variable-mu vs remote cutoff

[Opcom]

Looking at some triode specs, like for the 6BC8, which is a "medium mu twin triode with semi-remote cutoff characteristic", and others, it looks like those two terms almost mean the same thing.

I can see where increasing the bias on a tube reduces the mu, more in some than in others. These same tubes, of which the example is one, seem like many are the 'remote cutoff' types.

What is the real correlation between these terms "variable mu" and "remote cutoff"? My concept in this is kind of foggy. The datasheet is attached although there might be better examples.

On the next to last page, the curves indicate the mu changes with grid voltage. If this is the case, then it must change over the complete cycle of the signal, correct? And wouldn't that create distortion?

[KM1H]

Its a bit confusing since different manufacturers have different interpetations.

The 6BC8 is one of a very few triodes that can be AGC controlled but its range is much less than a remote cut-off tube in that it stops conducting at around -13V thus it is called semi remote.

The 6ES8 is another but at -7V cut off it has a sharper curve, but its still a semi remote characteristic.

Variable mu simply means that it can be controlled by the bias but Ive yet to see a standard defined. Radiotron Designers HB 4th edition refers to remote cutoff and variable mu (or gm) as the same thing. Ive also seen it defined in a triode as anything that deviates from a straight line charateristic.

Carl

[Tom WA3KLR]

(I’m going to answer this quickly, late at night, off the top of my head, a dangerous thing!)

Tubes that are one mu value, the sharp-cut-off receiving pentode for example, have the same given grid turn-to-turn spacing along the whole length of the grid. 

Variable mu tubes have the pitch of the grid turns changing spacing gradually along the way.   The original remote-cutoff receiving pentodes were called "super-control r.f. pentodes" such as the 34 and 35, gain-controlled by the receiver’s AVC bus.

You can think of a remote-cutoff pentode as an infinite number of small tube sections stacked vertically in parallel, with low-mu tubes sections at one end of the stack and high-mu tubes at the other end of the stack. 

Sharp cut-off receiving pentodes go from cut-off to full current in about 8 volts of grid bias range as I recall, and remote cut-off tubes go from cut-off to full current with a 32 volt range.  In the properly designed receiver applications it is expected then that the signal level applied to the grid is kept to a small swing of a few volts or less ideally, and then the grid bias is varied, setting the operating point for that amplifier stage, so that the gain is essentially the same over the r.f. cycle.  This will allow the variable gain characteristics to work properly.  When a strong signal is being received and with the AVC bus at greater than -10 volts, the sharp-cutoff/high mu end of the tube is cut off.

I’m not gong to take time tonight to find any statements on the semi-remote cut-off tubes but my guess is that they would be employed in the r.f. amplifier stage where low signal levels are expected.  This tube type would give a higher AVC loop gain (not explaining this now).

If a tube is given the “medium mu” classification properly, it’s grid should be wound with one pitch, in my opinion.  The semi-remote cut-off tubes may be in the medium mu range but must have a pitch variation.  If the tube does have a grid pitch variation, it should be called semi-remote and not medium mu.  But, you will find some tubes called medium mu in one data book and semi remote in others; sematics.

I hope this is a help.

[Opcom]

Maybe then a "good" variable mu characteristic would be a 50:1 range, not a 5:1 range. and it is regardless of the other characteristics as long as the tube is not cut off by a gain-controlling condition where the mu has been varied.