A very stupid mistake!!!!!

[Joel]

...No opinions about the cathode follower output idea?

I'm researching the idea of a 6L6GC as a triode cathode follower, with a choke cathode load, in a parafeed configuration, as we speak. What do you think about that?
I will use a Hammond 125BSE (very tiny) as the parafeed output transformer, with the 5k primary selected. 20H/100mA cathode choke.

Could be a nice n´grunty driver stage for a big triode amp...

Please don't say "grunty" - it's so Brett. Also, do a search on "slew rate distortion" and you'll see that the high current driver stage everyone thinks they need for the "big triodes" is actually a myth. The current required to neutralize their input capacitance is actually quite low.

Anyway, please post your CF output stage.

joel

 

[Fuling]

Wouldn´t a 125B (no SE) be a better choise for a parafeed amp?
No airgap should mean higher primary inductance, right?
By the way, I´m waiting for the parts for a 12B4 parafeed amp to arrive. I ordered them in february, but shipping from Canada takes time...
30H/40mA chokes and 125C OPTs. I also ordered two of those 150H chokes for the input stage, but those will probably end up in the driver stage in my 6B4G amp together with a pair of 6SN7:s replacing the current 6SL7:s.

Since there´s no airgap in my small surplus 2k/8ohms trannies I´ll
ditch the cathode follower idea for now. They will probably work alot better in a parafeed circuit, where there´s no DC through them.
Not that I like pentodes, but since I have a few EL86:s lying around why not put them in service? They seem ideal for this application.
Refer to the classical Philips SEPP OTL amp for 800 ohm speakers(You know the one, right?), but replace the lower pentode with the power section of a 6EW7 and add the other section of the 6EW7 section as an input stage. Should work, I think.

 

[7N7]

The 6EW7 looks alot nicer with a horizontal load line, which I guess represents a CCS or plate choke load.
Could be a nice n´grunty driver stage for a big triode amp or maybe even a hybrid

316A who frequently posts here has done active load parafeed experiments; I have no experience with these.

I agree that the second section of 6EW7 (as I wrote earlier) would make an excellent driver for a big valve.

I disagree that a weedy valve will do; you need current to drive big valves (this was all thrashed out in a long previous thread). Additionally, beefier valves have lower anode resistance, very important when your driver is looking at the low value of grid leak traditionally used with big valves - important in terms of distortion.

I am currently building a new stereo mid-range amplifier; this (like my INSTOCK project - previous thread) will use beefy DC coupled cathode followers to drive the output valves and as with the INSTOCK project I hope to couple without grid leaks: if I got away with it with 6528 with its very high gm, I should be able to manage with the triode-strapped dual beam tetrodes that I am using now.

Incidentally, at my operating point, 6528 has ra of about 390 ohms per section. Careful measurement showed that best distortion was achieved with a load of 6k a-a - far more than would be used traditionally.

7N7

 

[Joel]

...I disagree that a weedy valve will do; you need current to drive big valves (this was all thrashed out in a long previous thread).

Let's make a distinction here - you are saying that, subjectively, you think a high current driver sounds better, right? Surely you don't disagree with the statement that, mathematically high current is not required, do you?

 

[Joel]

Wouldn´t a 125B (no SE) be a better choise for a parafeed amp?
No airgap should mean higher primary inductance, right?

Yes, all things being equal - but in this case they are two different transformers, and the BSE has the better specs.

 

[7N7]

Let's make a distinction here - you are saying that, subjectively, you think a high current driver sounds better, right? Surely you don't disagree with the statement that, mathematically high current is not required, do you?

You have offered two areas in which I have to make a distinction:

First the observation that I think a high current driver "sounds" better and second a mathematical matter.

Neither applies I'm afraid; I doubt I would detect a difference aurally, and I certainly could not do the necessary mathematics.

However the arguments I referred to in my previous posting, made by those better qualified than I were very convincing.

Further, my point that one needs low output resistance: this I consider to be irrefutable since I have first read about this, second tried it, and third observed the results.

High current every time for me - it is worth bearing in mind also that high gm (usually a consequence of high current) does improve noise performance.

7N7

 

[Joel]

...However the arguments I referred to in my previous posting, made by those better qualified than I were very convincing.

I see. Well, could you explain one or two of those for us? (or point me to which ones you are referring) Because I have not run across any that held up under analysis.
The High Current Myth is a derivation of the use of tubes with low plate resistance as driver stages. The low Rp is what was originally sought, and the high plate current was an (unfortunate) side effect. Those not aware of the necessary calculations have since focused on the current as the important factor, which simply is not the case. A tube like a 2A3 needs only 2 mA in a driver stage, and this can be proven both experimentally, and on paper, quite easily.

 

[7N7]

I see. Well, could you explain one or two of those for us? (or point me to which ones you are referring) Because I have not run across any that held up under analysis.
The High Current Myth is a derivation of the use of tubes with low plate resistance as driver stages. The low Rp is what was originally sought, and the high plate current was an (unfortunate) side effect. Those not aware of the necessary calculations have since focused on the current as the important factor, which simply is not the case. A tube like a 2A3 needs only 2 mA in a driver stage, and this can be proven both experimentally, and on paper, quite easily.

Unfortunately, I cannot remember the title of the thread - I certainly did not understand the technical arguments (way above my head) but since I opt wherever possible for the lowest anode resistance (or at least high gm in my cathode followers) I am obliged to be on the high current side - a glance at the anode characteristics is usually enough to convince me.

7N7

 

[fdegrove]

PEACE.

Hi,

Please don't say "grunty" - it's so Brett. Also, do a search on "slew rate distortion" and you'll see that the high current driver stage everyone thinks they need for the "big triodes" is actually a myth. The current required to neutralize their input capacitance is actually quite low.

There we go again...hardly a month has gone by and Mr. Whimpy Driver rears his uggly head again?

There's more to life than overcoming the Miller capacitance, you know.

Why some people, like yourself, insist on taking pride in designing for mediocracy is beyond me.

Brett's going to love you for it, Joel, and so do I to name a few ...we often wonder why we go through all the trouble explaining it all just for you, just to see you adhere to the same old logic again and have the nerve to insult all of us in the process.

Whatever you do at home is utterly your own business, but must you really spread it as the ultimate "Truth" ?

Be a man for a change and admit you can't read beyond an RCA datasheet or better still prove all of us wrong?

Cheers,

 

[EC8010]

High current drivers

Hello Joel,

I'll disagree that a high current driver is not mathematically required. Slew rates were discussed ad finitum on a previous thread, but just because a stage can swing the required current without hitting cut-off or grid current does not necessarily mean that it can do so linearly. Capacitive load are particularly unpleasant because they force vertical movement of the loadline (turning it into an ellipse), and gm is non-linearly dependent on Ia. Once you start looking at the loadlines and real valve characteristics, the rule of thumb 10x doesn't seem quite so daft after all.

 

[Joel]

Re: High current drivers

...Once you start looking at the loadlines and real valve characteristics, the rule of thumb 10x doesn't seem quite so daft after all.

Hmmm... it seems more daft to me.

Frank - you feel that I act as though I have all the "right" answers, and that everyone else is ignorant? Sounds just like how I would describe you.

Perhaps you can see how I would think people telling me "always use a grunty driver" is just as dogmatic, and they bring the subject up far more than I do.

cheers.

 

[316a]

'drivers'

Mr 'Wimpy' is 100% correct , everyone else is daft , myself included . 10x is absolute overkill and will not improve the swing or distortion at all . I'm ripping out my meaty 2C50's and replacing them with 6SL7's . Do you think 100k anode loads with 175V HT will be enough to provide 100v p-p swing for my 6C33's ? 6C33's have 100k grid leak .


316a

 

[Fuling]

I changed the circuit into a parafeed with a pentode CCS today, and it worked. 1,5W out before visible clipping, but the output signal seems to be slightly unsymmetrical.
Did some listening tests and decided that if I can find a suitable power transformer in my junkbox or get one really cheap, then I´ll build it. I don´t wanna spend big bucks on this project.

 

[fragman56]

Good job.

I just get excited when my system works!

Rick

 

[fragman56]

BTW

Any chance you could post a simple schematic of your final design?

Thanks,
Rick

 

[Fuling]

Due to the graphic nature of this schematic, viewer discretion is advised :

 

[7N7]

Re: 'drivers'

Do you think 100k anode loads with 175V HT will be enough to provide 100v p-p swing for my 6C33's ? 6C33's have 100k grid leak .


316a

Possibly - but only with a dab or two of "Tube-o-lator" (that's gone very quiet hasn't it!)

Finsihed that clock yet?

7N7

 

[fdegrove]

Tube-o-later.

Hi,

LOL.

Bobken has suffered a blown Scanspeak tweetie that had to be ordered in from Denmark.
He must have had a bit too much "drive" somewhere along the line.

I expect he'll post his findings as soon asap.