Long Tail Pair Phase Inverter with Penthode ? - diyAudio
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Old 6th February 2006, 02:37 PM   #1
Yvesm is offline Yvesm  France
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Default Long Tail Pair Phase Inverter with Penthode ?

In another thread, SY wrote:
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Actually, my question was about using pentodes for the input tubes, since the desiradata there are high plate resistance and high mu. Seemingly, they would work better than triodes for this application.
I'm bothered about screen current(s) flowing thru the tail.

Does someone already built and checked one, are there special tricks ?

Yves.
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Old 6th February 2006, 04:18 PM   #2
ilimzn is offline ilimzn  Croatia
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The surefire way to remove the G2 current from the tail is to use a floating G2 supply, which is connected + end to the G2 of both pentodes, and - end to the common cathode point. However, it is a bother, not to mention capacitance coupling where you do not want it.
Fortunately, with some precautions, you do not bneed to do this.
Firstly, for Vg2 sufficiently lower than Vplate, G2 current is a virtually fixed potion of plate current. This means that in a LTP, G2 currents will also behave differentially, and you will only have a common mode error - as one pentode G2 current increases, the other's decreases. You just need to increase the tail current to account for 2xIplate + 2xIg2 flowing through the tail. Things do get complicated as plates swing closer to screen grids, current increases drastically and is not a 'fixed' portion of Iplate. Also, setting a relatively low amplification factor for the pentodes results in the common cathode node swinging up/down signifficantly enough to modulate effective Vg2, which introduces some error in expected performance.
In other words, if your pentodes are relatively high gm to begin with, and your chosen amplification factor is high, and your chosen Vg2 is low compared to Vplate, and your pentodes are well chosen so you can use the aforementioned Vg2 and Ig2/Vp relationships, you should not plan on any special measures due to Ig2, except for accounting on a constant 2xIg2 going through the tail.
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Old 6th February 2006, 09:50 PM   #3
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The Harman-Kardon Citation II used pentodes in its LTP splitter.

Radford used a pentode for the first half and a triode for the second half of the LTP, apparently to avoid Miller capacitance killing the high frequency response.
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Old 7th February 2006, 07:23 AM   #4
Yvesm is offline Yvesm  France
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TNX2U for answers.

Ilimnz,
That is exactly what I feared about.
According to your points, I'm looking for a penthode with lo screen voltage.
Probably, using a common screen resistor bypassed to the cathode may be the simpler way, screen currents trying to balance themeselves except at extreme "negative" plate swing where the cap could "smooth" by providing this extra current.

ray_moth,
Looking at H.K. it seems to me that the phase splitter has so many feed back loops around it that I'm not sure if it can still be called an LTP
Also, the voltage divider at the upper anode is somewhat confusing !
And that 12BY7 IS really beeffy
A very curious and nteresting implementation.


Still investigating,
Yves.
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Old 7th February 2006, 09:15 AM   #5
ilimzn is offline ilimzn  Croatia
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What you want is often in the first stage of tube oscilloscopes. The problem is, the input voltages there are on the order of tens of mV so the output swing is not very large. The typical solution is NO coupling cap to common cathodes, G2 of both pentodes tied together, and a common Rg2.
I have a scope with that sort of input stage here. The common Rg2 is quite large - 10kOhms. Two EF184 pentodes are used (typical gm is 15mA/V). Unfortunately I cannot tell you exactly what Vg2 is as the input stage is coupled in series (!) with the 3rd stage (4 stages total) in order to conserve power - it is a direct coupled amp of course. My guesstimate be about 100V or so.
Here you can again see that the constant current of the LTP is used to an advantage. The tail current is the same for both stages that are in parallel, and includes 2xIg2 of the pentodes for the second stage. The specs say less than 2% distortion so it seems to work quite well. Also, note high Rg2, for two reasons - to use Ig2 to get proper Vg2, and also to limit G2 dissipation in an event of overload. As Vp comes close to Vg2, G2 current increases and Vg2 decreases. This also decreases Ig2 of the other leg, so there is some balancing action. Presumeably, the idea is not to work into this area of operation under normal circumstances.
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