Driver valve discussion

Why is the 6SN7 bad ?
Lower gain and lower Rp and very linear.
Most important for some of us ......easy to get !
Cheers,
Ashok.

How about a 12B4, not many people seem to use this excellent driver triode.

I've used one to drive a 1:1 interstage transformer driving a DA100 output triode. (The DA100 is electrically similar to an 845, though physically very different) Driving a 300B is child's play compared to that.

Re: which driver?

Konnichiwa,

garbage said:

i am currently gather parts for my 300B amp.
i have a pair of 6SL7 which was to be used as driver, but after calculations, seems like the Rp of this tube is way to high.

Click to expand...

Yup. Agred. The 6SL7 can work, used as SRPP or Mu-Follower but I don't like it THAT MUCH either.

garbage said:

am considering a couple of dht tubes such as 10, UX201A, 26 and the telefunken re084 to replace the 6SL7. would any of these be a suitable candidate as a driver?

Click to expand...

They are all a bit high Ra and non have enough gain. And of course, they are triodes. If you must use a triode driver keep it single stage and go for something with reasonable Mu and Gm. Off hand, WE 417A/437A, Russian 6S45PE, E180F/E280F/E810F/D3a all triode wired come to mind.

Even better to my ears are Pentode drivers, something like an EL84, C3m or a russian 6E5P do a lot of things in a complete amp much better than any triode.

ashok said:

Why is the 6SN7 bad ?

Click to expand...

Because it sounds pretty uninspiring?

ashok said:

Lower gain and lower Rp and very linear.

Click to expand...

Well, let's just say it like this, shall we:

Too low gain, excessively high Ra for a triode and not very linear, especially at high levels of swing.

ashok said:

Most important for some of us ......easy to get !

Click to expand...

I don't think good 6SN7 are easy to get at all, actually. Considering the lacklustre technical and appaling sonic performance, why bother? Any equally easy to get 5687 wipes the floor with a 6SN7.

The only reason why people use the 6SN7 is because it is found in loads of expensive commercial gear and they think it must be good and hence have them around (if only from copying commercial stuff). Together with KT88/6550/EL34 and the like the 6SN7 is my pet peeve for serious DIY Audio.

Why bother with them, if you consider the investement in time and other parts, when there is so much drastically superior stuff available that is cheap and readily available if you look, by virtue of being unknown.

Ouroboros said:

How about a 12B4, not many people seem to use this excellent driver triode.

Click to expand...

12B4 needs another stage and has no advantage over any of the "High Gm" Family in terms of linearity and ability to drive things. The 12B4 makes sense daisychained as second stage after an old triode, like a 26/27, WE101 or 262 and the like.

Sayonara

Hi Garbage,
sorry to side track but do u happen to have a good source for RE084?


Regards,
gaga

Re: Re: Re: which driver?

Hi,

garbage said:

they are all IDHTs. would there be any DHTs that are worth a try here?

Click to expand...

Well, most DHT's lack the gain to drive a 300B, even if they make the grade on anode impedance and swing capability, so you always have to use two stages of gain. Plus you now have two (three, if you take that thought to conclusion) directly heated filaments to deal with. Life quickly becomes interesting that way.

There are two 300B projects that have found interesting that are all DHT. The first is the KATELELO from Ciro Marzio:

http://www.audiodesignguide.com/se/katelelo.html



The article is pretty good on giving an overview of both philosophy and issues.

The second is the 205/300B Amp from Susumu Sakuma.

http://www10.big.or.jp/~dh/work/89b3.html



Now here you have only two gainstages, but you also have an Input transformer with a lot of passive gain (1:9) which substitutes an active stage. So this Amplifier will only work from sources with a very low source impedance, such as a Preamp with a stepdown output transformer, which means you have brought the third stage back through the backdoor by now requiring an active preamp.

I prefer to make do with as few gainstages as possible, as it invariably sounds better to my ears. Usually I manage three transformers and four active stages between MC Cartridge and Speaker, which two fewer than common and the result is audibly superior.

Modesty obliges to link also to a little essay of mine:

Some thoughts about singleended Valve Amplifiers

A brief supporting document is here:

Harmonic Distortion Prodiles for Different Driverstages driving the 300B

The Article above was developed from some notes on the Joenet, preserved unedited and thus perhaps truer to intent here:

httpd.chello.nl/~m.heijligers/classic25/ nonstraigthforward/mod3/300Bampdrive.txt

Sayonara

Re: Re: Re: Re: Re: which driver?

Hi,

garbage said:

thanks for the explanation and also the links to those articles. i was wondering why there are not as many full dht diy amps.

Click to expand...

In the simplest terms, each added stage amplifies the noise and hum from the previous one. That means that not only DC is a must on drivers, but also that great care is required how that DC heater supply is implemented. If you look at that you can see costs, complexity and weight quickly.

garbage said:

i noticed on your website that you are using c3m as driver for your amp. these tubes indeed look great naked.

unfortunately, the heater voltage is 20v. way too high for my main trans to cough out.

Click to expand...

Why? You could allways use a Schottky Voltage doubler from a pair of nominal 6.3V AC Windings, look at the attached....

Sayonara

Lets stick to the facts...

Referring to 6SN7:

Kuei Yang Wang said:

..... Too low gain, excessively high Ra for a triode and not very linear, especially at high levels of swing. .....

..... Considering the lacklustre technical and appaling sonic performance, why bother? Any equally easy to get 5687 wipes the floor with a 6SN7. .....

..... The only reason why people use the 6SN7 is because it is found in loads of expensive commercial gear and they think it must be good and hence have them around (if only from copying commercial stuff). .....

Click to expand...

Where is the evidence for these assertions? Anyone failing to achieve good linearity from a 6SN7 is using it in an unsuitable circuit. Used in a mu-follower or differential pair, it has stunningly good linearity. I've just measured 0.2% THD at 180Vpk-pk between the anodes of a 7N7 (Loctal equivalent) differential pair. That's not lacklustre. With deliberate tweaking, I could undoubtedly improve that figure.

There's no such thing as "too low gain" or "excessively high ra", it all depends on how and where you use the valve.

I'm afraid that in a circuit optimised for low distortion (mu-follower) the 5687 produces 13dB more 3rd harmonic distortion than the 6SN7. I don't call that wiping the floor, I call it a measurement.

No. People use the 6SN7 because its a good valve. Just because it was traditional, cheap and popular doesn't mean that it's wrong.

I apologise to others for leading this thread away from its named topic, but the quoted assertions cannot be allowed to pass unchallenged. If people want to discuss relative merits of individual valve types, I will split this thread appropriately.

Re: Lets stick to the facts...

Konnichiwa,

EC8010 said:

Referring to 6SN7:
Where is the evidence for these assertions?

Click to expand...

Just listen.

EC8010 said:

Anyone failing to achieve good linearity from a 6SN7 is using it in an unsuitable circuit.

Click to expand...

That may be so. In those circuit that are commonly shown to use a 6SN7 as driver for a 300B it failes miserably to be linear and to sound good (to my ears). In any other application I have come across it it has also been very lacklustre. Now agreed, I never bothered to try designing a circuit myself for the 6SN7 and I am dismissing it on the basis of hearing and measuring it in circuits by others, commonly highly regarded, to me that is "good enough" though.

EC8010 said:

I've just measured 0.2% THD at 180Vpk-pk between the anodes of a 7N7 (Loctal equivalent) differential pair.

Click to expand...

Try 140V P-P in SE (not differential) and try getting 0.5% THD. I can do that with many of my favourite valves....

EC8010 said:

There's no such thing as "too low gain" or "excessively high ra", it all depends on how and where you use the valve.

Click to expand...

That is true. Shall I say then: "As 300B driver, Line Stage Amplifier and in many other audio applications it has unsuitable electrical charateristics and consistently fails to provide sound quality much above the lowest common denominator!"?

EC8010 said:

I'm afraid that in a circuit optimised for low distortion (mu-follower) the 5687 produces 13dB more 3rd harmonic distortion than the 6SN7.

Click to expand...

Then the circuit was not optimised for low distortion in the same way.... ;-)

EC8010 said:

No. People use the 6SN7 because its a good valve. Just because it was traditional, cheap and popular doesn't mean that it's wrong.

Click to expand...

That people use the 6SN7 because it is "good" is an opinion. In my view a wrong one, simply because most people never bother to look elsewhere. It was good enough for Williamson and Brooks (not for Bonavia-Hunt and WE though) so it's good enough for us seems to be the principle.

EC8010 said:

I apologise to others for leading this thread away from its named topic, but the quoted assertions cannot be allowed to pass unchallenged.

Click to expand...

Why not?

It happens to be true in extactly the way it was presented, namely in the context of a 300B SE Amplifier as Driver.

Now your truth may very well look different, though it changes nothing about the electrical parameters of the 6SN7.....

Sayonara

I don't see nothing wrong with the electrical parameterers of 6SN7 but the fact remains that every time i use it the sound becomes uninteresting, cold, grey. It could be the reason i hate the Williamson, i don't know. Every time i've replaced it with a 5687 (an even cheaper tube) there has always been an improvement. Of course one can do much worse than 6SN7; ECC82 anyone?

I can't comment on how you've used your 6SN7s, but my best amplifier uses lots of 6J5GT (half a 6SN7), and it sounds super (and measures well, too). I will say, though, that an awful lot of the older designs tended to run 6SN7 at low current and low anode voltage - they ideally need about 8mA through them, and that tends to require higher HT supplies than many of the older amplifiers could support.

unfortunately, the heater voltage is 20v. way too high for my main trans to cough out. do you think c3g is a good compromise for the c3m?

Click to expand...

It may be an improvement. The C3g is a completely different animal than a C3m, more than twice the Gm and more than twice the gain in triode mode.

John

Konnichiwa,

jlsem said:

It may be an improvement. The C3g is a completely different animal than a C3m, more than twice the Gm and more than twice the gain in triode mode.

Click to expand...

Wired as a triode the C3g is indeed the better choice. Using the C3m in triode mode is to waste an outstanding pentode to gain a mediocre triode....

Looking at the C3g in triode mode, very interesting. Have to get some for playing. Looks a lot like a 417A that way. I should get some before they are "discovered".....

Sayonara

Some driver circuits compared

Konnichiwa,

Prompted by all this "Is the 6SN7 a linear valve and a good driver for the 300B" I quickly put a few more or less common Driver circuits through P-Spice. All Valve models are pretty accurate and motly based on measured data, in some cases the basis is the manufacturers datasheet.

In my experience these THD (spectrum) simulations are quite accurate, if you use Norman Korens Valve Models and the Excel parameter finder for Norman Korens Models by Teodoro Marinucci

http://www.normankoren.com/Audio/Tubemodspice_article.html

http://web.tiscali.it/teodorom/

All circuits where considered with 70V Peak output into 220K (300B Grid resistor). This will drive a 300B to the complete full gridvoltage swing without grid current (Class A1).

I included the following circuits, without attempting to optimise any of them for either THD or harmonic spectrum. In fact, non are strictly speaking my own designs, they all come from publications, with the possible exception of the 6S45/417A circuitry.

In all cases assumed was a "bogey" 300B Amplifier, with 425V +B and most of this available to the Driver

Included are:

1) JE Labs 6SN7 DC coupled cascade Driver - as per schematic Diagram, slight variations are found in many commercial SE Amplifiers, including Audio Note UK, Sun Audio and Cannary (IIRC).

2) Aprilsound 5687 DC coupled cascade Driver from the Sound Practices 50 Amplifier. This was slightly adapted with 10K resistive load instead of Choke load but the ANode voltage was nearly retained at 305V.

3) Standard 6SL7 SRPP, +B 400V, Rk = 1k, Rk' = 1k - this is found in one variation or the other in many commercial SE Amplifiers, including Cary, the older Wavelength Audio Duetto and Opera/DIY HiFisupplies M500/Billie/Lady Day.

4) Russian 6S45PE as set up by me prior to using Choke Load, that was optimised "by ear" and used 250 Ohm Rk bypassed and 22K Anode load, +B 400V.

5) WE 417A plugged into the same circuit as the russian 6S45PE.

6) WE 310A in WECO 91B driver Circuit, as per original drawing.

For reference I have also included the data of the 300B at 79mA/350V/2K5 Load with 69V Peak Grid-drive. The 300B Model is based on measured Data from TJ 300B's and identical to a very high degree with a WE Datasheet based model.

So, how did they all stack up?


To interpret, the column "Higher" lists the highest level higher harmonic, usually the 5th but for the 6SL7 SRPP and the 300B it is the 4th Harmonic. The Column 2nd and 3rd list the second and third harmonics below fundamental respectively, the column THD

The results are intereating. In terms of raw THD the 6SN7 and 310A Circuits are by far the worst of the Field and 417A is best. Is that the whole story?

NOW, if one carefully observes what happens in an SE Amplifier we find that if we have a distorted drive signal we can cancel some of the Output Valves 2nd Harmonics if our Driver Stage distortorts a good bit.

http://digilander.libero.it/paeng/cascade_stages.htm

Now clearly we can reduce THD and IMD this way, but at a price. Higher order components will actually add on top of each other and thus while the THD is lower the higher order compone nts are boosted. If our driver stage has high levels of 3rd/4th/5th harmonics we must consider this to be "bad" as it will cause a distortion spectrum that is dissonant and unpleasant to the ear.

So, looking again we find that both 310A and 6SN7 have about as much 2nd harmonics level as the 300B and thus will cancel most of the 2nd harmonic produced by the output Valve. Yet the 310A has almost 20db less 3rd Harmonics and 16db less higher harmonics than the 6SN7, so overall it is highly likely that 310A Driver will combine a low THD for the whole Amp with low levels of high order harmonics, while the 6SN7 Driver will cause large levels of high order harmonics.

The 5687 in a similar Circuit as the 6SN7 is much more linear, around 7db lower 2nd/3rd harmonics and more crucially much lower higher order harmonics. So the overall Amplifier using this driver will have more THD, but comparably low levels of high order harmonics.

The 6SL7 also shows much better linearity and lower levels of high order harmonics compared to the 6SN7, performance is also slightly better than the 5687 Cascade.

The price for best overall linearity goes to my favourite, the 6S45PE, which shows the second lowest 2nd harmonics level (the lowest is the electrically similar WE417A) and among the lowest levels of higher harmonics and even 3rd harmonic in the test-field. I'm rather surprised at this as I never did a Distortion test on the original, I merely tuned by Ear towards what I percieved as great sound purity.

Well, some observations on some real world circuits used to drive 300B Output stages and their behaviour which is related to though not entierly determined by the different Valves. This is not to say that a given specific valve is a "bad valve", but it may a very bad valve for the given application, namely driving a 300B.

However, one thing it put's well to bed is that the 6SN7 is a linear Valve, certainly not in normal applications..... ;-)

Sayonara

code:

2nd 3rd Higher THD
300B -26.5 -38.0 -48.7 4.9
6SN7 -24.2 -40.7 -49.6 6.2
5687 -31.2 -48.0 -77.7 2.8
6SL7 -35.1 -50.7 -67.5 1.8
6S45 -39.1 -55.6 -69.7 1.1
417A -42.0 -49.9 -54.4 0.9
310A -23.0 -55.6 -65.1 7.0


It's interesting that the 310A has the highest distortion level since pentodes are more linear than triodes by a factor of five.

John