Is a Differential Pair (CCS tail) phase inverter suitable for a Class AB outputstage? - diyAudio
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Old 1st October 2003, 11:01 AM   #1
stigla is offline stigla  Norway
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Default Is a Differential Pair (CCS tail) phase inverter suitable for a Class AB outputstage?

Hi!

I'm having a little design phase for a small 3A5 -> EL84 PP amp. I'm using the 3A5 double DHT triode as a phase inverter and driver, with a CCS on the tail, so that I can just short one grid to ground for a SE input.

But after some time I began suspecting that this kind of phasesplitter is pretty senitive to the loads seen on its outputs, and it's important that they are equal at all times. (not just the grid resistors to the powertubes, but also the miller capacitance and other.)

Therefore, With a class AB outputstage, will there be any crossover troubles (making the miller-cap. change over a few seconds on one of the Powertubes grid, when one tube is conducting less?), or is this only if the OP-stage is biased in class B?

For example, Allen Wright's PP-1 have a cascoded phassplitter of this type, but the OP-stage is class A...

The Schematic is pretty old, and you can just ignor the PSU, as it won't work (VR'tubes have been omitted now) and it says CLASS A but it's not. it was my first outline of the amp.

And, any ideas of any nice operating points in REAL class A for the EL84?

Thanks for your time,

stigla

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Old 1st October 2003, 08:04 PM   #2
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Hi,

If your driver stage running in Class A , I don't see any problem using a CCS.

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Old 24th March 2007, 12:25 AM   #3
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How's the design going? I'm thinking of using something similar in my Leak Stereo 20. I use the 3a5 right now in my amp - nice sound!
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Old 24th March 2007, 09:21 AM   #4
stigla is offline stigla  Norway
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Wow, waking up an ooooooooold thread!

Well, lets see... This is some years ago. I experimented with one channel only. Got the design up and running back then. Gain was low, and the 3A5 was very sensitive to the PT's low physical humming, that vibrated the chassis. Sound was OK I guess, but nothing spectacular (given only one channel). I ended up experimenting with other tubes and feedback and stuff. More or less ending up with chokeloaded 5687 as the input, and running it from a balanced source (input transformer).

I also ended up with 6V6GT tubes in pentode mode at the output.
Tried some AL4 tubes and found they had a very similiar sound character to the EL84.

As with many projects, it never got buildt in full. I ended up using the parts for other projects. I never got wise on the phase splitter driving class AB output issues either. Had no measurment equipment to do any good research.

I still have plans to build the amp I experimented on back then, but who knows when I actually get around and do it. Nevertheless I plan to get a pair of Tamura F-685/485 that has an interesting, separate feedback winding i thingk it will be fun experimenting with.
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Old 24th March 2007, 12:16 PM   #5
SY is offline SY  United States
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Let's address the phase splitter issue. The problem is actually at the point where the amp is transitioning out of class A (if you want to be pedantic, it's at the point where the operating angle of the tubes falls below 360 degrees). At that signal level, the Miller effects of the two halves unbalance during the portion of the signal where one or the other output tube is cut off.

The split-load phase inverter is especially sensitive to this, but it can be taken care of in design if the designer is aware of the issue. A diff amp is somewhat less bothered, but it still does upset the balance for that portion of the signal. The formulas for gain and source impedance of diff amp and split-load inverters under balanced and unbalanced load conditions can be conveniently found in my now-tattered copy of Morgan Jones's "Valve Amplifiers" 3rd edition. For a given design, one can easily predict whether it's a real problem or not.
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Old 24th March 2007, 05:55 PM   #6
jane is offline jane  Norway
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Which speakers are you going to use? The output impedance will be quite high (50-60 ohm) with EL84 in pentode mode with no NFB.

The class A limit depends on the quiescent current and the reflected primary load.
Class A limit = ((Iq/1.414)*2)^2*(Rl/4)
2 x 33mA into 8k gives you about 4W class A.

It will be better for the standby switch if you put it in the AC domain. Less chance for sparks and arc over.

Jan E Veiset
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Old 27th March 2007, 06:38 AM   #7
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Quote:
Originally posted by jane
EL84 (triode) gives you about 4W class A.
And a glorious 4 W that is. The amp we built on a Scott LK48 donor, with the El Cheapo CCS diff driving Class A triode strapped EL84 is an amazing little amplifier. We hope to be able to compare it to one of SY's RLD soon (poke).

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Old 27th March 2007, 02:57 PM   #8
SY is offline SY  United States
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Hey, it's tough to get finished and packed up from 6000 miles away!
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