The OT secondary windings resistance is very low. It will be a near short reading on a DVM. That is normal. Connected to the circuit, you are reading through the winding to ground. Did you measure power tube voltages yet? If so, please post your readings.
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Marshall JCM2000 DSL100 Blown HT Fuse.
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Originally posted by The Dude View PostThe OT secondary windings resistance is very low. It will be a near short reading on a DVM. That is normal. Connected to the circuit, you are reading through the winding to ground. Did you measure power tube voltages yet? If so, please post your readings.
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Originally posted by gtrplayr1976 View Post
I WAS getting power tube voltages until I got to the anode (pin 3) of the first tube ,and that loud pop ,and arc came through the speaker so I stopped. That was with tubes out. .
Anodes are integral parts of the tubes. So with all 4 power tubes out there are no anodes.
Are you measuring at empty sockets?
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Originally posted by Helmholtz View Post
What? Power tubes pulled but still arcing?? And where?
No tubes no anodes.
Makes sense as there are 1R cathode resistors.Last edited by gtrplayr1976; 11-17-2022, 11:38 PM.
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If you have 16.4R between each pin 3 and OT CT, there's no need to measure pin 3 voltages.
With tubes out pin 3 voltages will be identical to B+ and with tubes in a couple of volts lower.
What is your B+ measured at the OT CT?- Own Opinions Only -
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Originally posted by gtrplayr1976 View Post
That cap I replaced on the fuse board may have been the cause of the HT blowing?
Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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Originally posted by The Dude View PostThe OT secondary windings resistance is very low. It will be a near short reading on a DVM. That is normal. Connected to the circuit, you are reading through the winding to ground. Did you measure power tube voltages yet? If so, please post your readings.
Pin #1: NC =0
Pin #2: Htr = 3.77 VAC
Pin #3: HV = 545 VDC
Pin #4: Scn = 535 VDC
Pin #5 Bias = -48 VDC
Pin #6: NC
Pin #7: Htr = 3.77 VAC
Pin #8 Grd = 0
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Originally posted by Helmholtz View PostIf you have 16.4R between each pin 3 and OT CT, there's no need to measure pin 3 voltages.
With tubes out pin 3 voltages will be identical to B+ and with tubes in a couple of volts lower.
What is your B+ measured at the OT CT?
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I got all the tube voltages checked with the preamp tubes in, and no arcing today ,but when I installed the power tubes I couldn't get the bias up to where it should be. When I first biased it after installing the new board I had it set at 70mA per side. Now it is at around 15mA. I even tried another quad of tubes ,but same result. Something must have fried ,but I didn't see any indication. Nothing burned.
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Your output tube (socket voltages) look reasonable. Do you have a scope? If so, I'd insert a test tone and follow it through the amp to see where signal is getting lost."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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Originally posted by gtrplayr1976 View Postwhen I installed the power tubes I couldn't get the bias up to where it should be. When I first biased it after installing the new board I had it set at 70mA per side. Now it is at around 15mA.
Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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