Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jhanly on January 05, 2025, 02:10:16 am
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Hi, I’m hoping for some guidance if possible regarding a PT that’s showing some strange readings.
I have a vintage vortexion power transformer that matches the specs of the PT I’m needing to replace in a jtm45 head (clone).
The main difference is the vortexion has one secondary winding more than I need. I’m wanting to simply tape it off and install - but the winding is reading strange - so I’m hoping for some wisdom to guide my decision-making.
Specifically, on this surplus secondary…
- R for the full winding reads 600k
- R values for the half windings are grossly asymmetrical - one side reads 300k to CT while the other reads 10 ohms to CT.
- I get a 3:1 voltage ratio between primary & the full winding (step-down).
- the high R side’s voltage ratio halves to 6:1 while the 10 ohm side shows closer to a 5:1 ratio with the primary.
Update: after double-checking all the above, the high R side of the surplus secondary winding is now consistently reading OL to both its CT and full winding. It’s still reading similar voltage ratios as before though.
From all the trouble-shooting I’ve done along the way, I feel confident to say this apparent increase to infinite R on this high R side of the winding doesn’t reflect any continuity problems WRT the actual leads for this secondary tap - ie, all readings have remained steady whenever I’ve physically manipulated these leads.
This anomalous winding reads OL to all other windings (primary and secondary) - and correspondingly my current limiter lamp revealed no shorts anywhere. All other windings are good for continuity, resistance and voltage ratios.
So, notwithstanding the dodgy internal resistance readings of this surplus winding, is it viable to tape off and install this PT into my amp? Or is that playing Russian roulette?
TIA
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I test transformers with reading output voltages, not dc ohms
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Thanks for that Latole, my experience with testing PTs is limited.
The voltage ratios I mentioned were based on 5vac fed to the primaries, and the resulting output voltages I turned into ratios - not sure if that’s the most helpful way to report.
Anyways, I seem to be getting reasonably sensible voltages from the secondary in question & my current limiter didn’t baulk - so am tempted to roll the dice on this one.
Cheers
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I suggest now to supply the primary via a light bulb limiter, fitted with a lowish wattage bulb, eg 40W rather than 400W.
My method is to fit all winding leads to separate terminals along a screw terminal strip and so able to safely measure the resulting voltages on all windings.
As all windings are unloaded, the bulb should go dim very quickly, and near full mains voltage should be across the primary.
(https://i.ibb.co/z65v7Dj/IMG-3188.webp)
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Hi pdf64, sounds like some actionable next steps.very much appreciated!
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This videos may help:
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Kind thanks Uki