When choosing a PT, is it the "Zero Signal" plate current that you need to consider.?
Just for example.....for a 6V6, seems like zero signal Ia is 70 ma. So the PT needs to supply 70 ma for every 6V6 in the amp.? Is that correct.?
Thank You
PT Specs
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Cliff Schecht
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Re: PT Specs
Each tube idles at a certain current. This means that in a push pull pair, 70 mA idle for a pair of 6V6's is 35mA a piece.
Cliff Schecht - Circuit P.I.
Re: PT Specs
Sorry Damnit.....yeah, value is for TWO Tubes. OK thank you, now I know what PT to look for.Cliff Schecht wrote:Each tube idles at a certain current. This means that in a push pull pair, 70 mA idle for a pair of 6V6's is 35mA a piece.
Thanks Again
Re: PT Specs
You gotta add the screens and preapm draw too, with a fudgefactor ...
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Cliff Schecht
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Re: PT Specs
Yes sorry I did neglect to mention other currents to add up. When spec'ing a transformer you have to look at the current draw of each branch (i.e. each separate gainstage is a branch). Each 12AX7 side draws about 1.5 mA, so 3mA per 12AX7 (phase inverter draws more current, maybe 5-10mA), then screen currents and plate currents. If you are still confused we can clarify further..surfsup wrote:You gotta add the screens and preapm draw too, with a fudgefactor ...
Cliff Schecht - Circuit P.I.
Re: PT Specs
No, you need to consider total maximum current draw, plus whatever extra you want for headroom and reliability.hired hand wrote:When choosing a PT, is it the "Zero Signal" plate current that you need to consider.?
Unfortunately, it's not quite that simple.Just for example.....for a 6V6, seems like zero signal Ia is 70 ma. So the PT needs to supply 70 ma for every 6V6 in the amp.? Is that correct.?
Thank You
The 70mA rating on the data sheet is for 2 tubes at maximum signal.
But then there's a few mils for screens and the preamps, driver and inverter to take into account.
The 70mA rating on that Princeton PT is possibly due to the output section not pulling the full rated capability of the tubes.
The Deluxe on the other hand puts out a bit more power from the same tubes but has a larger PT.
Only in the case of a true class A design is the zero signal draw the same as maximum signal.
rd
Re: PT Specs
Hey Guys.......
Appreciate the info. I understood about the pre tubes, forgot about screen current though. I was mostly confused about the power tube draw. I have a horrible memory, and should have known better. I just plain forgot about specs for push-pull being for two tubes. Not to mention, it says TWO TUBES right in the RCA Manual..!!
At any rate......I am all set now. Could not have done it without you. I hate to say it, because I am guilty of it so often.....but the Internet makes it real easy to run for help, before you have tried to "figure it out for yourself".
I know there are a lot of smart, educated people here, and I fall back on that too quickly sometimes. It is a fascinating topic (amps/electronics) and I am eager to learn. So I hit the keyboard pretty quick sometimes.
Thank You So Much
Chip
Appreciate the info. I understood about the pre tubes, forgot about screen current though. I was mostly confused about the power tube draw. I have a horrible memory, and should have known better. I just plain forgot about specs for push-pull being for two tubes. Not to mention, it says TWO TUBES right in the RCA Manual..!!
At any rate......I am all set now. Could not have done it without you. I hate to say it, because I am guilty of it so often.....but the Internet makes it real easy to run for help, before you have tried to "figure it out for yourself".
I know there are a lot of smart, educated people here, and I fall back on that too quickly sometimes. It is a fascinating topic (amps/electronics) and I am eager to learn. So I hit the keyboard pretty quick sometimes.
Thank You So Much
Chip