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Old 10th May 2010, 08:52 PM   #1
el156 is offline el156  Portugal
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Default Input stage

HI !
I have been spending lots of time to make the best input stage for my new amp! I want to use a 12AT7 (ECC81) ,and i need a plate voltage of 90V,because it is direct coupled with next tube (12B4A as phase splitter),and i want to have about -3v in the grid of the 12AT7, because i will use a CD player, an the output of it is about 2V. I have B+ of 280V! What value of plate resistor and cathode resistor should i use to get this values? And i would like to have plate current of about 3ma! Is this possible?
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Old 10th May 2010, 09:02 PM   #2
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How to "Screw Around" Your Tube Load Line

You can work out most stuff using Ohms law.
An anode resistor at 3mA dropping (280-90) = 190V will be:

R = V / I = 190/3E-3 = 63k

Now for a plate voltage at 3mA on an ECC82 lookup the load lines and locate Ia = 3mA and Va = 90V, so we get a Vg = 1V.

So to get 1V on the grid (actually -1V) you need a cathode resistor that gives 1V at 3mA, or R = 1/3E-3 = 330R.

Job done.
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Old 10th May 2010, 10:04 PM   #3
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Sorry to say but -3V and 90V is not gonna happen with ECC81, it is cut off.
I would suggest a more "low voltage tube" like ECC88/6dj8 where you could achieve a reasonable working point at -2 to -2,5 V on the grid with 90V on the plate.

/Olof
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Old 11th May 2010, 07:58 AM   #4
el156 is offline el156  Portugal
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Default Input stage

Quote:
Originally Posted by hemgjord View Post
Sorry to say but -3V and 90V is not gonna happen with ECC81, it is cut off.
I would suggest a more "low voltage tube" like ECC88/6dj8 where you could achieve a reasonable working point at -2 to -2,5 V on the grid with 90V on the plate.

/Olof
THANKS, that makes sense, but i need a higher gain tube for this input and working in class A all the time and no more than 90V on plate ! any suggestions?
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Old 11th May 2010, 09:31 AM   #5
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Use my post to calculate the operating point of an ECC88 - you just need the ECC88 load lines - google 'ECC88 datasheet'.

The ECC88 has higher gain than the ECC81 I think - by quite a bit.

ETA: Looked it up - you need -2.8V grid for 3mA at 90V, Rk = 2.8/3e-3 = 933 Ohms, pick 910R.

Last edited by Globulator; 11th May 2010 at 09:35 AM.
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Old 11th May 2010, 11:30 AM   #6
el156 is offline el156  Portugal
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Quote:
Originally Posted by Globulator View Post
Use my post to calculate the operating point of an ECC88 - you just need the ECC88 load lines - google 'ECC88 datasheet'.

The ECC88 has higher gain than the ECC81 I think - by quite a bit.

ETA: Looked it up - you need -2.8V grid for 3mA at 90V, Rk = 2.8/3e-3 = 933 Ohms, pick 910R.
Thanks Globulator!
I will make a few tests later on with ECC88, probably it is the best choice i have to do what i want! i will post the results! I was looking inside an AUDIBLE ILLUSIONS preamp, and they use ECC88 with parallel triodes with 22K to anodes
and 510R cathode to ground with 800mf parallel !
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Old 11th May 2010, 12:40 PM   #7
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Your other choice if you want to use an ECC81/12AT7 is to increase the plate voltage to accommodate the bias (2mA, BTW, is a good current for linearity) and then use a step network to couple to the next stage.
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Old 11th May 2010, 12:42 PM   #8
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Quote:
Originally Posted by Globulator View Post

The ECC88 has higher gain than the ECC81 I think - by quite a bit.
Other way around.

Problem with ECC88 is that you want to run fairly high currents to get decent linearity. 5mA minimum, 8-10 is better.
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Old 11th May 2010, 01:12 PM   #9
el156 is offline el156  Portugal
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Quote:
Originally Posted by SY View Post
Other way around.

Problem with ECC88 is that you want to run fairly high currents to get decent linearity. 5mA minimum, 8-10 is better.
Hi !
Yes,i would like to run ECC88 with higher current (about 5ma), but with grid bias
of about -3V ! How can i do that with Vb 280V and tha plate voltage cannot be more than 90V!Must i lower the Vb?
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Old 11th May 2010, 01:39 PM   #10
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Do not worry about that 90V anode V rating. I have run them safely for years with 150 anode V's, and none less than Tektronix have used even high voltages on them to military reliability. Go for it!

Regards, Allen
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