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In Reply to: RE: JJ EZ81 issues? posted by Eli Duttman on April 08, 2015 at 14:42:03
but to use a cap input of 50uf, the EZ51 requires between 150 and 310 ohms in series with each plate.With a critical inductance choke input, there is no series resistance requirement.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/08/15Follow Ups:
Thanks Tre. I assume I would measure between the plate and the CT of the transformer? (not "end to end"). I don't recall what the resistance of the secondary was, but I bet it is well under 300 end to end. Probably under 200. I don't have the amps in hand but that is something we will have to look at.
Just looked at the Sams for this amp. Each EZ81 has 2 plates connected in parallel, and each tube is connected to opposite ends of the transformer secondary. The resistance from each of those plates to ground is shown as 45 ohms. The AC volts on each side of the secondary is 300vac, and the resultant B+ is around 360vdc.
If I am reading the chart correctly, the design of the amp exceeds the spec on the tube quite a bit as it should have at least 150 ohms per leg of the secondary. Even an 8uf first cap would not have worked. Or am I missing something?
"If I am reading the chart correctly, the design of the amp exceeds the spec on the tube quite a bit as it should have at least 150 ohms per leg of the secondary. Even an 8uf first cap would not have worked. Or am I missing something?"What Steve said and....people have been abusing rectifier tubes for a long time.
It's not that it won't work......but those tubes are been hammered on (so is the input cap).
Critical inductance input choke and the hammering goes away.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/09/15
Since the two sections of each rectifier are paralleled, the min series impedance is 75 ohm, not the 150 from the chart. The stated sec resistance on the schematic is ~40 ohms so internal impedance is still less than spec. One detail often overlooked is that the primary resistance as reflected to secondary should also be added in to the meas secondary resistance. In this case it might add 10-20 ohms to to the 40 already present. One possible solution would be to add say 20-50 ohms resistor to each sec winding. Another would be to add an NTC IRL to primary for added reflected impedance.
"add an NTC IRL to primary for added reflected impedance."
Might not be the optimum solution... he did say the flashover occurs 8-10 seconds after power on. The fixed resistors you suggested would probably work, and I think he should try a different brand of rectifier tube as well (per Jim's advice).
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Buy Chinese. Bury freedom.
Flashover occurring 8-10s after power on is about when the tube begins to conduct. At this point the IRL will still possess considerable resistance because it's not fuly warmed up. Whether or not this prevents startup flashover using substandard rectifiers is unknown. But there are other benefits even if it doesn't prevent the flashover so it would be one of the first measures I'd consider if the amp were mine.
nada aqui
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Buy Chinese. Bury freedom.
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