Has anybody else installed a Plate Voltage External Test Point?

Discussion in 'Amp Tech Center' started by Platefire, Aug 17, 2018.

  1. Platefire

    Platefire Friend of Leo's

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    I have been installing the external test points for bias from power tube Cathode with 1 Ohm to ground on Banana Sockets and external bias pot on my amp builds for several years now but still had to open the amp up to get a current plate voltage reference. So I've been wanting to also have an external plate voltage reference so I could do the whole bias thing externally without pulling a chassis.

    So I finally did it after pondering it a while using this schematic/voltage divider. Not sure rather anybody is really interested in this so. I'll keep it short pending rather there being any interest in this. Platefire
    TP#1.gif
     
  2. Old Tele man

    Old Tele man Friend of Leo's

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    That method only works when both voltage-divider resistors are precisely ACCURATE.

    That's WHY the single cathode current resistor is preferred.

    Besides, why are you measuring the B+ when you SHOULD be measuring the voltage at the plate...on the OTHER side of the plate load?
     
    Last edited: Aug 18, 2018
  3. Platefire

    Platefire Friend of Leo's

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    Well with the known resistor tolerance offset/error measured and accounted for, you can pretty easily calculate to know the actual close enough to set bias. This will work for me but maybe not for everybody. Platefire
     
  4. Ten Over

    Ten Over Tele-Meister

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    That certainly sounds safer than dropping a tube and shoving a probe up the #3 hole. It could be more accurate, too, if you carefully select the resistors because the B+ rises when you drop a tube.
     
  5. gusfinley

    gusfinley Tele-Holic

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    I've installed 1 ohm resistors in the between the plate and the output transformer. mV = ma. Why not measure plate current only?

    I like to use "tip jacks" so you can just use your normal multi-meter leads, too.
     
    Last edited: Aug 18, 2018
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  6. Ten Over

    Ten Over Tele-Meister

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    Because then you are in HT land. Platefire is designing test points that are in mV land so that the end user doesn't have a shocking experience. He's such a good guy.
     
  7. Platefire

    Platefire Friend of Leo's

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    That's the reason I pondered it a long time(years) before doing it. I knew that having full B+ close in relation with the chassis in a Banana Jack was an accident waiting to happen. So honestly, I'm not to bright regarding electronics math and Ohms Law. A bright guy on the Hoffman amp site that goes by "PRR" produced the schematic is post #1 to make it safe. So once I got that, there was no holding back!

    I was also taught by a guy named "2deaf", same site, how to calculate to make up the difference
    in the "tolerance factor" in the resistors used.

    So I purchased five 3 Watt, 1%, 1Meg Vishay metal film resistor. I tested all five resistor and found one that consistently measured exactly 1Meg with my Fluke 135 Multi Meter. So I used that 1Meg. I also had a 1K metal film 2 watt resistor that measured .986 Ohms that I used.

    So the margin of error is this: Plate Voltage Direct was 416.5VDC and the reading through the voltage divider test point was 410.1 mV= 410.1 VDC. A 6.4 VDC error factor.

    Sooo-- here is the formula shared by "2deaf" to account for the resistor tolerance short fall flaw Referring back to the schematic in post #1 with the R-1 being the 1KR and R-2 being the 1Meg:

    R1/(R1+R2)=X 986/1000,986=X X=.000985

    Then .4101mV/.000985= 416.35 VDC

    Or you could just simply add in the known error factor of 6.4 to your reading of 410.1=416.5VDC.
    This is certainly close enough for my bias purposes. So I thought I would share what I've found out in case anyone else wants to use it. I will eventually be installing this on several other amps I previously
    built with the external bias test points and pot. BTW--I always use the MarVac Locking 50K pot.
    So I hope this can be of value to somebody. I'm certainly happy to get my first one installed. Platefire
     
  8. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I'd love to see a hi-lit drawing or gut shot once you get everything worked out.

    (I'm going to start adding external bias points and pot to my builds soon)
     
  9. Platefire

    Platefire Friend of Leo's

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    LudwigvonBirk

    I did the first one to a 1958 Dukane PA head I converted to a 5F6A Bassman in 2006 simply because everything is easy to get to. Here is a gut shot. You can see the clay red !Meg resistor going from Pin #3 of a 6L6GC to a terminal strip. Then a red wire goes from terminal strip to Banana Socket term.
    It just so happened I had a handy free ground term on a terminal strip adjacent to the Banana Socket,
    so it was pretty easy shoe in for the 1K resistor from test point to ground. If you look close you can see the layout---old style point to point.
    DSC03869.JPG
    Here is the test point on the exterior of the chassis to the far right. Try as I may to line it up perfectly with the two other test points, it's a little off but works fine.
    DSC03870.JPG
    Also here is a picture of my Multi-Meter readings. First reading the plate voltage direct from pin#3:
    DSC03873.JPG
    Next my MM reading going through the voltage divider circuit to the test point showing the voltage error of 6.4VDC that has to added back into your reading from the test point.

    DSC03874.JPG Here is what the amp looks like overall just to give you an idea
    DSC00416.JPG
     
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  10. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Thanks!^
     
  11. gusfinley

    gusfinley Tele-Holic

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    Ahhh.. Now that I've had some time to re-read this post a bit... you are looking to measure plate voltage.

    This does seem to be a good approach.

    There is always a risk with high voltages and a banana jack with high voltage seems like too big of a risk. This is why I use tip jacks. The tip of you multimeter are long and thin enough that its a little more difficult to accidentally get bit by the high voltage.

    Here are some pics from the Hoffman Amps Store:

    [​IMG]
    I have been known to run one of these jacks to a 1 ohm resistor between the plate and output transformer.

    The 1 ohm resistor in the plate circuit is helpful for tow reasons:

    1) Plate Voltage - Insert the positive multimeter lead and place the negative to ground and you get plate voltage

    2) Plate Current - Plug each multimeter lead into its respective jack and mV is now mV in plate current
     
    Last edited: Aug 18, 2018
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  12. dan40

    dan40 Tele-Afflicted

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    I used one of the banana jacks from Hoffman to measure plate voltage. I just wired it directly to pin 3 of one socket and mounted it between the two bias test points. It has worked fine for several years and makes taking the readings much easier. I also have the bias pot facing down below the chassis so all measurements and adjustments can be made without removing the chassis.
     
  13. RLee77

    RLee77 Friend of Leo's

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    Part of the reason your reading is measuring low is that you are using the wrong multiplier for that voltage divider. You should be multiplying by 1001, not 1000. I know it’s not a big diff, but may as well use the right math, it’s free. :D
    (The voltage across the 1k is going to be 1/1001, not 1/1000.)

    Your later post that describes how to compensate for tolerance errors uses the correct formula, summing the two R’s, so you’re good with that method.

    Also, I agree 100% with others that recommended tip jacks; it’s what they were made for. Safer, and they make it obvious what is supposed to be plugged into them.
     
    Last edited: Aug 18, 2018
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  14. Platefire

    Platefire Friend of Leo's

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    gusfinly---What you call tip Jacks looks like the identical thing I use to get from Radio Shack called a Banana Jack socket. I'm not understanding what you mean running a 1 Ohm between OT B+ in and one of the plate leads? I do that regular on the cathode pins for bias in mA measured in mV. BTW I just used my last Radio Shack banana socket so guess I'll start using Doug's since all the RS stores has bit the dust.

    dan40---I just always had a problem with the idea installing those banana jacks with full B+ plate voltage going to them. They always seemed kind of flimsy to me. That's why I hesitated until now. With the mV I feel a lot safer with that. That's just a personal hang up. After all if it shorted out it would probably just blow a fuse and everything would be OK. So glad yours hasn't been a problem and that proof the my fears were more likely unfounded:>)

    Bottom line a plate voltage reference point is just needed if you got the cathode test points also and with the external bias pot you have everything you need to check bias for curiosity purposes or if you install a new set of power tubes. Everything I got that needs biasing is two tube push pull with the exception of my Mesa Boogie Rocket 440 has four EL84's which hasn't got a bias pot. Platefire
     
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  15. RLee77

    RLee77 Friend of Leo's

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    This pic shows the difference. (I actually managed to find my old supply of jacks to take this pic — I got lucky.)

    The normal banana jack is on the left, tip jack on the right. This one is a mini tip jack, which I prefer since the panel hole it needs is smaller.

    3741D17E-30C8-4DB2-81F7-A38A7D2E4B7C.jpeg
     
  16. Ten Over

    Ten Over Tele-Meister

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    Sometimes there is the tendency to think that the black probe is reasonably benign because it is usually contacting ground. But if the red probe is connected to HT, the black probe can give you a pretty good jolt. More of a life-lesson event than a life-threatening event, but still a good reason to reduce the voltage before allowing a guitar player access to it.
     
  17. Platefire

    Platefire Friend of Leo's

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    Thanks! Well that settles it! Can't argue the difference in that view. Tip jacks it will be on my next order.

    Never been bit by the ground probe but I'll certainly be careful in of it checking HT.

    Another thing to watch. I was checking the turns ratio on an unknown OT the other night with
    a 14VAC power supply and got bit with the up-transformed voltage about 408V. That hurt!
    Them things truley do transform voltage:>) Platefire
     
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  18. gusfinley

    gusfinley Tele-Holic

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    I usually install this between the plate and the output transformer lead that usually is connected to the plate (these are often red or blue) and not the Center Tap (usually Red) that connects directly to the B+.
     
  19. wanderin kind

    wanderin kind Tele-Meister

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    you could install a small analog meter in the face plate, that way you could watch the needle jump when you crank out a low E string note,

    or you could buy a DMM on evilbay for 15 bucks, take out the pc board, build a DC supply off your heater supply, kludge in the LCD display into your cabinet, but that is getting a bit weird,
     
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  20. Platefire

    Platefire Friend of Leo's

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    Har! now that's going totally too far:D-----Just exterior contacts for an occational MM as needed.

    I did find out this plate voltage test point is not totally uncommon because some of you are
    using them. I really had not seen any discussion on this previously, so I thought I would bring
    it up. Also if someone is interested in a safer voltaged down safer contact, thought I would share mine.

    I'll have to get me some of those "tip jacks" so I can do my other two amps with external bias check points with the plate volt addition. Platefire
     
    Last edited: Aug 20, 2018
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