Out go the cathode's resistor and cåp - In goes the BATTERY

[Guitarist]

This might best apply to higher gain preamps.
From AudioKarma and AudioAsylum threads I've decanted a few notables for us. I'll drop these in soon and report back. As well, I think even AAAs would be just fine.

One more point is that a rechargeable battery in cathode circuit does pass AC current freely. However, since these batteries have very low internal impedance (typically less than 1 ohm when fully charged), a few mAs of AC current only generate a few mVs of voltage, about 1/1000 of the AC voltage when a 820 ohm resistor is used. As such, there is no need for bypass cap and there is no negative feedback in cathode circuit, leading to much higher gain.

To get the required 2V on the cathode, I had to raise a 1.2V battery above ground, with a 280 ohm resistor. I tried the "D" cell, against the "AA", and the "D" cell definitely sounded better, with the 6SL7 driver.

Use some double adhesive tape to secure a pair of single AA battery holders within proximity of the spot vacated by the 1K/100uf cathode resistors/bypass caps. Solder the red [+] wires to the cathode and the black wires [-] to ground, and then install the batteries.

Since you are bold enough to use battery bias for the input stage, you might want to replace the resistor with a series of rechargeable 9 volt (7.2 volts, in reality) batteries. This would also define a fixed bias voltage that would be renewed when the amplifier is in use! [this is for biasing of the output stage]

LEDS

In every case, a battery or LED is far superior to having a cap in the cathode. I'll never allow that to happen again.

The cheapest red ones are 2.1V/2mA, which is fine for a 6SL7; yellow, green etc. come in 20-60mA ratings. The shorter lead is the cathode, which goes to ground. Place it in the cathode circuit, turn on the amp and watch it light up! The 2 volt versions will give you about 1.8-1.9 volts bias.

The cheapest red ones are 2.1V/2mA, which is fine for a 6SL7; yellow, green etc. come in 20-60mA ratings. The shorter lead is the cathode, which goes to ground. Place it in the cathode circuit, turn on the amp and watch it light up! The 2 volt versions will give you about 1.8-1.9 volts bias.

[tbryanh]

"In every case, a battery or LED is far superior to having a cap in the cathode."

I am wondering if the battery, LED, and bypassed resistor are being compared at the same bias point?

It might be that you don't like the bias point the bypassed resistor is providing and that a different value resistor(s) would give you the bias point(s) you like.

[Enzo]

Oh dear lord, we are infecting our forum with audiophile stuff now.

In every case, a battery or LED is far superior to having a cap in the cathode.

I love this. In EVERY CASE a battery or LED is FAR superior. Two totally divergent techniques are FAR better. However superiority was measured.

[Jag]

LOL

Enzo, are you sure you're not my old high school electronics teacher?

[cminor9]

hahaha

I think after answering the many stupid questions that many of us have put out here, one gains the right to be a bit crusty.

Besides, blanket statements are ALWAYS wrong

[Enzo]

I never ever make blanket statements. You can never trust them, I always say. Everyone knows that.

[Robert M. Martinelli]

My background ( automation engineer ) unfortunately always brings me to consider the worst case, and, apart from all the "audiophools" crap, this design is not safe "per se" IMHO, and it can damage the tubes if the battery ( or batteries ) voltage falls down, bringing the cathode closer to GND potential ( and consequently making the grids less negative with respect to the cathodes ). There is nothing to protect the tubes should this occur, so I wouldn't feel comfortable having a "battery operated bias system" inside my amp with nothing to protect the tubes should the batteries fail.
Best regards
Bob

[Chuck H]

Just to add to what Enzo has stated...That these are two divergent techniques.

The use of a battery or any other "fixed" bias method will sound different from an RC on the cathode. Better becomes subjective since guitar amps are intended to color the input. I would dare to say that the alternatives to the typical RC bias system that give absolutely lower distortion and higher gain are definitely NOT always superior as their goal is sometimes contrary to our intent. Example:

"For the tone I want I need a gain stage that employs some local NFB in the low end but has a less restricted top end."

An RC can now do for you what a battery can't. You could immitate the effect with the decoupling circuit. But why bother. Also, there is a definite difference in feel relative to the specific impedance matching from stage to stage. So in that case you couldn't exactly duplicate the effect of the RC cathode circuit with the decoupling circuit.

FWIW it's a fun idea. And it should have been presented that way. It could show promise for some circuits in guitar amps. But to say it is always superior and imply that the RC cathode circuit is antiquated and crude is not only wrong, it's going to instantly alienate the subject from any positive outcome. And that is always vastly inferior on an information forum.

Chuck

[mooreamps]

It could show promise for some circuits in guitar amps.

Chuck

It already has promise for guitar amps. I have always biased my pre-amps this way. But, when ever I write about it, I always get flamed for it. So, the surrounding circuitry has to be adjusted to compensate for the extra bass responce you get for using a led ; and should use the led's that give you closer to 1.5 volts forward voltage drop.

-g

[Robert M. Martinelli]

Just to add to what Enzo has stated...That these are two divergent techniques.

The use of a battery or any other "fixed" bias method will sound different from an RC on the cathode. Better becomes subjective since guitar amps are intended to color the input. I would dare to say that the alternatives to the typical RC bias system that give absolutely lower distortion and higher gain are definitely NOT always superior as their goal is sometimes contrary to our intent. Example:

"For the tone I want I need a gain stage that employs some local NFB in the low end but has a less restricted top end."

An RC can now do for you what a battery can't. You could immitate the effect with the decoupling circuit. But why bother. Also, there is a definite difference in feel relative to the specific impedance matching from stage to stage. So in that case you couldn't exactly duplicate the effect of the RC cathode circuit with the decoupling circuit.

Chuck

Chuck,Enzo,
I think what you said touched the heart of the matter; in my previous post I intentionally focused on "tube safety" issues when using a "battery" bias; when it comes to sound, or better to TONE, the thing many often overlook is that guitar amp are not ( neither they have to be ) Hi-Fi amps, and using an LED indeed denies the possibility of shaping a stage's response/sound by means of the cathode RC network. As someone said, sometimes we DON'T want excessive linearity/fidelity, as the result would probably be a duller/flatter sound, and this is especially true when it comes to guitar amps IMHO.

Best regards

Bob

[Enzo]

In fairness, my objection was to the hype, not to trying alternative circuits.

Using a battery I think only has potential to damage tubes when creating a bias supply like for power tubes. certainly a preamp 12AX7 that gets its cathode shorted to ground won;t burn up.

Seymour DUncan was using diodes this way in their amps 20 years ago. And who knows how much farther back such application goes?

Of course one would have to configure a whole circuit when going to an alternative circuit, but then again, if you had a diode biased circuit, you'd have to reconfigure it to go to RC.

I don;t like the battery idea from a practical point of view. I don;t really want to rely on a component like that stuck in a chassis subjected to all that heat. The RC components rarely fail, they are under little or no stress, and if they do nothing bad happens except the circuit doesn;t work, but a battery is a little chemical saboteur waiting to leak all over your stuff.

[Chuck H]

but a battery is a little chemical saboteur waiting to leak all over your stuff.

I know a few people like that...Mostly under the age of two. Also some four legged friends. Batteries aren't cute enough to merit the same level of tolerance.

Chuck

[jrfrond]

Audiophile tweaking in guitar amps.

Screen doors on submarines.

Both useless. Good for spinning one's wheels though.

[Guitarist]

Thanks for all the insights. I guess I should've added a giant set of pull quotes to all those/seven excerpts - having just tried it a bit, I can say, that it's was a gainy move. I just created a brief introduction for their findings and opinions. In yesterdays case, this is a Fender/Alembic F2 type of preamp stuffed into a volume pedal that was meant to tubify things prior to the Guitar Rig 3 or Digitach GSP 11101 modellers it has given more levels than I can deal with. Another approach, is to use it with my rack, tube power amps and keep the preamps and overdrive pedals under foot.

Right after swapping in the charged AAAs, I did in hear something different. I'd say, sonically, an improvement - there is something to it. Unveiling is a perfect way to describe it. I'd love to hear your real world opinions. To say this is subjective is rather obvious right? But, I love good tone and spend hours/years working forward to this end from the perspective of a guitar player; as most of us likely do here. I think this would work well in, say, modding a JCM 800 preamp (or the B-stages of a Dµmble overdrive section); - that would be the acid test. The other would be the real acid test...

On this soldering session, I realized how bad a IRF820 coloured the sound and will never use one again. I should've taken Trace's advice from a few years back. The fet thingy was used as a CF. Using a 12au7 made it unneeded anyways.

As you can see from my horrible soldering skills, it is still in the prototype stage and I'm wrestling with a tiny bit of noise. By the way , I found pairs of AAAs for $2.50 but also grabbed a huge bag of 5uF caps for later. (Main Electronics, Vancouver- highly recommended.)

The mod was really easy to do. I think the main result is perceived clarity from the extra gain from not having the capacitor leaking levels - do I have that right?

†By the way, there is an EH toroid under the pedal with leads poking inside the pedal, 280 Volts- alright! I referenced the Hoffman approach because looking at the 2x sided EH 12AY7 preamp induces a headache.

[Bruce / Mission Amps]

On this soldering session, I realized how bad a IRF820 coloured the sound and will never use one again. I should've taken Trace's advice from a few years back. The fet thingy was used as a CF. Using a 12au7 made it unneeded anyways.

Bad?
For what it's worth... using all tubes is cool but I've used LOTS and LOTS of IRF 800 series Power MOSFETs for CFs in modified and hot rodded vacuum tube amps and with respect to them adversly altering tone or coloring the tone (etc) ... if setup correctly, their impedance is absurdly low and I think they are like 99% invisible with respect to doing anything negative to the tone.
I wouldn't be surprised if the tonal wiggle factor of using two different vacuum triode CFs would have a more effect on the overall tone then using a good IRF820-830 (etc.).