Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on November 12, 2015, 06:33:17 pm
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Im trying to find a different PT that will be suitable for my build. The one I am using now works just fine, however I believe it is overkill. Why change a PT? Price mainly. Weight possibly. But Ive been using my hammond 290mx. I dont want to leave that in the amp if I don't have to. This is the vox AC30 re-issue. The AC30 has A LOT more going on than my amp. I dont have tremolo or reverb. The mA rating on the 290mx is way higher than I need im sure. So I am hoping to save a few dollars if possible and replace it. I am also using a SS rectifier so theres that whole tap I dont need. Right now my output consists of four EL84's and the preamp/PI consists of 3 12ax7. I have an extra gain stage than the AC30. I dont know what all to take into consideration with my mA rating of the PT. I know the heater winding needs to be sufficient. And so does the mA of the secondary. But how to know what size.
My ASSUMPTION is this. Hopefully someone can correct:
EL84 heater is 760mA: 760mA x 4 = 3040
12ax7 heater is 300mA: 300mA x 3 = 900
900 + 3040 = 3940 = 3.94 mA
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EL84 plate current = 48mA: 48mA x 4 = 192
12ax7 plate current = 1.2mA: 1.2mA x 3 = 3.6
192 + 3.6 = 195.6 mA
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Ok so anyway this is just where I am with it. This may not be the way to go about it. Hopefully someone can point me in the right direction.
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I think that 290MX is perfect for your amp.
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Ah, I see. So my math was for just one half of triode?
I know the 290mx will work and be just fine. Im doing this for two reasons.
1. educational purposes. For when I am going to put an amp together in the future I would like to know how to size transformers accordingly.
2. Future builds like this amp. If I can find a suitable PT for 50, 60, or 70 bucks as opposed to the $100 of the 290mx, id like to do so.