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-   -   A question of bias... (http://www.diyaudio.com/forums/tubes-valves/19394-question-bias.html)

DrG 25th August 2003 08:30 PM

A question of bias...
 
A remark made in the 6336 thread got me thinking... why not use a current-source to bias an output tube? By-passed with the required capacitors of course. In a PP stage balance should be absolute, no need for adjustment... ever.

Anybody have any experience or thoughts on this matter?

SY 25th August 2003 08:37 PM

Bypass? Why?

dhaen 25th August 2003 08:37 PM

First thoughts...
 
Hi,

What kind of time constant will you give it? And what will happen if grid current flows?

Cheers,

Sch3mat1c 25th August 2003 08:52 PM

If you remove the bypass cap, you can drive from one side (ground or NFB to the other tube) and completely skip a phase inverter! Of course this necessarily burns up a good bit of power in the CCS and output tubes (class A only), so for any efficiency you must use a seperate splitter stage.
It won't balance the bias current because the way it works is 5+5 = 10 = 6+4. It won't stabilize bias, it will only provide a smooth, level base for it to operate on (for bias, a steep valley is preferable for stability: imagine a ball rolling around in a bowl, it wants to stay in the center).

Tim

DrG 25th August 2003 09:17 PM

but only class A
 
A single o/p LTP is possible with one CCS. But will sound crummy if driven into class AB/B when the CCS reaches the limit of it's current-sinking ability: ie tail-current>CCS, shutting down tail 2. Or am i wrong?

I was considering replacing conventional by-passed cathode resistors with by-passed CCS's.

Has anyone seen this done?

analog_sa 25th August 2003 09:25 PM

I am almost certain i've seen it done in a popular eighties british amp - parallel EL84s - name eludes me at present. I think they used 7805s hooked as current sources. My first thought was that the 'sound' of the CCS will prevail and 7805 would not be my choice of flavour in a valve amp.

PRR 25th August 2003 09:28 PM

> why not use a current-source to bias an output tube?

Using only hollow-state technology, the current source is either a fatter tube than the outputs, or wastes a lot of power.

With a sillycon device, it is quite practical.

This does of course force the stage to ONLY work in Class-A. If one side goes near cut-off, the output just clips. In contrast, most tube "Class-A" stages really do get into Class AB at full power, one side near cut-off and the other side covering the difference.

Distortion is in fact higher unless you bypass. When one side goes near cut-off, its cathode impedance rises. That reduces the effective gain of the other (active) side, causing soft-peaks. If it stayed fully Class-A, the variation of the two cathode impedances nearly cancels. Or with resistor bias and going just a little into cutoff, the resistor tends to be of a value to limit the loss of gain on peaks. Bypassing the resistor is not needed in true A operation but usually helps reduce peak distortion because most "A" amps do swing pretty near cut-off.

My gut reaction is that for audio and loudspeakers, it would be better to let the resistor and capacitor support AB swings. A transient at a speaker low-Z dip may swing into Class AB even if the amp has plenty of 8Ω power in Class A. Distortion rises as it goes into AB but not as bad as the clipping imposed by a constant current source.

However I have seen a guitar amp that in effect used constant current drive. It is certainly practical and may have unexpected advantages.

In a VERY different field, vari-gain circuits for recording limiters, I finally proved to myself that constant current biasing actually increases distortion and/or reduces usable input level, because of loss of Gm near peaks. If the CC source is bypassed with a resistor, it gets better. A good value for the resistor is very nearly the value of bias resistor you would use for a non-vari-gain stage. Which confirms why many P-P amps with common resistor and no bypass sound fine.

Try it. With and without bypass(es). At low level it will work fine unbypassed. At high levels you may (or may not) prefer the sound with a bypass. Don't stint on the bypass cap: you are not bypassing the resistor, you are bypassing the tube's cathode impedance (1/Gm) which may be 100 or even 50 ohms in fat tubes. And you really want to bypass small differences between the two tubes. 100µFd is usually a minimum, and 1,000µFd may be audibly better. With 6L6 et al, 1,000uFd 50V is not expensive.

> no need for adjustment...

With one CC source, a P-P stage is not sure to balance, as Tim says and I'm sure you realized. You could use two, but they must be bypassed.

> you can drive from one side

Sure. That's a long-known cheap trick. You need double the drive, but with pentodes that may not be a problem. The real issue is that it goes out of true push-pull at high level. In effect it morphs from push-pull at low level to a funny kind of Single Ended at high level. You retain the advantage of no-DC in the transformer which is nice. You may save a tube. Yet you just do not see this scheme in commercial designs from the Tube Era. Either you get a cheap SEP, or you get full push-pull.

DrG 25th August 2003 09:56 PM

thanks chaps
 
Thanks guys... esp PRR. Does analog_sa perhaps have that EL84 circuit handy??

Thanks

dhaen 25th August 2003 10:07 PM

Postscript
 
Hi,

In essence it is no different from any other auto-bias system. It does however have some disadvantages:
The signal must pass through it.
It assumes that the valve needs constant current, limiting it's application to A1.
A valve failure may take-out the bias circuit.

A very small sense resistor in the cathode, who's voltage is used to determine grid voltage, does not suffer these drawbacks, but lacks the simplicity.

Also, there is a view held by some, that PP sounds better when slightly off-balance.

Cheers,

fdegrove 25th August 2003 11:38 PM

Hi,

John,

Quote:

Also, there is a view held by some, that PP sounds better when slightly off-balance.
Those few, and you can count me in, are actually in the good company of RCA who actually described what was going on in their manuals.
I have the article somewhere in my archives...

Peter,

If that's the Beard I think it is, I can only say that it was almost impossible to bias it correctly for longer than a week.

That one was a techs' nightmare, no matter how well you paired the valves on the testers it just was no good.
To add insult to injury, B+ was higher than usual making it only fit for the more sturdy E84L/7320 which don't come cheap if you want a decent pedigree.

PRR,

Once again your posts are a joy to read.

Too bad it needed proof to realise what was happening when the cathode R isn't bypassed but you just confirmed what my ears often told me too all those years.

With cathode bypassed removed on gear that wasn't specifically and competently desisgned for it I often noticed abrupt clipping.
Not exactly a pleasant sounding experience I hasten to add.

Since I never came across a bypass cap that was sonically transparent I try to avoid them in stages where voltage amplification is high.

Thanks for the great posts.

Cheers,;)


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