JJ 6V6 bias ?

[daz]

Just wondering if anyone knows where these should be set. Weber's bias calculator says with 400v on the plate to bias a regular 6V6 to 18mA for average and 21 hot. But of course JJ's are a heartier animal. I have em at 18 right now but i want them as clean and efficient as possible so i thought to go 21, but i assume JJ's can go higher. Where would you all set em?

[Leo_Gnardo]

That's just about where I'd park 'em. I doubt you'd hear much diff between 18 and 21 mA. "Right around there," close enough for jazz, don't sweat a milliamp or 2 this way or that. You could dial up 50 mA with JJ's but I don't think there's any advantage to it.

[Justin Thomas]

If you want them as clean and efficient as possible, the LOWEST current you can get without objectionable distortion at your chosen volume is the way to go. The more current you are dissipating at idle, the more WASTED heat you're making.

Reference the Peaveys and Music Mans that bias as coldly as possible, and are the most efficient amps around. 10mA on a 6<L>6 is normal. Or a Super Twin.

High idle current = INefficiency.

Justin

[daz]

You could dial up 50 mA with JJ's but I don't think there's any advantage to it.

Literally? 50? Not that i will but curious.

[frus]

BTW, a guy had 4 x JJ EL84 in his Laney, and took out two tubes. In cathode bias.
He was running them like that (18W/tube idle) for 2 years, with no visible or audible detrimental effects (except the burnt JJ logo on them)

I know, they are not 6V6, but they evidently build tough tubes

[Leo_Gnardo]

Literally? 50? Not that i will but curious.

JJ's data sheet claims 14 watts plate power, that would put you at 35 milliamps. I've found they can stand about 19 in real life. Some amps have come in with JJ's installed, running that hot, and no meltdown. Of course I dial 'em down to a rational level.

[Mike Sulzer]

JJ's data sheet claims 14 watts plate power, that would put you at 35 milliamps. I've found they can stand about 19 in real life. Some amps have come in with JJ's installed, running that hot, and no meltdown. Of course I dial 'em down to a rational level.

Emphasis on "rational"! I pushed a JJ just little beyond that, and it lasted just a couple of minutes.

[daz]

If i dial them down to say 10 mA will it lose volume over say 25 mA or will the volume remain the same but just more compression/distortion at high master levels with 25 mA ?

[Leo_Gnardo]

If i dial them down to say 10 mA will it lose volume over say 25 mA or will the volume remain the same but just more compression/distortion at high master levels with 25 mA ?

Where we were last night, 18 to 20 mA plenty enough.

Of course at 10 mA you're choking the current badly, you'll have lots of crossover distortion and reduced gain.

25 mA & up, not much to be gained. It's like when you pour too much milk in coffee and it overflows. What good is that? Just a waste, no need to go there. Compression & distortion won't be the result. Shorter tube life & excess heat generated is what you'll get.

[The Dude]

Agree with Leo. I'll add, IMO, beyond that low end gets "mushy" and less tight and crunchy. So, I guess if you like mushy.......

[daz]

Agree with Leo. I'll add, IMO, beyond that low end gets "mushy" and less tight and crunchy. So, I guess if you like mushy.......

I was just going by what was said in post #3. I take it he was wrong ?

[The Dude]

I was just going by what was said in post #3. I take it he was wrong ?

No. I agree with what Justin said in post #3. I'm not sure how that contradicts my thought?

[Justin Thomas]

I used to bias my amps as hot as I could get away with. As The Dude said - bass got mushy, hum levels went up, no clean headroom at all, and eventually they redplated as the tube aged and it was a waste. Usually now I bias as cool as possible while still getting a good sound at the most extreme volume I can practically use. That usually ends up at around 50% max plate dissipation. It also allows some leeway as tubes and components drift to go up later.

If you're already redlining your car and your foot gets tired and sinks farther toward the floor, well, kablooie! Same idea. Do as you like, but I'd err conservative.

Justin

[mozz]

You can push them a bit more than a regular 6v6. 14w
I use this bias calculator recently.
https://robrobinette.com/Tube_Bias_Calculator.htm

[g1]

I was just going by what was said in post #3. I take it he was wrong ?

No, the key to the statement is "LOWEST current you can get without objectionable distortion" and meaning specifically crossover distortion.
Like Leo said, too cold bias will give choking effect, xover distortion, and lower gain.
Justin's point was that as long as you keep above this level, there is no point biasing hotter.