Modern "High-Gain" Amps

[trem]

I only see the few amps that some Local/Young musicians bring to me for "normal" fix and maintenance stuff.
But it seems like a lot of these "high-gain" amps run their power tubes pretty cold. It seems like:
Mesa
PV
and now Marshall
all like to do this.
When i say "cold" i mean in the 30%-50% range of dissipation for a class A/B amp.
But this Marshall JVM 410 (2x12 combo) is really cold. These tubes are drawing about 30 mA at 460 VDC. and that is for a pair, not for one tube.
I do not have a calculator, what is that about 8-9 Watts.?
I have NO IDEA when these tunes were installed or who biased them, but it is kind of what i have seen.
Anyway.....Finally to my question. Is there any reason i should not set these EL34 to the more "typical" range of 60% bias at idle.?
Thank You

[Enzo]

Same reason those OEMs went for cool. Longer tube life and better reliability. Mostly they are relying on the preamp for the tones.

And the alternate question: why SHOULD you do it?

The 5150 from PV if found often had idle current in the 11-15ma range.

You have a calculator on your computer by the way.

[nsubulysses]

People will probalby say to view it on a scope and if theres no crossover notch there's no problem. Cold bias means easier on the tubes which means less warrantee issues when new amps are sold. So you could see why companies favor this so their amp can make it past its 3 year warrantee or whatever.

People will also probably say to listen to the amp. Does it sound bad biased so cold. Does it sound any different or better biased at 60%?

SOme amps that are high votlage like ampeg V4 need to be biased cold or they will probably fail prematurely. I doubt any of these new gains amps you're seeing are 540V B+ but they probably just want to make it easy on their tubes so their warrantee department can get less phone calls. It should just be biased warm enough that there is no crossover notch. That may fall below the average 50-70% plate dissipation range that people always talk about. It is only a suggestion though. The sound of the amp and the reproduction of the test signal are more important than the calculations, I would guess, as long as it runs in a safe territory, which basically just means any bias setting that's not excessively hot bias for no reason, which may cause problems.

[Justin Thomas]

And given the reputation of new tubes, why should anyone take any chances? Those big companies are probably among the biggest buyers of tubes in the world, and they're businesses, not just in it for the art. They use a lot of tubes, and if this saves them money, they'll do it.

I don't know if it was a super-cold bias, but Music Man used a few "unconventional" tricks to eke out every last minute of use from their power tubes, and Peavey followed suit with the Mace/Deuce, correct? I don't know the mechanism, but saving tubes through extremely conservative measures isn't that new. And I'd imagine the tube engineers would be proud.

And would anyone notice crossover distortion at the ridiculous gain levels and (comparatively) low actual volumes these amps are often used at?

Justin

[nsubulysses]

FWIW, I've heard people even say things like, "crossover distortion is part of the 5150 sound." not really sure if that's true because I never cared to check it out and mess with different bias settings.

Some customers have asked me to put the bias in a "normal" range on 5150 amps and I have. Pretty minimal listening difference sitting in my tiny workspace with a 100W amp going through a 2x12 under my bench. But the customers said it "sounded better." It probably did slightly. But also they probably played their backup amp for a few weeks then got back to their main rig, "normal" bias setting and all, possibly new power tubes as well, and were pleased with the results

[Jazz P Bass]

Hotter bias messes with the 'resonance' control.

[trem]

Right.....i understand all of that, but i do not play guitar, so i always wonder.
Every time i have seen an amp like this, i have just kind of figured that The Manufacture has Designed/Set it that way, and i have simply followed their lead.
But not being a player.....i have no idea if a Marshall JVM sounds better when its EL34 are idling at 8 watts or 15 Watts.
To be honest, i sort of do not understand the purpose of an amp like this. There are 4 modes, and two of those are switchable for 3 sounds. Orange, Yellow, Green or whatever colors they are. To my ear, most of these options sound awful, but i guess i am kind of old fashioned at this point.
Thanks For All The Input

[Justin Thomas]

To be honest, i sort of do not understand the purpose of an amp like this. There are 4 modes, and two of those are switchable for 3 sounds. Orange, Yellow, Green or whatever colors they are. To my ear, most of these options sound awful...

I think half of the reason most of them sound "awful" is that people think a Master Volume makes any amp a bedroom amp... Gain 10 Master 1 = AWESOME BONE-CRUSHING METAL TONE!!! Not... Hear those amps in a proper context, at a proper volume (loud aggressive drummer, etc.), and I bet they don't sound so "awful!"

Well, I hear an awful lot of awful players making awful sounds on all those "awful" channels, too...

Justin

[J M Fahey]

But it seems like a lot of these "high-gain" amps run their power tubes pretty cold. It seems like:
Mesa
PV
and now Marshall
all like to do this.

Maybe there is a reason most (all?) amp *manufacturers* , who *designed* them and then proceed to build tens of thousands do things that way.

When i say "cold" i mean in the 30%-50% range of dissipation for a class A/B amp.

"Cold" ????? can you grab those tubes with your bare hands?

But this Marshall JVM 410 (2x12 combo) is really cold.

To you maybe, clearly not to Marshall.

These tubes are drawing about 30 mA at 460 VDC. and that is for a pair, not for one tube.

So?

I do not have a calculator, what is that about 8-9 Watts.?

???????????????

I have NO IDEA when these tunes were installed or who biased them, but it is kind of what i have seen.

Does the word "Factory" ring a bell?

Anyway.....Finally to my question. Is there any reason i should not set these EL34 to the more "typical" range of 60% bias at idle.?

1) "typical" for whom?
Factory or Gurus/Forumites?
2) Just considering averages, since tens of thousands amplifiers leave Factories being used as-is , while maybe 1% of them get rebiased "hot" following Forum lore, Statisttics say that average/normal way is the Factory way.

[trem]

Hotter bias messes with the 'resonance' control.

How does it effect that.?
I see the pot on the front panel schem, but i cannot really follow it after that.
Thank You

[Jazz P Bass]

http://music-electronics-forum.com/t43369/#post441793

[Chuck H]

I suppose I could participate in the cold bias with preamp distortion/hotter bias with power tube distortion debate... I have been on both sides of that one a few times. I'll just say that when I finally get around to designing my own uber gainer it will have a "relatively" cool bias and rely primarily on preamp distortion. I don't think I'd bias at 15%! Or even 20%. But I will be setting the amp up to make maximum power with relatively little clipping from the power tubes. So that's the sort of standard you're seeing with the colder biased, uber gain guitar amps. The preamps are set up as signal processors and the power amps only have the job of making it louder. No or little power tube clipping intended. For better or worse, depending

[Enzo]

I look at it this way also: you get the designed tone of the amp at a wider range of volumes. You don't need to crank the amp.

You CAN modify these cooler amps to run hotter, but you still have to be pretty loud to get power tube distortion. So then you either have to play loud or get into attenuators and stuff for that part of the tone.

[trem]

I suppose I could participate in the cold bias with preamp distortion/hotter bias with power tube distortion debate... I have been on both sides of that one a few times. I'll just say that when I finally get around to designing my own uber gainer it will have a "relatively" cool bias and rely primarily on preamp distortion. I don't think I'd bias at 15%! Or even 20%. But I will be setting the amp up to make maximum power with relatively little clipping from the power tubes. So that's the sort of standard you're seeing with the colder biased, uber gain guitar amps. The preamps are set up as signal processors and the power amps only have the job of making it louder. No or little power tube clipping intended. For better or worse, depending

Is there a Mathematical relationship between bias and the power output of an amp.?
That is to say.....if Marshall says that amp XYZ is a "100 Watt Amp"...running 4x EL34...but the power tubes are idling at 8 Watts, is it still a 100 Watt Amp.?
Or are those just round numbers that most manufactures use based on the potential of whatever power tubes a certain amp uses.?
Thank You

[trem]

http://music-electronics-forum.com/t43369/#post441793

Jazz P -
I see, interesting.
Like i said, i just follow what the factory does. Though all these years later, that can be hard to ascertain without input from them. I shot an email to Marshall and asked them what kind of bias number(s) they use when these amps are new. Perhaps they will respond.
I went ahead and biased these power tubes a few mA more than the old tubes were, but nowhere near what i would with a circa 1968 Fender/Marshall/Vox.
Thanks Again.....i appreciate the info