6BL7 as output tube for low power SE amp? - diyAudio
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Old 12th November 2004, 03:56 PM   #1
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Default 6BL7 as output tube for low power SE amp?

Anyone ever tried it? It looks like a couple of watts could be had from this tube working into a 5K load at 50mA per side with 14 volts at the cathode. Just curious.

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Old 12th November 2004, 04:45 PM   #2
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I've used them for driver cathode follower tubes and the similar 6BX7 has been used as an output tube. I don't have my data sheets here, but I recall the input capacitance of the 6BL7 as being pretty high.
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Old 12th November 2004, 04:58 PM   #3
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Hey SY! Long time. I checked the specs and it has less input capacitance than a EL84. Hold on. I see what you mean. I was looking at the wrong spec. So you would need a high current driver tube to use this as a power tube huh?
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Old 12th November 2004, 05:02 PM   #4
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You would indeed need to be able to swing a lot of current really quickly. A mu follower or CF might do the trick, but it won't be a trivial design task.

The Raiders are embarrassing.
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Old 12th November 2004, 05:52 PM   #5
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Quote:
Originally posted by SY
You would indeed need to be able to swing a lot of current really quickly. A mu follower or CF might do the trick, but it won't be a trivial design task.

The Raiders are embarrassing.

Don't rub it in Sy. I already walk around on Sundays with a bag over my head.
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Old 12th November 2004, 06:30 PM   #6
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Default 1-7

I'm a '49ers fan. How do you think *I* feel?
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Old 12th November 2004, 06:51 PM   #7
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Default Re: 1-7

Quote:
Originally posted by SY
I'm a '49ers fan. How do you think *I* feel?
Good point.
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Old 12th November 2004, 07:19 PM   #8
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*Checks data sheet* Looks like about 7k (per triode; 3.5k for a tube in parallel) will give 1.7W output (3.4W for both triodes), off 300V 20mA (40mA total). Just a SWAG loadline, YMMV.

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Old 13th November 2004, 05:31 PM   #9
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In trying to figure out the frequency response of a amp consisting of a 5687 gain stage with a gain of 14 and a output impedance of 1.64K driving a output stage consisting of a 6BL7 with a plate voltage of 360 and a current of 50mA per side operating into a 5K output transformer I am running into a lack of knowledge wall. I would like to calculate where the highs would rolloff in the circuit. Since I will be using a driver tube with a low output impedance and low gain factor I think the circuit may work in spite of the highish input capacitance of the 6BL7. If I can get a 22Khz signal out of the amp I will be happy. Any help would be appreciated. The associated tube data can be found here:

http://www.mif.pg.gda.pl/homepages/f...7/5/5687WA.pdf

http://www.mif.pg.gda.pl/homepages/f.../6/6BL7GTA.pdf
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Old 13th November 2004, 08:33 PM   #10
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You have three major poles: miller C of 5687, miller C of 6BL7 and capacitance of OPT. And anything else.

In the first case, F = 1 / (2*pi*Z*Cg-p*(5687gain+1) ), in the second, F = 1 / (2*pi*Rp*Cg-p*(6BL7gain-1) ), where Z is input impedance (grid leak, source impedance, etc.), Cg-p is grid to plate capacitance, gain is the actual gain (from grid to plate) of that stage, and Rp is the Zo of the 5687 (as you might've guessed).

1.64k into *1nF* is negligible.

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