New tube amps are biased cold because...

loudgtr

Member
Messages
187
the amp maker doesn't want to have power tube failer to deal with during the warranty period; this I understand. I guess what I'm really wondering is if all amps sound better biased even a little hotter than when they're new?

I have read how amps "sounded great biased hot but then they had to back down the bias because they noticed some "red plating" in one or more of the tubes. "but boy did it sound good" is the usual comment. Recently read this about a Bogner 101b as well.

What's the general consensus? Thanks.
 

Zim

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1,973
Its also for the floor models that get played all the time, so they will keep the tube life longer.
 

Blue Strat

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30,722
I wouldn't assume anything about this without taking a bias reading and trying different settings.

Generalizations like this are potentially dangerous from an informational standpoint and possibly to the health of your tubes and/or amp.
 

jcollins

Member
Messages
73
My buddy just bought a new hot rod deluxe...He's a bass player and doesn't even know the word tone.

Out of the box the thing was stiff, stiff, stiff. I checked the bias, 28mA. Sounds mucho better at 36mA.

Now we just need to wait until the speaker loosens up a bit and it should be fine.
 

dk123123dk

Member
Messages
3,890
I think its also because newer tubes aren't as robust as they used to be. So rather than "over work" the tubes, they set the bias cold, so they don't blow early. Plus a lot of manufactures use the cheapest tubes to ship with the amp.

dk
 

PRNDL

Member
Messages
496
Biasing amps is a VERY complex and hotly debated issue, and there are many opinions.

Here are some simple statements.
Modern amps tend to be biased on the cool side, which helps save tube life.
Biasing normal or slightly hot can make an amp sound much better.
Biasing very hot will shorten tube life considerably.
New tubes aren't made as well as NOS, and usually can't take hot bias.

That last statement is very controversial, but does seem to be true for EL34's.

A cold bias increases the crossover notch, which adds distortion. Distortion is a nasty word in HiFi, bit highly desirable in guitar amps. Once you start to play, however, crossover distortion is not an issue.

This is why some Marshall amps sound and play better with a cooler bias and at higher volume. A Marshall is not a HiFi amp - you buy them for the distortion sounds.

For the amp builder, increasing plate voltage improves tone, but that also has diminishing returns.
 

BudLite

Silver Supporting Member
Messages
3,026
What he said.......................................

I wouldn't assume anything about this without taking a bias reading and trying different settings.

Generalizations like this are potentially dangerous from an informational standpoint and possibly to the health of your tubes and/or amp.
 

KLB

Member
Messages
3,044
(snip...)
This is why some Marshall amps sound and play better with a cooler bias and at higher volume. A Marshall is not a HiFi amp - you buy them for the distortion sounds.

For the amp builder, increasing plate voltage improves tone, but that also has diminishing returns.

Define what you mean by cooler bias and high plate voltage. Please give some numbers. Thanks.

What about fixed vs. cathode-bias designs, which tend to run the tubes at or near max idle current.
 

mds

Member
Messages
1,189
Not all new amps are biased cold from the factory...Blue Jrs are super hot from the factory and a lot of people bias them cooler...
 

Steve Dallas

Member
Messages
8,344
It's also not true that amp companies use the cheapest tubes they can get their hands on. Amp companies use the most durable tubes they can get their hands on to reduce needless warranty work.
 

The Captain

Member
Messages
12,789
Sounds like motorbike performance to me. Honda applies it's R&D to a design, but that counts for nothging against the local expertise that can extractmore horsepower wiht a pipe or some crap. Trouble is, the balanced performance goes, ususally the midrange, and overall performance and durability is diminished.
Is it possible that amp makers actually know what they are doing ?
 

photios

Member
Messages
552
you just have to experiment and decide what you like best (which is why every git player should be able to bias his own amps)...I usually tend to prefer colder settings, but that can vary depending on the amp.
 

justonwo

Platinum Supporting Member
Messages
4,310
Biasing amps is a VERY complex and hotly debated issue, and there are many opinions.

Here are some simple statements.
Modern amps tend to be biased on the cool side, which helps save tube life.
Biasing normal or slightly hot can make an amp sound much better.
Biasing very hot will shorten tube life considerably.
New tubes aren't made as well as NOS, and usually can't take hot bias.

That last statement is very controversial, but does seem to be true for EL34's.

A cold bias increases the crossover notch, which adds distortion. Distortion is a nasty word in HiFi, bit highly desirable in guitar amps. Once you start to play, however, crossover distortion is not an issue.

This is why some Marshall amps sound and play better with a cooler bias and at higher volume. A Marshall is not a HiFi amp - you buy them for the distortion sounds.

For the amp builder, increasing plate voltage improves tone, but that also has diminishing returns.

Very true. I haven't tried NOS EL34s, but SEDs couldn't handle the 490 VDC plate voltage in my Super Lead even when biased to 50% at idle. JJs fared much better, but could still red plate under the right circumstances.
 

rockon1

Platinum Supporting Member
Messages
13,703
Very true. I haven't tried NOS EL34s, but SEDs couldn't handle the 490 VDC plate voltage in my Super Lead even when biased to 50% at idle. JJs fared much better, but could still red plate under the right circumstances.

My Supersonic runs at about 472v. Thankfully I havent had any issues with redplating running either the SED or JJ at 65% at idle. Heck I didnt even think it was an issue. Now youve got me wondering. Bob
 

Gumby

Member
Messages
380
What?!! I don't know of a single amp maker who has tubes under warranty. Most tubes are excluded with the exception of the occasional 30 day.
 

justonwo

Platinum Supporting Member
Messages
4,310
My Supersonic runs at about 472v. Thankfully I havent had any issues with redplating running either the SED or JJ at 65% at idle. Heck I didnt even think it was an issue. Now youve got me wondering. Bob

On my Super Lead, it only happens at about the mid volume setting, which is where plate dissipation is highest in a class A/B amp. It's pretty faint and hard to see unless you're in a dark room.

If you're really interested and have lots of time, check this thread:

https://www.thegearpage.net/board/index.php?threads/193340
 

Unabender

Member
Messages
684
New tubes aren't made as well as NOS, and usually can't take hot bias.

Well, then there's new JJ EL84's which take high plate voltages and power dissipation like a champ... I would dare to say better than many NOS EL84's.
 

Hacksaw

Time Warped
Messages
10,209
Very true. I haven't tried NOS EL34s, but SEDs couldn't handle the 490 VDC plate voltage in my Super Lead even when biased to 50% at idle. JJs fared much better, but could still red plate under the right circumstances.

Interesting to hear. I run a Splawn QR with 496v and haven't had an issue for a year now. (used to blow out the reissue mullards quite often though!) And running half power pop's it up to a little over 500v. still going great. bias is 63%-65% . My buddy runs a Bogner Extasy with over 500v and the SED's are ok as well.

SED's have always been my stable go to tube for reliability in high voltage amps.

Splawn biases a little cold. That is what Splawn likes the amps to sound like. I bias the QR's a tad hotter with a tube change in the amps I have.
 




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