to have leaked out where I pulled my hair out...

    Please feel free to ask any questions and I am thankful for any suggestions you might have.

    Thanks

    egdon
    Tags: None
  • #2
    Before you changed the parts, did the amp behave the same way? Possibly a cold solder joint in the replaced parts? I've had weird problems when I didnt read the color codes correctly too, 15k for 1.5k for example. At least,that's where I'd first start.
    "In theory, there is no difference between theory and practice. In practice there is."
    - Yogi Berra

    Comment

    • #3
      Before I changed *any* parts, the amp wasn't working so well at all...I had cleaned it up a lot and that kind of thing too. But before I replaced those resistors, it had that peaky sound, and I didn't like it, which is what made me start to think about what "blackfacing" or partially Bfing would do for it. Afterward, there was definitely something I liked a lot about the extra gain, but when I started to play it louder I realized things weren't as rosy as I originally thought.

      I should have mentioned that I have tried a lot of different tubes, including trying 12AT7s in the 12AX7 slots, just to see if that would be a major difference, and it really wasn't all that major

      I'm pretty sure the values are good but I will systematically check everything again and be 100% sure. thanks for the reply JoeM

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      • #4
        Lead dress in some SF models was questionable. Grid wires should be made as short as possible and raised up while plate leads should be pushed againt the chassis. Leads should be arranged so that they don't run parallel. Ninety degree angles are best IF leads MUST cross each other. OT leads should be twisted together up to their termination at the power tube plates for the primary or the output jack for the secondary. The NFB lead should be routed away from the preamp. Grounding may be an issue. Innard shots would help us to diagnose lead dress issues.

        Now... The amp has been worked on by you BUT didn't sound right before that. Perhaps it's been worked on before. It is possible that your negative feedback loop is a positive feedback loop if the PI plate OR power tube grid leads are swapped around wrong. This often causes violent oscillation but sometimes it's more subtle like the problem your having.

        Are the 470R "grid" resistors you replaced the "screen" grid resistors or the "control" grid resistors? The screens are indeed 470R but the control grid stoppers should be 1.5k.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #5
          OK...got a few of these things done...

          I've rechecked all the resistor values in the amp across the board - everything is well within tolerance. I also went over many solder connections and resweated anything that looked dull. The only really troublesome section was the solder welds on the brass grounding strip, which had visible cracks, and my iron was unable to melt them. I soldered over them anyway but I'm sure it would be best to get a bigger iron and remelt those. Continuity tests good though, between the chassis and brass strip.

          The lead dress was a little sloppy compared to some blackface innards I compared to...some of that seems to be the wires that run under the board in a BF...everything runs over the board in this one. I did tighten up some of the wound wiring, and tried to get the yellow wires from the board flat down to the chassis. I will follow up with some pics.

          I am sure this amp was worked on before - some filter caps had been replaced in a sloppy way [I redid them all], and definitely one of the screen-grid 470s had been replaced. I replaced both of those with new ones that I measured to be the same value. But I am assuming that they must have popped a 6L6 and burned a 470 at some point in here. Also the reverb xformer had been replaced. Reverb works ok, but I haven't had it connected while I'm doing all this, just keeping the trem and reverb knobs all the way down.

          The 470s are the screen resistors as you pointed out, Chuck, and I left the 1500s alone as they measured close to each other.

          The OT leads are twisted somewhat, brown going to the tube nearest the rectifier, and blue to the other. The grid leads from the 220K resistors also appear to go to the correct place - yellow to the tube near the rectifier, and red to the other. I tried to get them to look like the BF amps I have pictures of in terms of which wires are raised [heaters, and the yellow/green paired running laterally across the board]. The grid leads to the output tubes are pretty flat to the chassis, and I hadn't changed that because it looks that way on several other amps too. Do you mean the red/yellow wires to the 6L6 grids should be high off the chassis?

          I checked the PI grid leads, as that is where I replaced those resistors with the BF values, and I believe it is wired correctly. I get 229VDC on the 100K grid, and 226VDC on the 82K grid. A little lower than the schematic, but they are different values from the original 47K.

          Also I removed the NFB lead from the output jack, and it did not seem to have an effect on the oscillation symptoms, so I put it back.

          I also unscrewed the board and blew out any dust etc underneath it - nothing really came out but I wanted to check it.

          As I said, I did replace some coupling caps, and 1 or 2 of them are a little troubling to me due to being a slightly different size/shape, maybe someone can spot a problem there on the pics. The bias balance/adjust combination, where I just put a resistor and a trimpot on top of the existing balance pot, looks a little cheesy but has been working properly. Very easy to set the bias and balance a set of tubes closely. You'll see it in the pics...I don't have the sense that there is any problem there but I'm looking forward to hearing what others think

          Thanks again for the suggestions

          egdon

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          • #6
            One correction, the grid wires to the output tubes have nothing to do with any 220K resistors...the red/yellow wires I mentioned come from the big blue .1uF Paktron caps after the PI. Sorry, bad edit

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            • #7
              Pics as promised. I have more, and can take as many as needed















              More to come - thanks,

              egdon

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              • #8
                More pics:















                Thanks,

                egdon

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