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Thread: Simms Watt 100w Head

  1. #106
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    Quote Originally Posted by josegrad View Post
    The readings are 589, 587, 589 and 589 VDC.
    I think those should be the readings at pins 3, 4 or 6?
    Pin 5 (or either leg of the 10k grid stoppers) should be about -40Vdc.

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  2. #107
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    Quote Originally Posted by pdf64 View Post
    I think those should be the readings at pins 3, 4 or 6?
    Pin 5 (or either leg of the 10k grid stoppers) should be about -40Vdc.
    Good catch pdf64, yes that was Pin6 then.
    When reading at Pin 5 the values are:

    With mains ON:

    -48.2 -48.2 -47.2 -47.2

    With mains ON and Stand by ON:

    -47.2 -47.3 -46.2 -46.2

    -----

    Tomorrow I could maybe go and get a new tube and see if it makes any differences. In case that one tube is bad.

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    Last edited by josegrad; 12-06-2018 at 10:24 PM.

  3. #108
    Bent Member Chuck H's Avatar
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    Can you please, with the standby switch in the play mode (closed) measure voltage at pin 8 of any of the power tube sockets.

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    Quote Originally Posted by Chuck H View Post
    Can you please, with the standby switch in the play mode (closed) measure voltage at pin 8 of any of the power tube sockets.
    Click image for larger version. 

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    Pins 1 and 8 are interconnected to the bus that goes to ground.
    I can put the red probe in the Pin 8 but where does the black probe goes?

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  5. #110
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    I went and purchased a new set of matches valves, all four.

    Adjusting the bias I did set all four to be around 25mA using the bias probe.
    But I'm not sure what's the correct mA I should aim for.

    Hum is very low now when all valves are so even.

    I didn't play it yet, until that apparently high voltage issue is discussed.
    But apparently the other valves were not in optimal condition.

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  6. #111
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by josegrad View Post
    But apparently the other valves were not in optimal condition.
    You can say that again!
    Hum is reduced (cancelled) by 'matching' the pairs of output tubes. So what you've reported is good. Try NickB's web page http://bmamps.com/ivds.html to get an idea of what the idle current should be. Please note that colder is better until you have a grasp of all the operating conditions involved.

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  7. #112
    Bent Member Chuck H's Avatar
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    Wow! All valves but one failing to draw significant current and that one that does was very low. Those tubes must have ten thousand of hours on them. Oh well. No need to look for voltage at the cathode now.

    Ok... With the tubes drawing 25mA a piece in play mode please measure the voltage at pin 3 of the power tubes.

    You can take that number and multiply it by the current (25mA, that's "milli"amps so .025) to find the watt dissipation at idle for the tubes.

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  8. #113
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    Quote Originally Posted by Chuck H View Post
    ...You can take that number and multiply it by the current (25mA, that's "milli"amps so .025) to find the watt dissipation at idle for the tubes.
    Ok, so after swapping tubes around for a while I managed to leave them like this:

    Tube 1 Tube 2 Tube 3 Tube 4
    Bias probe reading 23.5 mA 25.4 mA 25.0 mA 25.0 mA
    Voltage at Pin 3 576 Vdc 576 Vdc 575 Vdc 575 Vdc
    Watt dissipation 13.536 14.630 14.375 14.375

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  9. #114
    Supporting Member eschertron's Avatar
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    Since I've never worked with a tapped OT, I'm curious where the screen (pin 4) sits with these tubes. Can you share? Thanks.

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  10. #115
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    Quote Originally Posted by eschertron View Post
    Since I've never worked with a tapped OT, I'm curious where the screen (pin 4) sits with these tubes. Can you share? Thanks.
    What do you expect? The voltage at the screen taps is more or less the same as at the plates, at idle.

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    Last edited by Helmholtz; 12-07-2018 at 11:12 PM.
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  11. #116
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Helmholtz View Post
    What do you expect? The voltage at the screen taps is more or less the same as at the plates.
    And would think so too, at least at idle. With the high B+ voltage All I see from the simulator I used is garbage. Way over dissipated under load. I'm guessing the tapped voltage looks much lower at AC with a signal applied. Something for which the simulator does not account.

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  12. #117
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    Quote Originally Posted by eschertron View Post
    And would think so too, at least at idle. With the high B+ voltage All I see from the simulator I used is garbage. Way over dissipated under load. I'm guessing the tapped voltage looks much lower at AC with a signal applied. Something for which the simulator does not account.
    By transformer action the instantaneous AC voltage at the taps is forced to follow to plate voltage at a percentage given by the turns ratio. This way momentary screen voltage always stays below momentary plate voltage, other than in non-UL designs. For this reason manufacturers allow higher screen voltages in UL designs for some tubes. E.g. KT88. Not sure about EL34s, though.

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    Last edited by Helmholtz; 12-07-2018 at 11:55 PM.
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  13. #118
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    Quote Originally Posted by eschertron View Post
    Since I've never worked with a tapped OT, I'm curious where the screen (pin 4) sits with these tubes. Can you share? Thanks.
    The reading at pin 4 is 574 Vdc

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  14. #119
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    Now the amp is very sensible (some clicks and pops) to any kicks, even hitting the wall in a nearby room causes some clicks. That's after the new valves.

    In any case the valves sit quite tight in the sockets, the old ones were are bit tighter though, but I wouldn't say the new ones are loose once they are in the sockets.

    But the socket terminals are a bit/quite loose, with and without the valves in place. I'm thinking if I should replace them.

    Of course there might be some bad solder around, which I'm checking again, but those can also be very well hidden.

    ---

    By the way, now that I was able to calculate the watt dissipation, should I use that number to recalculate the bias setting?

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  15. #120
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    The female contacts in the tube sockets are, necessarily / by design, fairly loose in the base. Otherwise they might put excessive mechanical stress on to a slightly misaligned tube pin.
    What's important is for the female contact to grip tightly on to the tube pin, and for the mating surfaces of both pin and socket contact to be reasonably clean and oxide free.
    Generally, if the grip of the female contact is tight, the wiping action of inserting / removing the tube a few times acts to establish sufficient electrical contact between the mating surfaces.
    It is often feasible to retension female socket contacts, eg using a dentist's pick tool; try a google search for more detail.

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  16. #121
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    Quote Originally Posted by pdf64 View Post
    ...It is often feasible to retension female socket contacts, eg using a dentist's pick tool; try a google search for more detail.
    Yep I did check google.
    In this amp the sockets are of this type:

    Click image for larger version. 

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    Adjusting the grip of the terminals is a bit hard since they are quite deep inside.

    I think if those were like in the next image that would be easier.

    Click image for larger version. 

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  17. #122
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    With a suitable tool, and manipulation of the contacts from the solder terminal side, it's often feasible to bend the contact forks in a little.
    Obviously , ensure that the amp is isolated from the mains and its power supply caps de-energised before attempting that

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    Last edited by pdf64; 12-09-2018 at 03:41 PM.

  18. #123
    Old Timer J M Fahey's Avatar
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    Just thinking aloud:

    Now the amp is very sensible (some clicks and pops) to any kicks, even hitting the wall in a nearby room causes some clicks. That's after the new valves.
    You always had them, or at least lately (measure that in years), but you didnīt notice them because dead/almost_no_emission tubes act as a cheesy "noise gate" , as in you need to drive them hard for them toproduce any output at all.
    And now they are sensitive as new.

    FWIW this causes some friction between techs and uninformed users: "hey, now the amp is noisier than before / it feedbacks too much / etc."

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    Juan Manuel Fahey

  19. #124
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    It's a prime example of mission creep; fixing the HT cap issue revealed that the power tubes were dead, replacing them revealed a noisy / bad connection somewhere.

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