JJ 6V6 bias ?

[Jazz P Bass]

I use this bias calculator recently.

I use a scope.

[daz]

Well, thanks all. I guess i'll just run em at a very middle number like 17 i got from the calculator mozz posted a link to above. It actually has JJ as a choice when you pick a tube and 17.5 is cool bias. Thats way i'm surely not too low and they should run cool and last. I only asked because i can't really experiment at home because theres no way to really hear the differences at the volume levels i'm limited to here. the outputs aren't even breathing slightly hard at the levels i can get away with.

[mozz]

I use a scope.

Use a scope to set bias? Explain, i thought that was a no-no. Or are you just joking?

[The Dude]

I can't speak for Jazz P Bass, but I'm pretty sure he wasn't joking. Lots of folks use scopes to bias. You turn up tube current slowly until crossover distortion is gone and then maybe give the pot a slight bump to make sure you stay out of crossover distortion in lower voltage conditions. Done. I'm not sure why that would be a no-no. Many techs have done it that way for years.

[Justin Thomas]

If you have the tools, use them. Biasing by scope is probably the fastest way, AND the best way to accomplish what the OP wanted: clean and efficient. Less power wasted as heat at the plates => more power available for signal! Or so I gathered from Juan, I think... and even if the saved power doesn't go to more signal, it doesn't go to melting plates and screens (the REAL tube killers) either.

I'm willing to bet that if I had a scope to bias with, I could save a good bit of power. I bet there's a wide margin between "a little bit of room for crossover" and "loud hum & mushy bass" that my ear could never detect, but a scope could. A scope is a technician, but the ear is an aesthetician.

Justin

[The Dude]

Let's remember that bias (idle current), has little to do with output power. Many are confused by this. Biasing your amp hotter doesn't make it louder. We are providing an idle current or voltage to the grid so that the tube conducts slightly with no signal applied to the grid. Without bias, it would take a larger signal to get the tube conducting and that is what causes crossover distortion with too little bias. Neither tube (push-pull) is conducting until signal swings past the level where the tube begins to conduct. So, you get a "notch" in the middle. Bias is somewhat irrelevant when the waveform hits peak and therefore has little to do with power output level. (all based on "normal" conditions)

[Richard]

Just read two conflicting reports on biasing with a scope. Duncan, on the scope biasing side, does mention it's not for amateurs.

Scope biasing

"Crossover Notch" Biasing - Why It Should Be Avoided

[Justin Thomas]

Exactly. But most people assume that if they bias a hot as possible then the sound will he louder.

I took my weird stance because I read somewhere recently that there is more power available in the whole system to do everything more efficiently when we're not wasting heat. In other words, heat ANYWHERE (other than the cathode, obviously) is an enemy to efficiency anywhere. It wasn't saying one is related to the other, but simply that excess heat anywhere is just waste with no added benefit to anything.

Add to that that all that heat just cooks every other thing in the amp. That bare chassis might be drilled with a thousand holes, but it's pretty airtight once you put in the parts, add faceplates, and bolt it in a chassis...

Justin

[The Dude]

Justin, that's why I used words like "somewhat", "little", "normal". It's a general description of things. If a person wanted to get nitpicky, you could argue that tube efficiency changes with temperature or that higher bias results in ever so slightly lower B+. All things that slightly effect output. But, certainly nothing you are going to even remotely detect as "louder".

[g1]

I use the scope method but double check cathode current when done. Once you are fairly experienced with the method, your particular test rig, and various common amp models, you will know what it 'looks like' when correct. The double checking of the cathode current takes care of the rare oddballs that end up too cold or too hot when you think they are right.
I'll note that with my rig I don't find the notch totally removed when bias is correct in terms of cathode current.

[Jazz P Bass]

Just read two conflicting reports on biasing with a scope. Duncan, on the scope biasing side, does mention it's not for amateurs.

Scope biasing

"Crossover Notch" Biasing - Why It Should Be Avoided

Where Aiken blew it is he says that you set the bias at full volume right before the signal flattens out.

That is BS.

I would never set up bias that way!

It is an idle setting.

So a small output signal will suffice.

[Richard]

It seems like the notch represents a larger portion of the output signal at lower outputs and would be more noticeable. In a side note, when I had a bad tube in my SRRI and was getting only one side the cutoff happened at nearly the same setting on the volume knob as when loud chords would start to distort. Would a non functioning tube change the characteristics of the other tube noticeably?

It seems like the notch would only truly disappear if the tube was never driven to cutoff (class A) or one tube started conducting at the same time as the other cut off.

[frus]

Where Aiken blew it is he says that you set the bias at full volume right before the signal flattens out.

That is BS.

I think he says so too:

The bias voltage was adjusted until the crossover notch "just disappeared", after first setting the input drive level to the point where the output was just entering clipping, as is claimed to be the "correct" procedure, by those who promote this method.

[Rob's Radio-Active]

I agree. There are good points raised on the Aiken white paper, and I am familiar with it from way back, but it assumes you are trying to bias while over-driving the output tubes and drawing grid current. Who would do that? Everybody knows that will cause a bias shift, anyway. And yes, you still need to check plate dissipation after.
cheers
Rob