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.
3 posters
filament transformer question
Guest- Guest
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!
Heck, you could go with 5A too....overkill though!
Guest- Guest
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- Posts : 473
Join date : 2011-02-07
Location : California
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 :-)
It is possible for smaller transformers to reduce voltage drop--it depends on your supply of liquid Helium :-)
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
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.
Guest- Guest
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
Guest- Guest
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- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
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 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.
Guest- Guest
deepee99 wrote: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 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.
aaahhh...the good ole times, when lead and asbestos were the leaders....
Guest- Guest
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.
Guest- Guest
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!
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.
Guest- Guest
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!
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.
Guest- Guest
you really cannot 'over spec' or 'over rate' a transformer, the only things limiting one to do that is size, price and common sense! 
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?
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?
Guest- Guest
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.
Guest- Guest
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- Posts : 473
Join date : 2011-02-07
Location : California
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.
Guest- Guest
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.
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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.
but you can compensate for that by using a low value resistor in series with your load.