Ever get no appreciable difference biasing hot/cold?

HotBluePlates

Member
Messages
10,632
Biggest impact should be on measured clean power output, and how high you have to turn the volume control to get to that power output.
... I understand and acknowledge your rationale (that reducing the bias voltage acting to limit signal swing at the grid and hence reduce power output), but it doesn’t seem to be working like that :huh ... the peak voltage being negligibly different to that achieved at high idle current.

If I've understood correctly, this sounds like my original post should have been, "Biggest impact should be how high you have to turn the volume control to get to the same power output."

Which makes sense as peak plate current is set by the supply voltage, OT primary impedance & tube characteristics.
 

oneblackened

Member
Messages
1,216
Several years ago I serviced up a few PV 5150 heads that my customers were using

I noticed that they were biased cold from the factory----- much colder than I ever would have biased an amp (at the time)
With regard to this: These amps are biased way, way cold. They're at the point where zero crossing distortion actually starts to show up on a scope visibly (an average set of 6L6s idles around 20% of max dissipation). Adjusting the bias slightly less negative (not hot, but more "normal" for AB1 amps at that kind of plate voltage - maybe -50 volts instead of -55, ballpark 45-50% idle dissipation) got rid of it. The amp got less buzzy and fizzy. Whether or not that's a good thing is up to the ear of the beholder - that fizz sorta helps with the aggression but it makes it a little bit less muscular and thick.


Be careful ! I said something similar about preamp tubes a while back myself. It was telling a group of unruly first graders there's no Santa Claus. :roll

I've heard subtle differences, but never had that truly "OMG wet my paints" moment from swapping a properly working 12ax7 for another properly working 12ax7.
Of course I've never spent 200 bucks on a holy grail NOS 12ax7. Lucked into enough of those over the years cheap enough that I judged them, without the euphoria one gets from spending a bunch of money.

Well crap.
I just knocked down that Hornets nest again.
Anyone who actually thinks different tubes will make a huge difference is listening with their eyes. It's always pretty subtle.
 

teemuk

Member
Messages
3,176
Some Peavey amps intentionally employ crossover distortion for enhancing dynamics of sustained overdriving so cold biasing definitely fits the scheme.
 

Badstrat

Gold Supporting Member
Messages
1,440
‘78 2203 JMP w 6550s

I decided to check bias drift seeing how the tubes are over 5 years old.

PV appears to be 464v from pin 3.

The tubes read @ 52mA and I decided to back them down to EL-34 dissipation values.

After a few back-and-forth trials through a captor X and headphones, I’m not getting any earth-shaking differences between cold (35mA) and hot (62mA). It all sounded Marshally to me.

About 10 min into adjustments, I am seeing the tube bias margins begin to deviate from within +/- 5mA of each other to 7mA, so I set it colder at the moment for the sake of longevity

- V4: @36mA
- V5: @35mA
- V6: @42mA
- V7: @38mA

Any thoughts experience on an amp that sounds fine regardless of how hot/cold it’s biased?
I have a 1978 JMP 2203 with 6550. I have read that because these amps were designed for EL34 tubes, when using 6550 42 watts tube , they should be biased as if they were an EL34. 25 watt tube. Something to do with the output transformer impedance I think. So 37mA is at about 70% of max dissipation of an EL34. If someone can confirm this is correct that would be good.
 

Badstrat

Gold Supporting Member
Messages
1,440
My Twin Reverb II is at about 40% of max dissipation. I think that’s how it’s supposed to be.
 

V-man

Senior Member
Messages
2,712
I have a 1978 JMP 2203 with 6550. I have read that because these amps were designed for EL34 tubes, when using 6550 42 watts tube , they should be biased as if they were an EL34. 25 watt tube. Something to do with the output transformer impedance I think. So 37mA is at about 70% of max dissipation of an EL34. If someone can confirm this is correct that would be good.
I have read that line of reasoning before in some threads over years past. Seems like I never read a definitive answer. All I know is that I have operated it well-above 34 spec for years and the transformers are ice cold except for a little latent heat transferred over by the 6550s.

Either way, I brought it back to the 36-40 range since there was no appreciable difference to my ears, since that increases tube longevity.
 

GT100

Member
Messages
3,829
I have a 1978 JMP 2203 with 6550. I have read that because these amps were designed for EL34 tubes, when using 6550 42 watts tube , they should be biased as if they were an EL34. 25 watt tube. Something to do with the output transformer impedance I think. So 37mA is at about 70% of max dissipation of an EL34. If someone can confirm this is correct that would be good.
No, its not about the OP transformer. Its about the power transformer.
The amp was engineered to deliver 100watts using EL34s.
If you bias only thinking about the capabilities of the 6550 -then you are going to pull down the plate and screen voltages and run the power transformer hot.
 

Tone_Terrific

Silver Supporting Member
Messages
32,984
You can fry tubes dropping them into a hot running amp when you change tube brands. For sure:cry:
Too cold sounds bad. Setting up with a tiny bit of crossover ripple showing sounds ok and I have not seen that run a tube into the danger zone.
My ears are not so good as to pick up where the edges of 'good' and 'bad' meet.

Also having run a bunch of random 12ax7 tubes in a pedal, I found that there are definitely differences.
Maybe not much amongst new tubes that are meeting spec but how do you know that unless you try a known good one?
 
Last edited:

ekafroops

Member
Messages
314
Recently biased my 5e8 for 5881 tubes (rather than 6L6GC), and colder vs hotter made a difference that I think laypeople would notice. Also got a Furman 117 to drop down the wall voltage (thus bringing the bias readings for the tubes downward), and that made all my amps sound pretty different.
 

Motterpaul

Tone is in the Ears
Messages
12,935
The funny thing is that I always believed tube biasing was settled science. Nothing is further from the truth. The more you look for a definitive and scientific "how to bias my tube amp" article the more arcane your reading will become.

There does not seem to be a settled way to bias tubes that is always the optimal approach, each approach has its drawbacks. Probably the most respected method is adding a 1-ohm resistor to your tube socket (and to ground) that you can measure across.

But aside from that - there are more people doing it in different ways than this and it really does not seem to be a big deal. Almost never do you hear anyone say "I think I have a bias problem" - if their amp has been recently serviced.

Of course, if you are hearing excessive hum, you may be overbiased, OR if you are getting ghost notes and some sound dropping out as a chord dies out, for example, you may be underbiased. But it is true that "in the ballpark" probably works a lot more often than people suspect.
 

J M Fahey

Member
Messages
2,370
I’m not getting any earth-shaking differences between cold (35mA) and hot (62mA). It all sounded Marshally to me.
In general, Manufacturers/Engineers bias tubes "cold".
All of them: Fender/Marshall/whatever.
Biasing amps HOT, or the silly "70% bias" or whatever (with no base in the real world) are a Musician/Guru/Forum thingie.
Power amps are designed by printing power tuve curves, and drawing various load lines on them, by hand, using ruler and pencil.
The so called "graphic" method, time and again used and honored from the 30´s or so, and still state of the art.
IF you use that, and superimpose "HOT biased" tube loadlines (yes, the loadline changes, of course) , you´ll notice you *lose* available power.
You lose even more, because now loadline slope has changed but you have NOT changed OT (so you have NOT changed rated impedance) so to add insult to injury, now you are mismatchingb the amp.

"But ... but ... now I like it better"
Well, that´s something different.
Overbiased amps both are "more sensitive" (lesser negative bias means lesser PI output needed to clip them) and clip earlier, a bonus to many.
But does not mean they are "better"; if anything, it´s something subjective.

Of course, biasing an amp VERY cold isn´t good at all: tube curves approach cutoff conditions and sound becomes "choppy", like a poorly adjusted noise gate.
 




Trending Topics

Top