• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

LED or fixed biasing?

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No veggie tossing here. With cathode bias, whether RC or LED, there's some loss of effective B+. Yes, with RC, the grid-cathode operating point will shift as the tube ages. But you can/will get blocking at the cathode. Soi-disant fixed bias will allow the full B+.

LED will have no blocking at the cathode, so splits that difference- at the same time, it gives a visual indication that the tube is conducting and when it gets closer to clipping. It's considerably cheaper than fixed bias. It's not a panacea, just a reasonable engineering choice.
 
I never thought this thread would go this deep!
As for RC cathode bias and it's merits go, I must say in HIFI use ( that's all I was/am bothered when I asked this question) it does not sound as good as fixed bias.
Some may argue this and that, engineering facts, temp. coef.'s - tube aging blah- blah - but if it reduces fidelity in a hear-able fashion, it is out for me and to many, who don't use their treasured tube amps just to play the guitar!
I understand now that LED biasing has it's merits in low-level sections, and even power sections as SY has successfully shown.
If we can not keep the interior of our amps in a ventilated and steady temperature state, keep summer sun off them, or avoid facing them to open fires - then I suppose there is always solid state!
Robust as a tank, no need for any of this mumbo-jumbo!
But if we are going "tube" - forgoing the cheapness, reliability of SS - then we must be after something different, so a bit more effort in temp. management, slightly more cost and regular re-tuning must be accepted as part of the game.
Sorry I can't quote any formulas to back my assertion.
If I am after the reliability of a Corolla, then I should not go near a vintage Mustang!
 
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Thermal issues are certainly present with solid state devices. What isn't always there is the size/cost/difficulty in using stable circuits, feedback paths (electrical or otherwise) and possibly multiple devices... or the inclination to take triodes too far that way without good reason due to other underlying views.

If you like fixed bias then I'd stick with it.
 
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Fixed bias works, and I'm good with it. In fact I suppose that I see a clean bias voltage and a proper grounding scheme, and the cathode at ground, as some kind of an ideal for what it's worth.

You can use your expected output and desired signal to noise ratio to trace back and discover what level of ripple you can tolerate.

A supply I'm working on now. HT at the back, bias supply on the left and heaters on the right. These will have reservoirs (and RF filtering) where they enter the amp chassis and decoupling at the valve socket, except the heaters which are AC (ballast loaded).

Bias supply uses three small (7-12VA) power transformers using two as chokes in a choke input supply [rcLCLCrc(dec)].
 

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Nice! good work.
I have three power-amps, two are EL84 pushpulls, one is an OTL 6C33C mono block.
My supply arrangements are very simple, but I think my grounding and filtering is good (some may disagree).
I have zero ripple problem on all three, well almost!
I mean you need a very quiet room, with an ear to within two inches of the bass driver, and concentrate to hear anything, I have just as much hiss.
Infact when I switch on my DAC connected directly to the amps, I can hear it
, I think that is quiet enough.
My problem/aim is getting maximum resolution, clarity and avoid compression of dynamics.
I do not listen very loud - I think people turn up their HiFi amps, because they can't hear what they want to hear, it is psychological.
If resolution is there in the first place, I don't feel like the need to go to 11 on my volume control!
 
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Thank you. When you turn up the volume you don't want it to sound any different except louder, and I happen to think that diffraction, and a few other speaker issues are most responsible for these clarity issues. These aside..

I once built a mu-follower pre stage using both halves of a 6SN7 per side with an LED for bias which I recall performed flawlessly from my point of view.

Even if I wanted some swamping of the cathode, say some 400 ohms on those 6SN7s with a low standing current I could still use LEDs to make up the rest of the bias voltage. Or normal diodes, rechargeable batteries etc. I've noticed some small differences here.
 
Maybe the mathematics give the reason to the led, maths give the reason to SS amp from tubes and I prefer 100 times tubes . For me the led biassing when listen the piano at high frequencies, the sound change to a little metallic or less natural than biassing with battery or resistor, resistor+cap the sound is more natural or gentle. IMO


Is possible that not all led give the same sound, I have a friend that confirm that difference brands give different sounds and same of them are very bad.

Maybe the SiC diodes open a new a more stable way, I tried with Ultra-fast HER207 and don´t like the sound, soon I will try the SCHOTTKY Cree CSD01060A. for the moment battery and resitor+cap win.

Any comment SCHOTTKY from Led or battery biassing?
 
For the easy to drive tubes with low bias voltage requirements like the EL84/6BQ5 to 7591 types, check out Dave Gillespie's EFB which is a fixed bias mod which floats off the B+ voltage and uses a simple inexpensive negative voltage regulator so no transformer is needed saving there also. It gives improved sonics over cathode bias and longer tube life as you can adjust the bias to the best operating point. If you google , you will find many happy users.

Dave's Lab
 
What you are actually referring to is "Grid bias". Grid bias is when you adjust a negative supply with a trimmer to reference the signal input. "Fixed bias" requires the cathode to be elevated with a (hefty) resistor, while the grid is referenced to ground.

Grid bias is a nice solution when you don't have enough B+ to do "Fixed bias". It has the disadvantage that you need an additional negative supply secondary from your mains transformer. It also has the disadvantage that you immediately introduce new AC noise into your audio signal.

Fixed bias : Has the advantage that you won't be introducing new AC noise into your signal, such as with "Grid bias". You also don't need an additional negative supply winding off your transformer. It has the disadvantage that it will need higher B+ than "Grid bias". So perhaps not something for the novice builder... Also has the disadvantage that designs often need to bypass the cathode resistor with an electrolytic capacitor. I won't go into all the things you might not want this electrolytic capacitor though. You also need a substantially rated cathode resistior to do Fixed bias.

In any case, I would chose Fixed Bias over Grid Bias any day for anything you aspire to become Hi-Fi. Just my opinion though...

Ian

This terminology is very wrong and confusing as it can be.

Grid Bias) The grid voltage is developed in one of two ways: one develops a negative potential across a very large resistor as stray electrons that are captured by the grid leak off. Often called "grid leak bias"

The other way is used with RF amps, and develops the bias voltage as the positive going peaks turn on the G-K parasitic diode, and the negative peaks are integrated into a DC voltage by the grid resistor and the coupling capacitor. If the master oscillator doesn't start for some reason, you risk a burn-out if no protective bias is provided.

The same effect can occur with capacitor coupled finals. In an audio amp, it's undesirable as it moves the total grid bias towards cutoff, and into less linear plate characteristics that has negative sonic performance consequences.

Fixed Bias) The grid voltage is "fixed" with an external, negative rail independent of applied signal. Used with audio and RF amps. User adjustable controls can be provided for setting Q-points ("univrsal" RF amps that can operate as "linears" for AM or SSB, or Class C amps for FM or CW or some other digital or quasi-digital modes) and/or balance between PP pairs.

Auto Bias) Grid bias developed across a resistor between the cathode(s) and DC ground. Has advantage of user friendliness as it's one less thing the end user need not concern himself. Main disadvantage is that all screen/plate voltages are reduced by the bias voltage. Often used in RF stages to provide protective bias.

LED bias is a type of auto bias that replaces resistors with the very low impedance to ground of forward biased solid state diodes. The degeneration caused by not including a bypass capacitor is minimal. Power finals will require a lot of LEDs in series/parallel, but can always be arranged in interesting patterns of blinkenlichten.
 
Neck Cramp ≡ LOL
The cap doesn't affect recovery 'cuz its on the "base" side of the emitter follower, and serves just as a low pass filter for the variable voltage drop potentiometer.

BTW: the combination I put up there that didn't include a Zener gets near-absolute (zener-like) regulation because of the near-constant VBE voltage drop. Kind of like a LED of low forward voltage. In the end, the constant-voltage is straight forward: Vreg = (R1+R2)/R2 × VBE of the darlington pair (1.26V in diagram).

If I leave the R1 (470k) alone and put a 15K "safety" resistor in series with a 25K Allen Bradley trimpot, one can vary the voltage from (15k + 0k) = 40.7 V down to (15k + 25k) = 16.4 V. A plenty broad range for output tubes of ordinary garden variety.

I've included a followup with the Class-B mode push-pull behavior. It demonstrates saturation/cutoff recovery. I used 1 μS sim steps… to catch the glitches.

GoatGuy

Hey Goat Guy, did you ever end up building and testing this vs. led?
 
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