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1987 plexi circuit - cathode follower issues

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  • 1987 plexi circuit - cathode follower issues

    I'm currently debugging an amp I put together, which is essentially a Marshall 1987 plexi circuit. It also has a "1 tube reverb" circuit added, but that's not important here as the amp behaves the same with the reverb circuit disconnected. The amp sounds good and has no noise issues, but does not have the volume it should - it's loud enough to hold its own in a rehearsal space but a 50 watt Plexi should be call-the-cops loud. So I set about re-checking everything against the schematic and measuring voltages. I found and corrected a mistake in the tone stack which would have been losing me a few decibels (wrong value slope resistor), but when checking DC voltages I came up with something which is puzzling me.
    The DC voltages at the valve pins are all within a few percent of the expected values except for V2b, the cathode follower. V2 is set up with the standard 100k plate and 820 ohm cathode resistor on V2a and a 100k cathode resistor on V2b. With a JJ ECC83s in place I measured the following voltages.
    JJ ECC83s:
    p6 256V, p7 146V, p8 86V

    It strikes me that pin 8 (V2b cathode) is far lower than it should be - all of the measurements I've found online place it within a few volts of pin 7 (V2b grid). I can find no errors in my component choice or layout around this stage, and the DC resistance between pin 8 and ground is 100k as it should be. Thinking it might be an issue with the valve, I tried a couple of old ECC83 valves which I had lying around. They are an old Mullard and a Brimar salvaged from used equipment, which pass signal but are otherwise untested. The voltages around V2b with those were as follows.
    Brimar:
    p6 251V, p7 146V, p8 112V
    Mullard:
    p6 252V, p7 137V, p8 94V

    Clearly the voltage at pin 8 does vary when changing valve, but all of these are still coming out lower than they should be. Because it varies between valves and I don't know how tired my spares might be, I think my next step may be to try a fresh, tested ECC83 in V2 and see what I get. I'd be grateful for any other pointers about this - am I barking up the wrong tree? I'm also not certain how much variation should be expected here.
    Last edited by Martin Beer; 07-11-2016, 06:25 PM.

  • #2
    Try to eliminate any possible measurement error by measuring the DC voltage different ways.
    Assuming you have your black probe grounded, try instead with black probe to cathode pin, red to grid pin. Also try one probe to each end of the cathode resistor.
    Also measure the 100K resistance.

    Example voltages can be seen here: http://mhuss.com/SmallBox/1987.gif
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      I always run my Preamp B+ rail on all pre-amp tubes higher than that.
      You can adjust the B+ rail at the two 10k resistors between the bias and PI coupling caps.
      I usually take one of the 10ks out, and add a strap.
      Also could something in the tone stack be dragging it down.
      Click image for larger version

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      "If Hitler invaded Hell, I would make at least a favourable reference of the Devil in the House of Commons." Winston Churchill
      Terry

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      • #4
        Originally posted by g1 View Post
        Try to eliminate any possible measurement error by measuring the DC voltage different ways.
        Assuming you have your black probe grounded, try instead with black probe to cathode pin, red to grid pin. Also try one probe to each end of the cathode resistor.
        Also measure the 100K resistance.

        Example voltages can be seen here: http://mhuss.com/SmallBox/1987.gif
        I measured it with the black probe clipped to chassis ground and the red probe pressed to the socket pin. I'll fire it up and re-check the voltages in the morning. The 100k V2b cathode resistor measures 100.6K, which seems to be well within tolerance.

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        • #5
          Yes, check the tone caps.

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          • #6
            Originally posted by big_teee View Post
            I always run my Preamp B+ rail on all pre-amp tubes higher than that.
            You can adjust the B+ rail at the two 10k resistors between the bias and PI coupling caps.
            I usually take one of the 10ks out, and add a strap.
            Also could something in the tone stack be dragging it down.
            [ATTACH=CONFIG]39839[/ATTACH]
            Is that just a tonal preference thing, running higher B+ on the preamp? The tone stack is probably something to look at - I've checked it over for wiring errors and can't find anything, but something like a leaky cap could conceivably throw the voltages off at V2b cathode. I guess I could check the DC voltage at the pot side of the 500pF and two .022uF caps in the tone stack, as any DC voltage there would indicate something was amiss.

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            • #7
              I try to find the sweet spot on the voltage.
              The headroom seems better with higher voltage.
              Yours seems a bit low.
              Just try adjusting it, and see what you think.

              T
              "If Hitler invaded Hell, I would make at least a favourable reference of the Devil in the House of Commons." Winston Churchill
              Terry

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              • #8
                Originally posted by Martin Beer View Post
                With a JJ ECC83s in place I measured the following voltages.
                JJ ECC83s:
                p6 256V, p7 146V, p8 86V
                It shouldn't be possible for the grid to be 60V higher than the cathode. I'm no valve expert but is it possible for the valve to be low emission on the 'b' side if its heater circuit isn't working correctly? e.g. because there's a dry joint or high resistance socket on pin 5.

                Also hanging meter probes on the valve pins sometimes causes a high frequency oscillation which throws off the meter reading. If you have a scope and x10 probe check the voltages with those to be sure there's no oscillation.
                Last edited by Dave H; 07-12-2016, 02:00 AM.

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                • #9
                  Originally posted by Dave H View Post
                  It shouldn't be possible for the grid to be 60V higher than the cathode. I'm no valve expert but is it possible for the valve to be low emission on the 'b' side if its heater circuit isn't working correctly? e.g. because there's a dry joint or high resistance socket on pin 5.
                  I think you might have put your finger on it, cheers!
                  I checked for oscillation with my rather basic digital scope and couldn't see anything untoward going on, checked the tone stack caps for leakage and nothing obvious stuck out. Then I re-flowed the solder connections on the socket, and I now have much healthier looking voltages, with the cathode 2V higher than the grid.

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                  • #10
                    Is it an earschplittenloudenboomer now or is it still not loud enough?

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                    • #11
                      Originally posted by Dave H View Post
                      Is it an earschplittenloudenboomer now or is it still not loud enough?
                      It's hard to tell until I can take it down to the rehearsal space at the weekend, as I can't play loud at home. It gets too loud for my flat much earlier up the dial than it did previously (I'm using a PPIMV), so that's hopeful.
                      I have access to a shared workshop with a dummy load and a better scope than mine, so I might take it down there and check whether it's making the full output.
                      Last edited by Martin Beer; 07-12-2016, 01:36 PM.

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