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Old 19th October 2013, 12:17 PM   #1
RollE2k is offline RollE2k  Sweden
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Default Higher gain twin triode!

I'm going to build a ES9018 DAC/Headphone amplifier, with Parafeed 6n6p output stage, and i have been thinking about using Twisted Pear Legato 3.1 as i/v converter - but then i started thinking that why not use tubes to increse the gain instead.

Since the DAC output have to be loaded really low (<1R) i have to have tubes with quite much gain. Will run the hole thing balanced, so there i will at least have a Little bit help.
But i was first thinking to use ECC88 as i/v gain tubes, but i seem to get a Little bit to low gain with these - so i started thinking. is there any suitable TWIN TRIODE with a bit higher gain than ECC88, that is still quite linear? ECC83 and those will of course not be an alternative since they just never sounds good.
Since i will drive headphones i will have 18:1 OPT, which is why i will need the gain.

Any help would be appreciated!
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Old 19th October 2013, 01:45 PM   #2
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Scope out the 12AY7.
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Old 19th October 2013, 02:10 PM   #3
RollE2k is offline RollE2k  Sweden
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12AY7 is one alternative - didn't think of that one. But another 10-20x gain wouldn't hurt. Any other ones?
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Old 19th October 2013, 02:30 PM   #4
SY is offline SY  United States
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A 12AX7 used in a circuit designed properly will be extremely linear, one of the best I've ever seen. Don't discount it just because many people have designed poor circuits with it.

You might also check the 6AQ4.
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Old 19th October 2013, 03:02 PM   #5
RollE2k is offline RollE2k  Sweden
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I actually have never heard any circuit with 12AX7 that i liked, and i dislike strong feedback, so no thanks to that tube. I rather use transistors than those tubes. But thanks for the tip anyway.

Last edited by RollE2k; 19th October 2013 at 03:09 PM.
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Old 19th October 2013, 05:17 PM   #6
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SY is right about the 12AX7/ECC83. You will use transistors to get every last drop out of the type. CCS load each section at 900 μA. Don't bypass the cathode bias resistor. DC couple ZVN0545A source followers to the anodes, which decouples the triodes from the driven load. Betcha the composite block sounds damned good, while providing "gobs" of gain.

The 'X7 triode is a WIMP! It can't directly drive any sort of load. The technique outlined above deals with the type's deficiencies and maximizes its strengths.

If you remain unwilling to use the 'X7, the same sort of scheme can be employed with the 12AT7/ECC81. The 'T7 triode performs well with 200 to 220 V. on the plate and an IB of 3 mA.
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Old 19th October 2013, 05:24 PM   #7
RollE2k is offline RollE2k  Sweden
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I will cascode ccs the circuit with two DN2540's per anode, and also it will be differential stage with common cathode bias resistor (without cap) to cancel a little bit of distortion.

didn't think of the 12AT7 - may be an alternative, is there any russian equivalent for that one? and wouldn't 6N2P-EV be better than 12AX7 if walking that path?
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Old 19th October 2013, 05:35 PM   #8
SY is offline SY  United States
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Quote:
Originally Posted by Eli Duttman View Post
Don't bypass the cathode bias resistor.
Agree with everything else, but my measurements indicate that keeping the cathode firmly at AC ground drops out all harmonics but second and a minuscule amount of third, and the second is already ridiculously low. I used a LED with a "helper" resistor from B+ to boost the current through it to 6mA or so.
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Old 19th October 2013, 05:42 PM   #9
oshifis is offline oshifis  Hungary
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This topology is well suited for balanced I/V:

Laszlo's Valve Output Stage with Lundahl transformer
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Old 19th October 2013, 05:54 PM   #10
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Remember, a differential gain block fed by a SE signal can produce only 1/2 μ of gain. Also, CCS loading of the LTP tail is "normal".

An unbypassed cathode resistor linearizes the triode, via local current NFB. Between local current NFB and a horizontal load line, SE 'X7 or equivalent triodes will be extremely linear. For all intents and purposes, the triodes in the 6H2П-EB (6n2p-eb) are equivalent to those in a 12AX7. The internal shield might be beneficial.

2700 Ω cathode bias resistors look good to me. The TO92 case version of the DN2540 will be fine at 900 μA.
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