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I want to bread board a pp 2A3 amp and to do so I need a heavy duty 2.5 volt filament transformer (10A) for the 2A3s. Just so happens I have a 5v/30A filament tranny that is center tapped. I hooked up my dvm to one side and the ct and when I brought it up on the variac it showed 2.5 vac.
Is this a legit way to get the filament voltage I need or am I missing something? Also, would I get the full 30 amps or only 15?
Thanks
Follow Ups:
Vinnie,
Are you breadboarding a stereoblock or only 1 channel?
Eli D.
Eli
I was going to do one channel first and then stereo if I liked what I heard.
He meant that if it is stereo, you can use both sides of the transformer. It is best to engage the full winding. Two 2A3 tubes in series is 5 volts. If your amp is stereo, this will work. Not so good as a mono.
Wouldn't two 2A3s in series still need 2.5 volts but 5A? I think he was saying I should use 1/2 of the tranny for each 2A3.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Are you saying if you wire the filaments of two 2A3s in series you will only draw 2.5 amps? Each one draws 2.5 amps doesn't it?
Yes.
All the parts in a series chain have the same amount of current flowing through them.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
So you could run six 2A3 filaments in series and only have to use a 2.5v/3A filament transformer?
No, 6 would take a 15 volt 2.5 amp transformer.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Very interesting. I guess its the same as with water, you need more pressure to maintain the same flow rate if you ad more hose length. Am I correct in thinking this is a result of I=E/R, where the R is increasing with each filament so you have to increase E to maintain I?
You've got it.
If you think in terms of watts, one tube...2.5 volts and 2.5 amps is 6.25 watts
Two tube in parallel....2.5 volts and 5 amps is 12.5 watts total, 6.25 per tube.
Two tube in series......5 volts (2.5 volt for each tube) 2.5 amps is 12.5 watts total, 6.25 watts per tube.
6 tubes in series.....15 volts (2.5 volts for each tube) 2.5 amps is 37.5 watts, 6.25 watts per tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Another one for the notebook. Thanks!
I'd like to see both sides of the 2.5-0-2.5 VAC secondary equally loaded. Put a resistor across the "idle" leg to simulate the presence of a 2nd 2A3. Also, put a resistance across the entire secondary to suppress regulation effects generated over voltage on the 2A3 filament.
If you are going to use that trafo for both channels in the finished project, a shared RC bias network connected to the CT is in order and hum balancing pots. are out.
Eli D.
I am afraid that is all Greek to me Eli. Anyplace that has diagrams of how all that is done? I understand the first half of your reply but you lose me after that.
Actually for now it will be temporary even with two channels. What size resistors are you thinking I should use?Also, I could just hook up the second 2A3 filament and have it balanced that way yes?
Edits: 11/24/14 11/24/14 11/24/14 11/24/14 11/24/14
Vinnie,
Generally most DHT tube amps or preamps use separate filament trannies. Then you can use a hum balance pot to null out any hum(to a certain amount). But because you are going to use both windings of the tranny with a common center tap(0) the balance pot is out and then you will have to incorporate an RC(resistor/capacitor) filter.
Or switch it to DC maybe? Thanks for the explanation. How would I calculate what I need for the R/C filter?
Thanks
Edits: 11/25/14 11/25/14
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