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Old 1st December 2007, 05:07 AM   #1
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Default Use for 5K:8 PP OPT?

I have a pair of 5K:8 push pull OPTs, and I am wondering if people have output tube recommendations for them. I think they were probably designed with 2a3's in mind, but I am too cheap to buy 4 of those. I thought about running them 20K:32 as a headphone amp with 6H30's, but it seems like a silly use, and I could frankly use a real power amp for once. Maybe 6AS7's? Any better ideas?
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Old 1st December 2007, 05:38 AM   #2
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Hi Dsavitsk

If you are going for the cheap tubes I would recommend you give a look at the sweep tubes (horizontal deflection in TV sets, if I am right). There have been several threads discussing amps with those, including a recent thread by Miles Prower using the 6BQ6GTB.
A Tee-Vee T00b Amp
He is using a 4k4 P2P OPT, so 5k should be fine as well.

These tubes find different uses: some triode them and use them low and hot (because G2 is generally rated for values up to 200VDC). Other (as Miles) use them in penthode mode, where one can keep G2 under the max.value. Still others use to apply the signal straight to a slightly negative biased G2, having both G1 and G3 grounded, aiming for a very linear class B operation (SY is an example). Download a good datasheet of a 6JN6 and look at the curves for G1 grounded (=0). THe curves are for the values of G2, in the left axis you will see the current draw through the plate, on the right (generally) the current drawn by G2. As you see the signal must be quite 'healthy': large amplitude fed from a low output source (cathode follower). www.davidberning.com has built several amplifiers with this configuration. Still other (Tubelab) do something similar, but bias G1 slightly negative, keeping G2 positive. The advantage of the last two circuits are large power with low dissipation in the tube, assuring a longer lifetime. In one of Berning's amplifiers he is even using only 5.7VAC on the 6.3V filaments of the 6JN6.

You see, lots of applications for those chaep tubes, and I have not seen anyone complaining of sound quality in any of the above mentioned configurations. I have not built any of them (yet), but that will come about after finishing other projects: I already collected a stash of 25DQ6B's and 22JF6's (not the everyday tubes, diyaudio search would probably not bring up much results).

Still, the 6AS7 sounds as a good idea to! Shoog has built a quite simple, completely differential, two stage, amplifier with them, with apparently quite good results.

Erik
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Old 1st December 2007, 05:45 AM   #3
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5K is way too high for a 6AS7.

4 x 6FD7 dual triode for a complete 8-9W stereo amp
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Old 1st December 2007, 05:56 AM   #4
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Yes, you are right Geek. When I was talking about shoog's amplifier I had already forgotten the 5k figure. Still, the 'maurits' amplifier by van der Veen uses a 8k P2P trafo as output tranny. I remember asking about the amplifier here at diyaudio and, I think it was positron, recomending to stay faaaaaaaaar away from it.

Erik
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Old 1st December 2007, 05:57 AM   #5
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5k P-P is an ideal load for EL34s in triode-mode AB1 PP with 400v plate-cathode voltage. If you want to use cathode resistor or CCS bias, you need about 440v B+.
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Old 1st December 2007, 06:10 AM   #6
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Thanks for the ideas.

Quote:
Originally posted by ErikdeBest
recent thread by Miles Prower using the 6BQ6GTB.
This looks really interesting, and looks like there is lots for me to learn as well. Plus, I have a few boxes of 12AU7's, 6sn7's, and 12AT7's that might get put to some good use here.

Quote:
Originally posted by Geek
5K is way too high for a 6AS7.
There is a possibility that this could end up being used with some 4 ohm speakers. Maybe it's more reasonable there?


Quote:
Originally posted by ray_moth
5k P-P is an ideal load for EL34s in triode-mode AB1 PP with 400v plate-cathode voltage. If you want to use cathode resistor or CCS bias, you need about 440v B+.
Seems too easy, and EL34's still aren't quite free. But, maybe worth a look.
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Old 1st December 2007, 07:26 AM   #7
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Hi,

Quote:
Originally posted by dsavitsk
There is a possibility that this could end up being used with some 4 ohm speakers. Maybe it's more reasonable there?
I think they loadline around 1.3K. So parallel 4 ohm speakers.... dipole, bipole, whatever they're called
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Old 1st December 2007, 11:44 AM   #8
7N7 is offline 7N7  United Kingdom
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Hello.

I think that I would use them with 829B or 5894 (dual RF beam tetrodes) paralleled and triode strapped. Ra of these is about 1k (I have some triode curves if you are interested) so I would think that your 5k transformers would be ideal.

7N7
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Old 1st December 2007, 12:49 PM   #9
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How about a pair of 6146B? They are cheap and could give about 100W.
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Old 1st December 2007, 06:25 PM   #10
DougL is offline DougL  United States
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What about the 6AQ5?
This was the tube that was substituted for a 6B4G, and gas a couple of threads going. Last I looked, they were $4 each.

As for the 6AS7, having a higher Primary that normal gives lower power and better distortion. The question you need to ask is: Can you live with a quarter of the normal power? If you can, its a perfect fit.

Good Luck,

Doug
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