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    filament transformer question

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    Captain Coconut

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    filament transformer question

    Post by Captain Coconut on Thu Feb 18, 2016 6:51 pm

    A 12AX7 (and its cousins) calls for 6.3VAC and 0.3 amps for the filaments. Would a transformer with a higher amp rating be beneficial? Less heat generated possibly? Or would too high a rating cause a reduction in voltage? Ohm's Law kinda' thing.

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Thu Feb 18, 2016 9:07 pm

    always go for higher rating. Say your total filament current is 1.2A, I would not go less than 1.5A, even 2A, as long as size and cost are within acceptable limits.
    Heck, you could go with 5A too....overkill though! Very Happy

    Captain Coconut

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    Re: filament transformer question

    Post by Captain Coconut on Tue Nov 01, 2016 9:43 pm

    Isn't there a risk of noise with too much current? Usually the input tubes are wired at the end of the string where current is lower - but still high enough to handle the current draw.

    j beede

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    Re: filament transformer question

    Post by j beede on Wed Nov 02, 2016 12:34 pm

    Filaments draw current, transformers don't push current. Replacing an undersized filament transformer with a correct/oversized transformer may increase current flow (and reduce voltage drop). Replacing an a correctly sized filament transformer with an oversized transformer should have little effect other than occupying more space, lowering the resonant frequency of your chassis, and raising your credit card balance.

    It is possible for smaller transformers to reduce voltage drop--it depends on your supply of liquid Helium :-)

    deepee99

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    Re: filament transformer question

    Post by deepee99 on Wed Nov 02, 2016 12:41 pm

    Perhaps it's time to revisit the tube ham gear era. My old Collins S-Line had a stand-alone (and fully shielded) power supply that plugged into the xmitter and rcvr via octals connections. IIRC, the only pwr supply components that were on the main chassis of the xmitter were the rectifiers. Wonder if such a concept could be applied to the M-125/ST-120 series. Outboard power supply would save a lot of grief, one would think.

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Wed Nov 02, 2016 12:50 pm

    j beede wrote:Filaments draw current, transformers don't push current. Replacing an undersized filament transformer with a correct/oversized transformer may increase current flow (and reduce voltage drop). Replacing an a correctly sized filament transformer with an oversized transformer should have little effect other than occupying more space, lowering the resonant frequency of your chassis, and raising your credit card balance.

    It is possible for smaller transformers to reduce voltage drop--it depends on your supply of liquid Helium :-)

    beat me to it. To put into somewhat simplified terms, current/current draw is a result of combination of load/resistive load and voltage. So if you're 'not using' or loading the power supply up, current will not increase. This applies to AC or DC. OHM's law comes into it, and here is quite a good article, https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Wed Nov 02, 2016 12:54 pm

    deepee99 wrote:Perhaps it's time to revisit the tube ham gear era. My old Collins S-Line had a stand-alone (and fully shielded) power supply that plugged into the xmitter and rcvr via octals connections. IIRC, the only pwr supply components that were on the main chassis of the xmitter were the rectifiers. Wonder if such a concept could be applied to the M-125/ST-120 series. Outboard power supply would save a lot of grief, one would think.

    physically separating the power supply from an amp or preamp is a good solution, it would sure help to keep unwanted noise down. As long as the power supply is not located too far way, requiring a long connecting harness. And that the wires used in that harness are of the correct gauge, plus a safety margin of course, to handle the currents, especially for the filaments. B+ current is usually very low, for a power amp that most of us have, no more than say 300mA, and thats on the very high end, for the preamp section, no more than say 25-30mA, again, thats on the very high side too.
    For the filaments, it can go as high as 8-9A!, depending on the preamp and power tubes.

    deepee99

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    Re: filament transformer question

    Post by deepee99 on Wed Nov 02, 2016 1:01 pm

    MontanaWay wrote:
    deepee99 wrote:Perhaps it's time to revisit the tube ham gear era. My old Collins S-Line had a stand-alone (and fully shielded) power supply that plugged into the xmitter and rcvr via octals connections. IIRC, the only pwr supply components that were on the main chassis of the xmitter were the rectifiers. Wonder if such a concept could be applied to the M-125/ST-120 series. Outboard power supply would save a lot of grief, one would think.

    physically separating the power supply from an amp or preamp is a good solution, it would sure help to keep unwanted noise down. As long as the power supply is not located too far way, requiring a long connecting harness. And that the wires used in that harness are of the correct gauge, plus a safety margin of course, to handle the currents, especially for the filaments. B+ current is usually very low, for a power amp that most of us have, no more than say 300mA, and thats on the very high end, for the preamp section, no more than say 25-30mA, again, thats on the very high side too.
    For the filaments, it can go as high as 9-10A!, depending on the preamp and power tubes.
    I think the Collins lash-up had a 1-meter cord (or shorter) and the pwr supply and xmitter basically sat right next to each other. Of course they were shielded heavily in cases made of metal the EPA probably has on its short list of toxic materials . . .

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Wed Nov 02, 2016 1:02 pm

    deepee99 wrote:
    MontanaWay wrote:
    deepee99 wrote:Perhaps it's time to revisit the tube ham gear era. My old Collins S-Line had a stand-alone (and fully shielded) power supply that plugged into the xmitter and rcvr via octals connections. IIRC, the only pwr supply components that were on the main chassis of the xmitter were the rectifiers. Wonder if such a concept could be applied to the M-125/ST-120 series. Outboard power supply would save a lot of grief, one would think.

    physically separating the power supply from an amp or preamp is a good solution, it would sure help to keep unwanted noise down. As long as the power supply is not located too far way, requiring a long connecting harness. And that the wires used in that harness are of the correct gauge, plus a safety margin of course, to handle the currents, especially for the filaments. B+ current is usually very low, for a power amp that most of us have, no more than say 300mA, and thats on the very high end, for the preamp section, no more than say 25-30mA, again, thats on the very high side too.
    For the filaments, it can go as high as 9-10A!, depending on the preamp and power tubes.
    I think the Collins lash-up had a 1-meter cord (or shorter) and the pwr supply and xmitter basically sat right next to each other. Of course they were shielded heavily in cases made of metal the EPA probably has on its short list of toxic materials . . .

    aaahhh...the good ole times, when lead and asbestos were the leaders....

    Captain Coconut

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    Re: filament transformer question

    Post by Captain Coconut on Wed Nov 02, 2016 4:03 pm

    j beede wrote:Filaments draw current, transformers don't push current. Replacing an undersized filament transformer with a correct/oversized transformer may increase current flow (and reduce voltage drop). Replacing an a correctly sized filament transformer with an oversized transformer should have little effect other than occupying more space, lowering the resonant frequency of your chassis, and raising your credit card balance.

    It is possible for smaller transformers to reduce voltage drop--it depends on your supply of liquid Helium :-)

    Yeah, I know transformers don't push current; I'm not that daft. Anyhow, thanks.

    Captain Coconut

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    Re: filament transformer question

    Post by Captain Coconut on Thu Nov 03, 2016 9:02 pm

    MontanaWay wrote:always go for higher rating. Say your total filament current is 1.2A, I would not go less than 1.5A, even 2A, as long as size and cost are within acceptable limits.
    Heck, you could go with 5A too....overkill though! Very Happy

    If the transformer is designed to deliver 6.3 volts at say, 5 amps, then wouldn't you have excessive voltage if you were pulling only 1.2 amps?  Seems to me that you wouldn't want to over-rate the current spec by too much.

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Thu Nov 03, 2016 9:18 pm

    Captain Coconut wrote:
    MontanaWay wrote:always go for higher rating. Say your total filament current is 1.2A, I would not go less than 1.5A, even 2A, as long as size and cost are within acceptable limits.
    Heck, you could go with 5A too....overkill though! Very Happy

    If the transformer is designed to deliver 6.3 volts at say, 5 amps, then wouldn't you have excessive voltage if you were pulling only 1.2 amps?  Seems to me that you wouldn't want to over-rate the current spec by too much.

    No. It will deliver 6.3V UP TO 5 amps, no more no less. Once you do go over the 5 amps, the 6.3V will start to drop, until you 'saturate' the transformer and it starts to 'cook' and eventually fail.

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Thu Nov 03, 2016 10:36 pm

    you really cannot 'over spec' or 'over rate' a transformer, the only things limiting one to do that is size, price and common sense! Smile
    It is always good to over spec by say 10-15%, more is really not needed. But if happen to have say a 5A 6.3V transformer lying around, and you only need 3A, and the 5A is not too big, then sure, use it!...what the heck...right?

    Captain Coconut

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    Re: filament transformer question

    Post by Captain Coconut on Fri Nov 04, 2016 10:03 am

    Thanks, Monty. Other sites say that if you're drawing less current than what your transformer was designed for, then you'll get above 6.3 volts on the heaters. You would think that this was black and white, but I guess not.

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Fri Nov 04, 2016 11:38 am

    Captain Coconut wrote:Thanks, Monty. Other sites say that if you're drawing less current than what your transformer was designed for, then you'll get above 6.3 volts on the heaters. You would think that this was black and white, but I guess not.

    a well designed and manufactured transformer should give you 6.3VAC, say +/- 2%, at least up to the rated capacity in amps. Most good transformers are already somewhat over rated, to give some head way, especially when the tubes are cold, as you will get a spike in current draw, but that is only momentary, and it will settle down to the total operating current draw.

    j beede

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    Re: filament transformer question

    Post by j beede on Fri Nov 04, 2016 12:10 pm

    It is possible to raise the filament voltage by replacing the transformer with a higher current capacity transformer. When this happens it says more about the "old" transformer than it does about the new one.

    MontanaWay

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    Re: filament transformer question

    Post by MontanaWay on Fri Nov 04, 2016 12:22 pm

    j beede wrote:It is possible to raise the filament voltage by replacing the transformer with a higher current capacity transformer. When this happens it says more about the "old" transformer than it does about the new one.

    if at all, only by a small percentage, and not enough to cause issues with the tubes, so lets not cause too much confusion here. Like I indicated, a well designed and manufactured transformer should not vary much at all, no matter what the load, up to its rating.

    tubes4hifi
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    Re: filament transformer question

    Post by tubes4hifi on Tue Nov 08, 2016 4:04 pm

    actually, in my many years of experience, yes, if you have a transformer rated 6v at 5A and are only using say 1A, you're likely to get more like 6.6v (about 10%) more voltage out of it,
    but you can compensate for that by using a low value resistor in series with your load.

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