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Harvard in a 5F2A Chassis Build

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Nov 22, 2020.

  1. BigDaddy23

    BigDaddy23 Tele-Holic

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    That plate current measurement is what you measure thru the precision 1 ohm resistor/s between pin 8 to ground on your power tubes. Pop that in and push calculate once you have entered Plate to cathode voltage as well.
     
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  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I assume you're using Rob's "Output Transformer Resistance Method?" You should be getting a current result somewhere on the order of ~20–30 mA.

    I don't use that method myself, so don't know which of your numbers look reasonable. Maybe give us the numbers you got for OT center tap and plate voltages? And someone else is gonna have to say if your OT resistance sounds right.
     
    Last edited: Dec 1, 2020
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    As I say, I don't use the OT resistance method, but if I understand Rob, it's also a way to figure out plate current.
     
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  4. BigDaddy23

    BigDaddy23 Tele-Holic

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    I don’t routinely either - I prefer the current shunt at pin 8 method as it is easier for me.

    I just took a look at James’ build pics again and see the 1R resistors aren’t insitu.
     
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  5. James Knox

    James Knox Tele-Holic Silver Supporter

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    I have some 1Rs on order. Be here in a couple days!
     
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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If you are using the OT resistance method...
    Warm up the amp so the OT is at operating temp. Turn off the amp and measure the resistance of the OT. Usual colors of the OT primary are red, blue, and brown. On JK's amp measure the resistance of the red and blue side of the OT by placing a probe on rectifier pin8 and the other probe on 6V6 pin3 (blue wire). Next measure the other side of the OT with probe on rectifier pin8 and the other 6V6 pin3 (brown wire).

    With the amp *on* and warm measure the voltage drop of the the same red to blue and red to brown sides of the OT. (Same pin location as finding the resistance.) There will be over 300VDC on those terminals but the meter will only measure the drop in voltage across the resistance of the coil. The DCV will measure a drop of about 4 volts.

    Voltage drop / resistance = current
    4.014 / 148.1 = .0271A
    = 27.1mA

    The voltage drop and resistance for each tube will be measured. One tube is almost always dissipating more than the other.

    For you guys using the 1ohm resistor. You are measuring the voltage drop across the resistor and dividing by the resistance but you are only dividing by a resistance of 1ohm which makes the math easy.
    Problem is, the 1 ohm resistor method measures plate current and screen current. You have to subtract the screen current. The OT resistance method only measures the plate current.
     
    Last edited: Dec 1, 2020
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  7. James Knox

    James Knox Tele-Holic Silver Supporter

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    Thanks LLC! I was doing this...

    But didn’t do this.... (I think I was measuring Screen to Cathode)

    That is why my math wasn’t adding up. I’ll go back in and get new measurements with the new Bias Resistors.

    Wait, looking over my notes, I was subtracting the Plate Voltage from the OT CT Voltage to get the Voltage drop, which was 4! Is that the same thing?
     
    Last edited: Dec 1, 2020
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Others suggest subtracting the large plate voltage to get the voltage drop. The meter is much more accurate measuring the actual 4.01 volts to a decimal place.;)

    Of course it doesn't have to be 4.01 volts. Hah.
     
    Last edited: Dec 1, 2020
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  9. James Knox

    James Knox Tele-Holic Silver Supporter

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    tru dat big brotha

    yes sorry, I meant plate rather than screen. I went back a corrected the post.
     
  10. James Knox

    James Knox Tele-Holic Silver Supporter

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    Hahaha - hard to stop playing it! It sounds so good and responds so well. I haven’t even hooked up any pedals yet!

    But yes, here are the voltages...

    E73198EE-1301-4081-A2AA-5D27085D6DB4.jpeg
     
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  11. gabasa

    gabasa Tele-Meister

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    Very cool, thank for posting this!
    Sound clips!!!!!!!
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    That's an excellent point. Still, I like the 1-ohm method a lot, since it's easy and conservative (you'll think your bias is a bit higher than it really is, so you'll be in no danger of redplating). I bias by ear anyway, and tho I believe Merlin that the AB safe range is at least up to 80%, my ear never gets above 70% per cathode current, which means I'm lower than 70% on plate current.

    Don't take my word for it, my 1-ohm brothers in arms. Here's the master, Steve Luckey, on the subject (from reply #3 in this thread):

    "So, the reading you get is always a bit higher than the actual plate current. This causes your calculations to be off a bit, but they err on the side of caution. Your actual plate power will always be a bit lower (cooler) than the calculated power. Less chance of accidentally red plating.
    I've always been happy using cathode current in my calculations. I install 1Ω resistors on the cathodes inside all my amps just to make it easy to check. If I ever need to have gnat's ass precision, I'd put a 1Ω resistor on the plate."
     
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hey, I am not knocking the 1ohm method. It is cheap and easy. I was just trying to point out in theory it is not much different than the OT resistance method. Both measure a voltage drop across a resistance. Both use Ohm's law to calculate the current across that resistance.

    I agree with biasing by ear. Most of the time I end up around 60% PD in fixed bias Fenderish amps.
     
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  14. BigDaddy23

    BigDaddy23 Tele-Holic

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    ^ This. The last couple I’ve built have sounded there best around 60ish%. I like the fact that I can make quick measurements with the 1R method. Either way, as long as you have a repeatable method that works for you, the amp sounds good and is not red plating, everyone kicks a goal
     
  15. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    I love the beer glass analogy in this thread!

    This has been a great build @James Knox, and a very instructional thread!

    Keep Sluggin’
     
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  16. Huddy

    Huddy Tele-Holic

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    Semi Rando Question regarding the 1 ohm resistor method, @King Fan & @Lowerleftcoast. Could you not put a 1 Ω Resistor between the plate load resistor and the plate to make it so that you're only reading the actual plate current and not the plate+screen current? Might be a little clutter/impractical but is the theory there?
     
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  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You sure could!
    @King Fan post #132 he quotes Steve Luckey. "If I ever need to have gnat's ass precision, I'd put a 1Ω resistor on the plate."

    I wouldn't have a resistor permanently in series with the plate-OT connection. If I were to go that route I would bypass that resistor with a wire for normal operation and lift the wire for measurement. OMMV.
    I suppose a port, for outside the chassis bias measurement, could be added. Both the Plate to Cathode measurement and the voltage drop could be measured from that port. When done with the bias measurement a jumper plug could bypass the 1ohm resistor and secure the port. It is a thought.

    On the other hand, I don't mind dividing the voltage drop by the resistance. Once the OT resistance has been measured, you can write that resistance on the chassis. It will not change during the life of the OT and can be referred to if the OT becomes suspect in the future.
     
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  18. Huddy

    Huddy Tele-Holic

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    :lol:
    doh! :lol: I must’ve skimmed over the last sentence!
     
  19. James Knox

    James Knox Tele-Holic Silver Supporter

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    I wanna kick a goal...I wanna kick a goal...I wanna kick a goal... (1 ohm resistors arriving today!)
     
  20. jsnwhite619

    jsnwhite619 Friend of Leo's

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    From my memory using the 275v and JJ's (which are supposed to be on par for NOS 5y3 tubes for voltage drop) you are probably biased pretty cold with it right now if you have a 324v B+. I was able to get mine dead-on with the schematic 305v but I can't remember what the dissipation was now - it was a safe level for sure, low to mid-50% I think. You can know for sure once your 1 ohm resistors get in there. It's probably at cold, safe level now, but you should be able to warm it up a bit and bring that down closer to the vintage level.
     
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