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Thread: Transformer questions

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    Transformer questions

    I am trying to restore a Gibson Stereo Bell 30 amp to some semblance of working order.
    It has no power supply and I am trying to use a very old transformer that I have lying around.
    This transformer was used with what I believe was a juke box amp. Used two channels of 6BQ5 push/pull output section. So it is nominally suitable. It has a 5V supply along with 6V supply.
    I measured the winding inductance with my DVM and got 4.3H for the HT secondary end to end and 1.1/1.2 H to the center tap. My issue is that I only have 195 mH for the primary winding and only 1.5 ohm DC res.
    Those values seem low to me.
    I also checked that all the windings have greater than 4 Meg ohm to the core.
    Nevertheless I hooked up the ac through my lightbulb(150W) rig and only got a 3-4V drop across the bulb with the trans secondary windings open. So the trans seems ok. Through the Lightbulb I got 303V - 306V AC to center tap on the HT winding.
    Is there anything I'm missing?
    Is that a resonable inductance for a primary winding?
    Also the powersupply schematic shows a choke in a pi configuration... what is a reasonable inductance for that choke or if I used a resistor what would you guys suggest?
    I'm going to use 33uf +33uf can for filtering.
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    Lifetime Member Enzo's Avatar
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    I have never measured the inductance of a power transformer in my life, and I been in electronics since 1954. Not saying you are wrong, but I wouldn't know what to do with that data. The transformer connects to 120vAC and produces reasonable secondary voltages? And it doesn't draw excess current? Sounds good to me.

    The Gibson expects 260vAC on the HT winding. Your have 300v. So your B+ will be higher than stock. 420vDC rather than 360vDC. More or less. Probably not a deal breaker. I don't stock chokes, I could probably find something well suited, but at least for a working project I'd stick a 400 ohm 20 watt, or something like that , in the spot. it is just a guitar amp, so you have a wide range of acceptable values.

    Northern VA? I grew up across the Potomac from you. Spent a year in Doswell for that matter.

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    Thanx Enzo. Maybe just need some hand holding.

    I see you are right about voltage being high though that was with no load. The trans was used with a tube rectifier.

    A 470 ohm resistor will help with lowering the B+. This Gibson amp has all the tubes(including the 7199) and if they are still good this could be a cool project.

    I'll hook this thing up and see what I get.

    Haven't been here in a while... I'll try to hang out more.

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    I only have 195 mH for the primary winding
    Inductance measurements with an LCR meter on transformers are not meaningful. Inductance strongly changes with (measuring) current and frequency and may be up to 10 times higher at full mains voltage.
    The low voltage drop across the bulb shows that the PT has no winding shorts.
    Also any shorted turns would kill primary AND secondary inductance.

    Through the Lightbulb I got 303V - 306V AC to center tap on the HT winding.
    What is the secondary voltage without bulb limiter? Something like 2x320V I guess?


    Edit: Just measured the primary L of a good PT with my LCR meter at 1kHz and got around 200mH.
    BTW, the ratios of your L measurements make sense. The full HT voltage is around 5 times the primary voltage, so the ratio of
    the turns must be around 5. As inductance is proportional to the square of turns number, the inductance of the secondary will
    be around 25 times higher than the primary inductance, which gets you in the ballpark with your inductance ratios. But as said absolute values are way too low.

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    Inductance strongly changes with (measuring) current and frequency and may be up to 10 times higher at full mains voltage.
    I new that the iron core could saturate and the inductance would go down but I did not realize that permeability could rise with increasing current/magnetic field.
    I have used the inductance measurements of my DVM meter to identify windings in unknown transformers. Mostly audio output trans. It makes a better distinction than the low ohm resistance.

    I'll install a fuse and the 1n4007's and report back on results. Part of my concerns were due to my knowing that this transformer has had a hard life. I used it in a converted stereo amp for guitar when I was a teen. I thought it was cool that I could play real loud and make the plates of the 6BQ5's glow red. The cathode resisters eventually exploded. Oh... Youth!

    The gibson amp itself is going to need some caps and I'll probably have some questions about that.

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