Amp problem: Ugly compressed distortion only during pick attack playing

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Flameout12, Mar 12, 2008.

  1. Flameout12

    Flameout12 Member

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    I have this Rivera R-55. It’s 9 years old. When I play in the hi-gain channel, there is this bad ugly distorted sound that it emits and it gets worse with more gain. It only does this when I pick and is much worse on the higher notes, especially high up on the neck. If I do hammer ons or pull-offs, it sounds great…it’s just there during picking attack.
    It sounds like something is clipping and I hope someone can help me with this. My suspicion is this is a cap or something that needs replacing.
    I currently have 2 other amps (1987 Marshall clone and Budda SD18) and neither of these have this problem.
    Here’s what I’ve done so far:
    - Tried several different picks and looked hard at my technique…it’s not me or my picks
    -I plug straight into the amp with a 10’ Fulltone cord. These are excellent cords. Nothing in the loop…just straight guitar-amp.
    -Tried several different guitars…HB PUs are much worse…and the hotter the PU, the worse this is. Both of my Les Pauls do the same thing.
    -Tried lots of new tubes….pre-amp and power as well as checking the bias. New tubes help, but it’s not tubes.
    -Made sure all tube sockets were clean
    -Tried this with 3 different speakers with good cords. Ohmage was correct.

    Guitar techs in my area are not aplenty. It would be helpful if I had an idea what was causing this cause this is really a nice amp that I like a lot.
    Thanks!
     
  2. Blue Strat

    Blue Strat Member

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    Have you tried a different guitar? Has anything changed like new speakers?
     
  3. Groovey Records

    Groovey Records Member

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    OP includes all the details!!
    the facts just the facts:)

    Groovey Records
     
  4. Blue Strat

    Blue Strat Member

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    D'OH! Missed the details:)

    Looks like it's "tech time".
     
  5. Rosewood

    Rosewood Member

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    It almost sounds like a bias problem, not biased wrong but like a coupling cap leaking and upsetting the bias on the following stage. You need to take it to a good tech for a problem like this before you spend too much money on it.
     
  6. TopBooster

    TopBooster Member

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    +1, that's exactly what I thought when I read the OP!
     
  7. Flameout12

    Flameout12 Member

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    Thanks....I'd bet money you're right. I got this from another forum I asked this on:

    It seems to be a problem with the bias of the last amplification stage in the preamp (HiGain stage). Incorrect bias leads to distortion, the cause for this could be a leaky coupling cap from the previous amplification stage or a bad bias resistor in the actual amplification stage. To sort this out, you'd need an oscilloscope to watch the signal, just a trace from the input up until it starts to distort.

    I have a tech guy who is Ok with amps, but I bet he can find this and fix it.
    You guys have been a great help cause I really like this amp.
    :BEER
     
  8. epluribus

    epluribus Member

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    Notion From Left Field Dept.: Does it play well with single-coils in other words? (Or have you tried the lower-output HBs in modern metal-type guitars?) It may just be the wrong flavor guitar and the amp is actually working as designed.

    Around the web I've noticed discussions popping up about using too-hot signals going into hi-gain amps. What seems to happen is that you can upset the carefully balanced gain structure in a cascading front end. Apparently in some circuits the first gain stage is intended to run fairly clean and the following ones do the rather delicately crafted dirt. When you hit those front ends too hard the results sound much like what you describe.

    In each case where I've gotten feedback from guys, going into hi-gain rigs with middling-output pups (like the bridge HBs used on later metal-type guitars,) fixes the problems with articulation and gainy mush. In my own experience, (even my Rivera-designed DR II) conventional HBs like the stockers on my LP strike me as too hot for good hi-gain definition. My metal axe has considerably lower output in its HBs, and though it sounds "thin" in a clean amp, it's incredibly articulate in a hi-gain circuit and the amp fills the tone in making it focussed, tight, and stout. In that environment, IMHO, I've concluded that an LP is just too much guitar.

    Anyhoo, once I learned this it saved me a lot of head scratching with trying to get good metal and shred tone from my amps. The amps were fine, just the wrong giutars.

    --Ray

    (BTW...early hi-gain tone was indeed produced with hi-output pups and clean boost, but going into crunchy older-style amps. So if early hi-gain tone is what you're after, that combination is likely the weapon of choice. For later stuff I find that low-output pups and true hi-gain circuits with their sort of built-in clean boost hits those modern sounds more squarely on the head. But the two definitely don't mix.)
     
  9. PRNDL

    PRNDL Member

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    9 years old ... it may be the electrolytics are fading, or a resistor has drifted.
    High gain channels are more susceptible to component values.
     
  10. Flameout12

    Flameout12 Member

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    Hey thanks...actually this was one of my suspicions early on with this since I've had this problem for quite a while. I just didn't remember it being there from the early days of me playing this amp.

    I do have JBs on my main LP and this is temp since I have something else due anyday. JBs are hot PUs.
    And yes, the amp does better with SCs as well as a few other oddities....higher treble/mid settings and this seems to help quite a bit. The gain channel on this amp just needs higher mids and treble, otherwise some of this comes thru as part of a darker sound which ain't nice. However the Budda has no issues with the JBs at all...it just sounds smooth with no unwanted distortion.

    I also have this issue with pre-amp tubes...Riveras are wierd in that they like the cheaper/brighter Chinese tubes better than some of the ones most folks would put in a Marshall. I swapped out my PI tube with some others and improved this quite a bit.

    But I'm still not sure we don't have something going on with a cap or resistor that may make this amp more sensitive to tubes and hi-powered PUs than it should (bias leakage is greater with more gain?).

    Thanks to all of you guys for helping me. :BEER
     
  11. pgissi

    pgissi Member

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    This could be bad news and is the worst case scenario and those suggestions of too hot of a signal from a humbucker etc are grasping unless your inputting a synthesizer with its 10volt output signal, maybe not even then.


    Your hearing some non-linearity or discharge occuring within the amp and only at higher frequencies

    -OT secondary is interally shorted due to insulation on some portion of
    windings burnt etc. could be why it only occurs on the upper registers since output impedance varys with frequency, the bad section of some windings in the OT secondary just happens to cause the higher notes to be affected by this only.

    -2nd worst case, output tube socket arcing

    -the bias problem mentioned earlier is also a logical conclusion but this would manifest across the fingerboard for all ranges and at all volumes.

    -hi freq bypass or coupling cap, the rivera is component laden and there may be a cap used in the signal chain that is charging and discharging abnormally and is used to shape tone somewhere


    Here are your 2 next moves

    1) inspect the amp for carbon tracks on the output tube sockets

    2) patch the pre-amp out only to another amps power amp in to isolate
    the problem between preamp or output stage

    3) if its an output stage problem testing the tranny is not possible without
    sophisticated test equipment and that does not mean a scope so you'll
    have to check the output stage up and down and if you find nothing wrong on the primary side, you got a bad OT


    I have an R30, the Rivera is a loaded PCB and is not easy to service.
     
  12. Flameout12

    Flameout12 Member

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    Hey thanks....I ran the amp into another via the loop, which should be pure pre-amp and the problem is there. It's almost certainly a pre-amp issue. I could take it to someone and aske them to check the caps.

    If you have any other suggestions, I'm listening. :dude
     
  13. Blue Strat

    Blue Strat Member

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    The the problem is in the preamp and it's not tubes, it's something else! :D

    This means bench, scope and technician.
     
  14. Swarty

    Swarty Member

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    The bias issue mentioned above may be able to be isolated without a scope as a leaking coupling cap should show DC on both sides. Not for the novice though. If you don't have experience diagnosing a powered on amp take it too a tech.
     
  15. Flameout12

    Flameout12 Member

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    Crap! I was hoping I could just order some special tubes from KCA and all my problems would go away. :)

    Thanks guys...I do have someone that fixes amps and I bet he can fix this. He's a little wierd, but he's cheap. :AOK
     
  16. pgissi

    pgissi Member

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    Well thats good news (pre-amp problem) I would think, its not an OT or in the power supply so far. I agree, take to a tech if your not experienced with working inside the amp powered up, it can kill you or the amp. But you should gather some more info and now that its narrowed down to the pre-amp-

    1) when it occurs when using the amp normally were you using the FXLoop? (normally meaning not feeding the pre-amp/fx loop send to
    another amp, using the whole amp)


    2) were you using reverb?

    The Rivera has an Op-Amp based Fx Loop and a Tube for Reverb Return and from what I can tell form this crappy schematic that seems to be the only one out there, its uses an Op-Amp based reverb driver.

    In addition to all that the Rivera uses LDR's to do signal routing.

    These amps are one of the most complex designs, difficult to work on and the more you narrow it down for a tech the less time to repair. That does not mean it will be any cheaper to repair as far as labor:bkw


    Here is my gut level diagnosis-

    you did not mention that the amp hums since this started so for me that rules out a coupling cap, they block DC and a bad coupling cap would make the amp hum.

    I am assuming you were not using the FX Loop and if you were, eliminate it by pulling the patch cords from the loop jacks, the fx loop op amps are bypassed when its not used. Likewise for reverb, unplug the pan and turn it down and test again.

    Aside from that

    The noise only occurs at high freq's and when you pick but not when you tap and why. Picking generates a higher voltage transient in the pickup with the naturally more aggressive attack where as tapping is less transient by nature, lower voltage generated in the pickup.

    When an amp has a problem responding to hard knee transients such as from pick attack versus soft knee transients of a finger tap, its is a sign of a possible gain and/or voltage problem possibly filtering if the problem was in the output stage. But its not

    The cathode resistor and its bypass capacitor of a gain stage sets a tubes gain characteristic (pre-amp tubes are cathode/self biased by the cathode resistor with lower values setting the gain higher) and the problem could point to either component being bad, intermittent or drifted off.

    One of your tube stages is going briefly non-linear on these hard knee transients during a pick attack and only at higher freq's where a cathode caps is used in parallel to (or to bypass) the cathode resistor to tailor the freq response and gain of that stage, typically to give that stage more OD in the higher frequencies.

    Its a gut level hunch and without making a more thorough inspection, it remains so.
     
  17. pgissi

    pgissi Member

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    Yes and your post could be no more wrong and confusing.

    Your generic "power supply would make amp hum" is the equivalent of seeing a fortune teller for the diagnosis and her telling you your going to die someday. Fact is hum comes from many sources and when and interstage coupling cap goes bad, dc is passed from one stage to the next and you get hum, maybe not powers supply hum, but hum nevertheless.

    HV caps are not coupling caps, not in audio amplification anyway and if you work in electrical distribution, you cant apply the terminology here to mean the same!

    Coupling caps "couple" audio stages either pre-amp or power amp and do the following-

    -block dc (which is the sdame as saying couple the signal to the grid at the proper dc level)
    -do some tonal shaping

    HV are just that, high voltage caps/filters, smoothing or regulating caps designed to keep B+ DC steady and clean with some reserve juice on hand to replenish the circuit during...drum roll please...transients

    Notice in the Geofex link he say signal coupling not HV coupling




    http://www.geofex.com/ampdbug/coupling.htm
    The biasing of any tube stage depends on having the signal coupled into the grid of the tube at the proper DC level. A leaky coupling capacitor lets current through from the preceeding stage and upsets the DC bias, usually turning the tube on so hard that no signal can pass through it. A power tube with a leaky or shorted coupling capacitor may blow fuses cause low power or excess hum, or kill transformers. A preamp tube will just cause little or no signal to pass through or ugly-sounding distortion of the signal that does get through.


    Remember, I said the OP since the OP did not mention hum my money is on a cathode resitor/cap issue with the slim possibility its an intermittent coupling cap that has drifted since its only in the pre-amp and only during hi-freq tones.
     
  18. Flameout12

    Flameout12 Member

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    As a follow-up to this, thanks so much for your efforts to help me here and thru PMs.

    Story goes like this...I'm still convinced the amp has some kind of issue, but the amp tech I use really didn't want to dig into my problem once I played it for him in his shop using one of my guitars. He wasn't convinced there was anything really wrong with the amp besides my overdriving it too much. It is a tough problem for someone like him to chase, especially since he's keyboard player. So anyway, it didn't get fixed.

    Basically, if I don't overdrive the amp with too much bass in hi-gain situations, I don't have a big problem. And the pick attack apparently is a bass signal (thump) and that's why it's a problem. Cranking up the treble/presence helps mask some of it as well as swapping tubes to get the best tone.

    But the best solution is running into some kind of stomp box & then into the amp. Apparently the stomp box even in bypass, helps buffer the signal enough to limit the problem.
    And one would think a longer cable work as well, but it actually makes it worse. :confused:

    There just is no way an amp should be this sensitive to input gain straight from the guitar, especially a Rivera. The guy made his living hotrodding Marshalls and Fenders in his early days.
    And there is no way I should have to try all kinds of tubes, cables and other methods to try and "fix" this without there being something wrong with the amp straight out.

    But since I have found some doable workarounds, I think I'm going to be Ok with this. The bright side of this is I have learned a lot in the last year about tone, tubes, impedance, speakers, stomp boxes, pots, fx loops, etc. That is valuable knowledge that cannot be taken from me.

    Once again, I appreciate all of the comments provided here...you guys have helped me out quite a bit and have saved me lots of $$. Now maybe I can regain some of my skills I lost to spending all my time on amp problems.
    :AOK
     
  19. Rod

    Rod Tone is Paramount Silver Supporting Member

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    I'd try a 12AX7-WB in the first V1 position
     
  20. Kenster

    Kenster Member

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    Maybe try the lower sensitive 2nd input?
     

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