High slope? Sharp cutoff?

[Diabolical Artificer]

I've often seen some low power pentodes described as "high slope" but looking further at datasheets and graph's I can't see this slope explained, same goes for sharp cutoff.

What do these terms mean and do they only pertain to the valve in question ability's as oscillators or mixers. Lastly what does high slope mean in terms of LF audio use?

TFL, Andy.

[paulsherwin]

High slope just means the valve has a high gm figure. Sharp cutoff means it isn't variable mu.

Neither term is directly relevant to audio use specifically, though most valves primarily intended for audio use will indeed be high slope sharp cutoff.

[Radio Wrangler]

Variable mu, the opposite of sharp cutoff, means that the valve grid characteristic shows an extended, non-linear region where the overall voltage gain (into infinite anode load) reduces. This function is used in radio receivers for AGC controlled amplifiers at RF and at IF. It's not the sort of thing you'd want at audio.

Gm is the valve's transfer function, mA of anode current versus volts of grid swing. The slope of a curve on a graph.

Mu, greek letter is related to Gm, but takes into account the valve's own effective anode impedance. To translate Gm into voltsge gain, you have to consider the anode current loaded by both the valve anode impedance (slope of another graph!) and whatever your circuit loads the anode with, all in parallel. So the Mu figure forgets the circuit load and uses the valve's own self-loading to give an ultimate voltage gain figure that acts as a limit for what the most favourable circuit theoretically possible could achieve (though they forgot negative resistance generators...)

David

[ms660]

Originally Posted by Radio Wrangler

It's not the sort of thing you'd want at audio.

Collins seemed to think otherwise:

https://www.steampoweredradio.com/pd...lins%2026u1%20
peak%20limiting%20amplifier.pdf

https://frank.pocnet.net/sheets/142/6/6386.pdf

Modern variants on the same theme using valves are also available.

[Jez1234]

Some variable Mu valves were used at audio in compressors and limiters such as made by Fairchild in the 60's.

[Diabolical Artificer]

Thanks Paul.

High slope just means the valve has a high gm figure.

So high conductance or should that be trans-conductance? The ability of a valve to shift current for Vgk.

Gm is the valve's transfer function, mA of anode current versus volts of grid swing. The slope of a curve on a graph.

Which is more linear for a pentode, EG a straight line across a Va/Ia delta Vgk graph.

Thanks Lawrence, I'll have a read of those.

Andy.

[Radio Wrangler]

Originally Posted by ms660

Collins seemed to think otherwise:

Collins weren't much bothered with hifi audio quality, but their customers were bothered about number of valve types and the effect on field serviceability. There were reasons.

David

[knobtwiddler]

The much-fabled Fairchild 670 (the last few I've seen for sale have had $200K+ price tags - see audiophoolery thread) uses a total of 8 dual variable-mu 6386 triodes (they double as an O/P stage, driving the TF direct). There are many modern replicas. One of the cleverer ones is the late Tim d P's one, which uses an easily found gain valve and SS side chain. Variable-mu comp / limiters remain a staple in studios and quite a few firms still make them. In fact, there is one with a valve signal path, and DSP side chain!

Here is an example of a modern unit: https://www.pendulumaudio.com/6386-es-8

[ms660]

Originally Posted by Radio Wrangler

It's not the sort of thing you'd want at audio.

Woz my point re: Collins and others using them....Audio is audio.

[DrStrangelove]

Across the pond "variable mu" seems to be referred to as "remote cutoff".

And there's also "semi remote cutoff" (though not very many of those).

[G6Tanuki]

Variable Mu was certainly used in tape recorders, the type that ran the tape at a low speed and were used to log voice traffic into airfield control towers, emergency control rooms for the police etc.

Also dictation machines. All used audio AGC in some form to releive the operator of the need to bother about audio levels.

[Radio Wrangler]

Yes, audio AGC, though some automatic level control used straight valves and suffered somewhat from their more rapid cutoff. Anyway, my comments in this thread were in terms of the original poster's main interest in hifi amplifiers. Tape recorder AGC recording amps and compressors (Even fabulously expensive ones) to prevent over-modulation of transmitters are not things anyone would want in their home audio systems.

Anyway, smooth, low distortion electronic gain control at audio is quite a difficult act to pull off. The better active approaches didn't come along until the Gilbert-cell multiplier came along (The full Gilbert cell with the diode current to voltage converters, not the simplifier RF mixer circuit) ~though it need only be a 2-quadrant version, not the full four. Even these have levels of distortion worse than is normally expected in a preamp, so motorised pots have appeared in some Japanese hifi gear, and lamp/photoresistors have been used.

The variable mu valve is a bit crude though it works satisfactorily in IF amplifiers where the harmonics created fall outside of the IF bandpass filtering, and unimproved diode detectors are the norm.

I'll say no mre.

David

[Leon Crampin]

The EF83 was a variable gm version of the EF86, used by Grundig, amongst others in their tape recorders featuring automatic level control.

Leon.

[knobtwiddler]

Originally Posted by DrStrangelove

Across the pond "variable mu" seems to be referred to as "remote cutoff".

And there's also "semi remote cutoff" (though not very many of those).

'variable mu' is a registered Trademark in the US (by a firm who make a delta-mu comp / limiter). See attached.

Should they have been given the TM? Well, many things are possible in the US IP world...

[Diabolical Artificer]

Thanks all, my questions have been answered, thread can be closed thanks mods.

Andy.