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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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    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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    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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    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 didn´t 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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  20. #125
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    Hi all. Happy new year!

    I have the impression I should replace all the pots. I've clean them with contact cleaner, but I think their effect in the sound is now what it should be.
    Channel two it not completely ok either so I should check it again.

    But. I've been thinking for a while if I should just sell it and get a small tube combo.
    I don't play around anymore so the amp wont ever get the use it deserves again.
    But I don't know if I will regret it later. Though it was stored for 15 years until few months ago.

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  21. #126
    Bent Member Chuck H's Avatar
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    I might pursue a completed repair before flipping it for another amp. For one thing, you're already way past getting compensation for your time back from any purchase price. For another, the amp was good to you. If you sell it "mostly" working it will continue to trouble the purchaser and possibly end up either coming back on you or worse, end up languishing on a shelf until age and disuse render your old amp a useless doorstop.

    At best though I suppose it's possible the amp will eventually find a tech that'll fix her right up. But I think that's the least likely scenario statistically.

    I have my own regrets about some of my old amps that correspond with my position.

    JM2C

    EDIT: Oh, Happy New Year to you too.

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    Quote Originally Posted by Chuck H View Post
    ...the amp was good to you...
    Yes. I mean now it would be sitting around and eventually used, at low volumes of course. So it would be kind of having a beast in a Zoo.
    I bet the amp wants to run free and scream loud :-)

    Of course IF I'd sell it I would make clear it should be properly serviced.

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  23. #128
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    I've decided to change the pots.
    Some do strange things, like operating then fading away, if I press them a bit then clicks and change in volume happens.

    But in the schematics I can only locate 5 out of 8. Any hints?

    https://music-electronics-forum.com/...7&d=1542389576
    https://music-electronics-forum.com/...8&d=1542389601

    Also there's no indication about linear or log variant of the ones I can see in the schematics.

    CORRECTION: I can actually see some details written on the pots, but I can't see all of them before I take them out.

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    Last edited by josegrad; 01-03-2019 at 07:39 PM.

  24. #129
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    Each channel looks to have treb, bass and vol controls; so with 4 channels that makes 12 total there, plus master vol and presence, 14 in all?
    I suggest log aka audio taper for them all, apart from linear for the presence.

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  25. #130
    Bent Member Chuck H's Avatar
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    Your amp has an additional "channel" that is only implied in the schematic. It would be the same pots as the preamp schematic X2. I can't know the actual taper percentages, but odds are it's all log (audio taper) pots except for the presence control, which would be linear taper.

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    I've located now all of them. Volume, treble and bass are replicated, so the same for both channels.
    I'm now taking note of the specs of each one, which is written on them.

    500V voltage rating is enough right?

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  27. #132
    Bent Member Chuck H's Avatar
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    You shouldn't need to worry about it for any standard size pots. The circuit design has them isolated from high voltages.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Just to have them listed here's the set of pots.

    CHANNEL ONE

    Volume 1M Ω LG A1 AB CTS
    Treble 250K Ω LN E0 AB CTS
    Bass 250K Ω LN M0 AB CTS


    CHANNEL TWO

    Volume 1M Ω LG A1 AB CTS
    Treble 250K Ω LN M0 AB CTS
    Bass 250K Ω LN M0 AB CTS


    COMMON POTS FOR BOTH CHANNELS

    Master 500K Ω Log This one is a different model, might have been replaced at some point
    Presence 5K Ω LN E0 AB


    So all clear.
    I just wonder what the A1 AB, M0 AB, E0 AB mean. I don't have access to such a selection variety, at least with easy access.

    UPDATE: I guess the letters are related to some characteristics of the pots. Which might even be obsolete today.
    I guess part of the ordering information like in this pdf: https://www.ctscorp.com/wp-content/uploads/026.pdf

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    Last edited by josegrad; 01-03-2019 at 11:02 PM.

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    Senior Member HTH's Avatar
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    Quote Originally Posted by Chuck H View Post
    Well... What I see is that somewhere along the line someone (probably on more than one occasion) kludged that amp with an odd assortment of filter caps (one of which is axial instead of radial) and added a series resistor to the B+ string after the standby switch putting exposed high voltage nodes within reach of the user from outside of the amp. All I can say is that this is a very bad thing that can result in death if such an accident were to happen. Soooooo...
    .
    That big wire-wound resistor on the top of the chassis is a stock arrangement - I have three of these amps (though they’re the earlier clear-panel ones). The resistor gets quite hot, so I’d imagine they put it there to dissipate the heat better, rather than keep it inside the chassis.

    I put a choke in one of mine and I’d recommend anyone doing this - I just prefer the feel.

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  30. #135
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by HTH View Post
    That big wire-wound resistor on the top of the chassis is a stock arrangement
    Yes. That was revealed in post #4 on page one. And I acknowledged in post #6. Thank you.

    Quote Originally Posted by HTH View Post
    The resistor gets quite hot, so I’d imagine they put it there to dissipate the heat better, rather than keep it inside the chassis.
    Of course. But IMHumbleO that's not a good reason to have HV accessible outside the chassis. And I said as much in post #6.

    Quote Originally Posted by HTH View Post
    I put a choke in one of mine and I’d recommend anyone doing this
    Good call. Being as josegrad was using his as a bass amp (IIRC) a choke would seem like a better choice than a resistor.

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  31. #136
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    Thanks for the comment HTH.
    Any recommendations for type/specs of suitable choke?

    Quote Originally Posted by Chuck H View Post
    Being as josegrad was using his as a bass amp (IIRC) a choke would seem like a better choice than a resistor.
    In theory this is a bass amp, but works nicely for guitar as well :-)

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  32. #137
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    While changing pots I have the impression this component got a bit too hot.
    Is there a way to test such a small cap with a normal multi-meter?
    Or would the only option to turn on the amp with the light bulb limiter in place?

    Click image for larger version. 

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    I'm not even sure if it is 220uF of pF...

    It is this one used in the Master pot.

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    Update: Must be 220pF. 220uF are not in this shape as I see.

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  33. #138
    Don't forget the joker g1's Avatar
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    Yes, it is 220pF.
    It is a 'bright' cap that allows some highs to bypass the master volume.
    If you find the amp too bright, you can leave it removed. Or you can try other values (47pF, 470pF for example) to let more or less highs through.
    It is probably ok, it should read very high or 'open' resistance on your meter.

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  34. #139
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    Quote Originally Posted by g1 View Post
    ...It is probably ok, it should read very high or 'open' resistance on your meter.
    Yes, resistance reads very high. I'll go on and keep it.

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  35. #140
    Senior Member HTH's Avatar
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    Quote Originally Posted by josegrad View Post
    Thanks for the comment HTH.
    Any recommendations for type/specs of suitable choke?

    In theory this is a bass amp, but works nicely for guitar as well :-)
    Check out this website, it shows you how to calculate the specs of the choke you’ll need (LC filter)... http://www.valvewizard.co.uk/smoothing.html

    You could just buy an off the shelf ‘choke for 100w Marshall’ and you’ll be fine. However, a little reading and calculating your requirements will teach you something and is more worthwhile imo.

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