Solderin' time!

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  • jpfampsjpfamps Frets: 2723
    ICBM said:
    Do you have an old 60W or 100W incandescent light bulb and a holder you can attach wires to? If so, power the amp up with the bulb in series with it. (Temporarily solder it across the fuseholder with the fuse removed, if you don't have any easier way of connecting it.)

    When you power up the bulb should flash brightly and then fade out. Then you can go around and check your circuit voltages are correct. If the bulb stays lit, you have a problem but the amp won't come to any harm since the bulb will absorb most of the power. Still keep a good look out for anything that looks or smells like smoke though!

    The modern BLS transformer may use a better grade of steel and thus can run cooler with a smaller stack.

    Increasing current carrying capacity requires thicker wire (lower resistance reduces I^2.R heating), and thus a larger winding window. A taller stack of the same laminations doesn't give you a larger winding window; in fact it may make the situation worse as you have a greater length of wire and hence more resistance.


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  • EdGripEdGrip Frets: 736
    A 60W bulb it'll have to be, I'm afraid.

    Thanks for your input, JPF - I thought it might be something like that. :)
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  • EdGripEdGrip Frets: 736
    edited January 2014
    You caused a complete stranger to make a thing. I hope you're all happy. 
    image
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  • ICBMICBM Frets: 71963
    Neat. You've shamed me into thinking I should make a proper job of my one now…

    Don't worry about the 60W bulb, it will only matter for higher-powered amps.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • EdGripEdGrip Frets: 736
    I love them loop-backed plugs... pure genius...
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  • ICBMICBM Frets: 71963
    EdGrip said:
    I love them loop-backed plugs... pure genius…
    They make it much easier for a rescuer to quickly pull the plug out of the wall before attempting to resuscitate you :).

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • EdGripEdGrip Frets: 736
    edited January 2014
    Exactly! Plus, as my brother pointed out, you can always cable-tie the flex to the loop, meaning in critical applications you can do the same job yourself...

    And they're just as tough as any Duraplug or Permaplug... they are my default plug now. Mmm, plugs.
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  • MistergMisterg Frets: 293
    EdGrip said:
    You caused a complete stranger to make a thing. I hope you're all happy. 

    Neat!

    I went all rack-mount, myself:

    http://i255.photobucket.com/albums/hh143/werdnayarg/webstuf/DSC_2902a_zps553cb5b7.jpg


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  • EdGripEdGrip Frets: 736
    Guard lamp works beautifully. Voltage from the rectifier is 394v. Onwards...
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  • EdGripEdGrip Frets: 736
    Schematic says there should be 420v coming off the rectifier - I seem to remember that in the amp before the transformer swap, this was more like 440v. Is this to do with the guard lamp?
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  • ICBMICBM Frets: 71963
    edited January 2014
    Yes, probably. Since it's clearly working OK with the lamp, now power it up (in the same way, just the rectifier and whatever else you have connected if anything) without the lamp and check the voltage again.

    Do the same at every step until you have the whole amp checked.

    Even if the voltage doesn't quite reach 420, don't worry about it. Lower voltages are less hard on the valves and that's close enough that you probably won't notice the loss of power - it's only about 5% low.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • EdGripEdGrip Frets: 736
    Come to think of it, when I measured that original voltage, the amp still had a modern-production GZ34 in it... 5U4GB now.
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  • EdGripEdGrip Frets: 736
    460v from the rectifier with no other load... that's more like it...
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  • EdGripEdGrip Frets: 736
    edited January 2014
    Right. Fully loaded up, with a wall voltage of 254v and the voltage selector set to the 250v tap...

    Plate voltage = 394v
    Heater voltage = 6.8v. 

    Neither is really ideal. FFS.

    Edit: Tell a lie - not plate voltage, just B+ from the rectifier. Plate voltage will be lower.
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  • ICBMICBM Frets: 71963
    What 5U4 do you have?

    Don't worry about the HT voltage. Far better 5% low than 5% high. 

    How many valves do you have on the filament circuit? If you have none in other than the rectifier yet, the filament voltage will be too high.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • EdGripEdGrip Frets: 736
    No, that's with them all in. Apparently there are plenty of people with amps with heater voltages in that sort of range, it's just I would like a new custom transformer to be a bit closer. Could a problem with the virtual-centre-tap resistors cause this?
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  • EdGripEdGrip Frets: 736
    ...can't find my bias probe now... Having to vent grumpiness in Off Topic handbag threads...
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  • ICBMICBM Frets: 71963
    EdGrip said:
    No, that's with them all in. Apparently there are plenty of people with amps with heater voltages in that sort of range, it's just I would like a new custom transformer to be a bit closer. Could a problem with the virtual-centre-tap resistors cause this?
    No. They don't draw enough current to affect it.

    In my opinion that's not really good enough for a new transformer, if you're connected to the correct primary tap for the wall voltage.

    What 5U4 is it? Some don't give a correct voltage drop either.


    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • jpfampsjpfamps Frets: 2723
    Most filament supplies are on the high side when I measure them.

    The spec for valves is +/- 10% so 6.8 VAC is OK.

    Bear in mind that your mains is a bit on the high side too, in fact 5% over spec.

    The transformer specs are given at a specific loading, so if the LT is specced as 6.3 VAC @ 3A, then you need to draw 3A from it to bring it down to 6.3 VAC (assuming the correct input voltage). 

    This is not an exact science (surprisingly) and often designs have to be tweaked in the field. Also the filament winding secondary has quite a low number of turns on it so you can only make relatively crude adjustments to the final voltage.

    We did this with one of our mains transformers, and we wanted a 5% reduction in LT, but had the choice of 3.5% or 7% (we went with 3.5%).

    In my experience 5U4Gs vary considerably in their voltage drop, so you may find a different rectifier will give substantially higher HT.

    However, as ICBM says I wouldn't be too worried about the HT being a tad on the low side; it's better for the amp.
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  • EdGripEdGrip Frets: 736
    Yo! Still no sign of the bias probe... it was here a week or so ago...
    There are 3 5U4s, a NOS Raytheon 5U4GB, an NOS Sylvania 5U4GB, and an old-but-good big-bottle RCA 5U4G. It's the Ratheon in there at the moment - first one that came to hand.
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