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Need info on how to bias jcm900 4100
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The schematic says to set bias voltage to -50V, but I'd actually measure current and do it right.
https://robrobinette.com/Tube_Bias_Calculator.htm
"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22 -
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Getting 49/47/49/45mA@500vdc. Is VR2 the bias adjustment for all four(4) power tubes?Originally posted by The Dude View PostComment
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Yes.Originally posted by ca7922303 View Post
VR2 sets the negative voltage for all 4 tubes.Last edited by Jazz P Bass; 02-11-2022, 04:11 PM.Comment
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When down to 30mA, now 510vdc. Is 510vdc safe with these caps rated at 500?Originally posted by ca7922303 View PostComment
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When 5881s are fitted, similar but not the same as a 6L6, set the bias to -42volts. For EL34s, set bias to -50volts.
For 6L6GC the bias will be between those values or may even be as low as -38volts, depending on manufacturer.
I always fit Winged "C", Svetlana output valves, for reliability.
The HT should not exceed 500volts. If it does, why.
Read the service manual!Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
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To get to 500v HT, I had to set at -25v @ vr2. 26mA w/bias adapter. These are 6L6s and the highest reading I could get at vr2 was -34v/515v HTOriginally posted by Jon Snell View PostWhen 5881s are fitted, similar but not the same as a 6L6, set the bias to -42volts. For EL34s, set bias to -50volts.
For 6L6GC the bias will be between those values or may even be as low as -38volts, depending on manufacturer.
I always fit Winged "C", Svetlana output valves, for reliability.
The HT should not exceed 500volts. If it does, why.
Read the service manual!Comment
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26/22/24/20mA 503vdc@bias adapter. Moved tubes around, since readings are following tubes, what order would be best for balance of tube strength, 1-3/2-4, or 1-4/2-3? Thanks.Originally posted by ca7922303 View PostComment
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Also, should I get to 500v at bias adapter or is 503 @bias adapter ok as long as I've got 500v HT from vr2 adjustment? Thanks.Originally posted by ca7922303 View PostComment
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What is the AC voltage across the high voltage winding that feeds the first smoothing capacitor. It should be 340v A/C, if bridge diode configuaration..
If it is too high, what is the mains input voltage and is it set to the correct primary tapping.Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
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Your readings don't make sense at all. Check the battery in your meter.
Higher idle current (mA) numbers at the bias probe should result in lower HT voltage at the plates. And a more negative number at the grids should reduce the idle current (mA).
Also, you can't measure the bias voltage at the VR2 trimmer, you need to measure at the junction of the 150K's, R26 and R27.
Originally posted by Enzo
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That sounds better and should be the most max. negative that should be needed.
When you measure that -60V, what is the plate voltage at the power tubes? And what kind of readings for current at the bias probes?
The reason I had mentioned the discrepancy before, when you had idle currents in the 45 to 50mA range, you said the plate voltage was about 500V.
Then later with idle currents at 30mA you said plate voltage was 510V.
Then with even lower idle currents, 20 to 26mA, you said the plate voltage went down again, to 503V.
Make sure your meter battery is good. Set bias to -50VDC at R26/27 junction. What are the idle currents and plate voltages from the bias probe?Originally posted by Enzo
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