5E3 low volume, low voltage on one pin only.

Discussion in 'Shock Brother's DIY Amps' started by Bristlehound, Jan 28, 2018.

  1. Bristlehound

    Bristlehound Friend of Leo's

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    Time to turn to my trusted TDPRI troubleshooters.

    My DIY 5E3 has gone rather quiet, and after checking voltages it seems to have developed a very low voltage on pin 7 of V2. According to Robrob it should be about 17V, and mine is 3V.

    The full V2 pin chart is (Rob's figures in brackets)...

    #1 (164) 179V
    #3 (1.2) 1.3V
    #6 (200) 189V
    #7 (17) 3V
    #8 (45) 45V

    All other B+ and pin voltages are close to Rob's.

    I've checked everything I can think of and tried two different working tubes in there, always with the same result.

    Any help greatly appreciated, and I'll buy you a pint of warm beer!
     
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  2. theprofessor

    theprofessor Friend of Leo's

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    As you know, Bristlehound, I am no pro. But: pin 7 on a 12A*7 is the grid for triode A, and pin 2 is the grid for triode B. There's a 1 Meg grid resistor at pin 7 of triode A that is connected to the 56k tail load resistor. Are the values of both correct? Is the 56k tail load resistor well-grounded along with the B+3 cap, as it should be?

    I look forward to what some more knowledgeable folks than I have to say...

    5E3_Layout._Annotated_Signal.gif
     
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  3. Bristlehound

    Bristlehound Friend of Leo's

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    Thanks Prof. I've checked all those values and grounds and they're fine. The amp's been working properly for many years and they've never been touched. I rewired the volumes the other day... I wonder if I messed up somewhere there. I'll check all those when I get the chance.
     
  4. theprofessor

    theprofessor Friend of Leo's

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    If you've done some re-wiring recently, that'd be the first place I'd check. See what you find...
     
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  5. clintj

    clintj Friend of Leo's

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    Pins 7 and 8 connect to a trio of resistors (1M/56k/1k5) between the last filter cap and first coupling cap. You say you have verified those to be correct resistance; check DC voltages at all the junctions in that trio and post that please. That'll give a more complete set of data on how the cathodyne is doing. The meter upsets the circuit too much to get an accurate reading of bias when used directly on the tube.
     
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  6. Bristlehound

    Bristlehound Friend of Leo's

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    Thanks Clint.... here it is;

    20180129_101308.jpg
     
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  7. clintj

    clintj Friend of Leo's

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    Ok, that puts the cathodyne bias at around 2.4V, pretty reasonable.

    I'd also look at the last work done, and do a quick bias check of the power tubes as a next step.
     
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  8. zook

    zook Friend of Leo's

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    Have you tried a different tube?
     
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  9. clintj

    clintj Friend of Leo's

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    The more I've had time to think about it, the more that low voltage at the pin puzzles me. Unless you've switched to a meter with much lower internal resistance, it shouldn't pull down voltage that far at the tube.

    With the amp's caps bled down (safe to enter the chassis IOW), what resistance do you read from that pin to ground? You may also have a solder joint starting to break down from handling and vibration in there, at either end of the 1M resistor/lead to pin 7, or even just an oxidized pin not making good contact.
     
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  10. Bristlehound

    Bristlehound Friend of Leo's

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    Thanks Clint. I've really been puzzling over it too and, coincidently, the amp has just developed an earth hum. So, on to the work bench it will go and I'll re-flow the solder joints around the 1M resister, as well as the pre-amp grounding, plus contact clean the pins. I wonder if re-tensioning the pin sockets might help.

    I checked the bias yesterday and got a plate-cathode voltage of 304V, a p.d. across the cathode resister of 18.2V giving 11.1W @ 92.5%
     
  11. Bristlehound

    Bristlehound Friend of Leo's

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    That was my first move. Two different tubes, just in case!
     
  12. RLee77

    RLee77 Friend of Leo's

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    What sort of a meter are you using? I know you've heard it before, but you really cannot use a standard meter to measure grid voltages, it will load that high resistance node down, giving you a wrong reading and also changing the bias on the tube. Do you know what the input resistance is on your meter for the voltage range you have selected? The manufacturer will usually provide this.
    Anyway, since the grid is biased negative, it draws no dc current, thus there is no drop across that 1 meg, meaning that the 46.4v on the tail R also appears on the grid. (Assuming no bad connections or components, but that's what we're looking for, since not everything adds up here.)
    You have nothing plugged into the inputs, I assume.
    With 2.4v across the 1.5k, the tube has 1.6ma plate current, seems normal, but the odd thing is we should have that same current flowing through the tail R, but we don't. If that tail R really has 46.4v across it, then it only has about .83ma, so where is the other .77ma going? So something seems off.
    I'd inspect that tube socket and pin connections carefully, and the component values -- measure the resistors. Maybe you have a leaky coupling cap (doubtful), or your recent wiring changes did something.

    I second everything clintj posted above, and I'd follow his advice to bleed down all the e-caps completely, let the amp sit for a bit, then measure those resistors, and grid to ground R, across the coupling caps, etc. It could even be that there's nothing wrong with the circuit and it's just your meter's low resistance causing these reading errors.
     
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  13. clintj

    clintj Friend of Leo's

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    That's what has me intrigued with the readings. 20-ish volts right at the grid is what a lot of folks get. It's not a valid reading, but it's repeatable across a slew of new build threads and consistent with what I get on a typical cathodyne. 3V is just, well, odd. Might be a red herring, might not.

    A meter with a 1M internal resistance should pull down grid voltage about halfway, as it'd make a voltage divider of sorts with the 1M grid leak. If he's got his original build measurements, and a new set with different voltages with the same meter, there's something weird going on. Bad joint, conductive board, yada yada yada.

    That meter has a good battery, right? And the probes are nice and clean? And it's not something like a Fluke set to the lo-Z voltage reading mode?
     
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  14. D'tar

    D'tar Friend of Leo's

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    Do you have a pre-pimv installed by chance?
     
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  15. Bristlehound

    Bristlehound Friend of Leo's

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    Thanks RLee77 and Clint. I have an ancient el-cheapo meter so that could be giving me false readings - I used the 20V setting.

    I've also futzed around with the amp constantly since I built it ten or so years ago, trying all sorts of changes "just because", so solder joints, capacitors, fibre-board etc could all be suspect.

    I've also developed an earth hum (which goes away when either of the volumes are maxxed) which I can't trace. I re-flowed the joints around the 1Meg, cleaned the valve socket... and still 3V. I checked all joints to see if there was any leak to earth.

    It might be time to build a new board with new componants.

    I did... but I took it out a few months ago. I didn't check any voltages after the change.
     
  16. King Fan

    King Fan Friend of Leo's

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    You don't need a Fluke, but a good MM with *auto-ranging* really made my life easier.
     
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  17. nsmith01tx

    nsmith01tx Tele-Meister Ad Free Member

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    I'm certainly not an expert, but it seems like if you check the voltage upstream from #7 (before each resistor) it should be higher at each step. If one of those steps seems like an unusually high drop that could be an indicator of a bad solder joint or component .. or, if you haven't already, you could just reflow every joint along that line, especially those you may have changed recently.
     
  18. ouaouaron

    ouaouaron TDPRI Member

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    this is a strange coincidence I have about 3 volts at 7 and all the resistors test correct and at 8 I have 47 volts, same as you. My problem is not low sound but no distortion. did you ever figure this out?
     
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  19. Bristlehound

    Bristlehound Friend of Leo's

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    I left it and then put a pre-pimv back in. I also took out all the changes I'd made (NFB etc) and re-flowed all my solder joints. New rectifier and 6V6s, plus a 0.02uF blocking capacitor and it's sounding good.
    I've just swapped the Jensen P12R for a C12Q. It's a bit soon to say, but initial impressions are very favourable.
     
  20. ouaouaron

    ouaouaron TDPRI Member

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    resoldering points, did that bring your v2 7 pin up from 3 to where it should be at around 18 - 20?
     
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