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-   -   6SL7 instead of 6N1P in this schematic? (http://www.diyaudio.com/forums/construction-tips/205687-6sl7-instead-6n1p-schematic.html)

bradberry00 31st January 2012 04:57 PM

6SL7 instead of 6N1P in this schematic?
 
Hello,

I am planning a build around this schematic:

http://webpages.charter.net/porkchop/tubeamp/se_kt88-full%20schematic.jpg


I would like to use an octal tube in place of the 6n1p. If I replace the 9pin socket with an 8pin, would i have to do any other changes to the components (values, etc) to run a 6sl7?

thanks much,
-david

SY 31st January 2012 05:28 PM

6SL7 is a very different tube. You would probably want to use a higher value plate resistor (150-180k would be a good starting point), then adjust the 1k cathode resistors for a current of 1-1.5mA.

bradberry00 31st January 2012 06:14 PM

would a value around 3.3k-4.7k be a good starting point for the cathode?

SY 31st January 2012 06:35 PM

Quote:

Originally Posted by bradberry00 (Post 2885924)
would a value around 3.3k-4.7k be a good starting point for the cathode?

Let's make a couple of assumptions, then take a look at the datasheet. You want the plate voltage to be at least 150V, since the tube will have to swing all of the drive voltage to the output stage. We'll also assume you've got a 400V supply to work with. Arbitrarily choose 1.5mA as the current (we can change that, but it's a reasonable starting point). To end up with 150V on the plate, we will have to drop 250V. So the plate resistor will be R = V/I = 250/1.5 = 166k. A nearby easy-to-buy value is 180k, but we'll have to reduce the current slightly (I = V/R = 250/180 = 1.4mA).

OK, 150V on the plate, 1.4mA current. Now we can figure out the cathode resistor. Looking at the plate curves on the datasheet, we can see that it will take something less that -1.5V grid-to-cathode voltage to hit that operating point; call it -1.4V. So we can calculate the cathode resistor as R = V/I = 1.4/1.4 = 1k. So there's your rough starting point. As a check, draw a loadline on the plate curves for a 180k resistor passing through the quiescent operating point and see what the total swing is and how symmetrical it is around the operating point.

As a side note, you really, really want to get, read, and understand the book "Valve Amplifiers" by Morgan Jones. There are a lot of free informational sites on the web of varying degrees of reliability, but Morgan's book is reliable, up-to-date, comprehensive, and easy to understand, It will give you a really good grounding in how circuits and tubes are chosen and designed.

Jebem 31st January 2012 07:04 PM

Or you could try a 6SN7 instead, if you can accept less gain (about 15 times with this tube).
The 6SN7 is better suited to drive an output tube, as the anode resistor will be of much lower value than what you have to use if you choose a 6SL7.

bradberry00 31st January 2012 07:14 PM

I am going to have to get my hands on that book.

So do you think it would be a waste of my time using the 6sl7? I want to use the 6sl7 primarily for pure tinkering reasons, an attempt to learn more by not just copying the schematic. Plus lets be honest octal tubes just look cool.

If I replace the 47k resistors with 180k resistors and leave the 1k Cathode resistors would I get a good sound? or should I just stick with the 6n1p?

is there a sound difference between these two tubes? good or bad?

thanks much for the help thus far!
-david

bradberry00 31st January 2012 07:16 PM

Quote:

Originally Posted by Jebem (Post 2885981)
Or you could try a 6SN7 instead, if you can accept less gain (about 15 times with this tube).
The 6SN7 is better suited to drive an output tube, as the anode resistor will be of much lower value than what you have to use if you choose a 6SL7.

would this be a mostly direct swap? what components would need changing? Good sound?

SY 31st January 2012 08:01 PM

Tinker away, you'll learn something and probably cook a few cheap parts along the way. :D

"Good sound" is a very relative term! It's like asking if a girl is hot- it depends on what your tastes are and what you expect.

bradberry00 31st January 2012 09:40 PM

haha, i would like to avoid cooking too many things. I am trying to find datasheets to compare the 6sn7 and 6n1p...from what i read the 6n1p was designed to replace the 6sn7. Does this mean is is a smaller version of a similar tube with similar specs??

alternately, is there any 2 kt88 single-ended schematics that run a 6sn7 or 6sl7 that i could build from?

Jebem 31st January 2012 09:48 PM

The 6SN7 octal tube is closer to the russian 6N1P than the 6SL7.
The 6N1P has got a amplifier factor of 33, against 20 for the 6SN7.

You could start using the 6SN7 with the indicated schematic anode resistor (47Kohm) but I would use a 3 Watt value, just in case.
You will get the same 4.2mA of anode current as proposed on the original circuit and about 200Volt on the tube anode.

For the cathode resistor, change to a 1.8Kohm (1/2 Watt) to have about 7,5Volt of bias, that corresponds to the 200Volt / 4.2 mA operating point according to the 6SN7 anode curves.

However, make sure you wire the tube with the correct pinout (check the net for the 6N1P and 6SN7 pinouts).

Then, I would recommend some reading, as SY has suggested before, to find out how to design a class A single end circuit using tube plate curves diagrams to start with.

You will get a smaller gain from the 6SN7 (about 17 with the above resistor values), when compared to the original circuit with the 6N1P ( about 30), so this can be a problem depending on the source signal You will use to drive the amp. Check this to calculate the gain Calculating Valve Voltage Gain

Assuming we will need about 15Vpp to drive the KT88 in single end, we need a minimum input signal at the 6SN7 of 1V. So a CD player (output of 2Volt standard) will drive it without problems, as one pre-amplifier will do.

Now, an older system giving just like 150mV will not drive the KT88 to full power, so here the original 6N1P is superior and a better solution.

Or You can try the 6SL7 as you mention initially, but it will give you an amplifier factor of 70 (!) that will be excessive if you use a CD player or a preamp to drive it, as it may distort due to excessive signal (think that the 6SL7 bias is about 1.4V, so anything above that will put the tube well out of linear region.

Have fun!


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