characteristics PT
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characteristics PT
Hello forum, I wanted to consult for the construction of a TMB superlite 18 watt, how many amperes (amperage) should the secondary of the PT 290+290 deliver to work properly and without getting too hot. Thank you and apologies for the possible errors in the English translation.
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- Bieworm
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Re: characteristics PT
Most 2x EL84 push pull amps have about 120 to 160mA on the HT secondaries. A pair of EL84 tubes will draw about 50 to 60mA and then there’s also the preamp tubes drawing some current. Imho 120mA for a SL will be plenty
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- Crabman
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Re: characteristics PT
Bieworm is right about current draw for a pair of EL84s.
You can do the calculations fairly easily. If the tubes are are dissipating 18W and the supply voltage is 360V then this will translate to about 50mA. You also have to consider the screens as well. You can consult a JJ data sheet to figure out the ratio of anode/screen current. Looks to be about a factor 8.7 so allow for 5.7mA at the screens (mine read a fair bit less than this at idle). Then allow for about 2mA for each 12AX7. So you would be looking at around 62mA if you had 3 x 12AX7's. If you look at Merlins ValveWizard website he shows this approach in the smoothing and filtering tab. He also recommends you design for maximum dissipation which is 22W for a pair of EL84s and this would increase your total current draw to about 75mA.
The next question is how much oversupply of PT current do you need. I'm not sure I can answer this but if you look at his Power Supply Design tab you will see that you need to allow for peak RMS ripple current versus load current. In the example he gives a ratio of 1.8 (not sure if this is a general factor or not). If you use this conversion factor then at 62mA load current you would need a transformer rated at about 112 mA. If you use the max value of 75mA you would need a transformer rated at 135mA.
The specs on the Hammond 290PAZ is for 140mA and that looks to be about right.
Hope this helps
You can do the calculations fairly easily. If the tubes are are dissipating 18W and the supply voltage is 360V then this will translate to about 50mA. You also have to consider the screens as well. You can consult a JJ data sheet to figure out the ratio of anode/screen current. Looks to be about a factor 8.7 so allow for 5.7mA at the screens (mine read a fair bit less than this at idle). Then allow for about 2mA for each 12AX7. So you would be looking at around 62mA if you had 3 x 12AX7's. If you look at Merlins ValveWizard website he shows this approach in the smoothing and filtering tab. He also recommends you design for maximum dissipation which is 22W for a pair of EL84s and this would increase your total current draw to about 75mA.
The next question is how much oversupply of PT current do you need. I'm not sure I can answer this but if you look at his Power Supply Design tab you will see that you need to allow for peak RMS ripple current versus load current. In the example he gives a ratio of 1.8 (not sure if this is a general factor or not). If you use this conversion factor then at 62mA load current you would need a transformer rated at about 112 mA. If you use the max value of 75mA you would need a transformer rated at 135mA.
The specs on the Hammond 290PAZ is for 140mA and that looks to be about right.
Hope this helps
1 x
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Re: characteristics PT
thank you very much for the responseCrabman wrote: ↑Tue 01/21/25 5:37 pmBieworm is right about current draw for a pair of EL84s.
You can do the calculations fairly easily. If the tubes are are dissipating 18W and the supply voltage is 360V then this will translate to about 50mA. You also have to consider the screens as well. You can consult a JJ data sheet to figure out the ratio of anode/screen current. Looks to be about a factor 8.7 so allow for 5.7mA at the screens (mine read a fair bit less than this at idle). Then allow for about 2mA for each 12AX7. So you would be looking at around 62mA if you had 3 x 12AX7's. If you look at Merlins ValveWizard website he shows this approach in the smoothing and filtering tab. He also recommends you design for maximum dissipation which is 22W for a pair of EL84s and this would increase your total current draw to about 75mA.
The next question is how much oversupply of PT current do you need. I'm not sure I can answer this but if you look at his Power Supply Design tab you will see that you need to allow for peak RMS ripple current versus load current. In the example he gives a ratio of 1.8 (not sure if this is a general factor or not). If you use this conversion factor then at 62mA load current you would need a transformer rated at about 112 mA. If you use the max value of 75mA you would need a transformer rated at 135mA.
The specs on the Hammond 290PAZ is for 140mA and that looks to be about right.
Hope this helps
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