(Silverface) AA270 - Other alternatives for 0.002uf capacitor from reverb circuit?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, May 6, 2021.

  1. itsGiusto

    itsGiusto Tele-Holic

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    I've been fixing up a AA270 Twin Reverb, and getting it to AB763 specs. One difference is that there is a 0.002uf capacitor from the grid of the reverb recovery triode (V4B) to ground on the AA270, bypassing the 220k resistor. There is no such cap on the AB763 specs.

    See http://ampwares.com/schematics/twin_reverb_aa270.pdf vs https://cdn.shopify.com/s/files/1/2...everb-AB763-schematic.pdf?5926397210715640943

    I've clipped this resistor in and out of the circuit, and I think it makes an audible difference to the tone (but barely). But definitely having it in the circuit reduces a lot of the buzz coming from the reverb tank. It's not intolerable without the cap, but definitely a little bit buzzier.

    Does anyone have suggestions for other ways to reduce the buzz that won't affect the reverb tonality? Are there any particular wires that better lead dress would help reduce the buzz?
    Or am I just hearing a difference where there is none, and I should put the .002uf cap back in?
     
    Last edited: May 6, 2021
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The cap would reduce some of the highs that made it through the reverb tank. Generally reducing some of the reverb signal lows are not a problem. What pitch is the buzz?
     
  3. itsGiusto

    itsGiusto Tele-Holic

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    Good question, I'll have to measure it, I don't know what frequency.
    The cap seemed to me to reduce some reverb highs, and definitely reduced the highs on the buzz, like putting a blanket over it.
     
  4. schmee

    schmee Doctor of Teleocity Ad Free Member

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    They added that cap in an attempt to quiet down the reverb background noise I've heard.
    Some amps they spiral wrapped a grounded white wire around groups of wires to quiet things too. Not sure how effective any of it was. But maybe they did it because people started playing a lot louder after the BF era/ in the 70's.
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    Hah, yeah, there are a few wires that are spiral-wrapped like that in my amp, but they're the ones coming out of the doghouse near the front-knobs.

    I guess I could try using shielded wire for the lead that runs from the reverb input to the grid, but come to think of it, I think probably the noise is picked up by the RCA wire running from the tank to the amp. When I unplug the RCA wire, I think the buzz goes away.
    Edit, this is incorrect. When I unplug the RCA wire, the buzz gets worse
     
    Last edited: May 6, 2021
  6. Ten Over

    Ten Over Tele-Holic

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    Actually, the opposite is true. The output coil and the .0022uF cap have a resonant frequency. The output coil has an impedance of 2250 ohms at 1KHz, which works out to 0.358H of inductance. The resonant frequency for .0022uF and 0.358H is 5.67kHz. The output of the tank has a sharp peak at 5.67KHz. The output at the frequencies on either side of the peak are pulled up just like a parametric equalizer. So the .0022uF cap makes the reverb a little brighter.
     
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  7. schmee

    schmee Doctor of Teleocity Ad Free Member

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    Hmmm. One thing to try is cleaning the RCA jacks, especially the outside of the jacks on the amp jacks (where the cable grippers go on) I had one amp that hummed badly and it drove me crazy. I finally found out it was that! Try just twisting them and see if it changes the hum.
     
  8. itsGiusto

    itsGiusto Tele-Holic

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    Interesting... But if that's the case, why do I hear the buzz just completely become so much more muted when I clip in the capacitor?
     
  9. itsGiusto

    itsGiusto Tele-Holic

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    Okay, I measured it, it's definitely buzzing based on 60 cycles. It has the 60 cycle overtone sound, and measures to have a period of about 17ms on my oscilloscope.
     
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  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    60Hz might be heater filament hum. Possibly the tube, the wiring, or the grounding. Do you have another tube? Push the wires around... again.
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am trying to wrap my head around this. Looking at a high pass LC filter, I can see how one might look at it as being brighter. What I see is the .0022uF cap is going to ground instead of the control grid. Am I not seeing the high pass signal going to ground?

    InkedLC Hi pass_LI.jpg
    LC High Pass Filter.

    EDIT: @Ten Over, I see it now. It is a Parallel Resonant Band Pass Filter. In my best Emily Litella... Never Mind.
     
    Last edited: May 7, 2021
  12. Dacious

    Dacious Poster Extraordinaire

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    Try a smaller value. If you clean up lead dress associated with the reverb circuit - push wires around with chopsticks.
     
  13. Forthefunofit

    Forthefunofit TDPRI Member

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    itsGiusto,
    I had a similar issue with reverb tank hum last fall when I built my AA1164 PR clone. When nothing else seemed to help, I tried the .002uf cap bypassing the 220k resistor at the reverb recovery tube grid (from AB1270 circuit) & guess what? Cleaned up the hum issue.
    Just out of curiosity, what tank are you using? I'm using an Accutronics 4AB3C1B in mine.
     
    Last edited: May 7, 2021
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