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In Reply to: Anyone else using a pentode as a driver tube? posted by Bas Horneman on February 19, 2007 at 04:51:31:
the reason they reportedly work so well as drivers in SE amps is actually due to their high distortion (relative to triodes in general) which cancels with some of the distortion produced by the load on the OP tube. I say this because I've observed (measured and heard) a similar improvement by biasing a triode driver to a less linear region through reduced current. One day I'll try it and take some measurements to prove or discount the theory.
Follow Ups:
hello, you are wrong with the high distortion of pentodes. Pentodes are very linear, compared at equal output voltage.But you are right with your experience. I have made many experiments with pentodes and triodes. The best results I have with combining one of both, with local feedback between both tubes.
In my Darling project (schematic is here: http://www.roehrentechnik.de/html/darling.html )I use your configuration pentode-triode, and it sounds much better than triode driver, compared with the same output tube, OPT and speaker.Combination of triode with pentode allows to work with only 2 tubes, married to one supertube because the local feedback, with high gain and low output resistance. And the distortions are similar to a triode, also if I use a triode driver with output pentode.
My theory says, there is 1 stage with high gain and no interesting sound and 1 stage with beautiful triode sound, but low gain. And combining this both tubes gives the characteristic of the tube with low gain. And this is the triode with dominant 2nd harmonics.
My last project was this combination in push-pull-configuration with cancelling 2nd harmonics in the OPT. (schematic: http://www.roehrentechnik.de/html/2a3-pp.html).
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.
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.
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 ;-)
Hi,See link.
Refering to a Komuro design Thorsten concluded: "Of some interest is here the very distinctive distortion of an "unloaded" triode wired pentode, which has a very strong 2nd Harmonic cancelling some of the distortion of the output triode if designed correctly."
So, you're not alone with this line of thought...
Regards,
NC
Thanks NC, pity I seemed to get on the wrong side of Thorsten early on in the piece (my fault I’m sure) because I share many (but not all) of his views and have had some very similar experiences. I too tend to run drivers at lower currents these days, although I still like higher currents for OP tubes. I also like grid chokes and prefer mu follower to textbook SRPP.On the subject of mu followers, I've done a lot of testing of different circuits recently and I'm surprised by the very different sound I hear when taking drive from the cathode of the top triode (mu follower) vs the plate of the bottom triode (CCS). And the cause can't be the difference in OP impedance because a high impedance load was used in all cases. Mu follower is subjectively much stronger at the frequency extremes particularly bass, whereas CCS tends to favour clarity in the mids. I can see why so many don't like SRPP in general because of the apparent loss of mid range detail causing a kind of muddiness, loss of precision and depth, or as Thorsten describes it "opaqueness". Perhaps it’s simply a by-product of their strong bass/mid bass. Paradoxically, they can also suffer from sibilance. But all that said I can also see that their strength might be of great benefit to restoring tonal balance in systems that need what they have to offer.
Hi.From the link, I can see 6SL7 SRPP driver topology exhibits the smoothest overall HD profile from very lo to hi O/P power.
He also said about the operation conditions of SRPP to deliver good sound:
"Most circuit benefit from a stiff regulated PS, the SRPP & most
other half assed PP circuits really NEED them.""It (SRPP) is a very linear & good sounding topology for as long as you have it working with a FIXED load".
So constant O/P load is crucial for such topology.
I would agree with Mr. Konichiwa (Thorsten's moniker here) that smooth & even decending profile of least orders of HDs would deliver good sound to our ears.
c-J
Hi c-J,Before you 'jump into conclusions' as I did when I first saw these graphs, maybe it is good to read part of a private e-mail conversation I had with TL about this (which he told me I could share).
Thorsten wrote:
First the graphs from the above, the perspective makes it difficult to see, but the peak at the 5th Hamrmonic at lower power levels on 417A and EL84) is from output valve (not sure why it did that, but it sure did), so if you do not see a distinct peak-up to the 5th it means the driver produces more 5th than the output did.
The 6SN7 cascade always does more distortion even at high orders than the 300B, in effecting swamping out the 300B's distortion, but it adds some more of it's own at the 5th causing that peak in the 2.5W graph.
When comparing the spectra I felt that the EL84 (and C3m) provided the most evenhanded (and low high order) distortion overall, arguably the 6SL7 SRPP is second and IMHO better than the 417A (same trend with 437A) purely from a harmonic profile.
That said I liked the various pentode driven 300B Amp's I have build about equally well as the 417A/437A/6S45/E180F/E810F/D3a TWGC Amplifiers (TWGC - Triode With Gridchoke), all BTW using rather low anodic current, usually around and below 10mA in the driver, feeling they offered a similar overall performance levels.
No, they do not sound alike, either between Pentode or Triode nor among the different types, but they share some overall charateristics, with the Pentodes offering more "Tone" and "Delicacy" while the Triodes tend towards "Detail" but also directness and notably greater macrodynamic robustness (maybe gridchoke plays a role here, they are not easily used with pentodes).
Compared to that the 6SL7 SRPP (with or without gridchoke, also when connected as mu-follower which I thought best) seems a little "further away from the music" sorta like "through a glass, darkly", giving a certain sameness to varied music, more so than the others).
The 6SN7 cascade seemed to combine the worst of all worlds, managing to sound opaque, without variation in the music (making all sound the same) while giving me headaches with edgyness at the same time. Didn't care for that at all.
A cascade with 6072A & 5687WB sounded more balanced and had more immediacy and directness, but not quiet up to the single pentode or triode levels.
Not sure if any of this of use.
I'm currently working (extremely slowly) on a topology provisionally called "Bushido" which aims to combine the best sonic features of the pentode driven amplifiers (the "delicacy", "lightness of touch", "speed" (whatever we want to call it) and the nice "tone" with the macrodynamic cojones of the "TWGC", it might see the light of day at some point.
Ciao T
Hi.Yes, it was TL's email to me yesterday with the same texts.
On reading TL's 3-D HD spectrum charts alone without other supporting papers showing the HD spectrum of the 300B involved, & sonic comparison, I would still find 6SL7 SRPP shows "arguably" better than EL84 pentode in overall HD figures from 0.66W to 11W except its 4th order HD levels at 0.66W & 2.5W
One thing I would agree with Thorsten in general sonic characteristics triode vs pentode. A triode sounds more "detail" & "macrodynamic robustness" & less "tone" than a pentode. Given proper component mating, a triode amp can deliver "lightness of touch" & "speed" too.
Hi.Was it from you or directly from Thorsten in U.K.??
c-J
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