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In Reply to: Re: Haven't tried it but I've a theory that ... posted by Gerd on February 22, 2007 at 00:03:25:
Hi Gerd,Interesting tube choice. I can't find curves for the pentode you use but generally speaking pentodes ARE less linear than triodes and that was the basis of my observation. Thorsten has apparently done some more research which elaborates on the subject. I'm the last one to argue that your combination doesn't sound good not having tried it; in fact there is plenty of evidence to support that indeed it does. However I haven't heard another theory that better explains the phenomena technically ... but I'm always willing to listen and to be educated.
Rgds,
Naz
Follow Ups:
Hello,with no feedback you are right with nonlinear pentodes, but comparing triodes and pentodes for gain stages needs similar conditions for output voltage and gain.
You can give a negative feedback with an voltage divider from plate to grid (Rk must be increased for making the correct Ug1) to reach the same gain. In this case pentodes are more linear than triodes.
For Output stages it is very simple. Power Triodes are more linear but needs higher grid swing than pentodes, and this gives more distortion in the driver stage. Thorstens experiments with cancelling 2nd harmonious are interesting, but it is very complex to find perfect matching driver tube.
Pentode with feedback can reach the linearity of a triode and need less voltage swing on the grid to drive, and the driver produces less distortion.
It is easier to use a pentode and feedback between both tubes.
The relationship 2nd and 3rd harmonious for pentodes can be changed through increased or made smaller the anode resistor. In this way it may be possible to cancelling 2nd harmonious.But there is another fact for using pentodes. Fullrange speakers have a nonlinear impedance curve, impedance rises toward both sides of the frequency range. Triodes with low output impedance (damping factor > 1) give less power to the speaker if the impedace is higher, pentodes with higher output impedance (damping factor < 1) give more power on higher impedances. It is very complex to fit Amp and speaker for best results.
Gerd
Hi Gerd,Good discussion here … a few comments for you to ponder.
> > You can give a negative feedback with an voltage divider from plate to grid (Rk must be increased for making the correct Ug1) to reach the same gain. In this case pentodes are more linear than triodes. < <
Partly agree but the use of NFB to lower distortion can give rise to an undesirable harmonic distortion structure and a correspondingly detrimental effect on the sound. 2ndH may end up better than triode but not high order distortion, particularly odd order.
> > For Output stages it is very simple. Power Triodes are more linear but needs higher grid swing than pentodes, and this gives more distortion in the driver stage. < <
True but most of the distortion is generated by the load imposed on the
OP tube and it's easy to design a low distortion driver anyway.> > Thorstens experiments with cancelling 2nd harmonious are interesting, but it is very complex to find perfect matching driver tube. < <
Agree in principle but I haven't tried it with a pentode driver. I was surprised how well it works with a triode though and in theory a higher distortion driver would work way better.
> > But there is another fact for using pentodes. Fullrange speakers have a nonlinear impedance curve, impedance rises toward both sides of the frequency range. Triodes with low output impedance (damping factor > 1) give less power to the speaker if the impedace is higher, pentodes with higher output impedance (damping factor < 1) give more power on higher impedances. < <
There are many issues here. 1) Actually, a high OP impedance only helps smooth frequency response through its inefficiency, ie higher attenuation of the mid band rather than by delivering more power into the high impedance parts of the spectrum. 2) The poor damping factor can cause bass problems 3) A high OP impedance (poor DF) implies a poor load line and therefore high distortion. 4) Even if high OP impedance was desirable then it can just as easily be achieved by loading a triode OP stage more heavily, the bonus being greater power and reduced turns ratio in the OP tranny.
To summarise, I have to agree that low distortion is possible with Pentodes but not without compromises, especially when used in OP stages.
Rgds,
Naz
Hello> > Partly agree but the use of NFB to lower distortion can give rise to an undesirable harmonic distortion structure and a correspondingly detrimental effect on the sound. 2ndH may end up better than triode but not high order distortion, particularly odd order.
yes, this is possible, but not probably. I think, NFB will reduce all harmonics because linearization of the entrance characteristic
> > .. and it's easy to design a low distortion driver anyway
and it is much easier to design this for less needed output voltage to drive a pentode with an 1/5 or 1/10 needed grid swing ;-)
> > but I haven't tried it with a pentode driver
than do it, I heard the difference, Darling with triode driver without NFB and Pentode driver with NFB.
> > There are many issues here. 1) Actually, a high OP impedance only helps smooth frequency response through its inefficiency, ie higher attenuation of the mid band rather than by delivering more power into the high impedance parts of the spectrum. 2) The poor damping factor can cause bass problems 3) A high OP impedance (poor DF) implies a poor load line and therefore high distortion. 4) Even if high OP impedance was desirable then it can just as easily be achieved by loading a triode OP stage more heavily, the bonus being greater power and reduced turns ratio in the OP tranny.
to 1: no, pentodes with high impedance are more efficient than triodes with low impedance, because of the small not controlable residual voltage. And you are wrong with higher attenuation of the mid band. Higher OP impedance behaves like current source, and higher load resistance have a higher power consumtion. For nominal load impedance in the mid band you get nominal output power, and the rising impedance at the ends of the frequency range makes another distribution of output power.
Triodes with low impedance are like voltage sources, at rising load impedance the current will be smaller and power is reduced.to 2: yes, this can be a problem ;-)
to 3: not necessarily, a loudspeaker is a magnetic transducer, and magnetic forces result from the current, which flows by the voice coil. And pentodes with current source characteristic delivers current..
to 4: no, it is not so easy. Example 300B with Ri 700 Ohms. To get low distortions ra should be around 3,5k. transformed to 8 ohm load, without any losses this gives a output impedance of 700/(3500/8), around 1,6 Ohms and DF=5. You can not wound an transformer for DF> 1. For DF=1 you get only ra 700 Ohms, this gives much power with much distortion, but it is still not a current source.
A pentode like EL12 works also on 3,5k Ra, I can use a transformer with the same impedance ratio, but plate resistance of the EL12 is 25kohms, I have an output impedance of 25000/(3500/8), around 57 Ohms, DF = 0,14 (without NFB). NFB reduces output impedance, but not so much, DF will be ever smaller 1.I think, this is a reason because EL84 sounds very good on fullrange speakers.
regards, gerd
( sorry for my english, it is very difficult to interpolate between my memory and babelfish ;-)
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