Blocking distortion in blackface era amps.

Discussion in 'Amp Central Station' started by JohnnyCrash, Jun 17, 2019.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I recently built a Super Reverb’s “reverb” channel into a Princeton chassis and cab. I encountered blocking distortion at high volumes — it happens in some builds.

    Seeing as how most amp “info” on the internet is usually “old wives tales” of sorts (people just continually passing on hearsay), I figured I’d start a thread here to ask for electrical explanations and solutions, rather than repetitions of hearsay found on an old website.

    Blackface amps seem particularly susceptible to blocking distortion. I’m hoping this thread can be of service to others experiencing this issue in new amp builds or vintage buys.



    1. GRID STOPPERS:
    My 6L6s grid stoppers are 1.5k. The amp is built to the AB763 schematic (with the Normal channel and its tube omitted). Is there an ideal size for these resistors? Everyone seems to guess blindly in the dark as to values that eliminate blocking distortion while not dragging tone and treble down. I don’t want to leave tone up to guesswork.

    2. PHASE INVERTER INPUT CAPACITOR:
    Phase Inverter input cap is 0.001uF. Is there an ideal PI input cap size? We can do better than blindly guessing on values here. Some bf Bassman had a 500pF PI input cap. Is there a better way to calculate the ideal cap size here (given that we already know the guitar’s lowest frequency)?

    3. REVERB DRIVER TUBE:
    The reverb driver tube is brand new (JJ tube)... but given how hard these tubes are driven (generally around 400 volts for a 12AT7), blocking distortion is often derived from this part of the circuit. Are there any improvements to this circuit to eliminate it as a source of the problem?



    These are the usual three suspects, but feel free to help us all out if you have more tips/tricks (I already used shielded wire on the inputs).
     
  2. Uncle Daddy

    Uncle Daddy Tele-Holic

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    Did you see this?

    https://www.aikenamps.com/index.php/what-is-blocking-distortion

    Fender amps are particularly susceptible to this because of the large values of coupling capacitors on the grids of the power tubes (0.1uF). Blackfacing your Super Reverb can actually make the problem worse, because you change the grid bias feed resistors from 100K to 220K, which increases the time constant of the AC coupling to the output tube grids. You will note that most Marshalls use 0.022uF coupling capacitors and 100K resistors, which gives a much faster time constant. In addition, the preamp stages have a much more rolled off low frequency response. This is why they sound tighter when played wide open.
     
  3. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Yes.

    The problem is, I don’t have 220k grid bias feed resistors... instead I have a dual pot 250k Post Phase Inverter Master Volume.

    The blocking distortion is clearly occurring before the power tubes since it happens when the volume is turned up high, even if the PPIMV is set very low... precisely where these grid bias feed resistors would be doing their thing.
     
  4. JD0x0

    JD0x0 Poster Extraordinaire

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    That's worse. That's like having 250k grid leak resistors, which makes it more susceptible to blocking.

    1. No guessing. You can calculate the low pass frequency. In many cases you can go up to 90K before it's even audible for guitar.
    2. Again, no guessing, you can calculate the high pass of the coupling cap. Personally, I like a larger value here, and cut more bass earlier in the circuit.
    3. Never heard of blocking at the reverb driver. Especially because it's fed by a very small coupling cap. If you feel it's blocking, you can again, just add a grid stopper resistor. You may be able to move the B+ node of the driver tube to a later part of the rail, to reduce the B+ it sees. Just be aware of decoupling issues. If you use a good AT7 and don't bias it too hot it shouldn't be too much of an issue as the tube wont be dissipating that much wattage. The bigger danger is probably when you initially flip the HT voltage on. You can see voltage spikes over 2x the normal B+ voltage, and that could damage the tube, since it has no protection or plate resistor to limit things during those spikes.


    Don't assume Fender had things perfect. He didn't, and he was very cheap, which meant compromises.



    You're looking in the wrong places then.
     
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  5. schmee

    schmee Poster Extraordinaire

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    Going to single channel did you change the 820 to 1500 resistor off pin 8 of your V1? (was V2)? I doubt that's your problem though. Also, add the 220k mix resistor to ground for the missing channel.... but maybe with the MV you cant?

    I'm not sure what Blocking Distortion is. But I did a SR in my PR chassis though years ago and did the above.
     
  6. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    It would be, if the MV is cranked. I’m getting blocking distortion at low MV levels though.

    Then why don’t amp designers just routinely use 90k?

    You can always cut early, but you can’t add bass later. With high gain amps however, I take the trim early approach. I want full range clean out of this one, though.

    Sorry for the misunderstanding. I don’t think it’s the reverb driver that is *causing* the blocking distortion. I think it’s *adding* to the problem.

    Some folks chasing this perennial problem note how pulling the verb tube eliminates oscillations and blocking distortion. With the verb control at higher settings I can hear an audible increase in noise.

    My B+ might be a bit high. I appreciate this advice.

    I never have. He was a radio repair guy who didn’t play guitar in a time where overdriven sounds were considered not musical.

    When I build tweed Champs, I trim all the fat (smaller cathode bypass caps to reduce lower frequencies below the guitar, smaller coupling caps, etc.). Apparently Leo thought every amp needed a radio’s full frequency response. I’m not plugging vocals mics into my guitar amps :)


    Aiken and Rob (among others) have been the go-to websites for amateurs for the last 10 years. They know what they’re talking about and explain technical terms for the layperson. But I’m hoping we can leave a better thread here for the next bunch of builders.

    Short of posting blocks of electronic textbook text, I was hoping for a quick and comprehensive guide for others.



    As for this particular build, I’m going through the usual suspects.

    I have shielded wire from the input jacks. Shielded wire at the Reverb pot. A 0.002uF across the return grid’s 220k.

    My lead dress and voltage might be better... and this is where I’m working next.

    My last choice are the large power tube coupling caps... but I love the amount of bass I have and I don’t want to possibly reduce it yet.

    PS
    I have SS rectification and a slightly overrated output transformer precisely so I can have tight and deep bass... but if the couplers need to be smaller at the PI, so be it.
     
  7. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Great minds...

    Yes, I remembered to give my new V1 its own cathode (1.5k).

    I also have the 220k to ground for the missing channel at the mixing resistors spot.

    Good looking out.
     
  8. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Is this issue only happening with the channel volume high and the ppimv turned down low? In that situation, the nfb loop is collapsing and the phase inverter tube is being slammed. I try not to turn my ppimv down below halfway as the resulting tone becomes very unpleasant when the PI begins to distort.
     
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  9. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    This doesn’t happen with my other PPIMV (and NFB/Presence) equipped builds. Interesting.

    It also happens with the MV maxed.

    Ironically, it is much less when I engage the tonestack-dump gain-boost... this switch also reduces the first preamp stage’s cathode bypass cap to 1uF.

    Sounds like it may be too much bass in the pipe after all?



    My voltages look ok... except for the tremolo tube’s plates. They’re a little high. The trem still works though, but it’s not supposed to be a part of the signal chain.

    The trem circuit/tube can’t be contributing to the problem, can it?



    I’m now looking at lead dress.

    Next up, PI couplers...
     
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  10. Snfoilhat

    Snfoilhat Tele-Afflicted

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    If the input impedance of a long-tailed pair PI like those in the AB763 family of amps is around 2M ohms, and we want a cutoff frequency of 82 Hz (low E on a standard tuned guitar), then the coupling cap calculates to 971 pF. Just a bit under the open-backed cab Fender standard of 1000 pF (0.001µF).
    http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

    And that's still knocking low E down -3 dB. To pass full frequencies without attenuation I suppose the cap should be a little larger. But didn't the other AB763 amps intended for closed back cabs still have plenty of bass with 500 pF coupling caps at the PI? I recall reading about this on robrob's site, though I may be relating it wrong.

    http://www.valvewizard.co.uk/acltp.html
     
  11. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Sounds about right. I’m using the stock value there (0.001uF).



    OK, so I spent some time on it...

    I cleaned up all of my lead dress. Shortened wires where I could, rerouted grid wires away from plates, and used a chopstick to listen for crackly wires or cold solders.

    I replaced the 1.5k with the only value I had in my junk drawer: 2.2k. I know, not much of a jump in values.

    And I’m still looking at blocking distortion at around 6 or 7 on the volume.

    Also, possibly related: my reverb is noisy once it gets to 4 on the dial. I know reverb adds a bit of hiss, but this might be too much.



    So, I just pulled the reverb tube and NO oscillation. I can crank it on the volume and turn the MV real low and still have no problems. I can even use the tonestack dump switch at full blast with the MV on 1 and it sounds fine (actually, it sounds glorious!).

    This seems to be one of the common reasons for oscillations and blocking distortion in blackface Fenders, even if the reverb knob is rolled all the way off.

    Interestingly, this tube didn’t seem microphonic. It’s just a noisy sonofabitch, apparently.
     
  12. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    ... OK, so I tried three different 12AT7 tubes.

    Still have a problem.

    1. I’m using an old reverb tank. Maybe 5-10 years old. I’m wondering if maybe the RCA jacks are oxidized...

    2. My volume is actually a mini pot. I had to use a 1m push-pull DPDT switch/pot for my tonestack-dump gain-boost. I’m wondering if my Weller soldering iron fried the little fella a little bit...
     
    Last edited: Jun 18, 2019
  13. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    V2 seems ok then (V2 = reverb driver).

    Have you tried swapping V3 with another, known-good 12AX7? That tube is reverb recovery + reverb mix. V3 might not be the culprit... it will be easy to verify that with a tube swap.
     
  14. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    With the 12AT7 out, the problem disappears. Which led me to believe it’s the driver or the tank... but you might be onto something. If the driver is hitting the recovery stage with a bad tube there, maybe that’s the cause...

    I haven’t swapped the recovery tube intentionally — probably on accident though, since I’ve pulled the tubes and put them back in randomly during this troubleshooting several times.

    I’ll check that tube.



    In the meantime, I used a similar Heyboer PT about 10 years ago. I vaguely remember a discussion about how the PT shield (orange wire to ground) might somehow cause problems. My memory isn’t what it used to be, though.

    Maybe something to do with ground loops and the reverb tank? I don’t recall.

    Does this make any sense? Can a transformer’s shield cause problems? I’m tempted to clip that shield wire to ground, just to see...
     
  15. Greg_L

    Greg_L Tele-Holic

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    Check that. I had a minor grounding issue on a build in the reverb circuit and I had some minor oscillation. Fixed the ground....actually just moved it elsewhere...problem solved.
     
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  16. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Why is this lead on some transformers? I've had it on a couple that I've used, but most do not. Anyone have this intel?
     
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  17. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    It’s supposed to keep the transformer’s own hum down. Given how close together everything is in my build, I figured the PT shield would help.

    But, if I remember that ten year old build correctly, the grounded shield caused noise issues for some reason or another. I don’t remember why/how, though.
     
  18. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Time to go back over my ground scheme...
     
  19. JD0x0

    JD0x0 Poster Extraordinaire

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    You start the thread with a blocking distortion issue, now you're talking about an oscillation. These are two completely different issues.
     
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  20. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Forgive me for using the incorrect term. It’s blocking distortion. Hits around 6 on the volume. Not oscillating.

    I’ve had some recent problems with my cognitive functioning this year due to some personal health issues. Which is terrifying for me as an academic (archaeologist and historian). I have difficulty remembering (words/terms and events) and sometimes even forming sentences.

    I’m not trying to piss anyone off, I’m just an amateur trying to figure this out.

    The reason I picked up building amps again is for therapeutic reasons. Besides my cognitive problems, I’ve also been suffering from clinical depression and anxiety. Having something “fun” to focus on helps.
     
    Last edited: Jun 19, 2019
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