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In the Forum: Analog Playback
In the Thread: 7788 tube mic pre design
Post Subject: E55LPosted by Lucian on: 1/2/2008
Hello first, as I am new on this site...

I have tried a number of high Gm pentode/triode tubes, including E55L and, of course, 7788. All are very linear when used as triodes, but do not bet on very low noise, if that is what you would be after. These are very high bandwidth tubes, so be on the watch for RF behavior (see grid stoppers, decoupling, heater RF filtering, etc).
Great linearity for E55L/8233, not necessarily at very high anode currents (20-30mA usually are enough if a bit of conductance loss is tolerable, but maximum linearity is reached at about 50-60mA and 125-130V on the anode, whether triode or pentode connected). Great line driver and, why not, can be used for spud SEs (E55L can be run at 200V maximum). Also great perveance, E55L will swing between 25V and 200V at 50mA, if pentode (was used by Tektronix as a deflection amplifier up to 30MHz and more). Good current source too, but relatively noisy.
The only drawback: they are VERY expensive and fabricated no more. For fans of high Gm tubes, a few recommendations:
1. 6J51P (EF184) - Russia
2. 6J9P (E180F) - Russia
3. 6J52P - Russia
4. 6J53P same
This list is only a very small one out of the many tubes like this out there.

While there is no known correlation between Gm and sound quality, the increased conductance/perveance guarantees driving difficult (read reactive/complex) loads and also increases the efficiency of the smallest amount of current feedback, if used to increase the linearity. As a matter of my own personal taste, I do not use tubes that have less than 10mA/V and plate resistances larger than 10K, so I am basically confining myself to triode connected pentodes and high Gm triodes.

As a last comment, many reputable designs have used pentodes as drivers, but there are design rules to that, if the very high output impedance of a pentode is not to become a problem. See Citation II, Radford STA100, etc. The fine point is to use pendodes into a "zero impedance" NFB summing point: thus a pentode is used in the ideal way - a constant current source into an integrator input guarantees maximum bandwidth, lowest possible distortion, but noise can become a problem because it is presented also as a noise current source to the said summing point... but I should keep that rant for another occasion :-)...

My two cents.

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