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Old 22nd February 2009, 02:49 PM   #1
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Default Phase splitter which tube?

Hello to all

I would have to choose between some valves for a phase splitter
(For directly drive a couple of EL84 or EL34)

In Long Tail Pair it is better a hi mu or a low mu triode ?

12AT7
(Plate 250 V, 10mA)
u = 60
Ri = 10k Ω
gm = 5.5 mMho

12AU7
(Plate 250 V, 10.5mA)
u = 17
Ri = 7.7K Ω
gm = 2.2 µMho

12AX7
(Plate 250 V, 1.2mA)
u = 100
Ri = 62.5K Ω
gm = 1.6 µMho

Perhaps it would be better a low mu and low plate resistance as 12AU7 ?


Thanks.
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Old 22nd February 2009, 03:50 PM   #2
owen is offline owen  United Kingdom
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Simple answer, none of the above.

The challenge of phase splitting is the variation of output impedance between anode and cathode produces all sorts of drive/slew related harmonics, pushing distortion up.

Much better (Imo) to have a unity gain splitter (any of the above), feeding a pair of either anode followers, so that the impedances match up.

This is but one reason why a Transformer phase splitter is very handy

Have fun



Owen
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Old 22nd February 2009, 04:14 PM   #3
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by owen
Simple answer, none of the above.

The challenge of phase splitting is the variation of output impedance between anode and cathode produces all sorts of drive/slew related harmonics, pushing distortion up.

Much better (Imo) to have a unity gain splitter (any of the above), feeding a pair of either anode followers, so that the impedances match up.

This is but one reason why a Transformer phase splitter is very handy

Have fun



Owen
He was actually talking about a long tail pair which with a CCS on the bottom is very symmetrical in terms of source impedance and ac parameters. I would tend to choose a type with a relatively low rp for this sort of service, of the types listed either the 12AT7 or the 12AU7A could serve. The 12AT7 will give you about 3X the overall voltage gain of the 12AU7A, neither type IMO is the best in terms of linearity but in PP service all of the even harmonics are largely cancelled - I think the 12AT7 might have a slightly more desirable spectra in this application. (Eli and SY both think highly of this tube for this sort of service, good enough for me IMHO.)

Another very interesting type to consider for this application, particularly with EL34 where the drive requirements are greater is the 5687, rp would be much lower as well - useful in particular if you are contemplating PPP triode connected EL34..

For a very interesting discussion of the problems you can encounter using IT in PP amplifiers see this thread: A pair of Visaton RHT 12S Ribbon tweeters for sale Note also that even with a single IT if you intend to use negative feedback in this amplifier design getting good stability with meaningful levels of feedback and a "bogey" load is going to be difficult. ("bogey" in this case = any typically random speaker load)
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Old 22nd February 2009, 04:37 PM   #4
SY is offline SY  United States
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12AU7 has terrible linearity, so I wouldn't use it outside of guitar amp service. If you want a medium mu LTP, 6SN7 or 6CG7 are much better choices.

12AX7 has issues with any loading heavier than a CCS, and the high mu means high Miller capacitance. But if you can use very high plate resistors and a high supply voltage (say, 300k and 400V), buffer the output and drive the input from a low source impedance, it is surprisingly linear.

12AT7 tends to be less linear than a 12AX7 run optimally, but... running the 12AX7 optimally is a challenge. For most practical circuits, the 12AT7 will show better linearity.

Using a good CCS in the tail is a must. That will take care of any balance issues.
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Old 22nd February 2009, 06:28 PM   #5
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Quote:
12AU7 has terrible linearity, so I wouldn't use it outside of guitar amp service. If you want a medium mu LTP, 6SN7 or 6CG7 are much better choices.

Agreed! IMO, the 'U7 triode is suitable only for cathode follower and "concertina" phase splitter duty, in HIFI service. Add JJ's ECC99 to the list of suitable medium mu candidates.
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Old 22nd February 2009, 09:47 PM   #6
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hey-Hey!!!,
Type 12A4 is another single triode with good gain/gm combination. Also has good anode dissipation and voltage tolerance...extremely linear too. Also consider 6H30, perhaps also 6S45( a single triode so you'll need a pair just like 12A4 ).

And then there's pentode LTP too...
cheers,
Douglas
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Old 22nd February 2009, 10:59 PM   #7
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Kevinkr,

Wrong thread reference in your post; that comes up as for the sale of a tweeter.
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Old 23rd February 2009, 03:52 AM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Johan Potgieter
Kevinkr,

Wrong thread reference in your post; that comes up as for the sale of a tweeter.
Try this one: New 300B PP amp completed, phase shift questions
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Old 23rd February 2009, 06:17 AM   #9
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I feel sure the 12AT7 is the right choice here; second would be 6SN7/6CG7/6FQ7. I agree the 12AU7 is unfit for audio purposes (yet people still use it!), and the 12AX7 is too lightweight for driving EL34s, especially if you use fixed bias because that will mean a low-ish value for the grid resistors.

I am conscious of Morgan Jones's warning in his Valve Amplifiers against using an LTP splitter to drive OP tubes in Class AB, but I'm unsure what problems it is likely to cause.
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Old 23rd February 2009, 08:36 AM   #10
kmaier is offline kmaier  United States
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Default the lemmings syndrome

In retrospect, it's a true shame that many of you weren't around decades ago to guide and mentor all of the successful designers who unleashed hundreds of inferior audio products that used this horrible tube to the general public, who in turn, had no idea what horrible sounding equipment they were getting stuck with. The list of these shameless manufacturers is long and includes names like Altec, Bogen, Eico, Fisher, Harmon-Kardon, Heathkit, Jadis, Leslie, McIntosh, Pederson, Pilot, RCA, Scott, Sherwood, Stromberg-Carlson and untold others. What they hell were they thinking?? Oh, the "audio crimes" you could have prevented! For those of you who were unfortunate to have fallen victim and purchased any of these products based on sound quality or marketing hype, do yourself a favor... dump it quick!

Regards, KM
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