Biasing

[fauxpas]

Is this a good guide for biasing?

[kevinkr]

Originally posted by fauxpas
Is this a good guide for biasing?

This is intended for biasing guitar amplifiers using fixed bias and is fine for the stated purpose.

Hifi might actually be a little more complex. Instead of just listening by ear (still needs to be done) you might actually want to measure overall distortion and the related spectra, and select a safe operating point which might typically be chosen to reduce 3rd harmonic in the output of either SE or PP amplifiers, in the case of an SE amplifier you might favor an operating point that resulted in slightly higher 2nd harmonic in order to reduce 3rd harmonic as much as possible.

A pc sound card based spectrum analyzer is ideal for measuring distortion and spectra for hifi amplifier tweaking.

These techniques are applicable to whether the amplifier is fixed or cathode biased. Cathode bias requires more work as you have to physically change bias resistor values, whereas in the case of fixed bias it is usually a quick adjustment via the bias pot. (if present)

I'm out of time, others I'm sure will add a lot more..

[ray_moth]

As Kevin says, Duncan is all about guitar amps. For hi-fi, many people prefer to set higher quiescent current/dissipation than Duncan proposes as the maximum for each tube type.

What I find strange is the difference in results that I get from LTSpice, when trying different bias arrangements using EL34s in pentode-mode PP.

1. I find that the lower the quiescent current, the better the distortion behavior seems to be. In almost Class B (quiescent current about 10mA per tube), the harmonic distortion has fallen to zero.

2. With a shared CCS in the cathodes (i.e. strict Class A), odd order harmonic distortion is worse than in Class AB, and it starts to show up some 2nd harmonic distortion.

I just put these anomalies down to imperfect simulation. There are definitely quite a few things wrong with the simulation 'engine' in LTSpice, and this is probably just another of them.

[kevinkr]

Originally posted by ray_moth
As Kevin says, Duncan is all about guitar amps. For hi-fi, many people prefer to set higher quiescent current/dissipation than Duncan proposes as the maximum for each tube type.

What I find strange is the difference in results that I get from LTSpice, when trying different bias arrangements using EL34s in pentode-mode PP.

1. I find that the lower the quiescent current, the better the distortion behavior seems to be. In almost Class B (quiescent current about 10mA per tube), the harmonic distortion has fallen to zero.

2. With a shared CCS in the cathodes (i.e. strict Class A), odd order harmonic distortion is worse than in Class AB, and it starts to show up some 2nd harmonic distortion.

I just put these anomalies down to imperfect simulation. There are definitely quite a few things wrong with the simulation 'engine' in LTSpice, and this is probably just another of them.

Hi Ray,
It is more likely that the tube model is the culprit. I get much better results with triodes than pentode.

[ray_moth]

Sorry, I didn't mean to hijack fauxpas's thread.

I should have added that I am impressed with the Duncan Amps website and I think his advice is generally pretty solid. You might find, though, that after some tweaking by ear you end up preferring a higher bias current than a guitarist would choose. This means hotter running and shorter tube life, I suppose, but that's often the price we pay for warmer sound.