Hmmmm, bias point...

So true! In one numbskull moment I found myself thrown across the room. Can feel it just thinking about it. LOL
120 volts will cause an ugly sensation that you won’t soon forget.

220 volts will make your muscles spasm, and your hand will grab and hold onto whatever is shocking you.

440 volts will throw you across the room.
 
For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.

Same with me; always biased at 60% for years. I've liked what I heard/felt when I went to ~70% and that's as far as I went but never left it there for fear of eating tubes quicker. Did they say why they think 70% is optimal? Curious....
 
Same with me; always biased at 60% for years. I've liked what I heard/felt when I went to ~70% and that's as far as I went but never left it there for fear of eating tubes quicker. Did they say why they think 70% is optimal? Curious....
The didn't say but when I ran my MATLAB simulation the curve was straighter near the origin and "rounder" going into saturation. So I'd say the reason is smoother distortion and less crossover distortion.

I should probably do a plot of the first derivative to compare the crossover distortion.
 
Not entirely true, I zapped myself with a home-built tube headphone amp operating at 500 V and it merely jerked my arm around ;)
Mostly true. And just about universally true in situations involving electrical power, which can supply multiple amperes of current.

In other words, you got lucky, brother! :)
 
I only sorta do... but some amp companies bias their amps cold to get maximum tube life. To my ears, this sounds harsh and cold, almost shrill. At 70 % you'd get less tube life, but likely warmer tone.

And of course in the virtual world, there are no tube concerns!

They also do this 5150 KSR so the poweramp feels a little stiffer
 
For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.

Set Power Tube Grid Bias to 0.7 to do this.

I and many, many others have all been designing amps around a 70% dissipation with modern tubes like JJ 6L6 for at least ~18 years and all or most non-static biased amps from major companies are the same. Some much higher if the B+ voltage is acceptable and there's little or no red plating.
Although it should be pointed out that amps like 5150 and rectos signature sound is from biasing closer to class B heh.
 
For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.

Set Power Tube Grid Bias to 0.7 to do this.

The issue with this 70% rule is, that it's based on the old tube production. Newer tube production is no longer made for military usage, therfore the safety operation area of these tubes is not that superior as it was back in the days. The most troubles with newer production tubes were screen voltage issues - for example, EL34s should not take more than 450V on their screens, says the datasheet - in the older days, some EL34s were able to take more than 500V at the screen grids, today we run in various issues with these high voltages.

The 70% rule was just based on fixed bias Class A/B amps. On cathode bias amps (such as the Tweed Deluxe 5E3), the maximum plate dissipation can reach up to 100%....nominal it's between 85 to 95. Some modern fixed biased amps have only 27% stock setting - such as the 5150 or between 30 to 45% on various Mesa/Boogie amps (they said they use special long life tubes, but in reality they just use them with lower bias settings) - I recentely had a a Mesa 395 Simul Stereo amp in, which had 25% stock setting and 35% on their (so called Class A - which isn't Class A just a bit warmer biased Class A/B) tubes......
On the other hand, there were several cathode biased Class A/B designs which go over the 100% mark - such as the Vox AC30 (yes this is no true Class A amp) - I saw 130 to 140%, the newer production Supro amps, some Mesa Boogie amps (Studio Caliber .22/ DC3 etc.) - this wear out the tubes much faster and produce a lot of 120 cycles hum (for US - 100 cycles for EU) - some techs said, this would make them sound better, but that is just stupid......

I normaly bias amps with modern production tubes between 55 to 65% (55% for cold, 60% normal and 65% for hot). I learned the 70% in my EE days, 25 years ago. They also did the biasing with an oscilloscope back then - today I go along with the shunt method - much more accurate.

all the best, cheers
P
 
JJ's EL34 can handle insane voltages and while their 6L6GC is listed at 450 - 500, it can go way higher. Not that you even need to. IMO most current tubes sound best under 450V (assuming from scratch with transformer options), with a hotter bias, in AB. But given something like a stock jubilee at 550+ plate voltages, it's definitely going to eat cheaper Chinese and Russian tubes if you overbias.
Most engineers design around what's available, not around NOS, so given a datasheet for a known good current tube and wide field provability, new designs are definitely using 70% or hotter in cathode biasing. Like VOX AC30 reissues.
Also companies like Mesa Boogie importing and restamping power tubes that can survive their behemoth testing machine and then they give it a cold to hot bias number, and Groove Tubes, TAD, Ruby. You never really know who made them originally, so it's not possible to argue on solid ground about modern tube survivability at higher voltages with hotter bias, in a completely generalized way at the design stage.
JJ's can handle it and generally are the benchmark when designing. Not that they sound amazing as drop-ins from GEs or RCAs, unfortunately.
Also shunt bias every day. I wish all amps had probe inputs.
 
Dumb question maybe, but what makes this 70% of maximum dissipation value 'optimal'? What are the pros and cons of going even higher? I still love using my tube amps a lot so this is something I'm interested in for sure :O
 
Dumb question maybe, but what makes this 70% of maximum dissipation value 'optimal'? What are the pros and cons of going even higher? I still love using my tube amps a lot so this is something I'm interested in for sure :O
The optimal bias point (I don't know if it's really 70%) can be defined as the best compromise between tone and tubes' longevity.
If the bias is 'cold' tubes will last longer but audio performance won't be the best, there would be a gating effect which causes crossover distortion.
 
120 volts will cause an ugly sensation that you won’t soon forget.

220 volts will make your muscles spasm, and your hand will grab and hold onto whatever is shocking you.

440 volts will throw you across the room.


Well, that two to three feet felt like "across the room" :) It definitely knocked me on my ass.
 
Well, that two to three feet felt like "across the room" :) It definitely knocked me on my ass.
Drain those caps son.

70% is a rule of thumb when you're making a loadline on a tube datasheet to determine the operating point, not a law.
 
Dumb question maybe, but what makes this 70% of maximum dissipation value 'optimal'? What are the pros and cons of going even higher? I still love using my tube amps a lot so this is something I'm interested in for sure :O
Basically, it's the point where you get the "best" performance from a set of tubes, typically in the 60-70% range. If biased by ear, they will typically be in or very close to this range. The benefits of going higher is similar to some amps master volumes. There's usually a sweet spot that can be found, going higher doesn't really help the tone anymore and in a lot of cases can be tonally detrimental.

If you're trying to achieve tones associated with amps that are biased correctly, it makes perfect sense to try this to see if it makes any difference.
 
Dumb question maybe, but what makes this 70% of maximum dissipation value 'optimal'? What are the pros and cons of going even higher? I still love using my tube amps a lot so this is something I'm interested in for sure :O
The higher the bias the more linear the transfer function (to a point) and the smoother the transition into saturation. At one time I posted transfer functions for a half-dozen or so bias values. Probably in the Tech Notes section.

The problem is that as you increase the bias eventually you'll exceed the Safe Operating Area (SOA) which can shorten the tube life or even damage it. For a typical Class-AB amp you can't go much beyond 70% without exceeding the SOA at some point in the transfer function.

Class-A amps typically run lower voltage and/or higher plate impedances which allows biasing the tubes at or near 100%.
 
As a (still) AFX II user who doesn't check this sub-forum too much for FOMO, I've been playing with this, especially for low amd mid gain drive tones. Moving many amps to 0.7 like plexis and tweeds has added some punch for lack of a better term. Modern Amps and High Gainers still seem best at their default and colder settings.

Thanks for the heads up Cliff for looking at a oft ignored parameter!
 
120 volts will cause an ugly sensation that you won’t soon forget.

220 volts will make your muscles spasm, and your hand will grab and hold onto whatever is shocking you.

440 volts will throw you across the room.

The current across your heart is what will kill you. Higher voltage isn’t the primary concern

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The didn't say but when I ran my MATLAB simulation the curve was straighter near the origin and "rounder" going into saturation. So I'd say the reason is smoother distortion and less crossover distortion.

I should probably do a plot of the first derivative to compare the crossover distortion.

Back when I used to do such things, I always biased with a scope and would dial in a bit of crossover distortion as I favored the sound. Cliff, at around what % does the notch start appearing on EL34s? Just a rough number so that I have some correlation in my head.
 
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