Pentode Tests

[mjrippe]

RDH4,  Page 511, states that for a first stage audio amp with outputs below 10Vrms, a pentode gives one eighth of the intermodulation distortion of a triode, when both are adjusted for minimum distortion.

Best
DaveP

I believe we have different editions of RDH4. In mine, p.511 is oscillators...
In what chapter and section have you found this quote?

I have the RDH4 1960 printing and my copy has the same p511 as Dave's.  Part 3 Chapter 12, about 30 pages in.  Oscillators are in Part 4 Chapter 24 starting at p947. 

 

[lassoharp]

There is a section in RDH3, chap 6, p 41 that discusses pentodes being preferred over triodes  in preceeding stages of resistance coupled NFB circuits and citing the reasons.  Seems common in a number of American broadcast amps and vari-mu compressors.

 

[abbey road d enfer]

RDH4,  Page 511, states that for a first stage audio amp with outputs below 10Vrms, a pentode gives one eighth of the intermodulation distortion of a triode, when both are adjusted for minimum distortion.

Best
DaveP

I believe we have different editions of RDH4. In mine, p.511 is oscillators...
In what chapter and section have you found this quote?

I have the RDH4 1960 printing and my copy has the same p511 as Dave's.  Part 3 Chapter 12, about 30 pages in.  Oscillators are in Part 4 Chapter 24 starting at p947.

Silly me. I opened the Radio Engineer's Handbook! Doh!

 

[abbey road d enfer]

RDH4,  Page 511, states that for a first stage audio amp with outputs below 10Vrms, a pentode gives one eighth of the intermodulation distortion of a triode, when both are adjusted for minimum distortion.

Best
DaveP

That's interesting. There is no explanation for that, unfortunately.
For an input stage without NFB, that may be a significant motivation.
In most cases, a preamp would operate with less than 1V rms output level, so I'm not sure it's that important. For a phono pre, noise is more annoying than IMD, IMO.
For a two-stage mic pre with overall NFB, the IMD level should be negligible.