Correct termination of a 6SN7/LL 1660 combination?

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Correct termination of a 6SN7/LL 1660 combination?

Postby cpdoc » Sun Jun 23, 2013 12:00 pm

Hi folks,
I play the above combination in a 300 B-Xls amp. What will be the right termination of LL 1660 to correct the mismatch? In divers forums you find values from 10K to 1ooK or even no load. So what is the ultimate value?
A second question: since I use the LL1660 there is a loss of 20 Hz in deep bass instead of former 25 Hz I get only 45 Hz (-3dB). Any idea why?
Wolfgang
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Re: Correct termination of a 6SN7/LL 1660 combination?

Postby Doc10 » Sun Jun 23, 2013 3:21 pm

LL1660 is a very good transformer. Termination is only needed when you have ringing or overshoot. You can test that with a 1k block wave. You may not have ringing when the driver tube is low enough impedance.

Which type LL1660 are you using? How my mA? You need to know the inductance, because that plays together with the drive tube's plate impedance.

When a 6SN7 has problems drive an interstage, you can just parallel two of the triodes inside one glass. Such a solution is nice and elegant. This will give half the impedance, that helps in many ways. You end up with 2x the number of 6SN7, but the tube is not expensive. That will bring you from 45 Hz@ 3db to 22Hz at 3dB. This is still not very good. You need 20Hz at 1dB. If you can post a circuit diagram, it is easier to understand the situation. At least you have to say how you connected the 1660? So from the datasheet, what "Alt". Problems can have another reason as one expects, so the more detailed the question, the better it is.
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Re: Correct termination of a 6SN7/LL 1660 combination?

Postby cpdoc » Sun Jun 23, 2013 5:37 pm

Hi, thanks for the quick reply. Since there is no circuit diagram existing I will describe in writing. One system of CV 181 (6SN7) is driven directly by a CDP. Plate voltage 250 V, Ik 9 mA, Rk 1K, Ck 220µ bridged with 22µ MKP. Into LL 1660-25mA (may this is too high) Alt S, 4:4.5. Into EML 300B-XLS at 400 V 80 mA, same Ck as before. At the moment I use the amp without termination of LL 1660 and treble and mids are coincident with my taste of listening. The only prob seeems to be the limited bass. Before I used a Mundorf Ag/Au/Oil 0.47 µ for coupling and the measurements of what comes out of my TML's (fu 25 Hz at -3db) showed 28 Hz. Now it's as mentioned only 45 Hz. So some help will be appreciated.
Best
Wolfgang
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Re: Correct termination of a 6SN7/LL 1660 combination?

Postby Doc10 » Sun Jun 23, 2013 7:52 pm

I checked the 1660 datasheet. http://www.jacmusic.com/lundahl/datasheets/1660.pdf

The 10mA Version is specied with Alt S connection: 25Hz @-1db when driven with a 14k source. It has 130H in Alt S.

You use the 25mA version so that has only 130/2.5 = 52 Henry. So you need a source of 14k/2.5 = 5k6 to get 25Hz @-1db.

The 6SN7 itself however is 7k7 at 250V plate, 9mA. Add to this, any load on the secondary side, it will be transformed to the primary also, and the 6SN7 has a harder job even.

So what you should try, use no secondary load. The output winding can be used directly to drive the 300B grid, you need no resistors there. So one end of the secondary to ground, and the other end to the 300B grid.

Still that is not enough, because you need 5k6 MINIMUM . For that connect two systems of the 6SN7 in parallel, and you are ok. I recommend you to give each tube it's own Rk of 1000 Ohms. So the cathodes are not parallel. Only the grids and anodes. That delivers 18mA into your LL1660. Even you can try it with three in parallel. In that case 3x 8,3mA will give you exactly this 25mA for the LL1660k, and frequency range will be even better than the LL1660-10mA driven by just one 6SN7 triode. You should it with two 6SN7 in parallel, probably that works very good already.
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Re: Correct termination of a 6SN7/LL 1660 combination?

Postby cpdoc » Sun Jun 23, 2013 9:13 pm

Thanks for the advice and especially for a little simple math that makes it clearer to me than only "do that and that".
Will take a while but I'll post the results.
Wolfgang
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Re: Correct termination of a 6SN7/LL 1660 combination?

Postby goetz » Thu Oct 31, 2013 6:21 pm

cpdoc wrote:Thanks for the advice and especially for a little simple math that makes it clearer to me than only "do that and that".
Will take a while but I'll post the results.
Wolfgang

We are waiting for the results, Wolfgang ! ;)
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Re: Correct termination of a 6SN7/LL 1660 combination?

Postby cpdoc » Fri Nov 01, 2013 6:08 pm

Sorry,that I have forgotten to display my results. The advises were helpful. I now use both systems of the CV 181 in parallel with 8.5mA/250V/-8V each (so LL 1660-18mA shall be more correct than the actually used LL 1660-25mA).Deep bass is back, treble is detailed and not too crystalline, voices (mids) are well pronounced. As often cited in literature the CV 181 (6SN7) cannot sufficiently drive a 300B (XLS) though it is pretty loud. Since this amp was an experimental one to get an impression what a 300B amp sounds like I will install an additional transformer loaded 300B stage and see what happens. Again thanks for the help in this qualified ( in comparison to others) forum.
Wolfgang
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