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    My ignorance runs deep...

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    Chris Scott

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    Join date : 2017-05-09

    My ignorance runs deep...

    Post by Chris Scott on Wed Nov 01, 2017 2:28 pm

    ...so I'm hoping someone can explain this to me. Cool

    After posting my last query, I decided to just set the bias current to the -37v figure called for on the voltage check points page from Dynaco, and now the B+ voltages are good, and the power tubes appear to be biased within range as the plates show no overheating when viewed in a dark room...

    ...yet they're drawing around 70 milliamps of current when I interface a bias probe (same as taking a reading across a 1ohm resistor connected from pin 8 to ground) which I use when setting the bias current on a guitar amp.

    In a guitar amp with +/- 440v on the plates, I'm pretty sure that an EL34 is going to be already starting to redplate pretty severely at this setting, yet they appear to be happy in this amp...

    Yeah, I'm missing something...
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    corndog71

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    Re: My ignorance runs deep...

    Post by corndog71 on Wed Nov 01, 2017 4:20 pm

    Chris Scott wrote:...so I'm hoping someone can explain this to me. Cool

    After posting my last query, I decided to just set the bias current to the -37v figure called for on the voltage check points page from Dynaco, and now the B+ voltages are good, and the power tubes appear to be biased within range as the plates show no overheating when viewed in a dark room...

    ...yet they're drawing around 70 milliamps of current when I interface a bias probe (same as taking a reading across a 1ohm resistor connected from pin 8 to ground) which I use when setting the bias current on a guitar amp.

    In a guitar amp with +/- 440v on the plates, I'm pretty sure that an EL34 is going to be already starting to redplate pretty severely at this setting, yet they appear to be happy in this amp...

    Yeah, I'm missing something...

    One way David Hafler caused a lot of confusion was by relabeling the idle current as "biaset" or what's commonly referred to as bias. Yet the actual bias is an internal circuit and should be a negative number.

    The idle current or "Biaset" number when using the original 15.6 ohm resistor came to 1.56V which equates to 50mA per tube. It hurts my brain to convert that using a 1 ohm resistor but I think you should measure 0.1V .

    Can someone confirm this?
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    peterh

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    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: My ignorance runs deep...

    Post by peterh on Wed Nov 01, 2017 4:59 pm

    Chris Scott wrote:...so I'm hoping someone can explain this to me. 8)

    After posting my last query, I decided to just set the bias current to the -37v figure called for on the voltage check points page from Dynaco, and now the B+ voltages are good, and the power tubes appear to be biased within range as the plates show no overheating when viewed in a dark room...

    ...yet they're drawing around 70 milliamps of current when I interface a bias probe (same as taking a reading across a 1ohm resistor connected from pin 8 to ground) which I use when setting the bias current on a guitar amp.

    In a guitar amp with +/- 440v on the plates, I'm pretty sure that an EL34 is going to be already starting to redplate pretty severely at this setting, yet they appear to be happy in this amp...

    Yeah, I'm missing something...
    Tubes are no exact devices. They differ as much as +-25% in parameters.
    Setting bias per voltage can only be done if you have tubes within a very narrow spec.
    The sweet working point is decided by the current through the tube, and as tubes
    differ wildly , the voltage to get that current differs.

    Do set bias to a specific current. Don't mind the voltage on g1.
    In a dynaco amp, bias should be set to 1.56V on the biaset probe. On vta amps
    it should be set to 0.4 or 0.5V, the difference is there as the amps has different
    cathode resistors.
    And don't use "bias probes" on dynaco-style amps, the are not as accurate
    as the built-in resistor and also needlessly demands removing and inserting
    the tubes.
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    corndog71

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    Re: My ignorance runs deep...

    Post by corndog71 on Wed Nov 01, 2017 5:41 pm

    To clarify what peter said, the VTA70 uses 10 ohm cathode resistors for each tube and each should measure .4 - .5 vdc.

    In the classic dynaco amps the cathodes of both output tubes were tied together so the measured number would be for both tubes.

    Here's how I worked it out.

    1 ohm cathode resistor : 0.050V which is equal to 50mA of idle current per tube or 0.100V / 100mA for 2 tubes.
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    PeterCapo

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    Join date : 2008-12-05

    Re: My ignorance runs deep...

    Post by PeterCapo on Wed Nov 01, 2017 10:54 pm

    Around 70mA (actually, 65mA) through each EL34 is normal for the Mark II.  If you are using JJ E34L, do I remember something about them being more rugged than the average EL34?

    Chris Scott

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    Join date : 2017-05-09

    Re: My ignorance runs deep...

    Post by Chris Scott on Thu Nov 02, 2017 11:23 pm

    PeterCapo wrote:Around 70mA (actually, 65mA) through each EL34 is normal for the Mark II.  If you are using JJ E34L, do I remember something about them being more rugged than the average EL34?

    Thanks Peter, and it certainly explains why they weren't redplating...

    Chris Scott

    Posts : 20
    Join date : 2017-05-09

    Re: My ignorance runs deep...

    Post by Chris Scott on Thu Nov 02, 2017 11:25 pm

    corndog71 wrote:To clarify what peter said, the VTA70 uses 10 ohm cathode resistors for each tube and each should measure .4 - .5 vdc.

    In the classic dynaco amps the cathodes of both output tubes were tied together so the measured number would be for both tubes.

    Here's how I worked it out.

    1 ohm cathode resistor : 0.050V which is equal to 50mA of idle current per tube or 0.100V / 100mA for 2 tubes.

    Chris Scott

    Posts : 20
    Join date : 2017-05-09

    Re: My ignorance runs deep...

    Post by Chris Scott on Thu Nov 02, 2017 11:34 pm

    corndog71 wrote:To clarify what peter said, the VTA70 uses 10 ohm cathode resistors for each tube and each should measure .4 - .5 vdc.

    In the classic dynaco amps the cathodes of both output tubes were tied together so the measured number would be for both tubes.

    Here's how I worked it out.

    1 ohm cathode resistor : 0.050V which is equal to 50mA of idle current per tube or 0.100V / 100mA for 2 tubes.

    So this explains why I get the high figure off the bias probe...though only one probe's installed, it's reading both tubes. There's no 10 ohm resistors on this amp, but the both cathodes are in fact tied together.

    Sound right?...
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    PeterCapo

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    Re: My ignorance runs deep...

    Post by PeterCapo on Fri Nov 03, 2017 12:11 am

    If your bias probe plugs into a single socket with a single E34L plugged into the probe, then you must be reading the current through the one power tube (again, 65mA per power tube is right for the Mark II).

    In the early Mark II, IIRC the cathodes were tied together directly to ground.  In a somewhat later Mark II, both power tubes have their cathodes tied together through a single 12Ω resistor to ground.  1.56VDC/12Ω=0.130A for both tubes total.  Divide this total current by two and you have 65mA per power tube.

    Do you have a copy of the correct manual corresponding with your particular Mark II?
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    corndog71

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    Re: My ignorance runs deep...

    Post by corndog71 on Fri Nov 03, 2017 11:42 am

    I wonder if it really matters what the value is of those cathode resistors. Some guys use 1 Ohm, some use 10 Ohms, and vintage guys prefer the original value of the circuit.
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    peterh

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    Re: My ignorance runs deep...

    Post by peterh on Fri Nov 03, 2017 12:00 pm

    corndog71 wrote:I wonder if it really matters what the value is of those cathode resistors.  Some guys use 1 Ohm, some use 10 Ohms, and vintage guys prefer the original value of the circuit.  
    No, it does not matter what value of measurment resistor as long as one is aware
    of it's value when measuring the current. The resistor should be small enough not
    to affect the tube's working point. Anything < 25ohm is ok here.

    1 ohm is convenient as it converts to 1mV = 1mA
    10 ohm "flameproof" 1/4w is convenient as they will blow when a tube flashes over,
    this save output transformer and other stuff ( a fuse won't open as fast !)
    Dynaco choose an odd value in order to have the biaset value 1.56V , same voltage as
    a drycell. Instrument of unknown accuracy could be used since the cell and biaset
    should give the same reading.


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    PeterCapo

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    Re: My ignorance runs deep...

    Post by PeterCapo on Fri Nov 03, 2017 1:37 pm

    That's fine, but I think what Chris Scott is asking about is how to make sense of the readings he gets from using his bias probe on his Mark II.
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    corndog71

    Posts : 575
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    Re: My ignorance runs deep...

    Post by corndog71 on Fri Nov 03, 2017 2:29 pm

    peterh wrote:
    corndog71 wrote:I wonder if it really matters what the value is of those cathode resistors.  Some guys use 1 Ohm, some use 10 Ohms, and vintage guys prefer the original value of the circuit.  
    No, it does not matter what value of measurment resistor as long as one is aware
    of it's value when measuring the current. The resistor should be small enough not
    to affect the tube's working point. Anything < 25ohm is ok here.  

    1 ohm is convenient as it converts to 1mV = 1mA
    10 ohm "flameproof" 1/4w is convenient as they will blow when a tube flashes over,
    this save output transformer and other stuff ( a fuse won't open as fast !)
    Dynaco choose an odd value in order to have the biaset value 1.56V , same voltage as
    a drycell. Instrument of unknown accuracy could be used since the cell and biaset
    should give the same reading.



    Roger Modjeski recommends a certain kind of fuse for a high voltage DC circuit. A high braking sand-filled ceramic fuse will work between the output tube cathode and the resistor connected to ground. 200mA slo blo is recommended for typical output tubes. No fancy hifi fuses even if they're ceramic! Littlefuse makes legitimately tested and rated fuses that will work. He's been fusing the output tubes of his amps for decades. Tube shorts don't always mean the death of a tube and replacing a fuse is easier than replacing the resistor.

    "When a fuse interrupts in a DC circuit the interrupting causes the element to turn to plasma which continues to conduct long after it should. This extended time causes damage to the circuit. Ceramic fuses are often filled with sand which helps prevent the plasma from forming."
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    Peter W.

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    Location : Melrose Park, PA

    Re: My ignorance runs deep...

    Post by Peter W. on Fri Nov 03, 2017 4:04 pm

    "When a fuse interrupts in a DC circuit the interrupting causes the element to turn to plasma which continues to conduct long after it should. This extended time causes damage to the circuit. Ceramic fuses are often filled with sand which helps prevent the plasma from forming."[/quote]

    Yep - DC does arc far more than AC, and why items such as fuses, switches and similar often have different ratings for AC and DC.

    Why use a high-quality fuse rather than a cheap fuse? With high quality fuses the element material is very carefully designed and typically "eutectic" in behavior. Meaning that when it melts, it *all* melts at once. maximizing the cap, reducing the tendency to arc. Sand and oil-filled fuses address this issue more effectively.

    There is no particular reason for a sand-filled fuse on an AC (or signal-carrying) circuit as the tendency to arc is greatly reduced.

    And, fuse tables are very revealing. Example: look up a Slow-Blow fuse one day - the type that has a spiral element wound around a small ceramic rod. It can take a 50% overload almost forever, and why I will not used them in anything I own.
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    deepee99

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    Location : Wallace, Idaho

    Re: My ignorance runs deep...

    Post by deepee99 on Sat Nov 11, 2017 12:46 pm

    Peter W. wrote:"When a fuse interrupts in a DC circuit the interrupting causes the element to turn to plasma which continues to conduct long after it should. This extended time causes damage to the circuit. Ceramic fuses are often filled with sand which helps prevent the plasma from forming."

    Yep - DC does arc far more than AC, and why items such as fuses, switches and similar often have different ratings for AC and DC.

    Why use a high-quality fuse rather than a cheap fuse? With high quality fuses the element material is very carefully designed and typically "eutectic" in behavior. Meaning that when it melts, it *all* melts at once. maximizing the cap, reducing the tendency to arc. Sand and oil-filled fuses address this issue more effectively.

    There is no particular reason for a sand-filled fuse on an AC (or signal-carrying) circuit as the tendency to arc is greatly reduced.

    And, fuse tables are very revealing. Example: look up a Slow-Blow fuse one day - the type that has a spiral element wound around a small ceramic rod. It can take a 50% overload almost forever, and why I will not used them in anything I own. [/quote]
    Fluke won't use 'em in their meters, either.
    There are better types of fuses out there that will actually blow as advertised. I'd give Mouser a yell.
    BTW, I ran 3-amp slow-blow fuses in my M-125s and never blew one. Depends (I think) on speaker efficiency.

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