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Input stage

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el156

el156

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Joined 2010
#1
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?
Thanks
 
#2
Check out this link:

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.
 
#3
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
 
#4
hemgjord said:
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
Click to expand...

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?
Thanks
 
#5
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.
 
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#6
Globulator said:
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.
Click to expand...

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 !
Thanks
 
#9
SY said:
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.
Click to expand...

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?
Thanks
 
#11
You can't. The plate voltage will certainly have to be higher, probably 120V or so. Again, you have to consider how that fits into the overall design- either rework the following stage or use a step network to couple.

edit: The max rated plate voltage is 130V according to the Philips datasheet. So you're fine at the higher voltage.
 
#12
Allen Wright said:
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
Click to expand...

HI !
My problem is that i cannot have more than 90V on plate ,because the next stage is a concertina phase splitter with 12B4A, and the plate of the ECC88 connects directly to the 12B4A grid !
And how can i do that with -3V on the grid of the ECC88?And no more of 90 V on the plate with VB of 280V?And with current plate of 5ma ?
Thanks
 
#14
el156 said:
HI !
My problem is that i cannot have more than 90V on plate ,because the next stage is a concertina phase splitter with 12B4A, and the plate of the ECC88 connects directly to the 12B4A grid !
And how can i do that with -3V on the grid of the ECC88?And no more of 90 V on the plate with VB of 280V?And with current plate of 5ma ?
Thanks
Click to expand...

And with a step network, i will loose some gain,wich is something i dont want!
I need the highest gain i could get of the ECC88!
Thanks a lot
 
#16
Get anode curves on data sheet and draw load lines !

At first, if you target 5mA and 90V with 280VB, you must have (280 -90=) 190V accross the load resistor so that its value must be (190 / 5=) 38 Kohms.

All that remain to do is to look at the datasheet to find wich bias voltage will allow the tube to pass 5mA with 90V on its anode.
On the last page of this:
http://frank.pocnet.net/sheets/030/e/ECC88.pdf
you can see that this occurs with -2.5V on the g1, or, if it is grounded, +2.5V on the cathode that you will obtain using a (2.5 / 5=) 500 ohms resistor btwn cathode and ground.

Yves.
 
#17
SY said:
No. If you bypass the top resistor and have the resistors sized in the 1M range, the gain is nearly unchanged.
Click to expand...

HI again !
How about a different tube? To meet this purpose wich kind of tube would be the best for this?Considering that i really want no more than 90 V plate voltage,good linearity,and grid bias of about -3V ?
Thanks again
 
#18
You can't have butter and money for butter !

Either you follow the SY suggestion to allow you freely choosing the "best" plate voltage for any tube, either you choose one linear enough at 90V (ECC88 is a good candidate) and trim the values around it including grid bias as explained above 😎
None of the classical '81, '82 or '83 are able to work correctly at such lo plate voltage.
However some designers did that with an '83 to obtain a "caricatural" tube sounding amplifier . . . not my personal taste, but . . .

Yves.
 
#19
Yvesm said:
You can't have butter and money for butter !

Either you follow the SY suggestion to allow you freely choosing the "best" plate voltage for any tube, either you choose one linear enough at 90V (ECC88 is a good candidate) and trim the values around it including grid bias as explained above 😎
None of the classical '81, '82 or '83 are able to work correctly at such lo plate voltage.
However some designers did that with an '83 to obtain a "caricatural" tube sounding amplifier . . . not my personal taste, but . . .

Yves.
Click to expand...
Thanks again!
OK ! I think if we dont have a dog,we go hunting with a cat ! I will make some tests, and i will have a go with EF86 triode connected!
I saw some designs where 60V plate voltage was used!
Probably is a good choice except for microphonics!
Thanks you all
 
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