Dear people,
I have been looking at power transformers for a 100 watt Marshall style build and I am confused about the mA rating of the HT.
What would be the effect of over- or underspeccing the mA rating of the HT?
Classictone 175 0 175 @ 290 mA HT
TAD 173 0 173 @ 300 mA HT
Hammond 175 0 175 @ 420 mA HT
From my understanding the amp will draw the following current:
- preamp tubes 6x 1 mA = 6 mA
- el34's @ 450V = ( 25W/450) x 4 = 222 mA
- screen current 222 mA / 6.5 (screen-to-anode current ratio in datasheet) = 34 mA
Total amp draw 6 + 222 + 34 = 262 mA
Then there is also the RMS ripple current that flows around the transformer and rectifier which I cannot calculate because I do not know the source resistance of the transformer.
Necessary transformer mA is thus 262 mA x unknown ripple current ratio.
Is there something else I am missing in the amps current draw on the transformer?
I have been looking at power transformers for a 100 watt Marshall style build and I am confused about the mA rating of the HT.
What would be the effect of over- or underspeccing the mA rating of the HT?
Classictone 175 0 175 @ 290 mA HT
TAD 173 0 173 @ 300 mA HT
Hammond 175 0 175 @ 420 mA HT
From my understanding the amp will draw the following current:
- preamp tubes 6x 1 mA = 6 mA
- el34's @ 450V = ( 25W/450) x 4 = 222 mA
- screen current 222 mA / 6.5 (screen-to-anode current ratio in datasheet) = 34 mA
Total amp draw 6 + 222 + 34 = 262 mA
Then there is also the RMS ripple current that flows around the transformer and rectifier which I cannot calculate because I do not know the source resistance of the transformer.
Necessary transformer mA is thus 262 mA x unknown ripple current ratio.
Is there something else I am missing in the amps current draw on the transformer?
The transformer you need is at least this one :
Hammond 175 0 175 @ 420 mA HT.
At full power output you can easily double the plate dissipation of the EL34.
I use this one with a push-pull of 6L6GC for a 95WRMS clean output :
Tube-Town Store - TT Powertransformer 100 Watt for Marshall / JCM 800 / Plexi
A+!
Hammond 175 0 175 @ 420 mA HT.
At full power output you can easily double the plate dissipation of the EL34.
I use this one with a push-pull of 6L6GC for a 95WRMS clean output :
Tube-Town Store - TT Powertransformer 100 Watt for Marshall / JCM 800 / Plexi
A+!
Just for planning: ClassicTone has ended production. You can try to get stock from their many distributors but I think it may already be snapped-up.
ClassicTone Going Out Of Business
ClassicTone Going Out Of Business
Heater current for ONE 12AX7 is ~ 150mA. I don't know what tubes your preamp uses, but it certainly consumes much more than 1 mA for the heater alone.
> Heater current for ONE 12AX7 is ~ 150mA.
At 12V. We often wire and feed 12AX7 for 6V 300mA.
> certainly consumes much more than 1 mA for the heater alone.
Right on; but maybe this was meant to be Plate current??? Since 225mA is mentioned for EL34 (heater current 1,600mA), it may be.
I have seen 12AX7 take more than 1mA/section, but in context of giant 225mA bottles it usually does not matter for PT selection.
At 12V. We often wire and feed 12AX7 for 6V 300mA.
> certainly consumes much more than 1 mA for the heater alone.
Right on; but maybe this was meant to be Plate current??? Since 225mA is mentioned for EL34 (heater current 1,600mA), it may be.
I have seen 12AX7 take more than 1mA/section, but in context of giant 225mA bottles it usually does not matter for PT selection.
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Thanks for all the responses.
I have found something else online, it is a datasheet of the el34 from Mullard shown below.
In my calculation 4x el34 will draw 256 mA max
Mullards datasheet says 532 mA max at 400V
This is a big difference, am I reading the datasheet correct?
Also, what would be the effect of under- or overspeccing a transformer?
I have found something else online, it is a datasheet of the el34 from Mullard shown below.
In my calculation 4x el34 will draw 256 mA max
Mullards datasheet says 532 mA max at 400V
This is a big difference, am I reading the datasheet correct?
Also, what would be the effect of under- or overspeccing a transformer?
Attachments
> In my calculation 4x el34 will draw 256 mA max
How is that figured? Plate dissipation?
A Fix-Bias amp will be designed to idle not-hot and suck more current when output is demanded. In tubes it is utterly normal for current to double from idle to full roar.
In Self-bias, a large rise of current with signal will un-bias the power amp, high distortion and possible blocking after loud transients. Self-bias is normally designed for 10% to 40% rise from idle to full roar.
In HiFi *listening* the average is <10% of max so the PT may be selected for the idle, EXCEPT it is customary to Test at 33% to 100% for milliSeconds to Hours, and a wee PT will suffer in testing.
In guitar you better buy a large fraction of that MAX current demand.
How is that figured? Plate dissipation?
A Fix-Bias amp will be designed to idle not-hot and suck more current when output is demanded. In tubes it is utterly normal for current to double from idle to full roar.
In Self-bias, a large rise of current with signal will un-bias the power amp, high distortion and possible blocking after loud transients. Self-bias is normally designed for 10% to 40% rise from idle to full roar.
In HiFi *listening* the average is <10% of max so the PT may be selected for the idle, EXCEPT it is customary to Test at 33% to 100% for milliSeconds to Hours, and a wee PT will suffer in testing.
In guitar you better buy a large fraction of that MAX current demand.
Thanks PRR. I calculated the el34 current draw as such:
- el34's @ 450V = ( 25W/450) x 4 = 222 mA
- screen current 222 mA / 6.5 (screen-to-anode current ratio in datasheet) = 34 mA
The amp is push pull, 100 watt sine, 4x6ca7 ( el34) with fixed bias.
Should I better use Mullards datasheet for the max current draw ( Ia max. sig. and Ig2 max. sig.) ?
For a guitar power amp that is working with a square wave-ish output from the preamp, what would be a good fraction of the max current demand when designing the power transformer?
I would like to design for the possibility to turn the amp up full for a couple of hours with a vivid playing style.
- el34's @ 450V = ( 25W/450) x 4 = 222 mA
- screen current 222 mA / 6.5 (screen-to-anode current ratio in datasheet) = 34 mA
The amp is push pull, 100 watt sine, 4x6ca7 ( el34) with fixed bias.
Should I better use Mullards datasheet for the max current draw ( Ia max. sig. and Ig2 max. sig.) ?
For a guitar power amp that is working with a square wave-ish output from the preamp, what would be a good fraction of the max current demand when designing the power transformer?
I would like to design for the possibility to turn the amp up full for a couple of hours with a vivid playing style.
Put away the calculator. For-purpose "Marshall 100W" replacement transformers are readily available and known to work on stage.
Marshall 100 W Power Transformer / Netztransformator for Plexi & JCM 800 Amplifiers
290HX - Hammond Mfg.
290HX Hammond Mfg. Enclosures | Hawk Electronics
Marshall 100 W Power Transformer / Netztransformator for Plexi & JCM 800 Amplifiers
290HX - Hammond Mfg.
290HX Hammond Mfg. Enclosures | Hawk Electronics
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^^^^^^ That.
These designs are 50 or 60 years old, always the same, mature well known technology and then some,why reinvent the wheel?
Just pick one of the dozens commercial offerings available or copy specs if you want a custom wound one.
These designs are 50 or 60 years old, always the same, mature well known technology and then some,why reinvent the wheel?
Just pick one of the dozens commercial offerings available or copy specs if you want a custom wound one.
Thanks.
The reason I am asking is that I am using a 100 watt replacement Marshall transformer in a amp and it runs so hot at full volume that it starts to smell to varnish and is too hot to touch. It is also fairly noisy. Made by Dagnall.
I have blown the original PT before I installed this one, also by diming the amp.
HT is at 300 mA.
For a new amp I would like to spec a custom transformer that solves these issues, and for that I need to calculate the max current draw of the amp.
Marshall originally made these amps for use without distortion, as stated in the original manual "turn amp down if ugly distortion starts to appear".
I suspect the old transformers are not specced to support dimed playing.
The reason I am asking is that I am using a 100 watt replacement Marshall transformer in a amp and it runs so hot at full volume that it starts to smell to varnish and is too hot to touch. It is also fairly noisy. Made by Dagnall.
I have blown the original PT before I installed this one, also by diming the amp.
HT is at 300 mA.
For a new amp I would like to spec a custom transformer that solves these issues, and for that I need to calculate the max current draw of the amp.
Marshall originally made these amps for use without distortion, as stated in the original manual "turn amp down if ugly distortion starts to appear".
I suspect the old transformers are not specced to support dimed playing.
Oh, the OLD ones could stand anything, are still working 40/50 years later.
Only way to burn one is to have a gross short, either tubes, diodes, supply capacitors, and replace mains fuse with a much larger one which never blows or even wrap it in cigarette aluminum foil.
Now "modern" ones, (think 90´s and later), for (their) convenience are not wound with old style synthetic varnish or epoxy but with new fangled "self stripping" enamel, the kind which *evaporates with no residue* at soldering iron temperatures.
Convenient to save, oh, 5 minutes at transformer assembly but a ticking time bomb, you just overload them a little and they short.
So you don´t need to find mA consumption, that has already been worked out, but have one wound using "Class H - Class F" wire, made to wind macho man electric motors, not sissy transformers, which stand 150-180C under high vibration.
I use that to wind my own, never have a problem.
Only way to burn one is to have a gross short, either tubes, diodes, supply capacitors, and replace mains fuse with a much larger one which never blows or even wrap it in cigarette aluminum foil.
Now "modern" ones, (think 90´s and later), for (their) convenience are not wound with old style synthetic varnish or epoxy but with new fangled "self stripping" enamel, the kind which *evaporates with no residue* at soldering iron temperatures.
Convenient to save, oh, 5 minutes at transformer assembly but a ticking time bomb, you just overload them a little and they short.
So you don´t need to find mA consumption, that has already been worked out, but have one wound using "Class H - Class F" wire, made to wind macho man electric motors, not sissy transformers, which stand 150-180C under high vibration.
I use that to wind my own, never have a problem.