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diyAudio Member

Join Date: Jun 2008

Hi tubes world!

I have a chance to work on a friends 45 amp. He wanted to see if I can rebuild it for him, so I took some measurement to see what I have to begin with. Anyway, I've also been trying to learn how to plot load line and when I try to plot a load line for the 417a I notice that it's not where I think it should be. The plate voltage seem high and the B+ seem high also, if I'm right the load line seem to be way down on the bottom of the curves so gain and linearity are not good.

So, please help me learn how to figure this out.

My plan is to load the 417 with a CCS set at 25ma, -1.5v on the grid B+ @ 220 and plate voltage of 140.
With these numbers I try to do some math and I got gain of 40 and plate dissipation of 3.4 watts.

If someone can plot these two plots for me so I can confirm if I got this right.

Thanks!
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__________________
Tim
" perfect imperfection "

 27th January 2014, 05:30 PM #2 diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6 I suspect 25mA is a bit ambitious, and tube life will be short from experience. I generally run 5842/417A at 10 - 15mA with excellent linearity. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan
 27th January 2014, 05:40 PM #3 diyAudio Member     Join Date: Jun 2008 Thanks Kevin, I thought keeping the tube from within plate dissapation limit it should be ok. No? __________________ Tim " perfect imperfection "
 27th January 2014, 05:49 PM #4 diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6 There is more to it than that, you want to consider tube life and also determine what is best for linearity over the intended operating range. If you have LTSpice (you can download it for free from Linear Tech) you can approximately optimize performance using a circuit model before you build. See the tube models sticky here for more details. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan
diyAudio Member

Join Date: May 2006
Location: Lancashire
Quote:
 Originally Posted by tim614 My plan is to load the 417 with a CCS set at 25ma, -1.5v on the grid B+ @ 220 and plate voltage of 140. With these numbers I try to do some math and I got gain of 40 and plate dissipation of 3.4 watts. If someone can plot these two plots for me so I can confirm if I got this right.
Your numbers are correct. But 25mA is indeed pretty hot for a valve rated only for 40mA.

 28th January 2014, 01:20 PM #6 diyAudio Member     Join Date: Jun 2008 As I was saying, I am trying to learn how to plot ( understand ) plate's curves. I though you need to calculate for linearity by aiming for the straighten part of the curves where it is spaced out evenly and as long as I don't go over the dissipation limit. Can you guys elaborate more on how to plot these curves for the 5842/417a for this amp? As for the measurement according to the schematic I posted, the 417a is @10ma but when I try to draw a load line using the measurement I have it's seem that the b+ and plate voltage seem high. As for gain. How much gain do I need to consider for this design? I was thinking of using 5687 instead but didn't think there is enough gain by itself. __________________ Tim " perfect imperfection "
diyAudio Member

Join Date: Jun 2008
417a operating conditions
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__________________
Tim
" perfect imperfection "

diyAudio Member

Join Date: May 2006
Location: Lancashire
Quote:
 Originally Posted by tim614 As for the measurement according to the schematic I posted, the 417a is @10ma but when I try to draw a load line using the measurement I have it's seem that the b+ and plate voltage seem high.
Your measurements and the loadline look pretty spot on to me... The supply voltage is indeed beyond the maximum rating given on the data sheet, although the data sheet figure (200V) is suspiciously low.
http://www.westernelectric.com/spec_sheets/417A.pdf

Quote:
 As for gain. How much gain do I need to consider for this design?
Your output tube is biased to 56V, so for maximum swing you need 112Vpp. Probably less in reality, as the output tube probably isn't biased dead-centre. Call it 100Vpp, or 35Vrms. If you want the amp's sensitivity to be 1Vrms then you need a gain of 35. I think you have about 40 at the moment, which is also fine (a bit more sensitive).
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 417aloadline.jpg (60.9 KB, 158 views)

Last edited by Merlinb; 28th January 2014 at 01:50 PM.

 28th January 2014, 04:16 PM #9 diyAudio Member     Join Date: Jul 2007 While the 45 is not an overly difficult tube to drive, with the current supply voltage, I would recommend increasing the cathode bias resistor to 1.74K. This will get the static current around 34ma which is much better for the 45 and plate dissipation will be at 10 watts (just at spec). The cathode/plate voltage will be pushing 300V (right around spec). I run my 45 SET amps with this biasing and they are very clean and will push 2.25-watts before clipping (I also use a grid resistor value of 249K and a 0.22uF coupling cap). Having said that, your driver requirements still are not trivial. Voltage swing at a minimum is 120 volts peak-to-peak (and you can easily use all of it). Adding a 25% margin to ensure linearity at the driver puts the requirement at 150 volts peak-to-peak. Divide this by the actual in-circuit gain of the driver (likely to be closer to 35) and the input signal at the 417a grid is ~4.3 volts peak-to-peak. Do the math here with some margin and you'll want around -2.75 volts of grid bias. Also look at the plate loading to ensure you can get 150 volts of swing while keeping it in a linear region. Regards, KM __________________ ... just because you're paranoid, it doesn't mean they're not after you...
 28th January 2014, 04:22 PM #10 diyAudio Member     Join Date: Jun 2008 Thanks alots guys! I recently got a book to learn about this stuff so I'm kinda fresh on this subject, I think I'm getting it as I see the explaination from you guys more clearly. I will study this so there will be more question on the way. __________________ Tim " perfect imperfection "

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