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Thread: Measuring Plate Voltage

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    Measuring Plate Voltage

    Is there any reason why I can't measure the plate voltage of a 6L6 socket by removing the tube and placing the DMM probe into the socket for pin 3? Will having the tube removed change the plate voltage supply by enough to change my dissipation calculations by that much? There would be 3 other 6L6 tubes still in the amp?

    thank you

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    Lifetime Member Enzo's Avatar
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    You will lose 25% of the power tube draw from the B+.

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    Shoot... forgive my ignorance.

    I would lose current draw from that tube being removed. The voltage potential would stay the same? I'm not sure what to deduce from what you wrote.

    I have a bias probe on order for reading plate current. Wish I would had just bought the probe that reads both plate current and voltage...

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    Supporting Member The Dude's Avatar
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    Just measure the B+ node. No reason to remove any tubes.

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    thank you for the replies. I was hoping to be able to do this without taking the amp apart from the head. I'm working with my Peavey XXX and it has the bias voltage adjustment screw located on the outside of the amp. If it comes down to it I'll pull the chassis out when I get the probes. Just wondering. how off the readings would be.

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    Lifetime Member Enzo's Avatar
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    If you remove all the power tubes, the B+ will rise to its maximum level, because there will be no current draw from the supply. If you pull one tube of four, then the B+ will indeed rise some - don't ask me for a figure - because the current through the B+ winding of the transformer faces resistance there.

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    Supporting Member The Dude's Avatar
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    Gotcha. I wan't aware you didn't have the chassis out. That said, I wouldn't bother taking the amp apart if you don't need to. You can measure the plate voltage as you said. It ought to be close enough, IMO. A few mA one way or the other isn't going to kill anybody. As Enzo says, it isn't lab equipment.

    Edit <simulpost> As I'm typing his name.

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    Supporting Member The Dude's Avatar
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    BTW, I think it's silly that Peavey went through the trouble to add bias test points and an external adjustment and then gives you a rather meaningless bias voltage reading instead of bias current.

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    Thank you very much. I'll give it go. This forum is great by the way. Lots of helpful information here.

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    Yeah, I wish I could hear directly from James Brown about the theory / reasoning / procedure was on this. He seems like a pretty stellar amp designer. You wouldn't think he would do something with no good reasoning behind it.

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    Supporting Member The Dude's Avatar
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    Welcome to the place!

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    Lifetime Member Enzo's Avatar
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    There is plenty of good reasoning behind it. Their silly spec ensures long tube life and reliable operation. They bothered to make a bias adjustment available for marketing, people have come to expect it. Their amps didn't have an adjustment at all for years. They also limit the range of these controls so you typically cannot heat the amp up far anyway. If you set the bias per their instruction, just about any set of tubes you install will work well and not fall outside the bounds of reasonable bias. Their goal was never to hot rod the amps, it was to set cool power tube operation.

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    I think if you check it with 3 of 4 power tubes installed, the reading will be close enough.
    A slight change in plate voltage will have very little impact on your dissipation calculation.

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    thank you again.

    armed with some of the above information I garnered up enough courage to dial the bias voltage pot. while plugged into the amp and playing. At most the bias voltage would go up to -50VDC and lowest it would go to -60VDC.

    As soon as I get my plate current probe I'll make dissipation calculations.

    So the bias gets hotter and volume and or bass goes up. As a soundman I know that the brain perceives apparent volume increases with a knee-jerk "better".

    Is all the talk about hotter bias into the "70" range or beyond really that big of a difference. If it was why wouldn't J.B. make his amps to sound "hotter".

    At this point I'm just curious. I can't stand gear mythology. I'm more of the practicle type. I had watched an interview with J.B. where they referenced the "cold" bias of some of his amps and he really didn't seem overly impressed or concerned about the topic. As if the topic is more dramatic than it needs to be.

    Any thoughts?

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    Supporting Member The Dude's Avatar
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    Those amps are intentionally biased to the cold side. The distortion comes from the preamp. FWIW, when you bias an amp, it is with no signal and volumes down. You are setting idle current. Don't try to adjust bias while playing.

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    Quote Originally Posted by The Dude View Post
    Those amps are intentionally biased to the cold side. The distortion comes from the preamp. FWIW, when you bias an amp, it is with no signal and volumes down. You are setting idle current. Don't try to adjust bias while playing.
    I had read that. I had also read that you could bias by ear within the margins of the operating range. I also garnered from the above and some other research that the bias adjustment on the XXX is narrow enough to not quickly destroy the tubes. So I just went for it.

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    You can adjust the bias in this amp by ear, but in that case you are not using any measurements. You didn't hurt anything.

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    Supporting Member loudthud's Avatar
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    Is this one of those Peavey amps where all the power tube heaters are wired in series? Pulling one tube cuts off all the others.

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    Lifetime Member Enzo's Avatar
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    No, this is the XXX, all four 6L6 heaters wired in parallel across 6v supply.

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    For the sake of someone else like me searching for info. And if some of you who know more want to comment on my readings that would be appreciated if you ha anything helpful to add.

    With one 6L6 removed from the circuit I measured;

    pin3 to ground = 560VDC
    pin3 to pin8 = 504VDC

    I'll update this when I get my plate current probe.

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    One thing I recall seeing posted here was that JB said the resonance control (for 5150 but probably applies here too) depends on cold bias to work properly.
    I would guess the bias range is limited to an area that allows for proper resonance function. But if you are biasing by ear, it would be interesting to take note of how it affects the resonance circuit. (especially for lower frequencies, I would think)

    Quote Originally Posted by ultra-gain View Post

    pin3 to ground = 560VDC
    pin3 to pin8 = 504VDC
    Pin8 should be grounded so those readings should be the same. Double check them. You may have to dig in more with your probes to get a good reading.

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    thanks for the reply g1. I wonder if I made a mistake with regards to my "ground" on the first measurement. I just assumed the negative terminal on the "bias test" points(for reading negative bias voltage) was connected to ground. I bet ya I'm wrong and my 560VDC is an error.

    I ordered a plate voltage probe from ebay so eventually I'll have this down when combined withe current probe.

    I'll have to listen to the resonance control while tweaking the bias. I think it will be hard to tell. Here is why. The bias cold volume goes down significantly. So while it sounds like the bass range comes up drastically when biased hot its hard to tell if I'm not just hearing the overall volume go up, because its significant. Guess I could record a couple licks and normalize it; two of the resonance control positions and the two extremes of the bias settings. That would be entertaining.

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    Lifetime Member Enzo's Avatar
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    Remember, bias is a negative voltage. Note that your ground versus pin 8 difference is exactly the amount of bias voltage. I suspect the red probe terminal is ground and the black is bias. Easily verified whichever.

    Use the bias test points for measuring bias voltage only, use a real ground for everything else.

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