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Topic: Can you help me confirm the right PT replacement? |
Steve Waltz
From:
USA
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Posted 26 May 2020 5:25 pm |
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No schematic and no markings on the power transformer. Noise led me to check heater voltage over and over to find it moving a bit but mainly above 25 volts. So the heater wires are getting additional power inside the transformer and has something wrong in it?
The amp is two EL34 power tubes with plate voltage at 420 and screens around 370ish. It is cathode biased with a center tap off of the heater winding going to the cathode resistor location. There are four 6SN7 preamp tubes and one 6SJ7 tube. Tube recitfier.
I found Hammond 379X which is 850V at 173ma with a center tapped heater winding. So that would get my plates back to 425 but the ma might be a little low? The Hammond 278X has higher ma at 230ma but lower 800v which would only give me 400 at the plates. These are the right size.
Is the 173ma going to be OK on this amp?
Thanks,
Steve |
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Fred
From:
Amesbury, MA
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Posted 26 May 2020 6:28 pm
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I would expect 25 volts on your heaters would burn them out. The center tap on the heater winding going to the EL34 cathodes puts a DC bias on the AC heater voltage. The correct way to measure the heater voltage is directly across the heater winding (not to ground) with the meter set to AC. I would expect 25 volts DC on the cathodes.
If you already know all this I’ll shut up now 
Fred |
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Brian Hollands
From:
Geneva, FL USA
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Posted 26 May 2020 6:29 pm
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The PT secondary voltages are AC and the sum of both sides. Plate voltage is DC after the rectifier. It's not a 1 to 1.
I'm assuming this is a homemade amp and that's why there's no schematic. If not, what is the amp?
Either way the existing PT should have a model number stamped on it. That would allow you to find it's specs.
If no model number I think your rectified DC voltage should be about 1.4x the AC voltage of either side of the PT secondary. (It's been a while so let someone else confirm that). If that's right it would mean the PT should be more like 300-0-300. That sounds a bit low to get 420 on the plates but the math is right, just not sure the equation is...
Google each tube to get its current draw numbers. Sum those and you'll have the current draw.
Edit: what tube is it using as a rectifier? There's voltage drop from a tube vs. SS rectifier which is likely why that 300v number seemed low to me.
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'81 Sho-bud LDG, 2 EMCI's |
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Tim Marcus
From:
San Francisco, CA
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Posted 26 May 2020 8:33 pm
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are you measuring AC or DC voltage on the heaters?
They might be elevated with 25VDC which is fine but if you are measuring 25VAC you are going to burn out the filaments pretty fast if not immediately
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Milkmansound.com |
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Steve Waltz
From:
USA
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Posted 27 May 2020 4:21 pm
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Yep....I'm an idiot.......meter was set to auto so it was seeing the cathode dc. With the meter set to AC heaters are now 6.6 volts which is .3 higher but not really an issue, I think?
You guys saved me from buying a PT I didn’t need.
Steve |
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