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Old 13th August 2008, 03:42 AM   #1
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Default page Eli Duttman's Simple p-p with 5965 MQ EXO-173 and 6AQ5

Hello Eli,

I was wondering if you or anyone else has experiemeted with the MQ EXO-173 phase splitter in your simple p-p amplifier design?

My thought was to substitute the 12AT7 driver with a 5965 and splitting phase with EXO-173 to a pair of 6005's.

Any ideas or help would be appreciated?

Norris
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Old 13th August 2008, 11:15 AM   #2
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hey-Hey!!!,
That choke( or more exactly half of it ) is a bit more load than I think a 5965 would be happy with. You have some options given your output tube choice. You can drive the choke with one section as a cathode follower. The 6AQ5's are not going to need a lot of swing and the cathode follower should be far more comfortable with the choke.

There is one more consideration, that choke seems to do best with a large coupling cap( ~5 uF ), so you may wish to avoid the use of such large caps and do phase splitting with a tube circuit. Again, the small drive requirements are working in your favour, even a split-load will work here( even with Class A B+ levels ).
cheers,
Douglas
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Old 13th August 2008, 03:08 PM   #3
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You're referring to the highish Rp of the 5965 (compared to, say, a WE 417a)? If so, I had my EXO-173s done with nickle with the understanding that this will open up use of drivers with comparatively higher Rp. So that might be an option for Norris. Another option would be to use the driver specified in the SETH design. IIRC the SETH uses a 3uf cap, not 5, but the point about a big cap in there is well taken. I will say, however, I built a SETH variation using a 417a --> 3uf --> driving 6550's and the amp received great reviews. I felt it was a very musical and powerful sound, it was my first venture into using a transformer phase splitter (well, autoformer) and it worked very nicely. Elegantly simple.
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Old 15th August 2008, 02:57 AM   #4
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Thanks Douglas, Eli and Steve,

Douglas:

I agree that a large value capacitor in the signal path would be a compromise, even 1uF seems large.

Looking at the Electra-Print EL84 p-p circuit using their SMA-2 splitter choke and 6J6 driver. I see that it only require a 1uF coupling capacitor. I was curious why they can get away with using a driver tube that has an Rp of 7.1k ohms, versus 1.8k of the 5842?
Where the 5965 has an Rp of 7k.

http://www.electra-print.com/el84psapp.php

Would using a quality coupling transformer like the Lundahl LL1660S for push pull with a differential driver stage be a better solution over the choke, or vacuum tube phase splitter with a SE driver to p-p?

If so, is there a circuit available that I might be able to easily adapt to use with the 6005?

Steve:

If using the MQ EXO-173 with nickle, what other driver tubes from your experience might work well with the 6005, but require a much smaller value of coupling capacitor than 3uF?

Thoughts from Eli about how to make the 5965 work with the EXO-173:

"Buffer the 5965 common cathode voltage amplifiers with DC coupled IRFBC20 MOSFET source followers. Build a reasonably robust B+ supply and run the FETs with 10 mA. of drain current. Believe me, the O/P impedance will be LOW."

And finally, would anyone of you have suggestions of the best way to split phase, either with iron, vacuum tubes, differential, or SE to p-p?

Any and all suggestion will be appreciated.

The simpler the better for my knowledge level.

Thanks

Norris
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Old 15th August 2008, 10:35 AM   #5
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Norris,
The inductance of that half of the coil sets the coupling cap size. Then the tube has to live with that low( numeric) load at LF. I think the ability of triodes to live with eliptical, reactive load lines is a bit overstated...they just sound better when the line is straight.

I use 1.5 uF coupling caps in my big amp and it has a balanced input to the grid choke. That choke is ~700 Hy and has a gap so that the inductance does not vary too much with signal. Each phase works into ~350 Hy. My driver/LTP is a 6H6 cascode with 15k plate loads. I'd rather go for 10k but that would put my OPT even further over the edge in terms of idle current through the primary( it is E-Linear rigged ).
cheers,
Douglas
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Old 15th August 2008, 10:48 AM   #6
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By the way guys. Should this thread not be called Eli Duttman's El Cheapo with 5965 and MQ EXO-173?
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Old 15th August 2008, 12:41 PM   #7
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Thanks Douglas,

I will study your amplifier arrangement and see if I can figure out how to impliment it to my needs.

Bas,

Yes, I thought about that after I posted, but did not have the edit option available. Or, did not figure out how to execute it.

Eli Duttman's amplifier was the inspiration for this thread.

Could you change the title for me, or tell me how to edit it myself?

Duh!

Thanks

Norris
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Old 15th August 2008, 12:45 PM   #8
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@Norris:
Thanks for your reply. I've changed the title.
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Old 15th August 2008, 12:50 PM   #9
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Norris,
One more thing, the balanced drive to the grid choke does better without an interleaved choke. The additional capacitance or perhaps the coupling itself works against you. Seemed counter intuitive( matter of fact so counter I went to the trouble of doing the interleaved experiments and found the negative result ). The 173 iron phase splitter has not enough inductance in half its coil to be a pleasant load just in case you needed another issue to overcome.
cheers,
Douglas
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Old 15th August 2008, 12:55 PM   #10
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The thing is. Once you start using something like the MQ EXO-173 it won't be an El Cheapo anymore. Or does he have another design with the EXO-173 somewhere? Anyone have a link?

But ofcourse the EXO-173 is one way of keeping the phase splitting simple.

And Douglas is correct that you'd need something strong to drive the choke or a transformer for that matter. I noticed John Atwood's Artemis Labs DP-2 amp using a KT-88 for that. Since I saw that I've been thinking of driving my EXO-173's with a KT-88 as well.
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