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Old 24th January 2012, 01:57 PM   #1
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Default Line output tube

Hello,
having a line output trans with 15K:600 Ohm and max. Ia=8mA ,I wonder what tube will be best to use with that trannie.
I would like to use 6085 or 6DX8, 6072 but no one of those seems to be the perfect match.
Dont know the exact working point of 6Dx8 either.
Will it work with one of the above or are there better solutions?
It should be noval socket and preferable high quality long live tubes, please.
Thanks,
Uwe
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Old 24th January 2012, 04:12 PM   #2
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I don't really know of any potentially suitable novals other than possibly the 6CG7/6FQ7, in octals the 6SN7, 6J5, 6C5 might be worth a look, in the DHT world 01, 26 and 12 all come to mind. All of this of course depends on having sufficient primary inductance for good bass response down to 30Hz or so. What transformers do you have?
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Old 24th January 2012, 04:15 PM   #3
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The 8 ma are DC or AC??
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Old 24th January 2012, 04:21 PM   #4
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Quote:
Originally Posted by Osvaldo de Banfield View Post
The 8 ma are DC or AC??
The 8mA may be the DC rating but often must include the peak AC current as well. I have HA-133 rated at 8mA primary current which I run at 6mA quiescent, and the maximum audio current is much less than the 2mA of headroom provided. Running at 8mA results in audible distortion with these transformers as they start to saturate on LF (bass) current peaks.

It depends heavily on how the manufacturer specified the primary current. Sometimes it is just static DC current, and they have built in what they think is a reasonable margin for the AC component on top of this, while others specify a combination of DC + AC primary current. Unfortunately this is not always clear from the data sheet. You'll know if you are running it too hard.
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Old 24th January 2012, 05:03 PM   #5
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Thatīs OK, why donīt try a stage that doesnīt put any DC in the primary of the Xformer, like a WCF or SRPP?
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Last edited by Osvaldo de Banfield; 24th January 2012 at 05:09 PM.
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Old 24th January 2012, 07:07 PM   #6
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It is UTC LS-27 and 8mA is max. unbalanced DC in primary, so how about using an 6085 tube? Its minimum Ri is 7K and it is not so far away from 6CG7 but sounds better to me. And 6072 should be usable with Ra=20K, so its a little under the rating but 15K should work fine? Its a five star tube, too with low noise. Which one of those will fit better or other suggestions out there?
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Old 24th January 2012, 08:20 PM   #7
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Hi

I would suggest tubes with low internal resistance. This will ensure a good -3 dB point at both the frequency extremes.

You need a LOT of inductance to ensure good bass response with 5 .. 10K internal resistance tubes. This leads to a large number of turns which gives problems at the higher frequencies.

But it is possible, even at Rint=10K when using very large cores.

grz, //WDC
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Old 24th January 2012, 08:38 PM   #8
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Sorry, but I dont know the inductance of those LS27 but they maybe have a big core with loads of inductance?
So it is preferable to have a lower resistance valve like 6DX8 pentode triode wired (as Pentode Ra= approx 12K but as triode possible lower, is that right?
But as I have no chart of this noval tube it is not shure if running with 6mA current it is in the linear region. As it usual have Ia= 18mA as Pentode I think this could the curved region of the triode mode.
How about 6G6 if there is a noval version of this tube. It has 5K =Ri and pretty linear curves as pseudo triode.
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Old 24th January 2012, 08:44 PM   #9
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LS-27 has something like 200H primary inductance. This makes it possible to use a high-Ri tube like 6085 or the DHTs spoken of above. Still I wouldnīt use it due to its mediocre linearity. You will get better results with a run-of-the-mill tube like E88CC.
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Old 25th January 2012, 07:15 AM   #10
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Have a look at DHTrob’s EF6/EF37A linestage. Maybe that could be a candidate?

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