Bias your amp...really?

[electron transl]

i've got a '65 twin reverb RI. looking to change out all of the tubes. i've been told two different things regarding the changing of the power tubes:

1. if you change the power tubes, you MUST bias the amp correctly to match these new tubes.

2. if the new tubes you get are a matched quad (6L6's) then you don't need to worry about biasing your amp. just put 'em in yourself and enjoy.

fact or fiction?

[John Phillips]

1. Probably. The factory bias is quite cool on these, so putting in a new set without rebiasing will probably be OK... but not necessarily (especially if you don't use the same brand of tubes, and some of the modern ones vary significantly from each other), and in any case you won't necessarily get the best performance. IMO biasing is generally over-rated - ie it isn't critical, and if the amp is properly designed to run conservatively for an in-spec set of tubes, it often isn't necessary - but it's still important to at least check it. Especially as many modern tubes are not correctly in spec for real 6L6s.

2. Definitely not. Just because the tubes are matched to each other - which is what a 'matched set' means - there is no guarantee they're matched to the amp's bias setting.

[rockon1]

1. Probably. The factory bias is quite cool on these, so putting in a new set without rebiasing will probably be OK... but not necessarily (especially if you don't use the same brand of tubes, and some of the modern ones vary significantly from each other), and in any case you won't necessarily get the best performance. IMO biasing is generally over-rated - ie it isn't critical, and if the amp is properly designed to run conservatively for an in-spec set of tubes, it often isn't necessary


30 years and havent biased one of my amps yet. No problems. Must be overated. I DID try to about a 1 1/2 ago. The irony is that I almost blew up my amp trying to "do things the right way" . Got a misassembled GT probe and immediately blew an internal fuse when I took the amp off stand by! Well,ignorance isnt bliss and I will have a crack at it again(once bitten twice shy) but it is overated for sure IMO. Bob

[Jef Bardsley]

This is a deep subject, and a lot has been said before, so I'll just agree with John. :)

I will point out though, a lot of internet tube lore is lifted from the hifi guys, who have very different concerns from those of us making music. They want the lowest possible distortion, and to get proper imaging they need both amps to have matched transfer curves, so biasing is essential for them, and not just with new tubes, but as the tubes age.

For a guitar player, it's pretty much a choice between longevity and tone, and since that's a tough choice, most of us just aim for the middle.

[Wicksy]

Newer tubes often need biasing i've found. Also, some modern amps need work/rebiasing when installing old stock tubes. Always best to check i think.

[Blue Strat]

At the very least, biasing your power tubes will give you somewhat predictable results as opposed to wondering why set A of brand X tubes sounds different than set B of the exact same tubes or why one set lasted a lot longer than the other.

Emulating ignorance of the past generally isn't a great idea.

[greggorypeccary]

Emulating ignorance of the past generally isn't a great idea.

Thanks, now I have to rethink my whole way of doing everything!

But seriously, this only applies to fixed bias right? Cathode bias will take a pair of matched tubes and be fine?

[Blue Strat]

Thanks, now I have to rethink my whole way of doing everything!

But seriously, this only applies to fixed bias right? Cathode bias will take a pair of matched tubes and be fine?

Not always. Most "self biased" EL84 amps run their tubes too hot by design. Not sure why amp "desingers" violate design specs of the tubes. I'm sure they wouldn't do this with resistors or capacitors.

Since tubes have a wide range of operating characteristics, nothing is ever guaranteed and it's best to at least check.

[WailinGuy]

If you care about your amp sounding and performing at its best, then setting output bias is very imprortant. Even if you ignore the tube longevity issue (tubes biased too hot), I find amps with output tubes biased too cold to be rather ungratifying to play. You can hear the crossover distortion, and it's irritating, not at all musical. What's the point of taking on the extra expense and upkeep of a tube amp if you're not getting all the fatness and warmth that you expect from a tube amp?

This wouldn't be such a controversial subject if all newer guitar amps came equipped with externally accessible bias trimpots and DMM test points (jacks for inserting multimeter test leads). Then biasing would be more of a user-oriented servicing operation, with a big reduction is safety concerns (no exposed chassis). Yes, it is a pain to have to take your amp to a tech every time you want to try out a new set of power tubes. The tubes are expensive enough by themselves without the added cost of bench labor charges. (Kudos to Allen Amplification and a few others for making biasing easy for most users.)

[kurtsstuff]

Biasing is important but....I sure long for the days when I was young and ignorant of such things. Life was soooo much easier back then...slam a new set of tubes in an amp and let er rip!! Nowadays since "I've aquired this knowledge" it's turned into a frickin nightmare...pull the chassis,install the ol biasrite install the tubes into the bias rite let simmer for 10-20 minutes,check bias , agonize over the 3-6MA difference that 1 tube will enivetibly(sp) have between the other 3 (100 watt heads) do the calculations 10 times to get the exact difference between each tube due to the 1 tube variance. But of course....you only start this procedure after checking the plate voltage and yes!! there will be one socket getting a different reading than the other three and it's not the one showing the difference in bias so....you pull the tubes and start all over only to find that everything has shifted and it's all different now!! But....it's what we do cuz in my case..I love High maintenance women too....lol!! ;)

[rockon1]

Biasing is important but....I sure long for the days when I was young and ignorant of such things. Life was soooo much easier back then...slam a new set of tubes in an amp and let er rip!! Nowadays since "I've aquired this knowledge" it's turned into a frickin nightmare...pull the chassis,install the ol biasrite install the tubes into the bias rite let simmer for 10-20 minutes,check bias , agonize over the 3-6MA difference that 1 tube will enivetibly(sp) have between the other 3 (100 watt heads) do the calculations 10 times to get the exact difference between each tube due to the 1 tube variance. But of course....you only start this procedure after checking the plate voltage and yes!! there will be one socket getting a different reading than the other three and it's not the one showing the difference in bias so....you pull the tubes and start all over only to find that everything has shifted and it's all different now!! But....it's what we do cuz in my case..I love High maintenance women too....lol!! ;)


Or not.:)

[carbz]

For whats its worth I threw a new set of new power tubes in a Fender Prosonic head I had a few years back without biasing. The amp seemed to run and sound fine to me. It may not have been an even current running through both tubes but I figured paying a tech $50 to $60 to do it I'd rather take my chance and at worst case the tubes wouldn't last as long as they would have being properly biased. Unless its way off I believe you can get away without biasing but what do I know.

[Randy]

Biasing is important but....I sure long for the days when I was young and ignorant of such things. Life was soooo much easier back then...slam a new set of tubes in an amp and let er rip!! Nowadays since "I've aquired this knowledge" it's turned into a frickin nightmare...pull the chassis,install the ol biasrite install the tubes into the bias rite let simmer for 10-20 minutes,check bias , agonize over the 3-6MA difference that 1 tube will enivetibly(sp) have between the other 3 (100 watt heads) do the calculations 10 times to get the exact difference between each tube due to the 1 tube variance. But of course....you only start this procedure after checking the plate voltage and yes!! there will be one socket getting a different reading than the other three and it's not the one showing the difference in bias so....you pull the tubes and start all over only to find that everything has shifted and it's all different now!! But....it's what we do cuz in my case..I love High maintenance women too....lol!! ;)

I know the feeling... I've just started biasing myself and of the 2 amps I've checked, one was too hot and the other too cold, but both their bias pots were maxed out in the wrong direction. Now I've got to hunt down tubes for them that are at the far end of the spectrum, or get the bias resistors changed... I should have left well enough alone... :puh

[John Phillips]

Biasing is important but....I sure long for the days when I was young and ignorant of such things. Life was soooo much easier back then...slam a new set of tubes in an amp and let er rip!! Nowadays since "I've aquired this knowledge" it's turned into a frickin nightmare...pull the chassis,install the ol biasrite install the tubes into the bias rite let simmer for 10-20 minutes,check bias , agonize over the 3-6MA difference that 1 tube will enivetibly(sp) have between the other 3 (100 watt heads) do the calculations 10 times to get the exact difference between each tube due to the 1 tube variance. But of course....you only start this procedure after checking the plate voltage and yes!! there will be one socket getting a different reading than the other three and it's not the one showing the difference in bias so....you pull the tubes and start all over only to find that everything has shifted and it's all different now!!
One important thing to remember when doing this, which will make things much simpler, is that the plate voltages cannot be different between the two tubes on the same side, and can't be more than a volt or two (at the outside) different even from one side to the other.

Any readings that show otherwise are due to metering error, so it's best to just take the readings, average them, and then use the same number for all further calculations - it doesn't vary significantly (again, normally no more than a volt or two at most, ie less than half of one percent) with changing tube current either.

You can't trust any of the readings to better than +/- 1% anyway, since neither the meter nor the resistors used are any more accurate than that. That's really quite a big difference (up to 10V, from -1% to +1%) on plate voltages.

[scopeboy]

Bias test points are cool, but I put a permanently installed cathode current meter on the last amp I built. I got a tiny round analog meter at a hamfest that probably used to belong to some Collins military radio or something.

It seems like a geeky thing to do, but it really makes biasing and swapping tubes easy and fun. I also installed independent bias pots with a very wide range, so the amp can be biased up with any tubes from 6V6s to KT88s, or weird combinations like one 6L6GC and one EL34. It takes about 5 minutes to change from any set of power tubes to any other. It's also reassuring to see the needle pop up to whatever bias current you set every time you flip the standby, and on the other hand, if it pegs the needle, you know to turn off immediately...

[Blue Strat]

For whats its worth I threw a new set of new power tubes in a Fender Prosonic head I had a few years back without biasing. The amp seemed to run and sound fine to me. It may not have been an even current running through both tubes but I figured paying a tech $50 to $60 to do it I'd rather take my chance and at worst case the tubes wouldn't last as long as they would have being properly biased. Unless its way off I believe you can get away without biasing but what do I know.
You may be fine but you'll never know.

The other alternative to a tech is getting a bias probe and learning to bias yourself.

[electron transl]

wow. i did not anticipate the wide response. thank you all for chiming in. i believe that on the twin it's relatively easy to get to the bias pots. i have a friend who has his EE Masters degree and happens to be a guitar player. chances are high he has a multimeter and the knowledge to bias. i won't ask questions on how to do so here - i'm sure i can google that.

one last thing i'd heard (and it seems like this was somewhat echoed by john p) that if you are replacing the tubes with the same brand that can help you skip the bias process. ie groove tubes chinese 6L6's for new GT chinese 6L6's = no need to bias.

this may not apply in my situation as i am probably going to put some new JJ's in there; but what say you about that sort of situation? same brand and/or model tubes = no need to bias?

[drfrankencopter]

Not always. Most "self biased" EL84 amps run their tubes too hot by design. Not sure why amp "desingers" violate design specs of the tubes. I'm sure they wouldn't do this with resistors or capacitors.

Since tubes have a wide range of operating characteristics, nothing is ever guaranteed and it's best to at least check.

I'd agree with this statement....most EL84 cathode biased amps that I've seen are essentially biased wrong. Leads to early tube death....probably more so with new production EL84s vs old stock.

Cheers

Kris

[John Phillips]

one last thing i'd heard (and it seems like this was somewhat echoed by john p) that if you are replacing the tubes with the same brand that can help you skip the bias process. ie groove tubes chinese 6L6's for new GT chinese 6L6's = no need to bias.

this may not apply in my situation as i am probably going to put some new JJ's in there; but what say you about that sort of situation? same brand and/or model tubes = no need to bias?
Still not quite true, since the tolerance quality on new-production tubes is so poor. It also depends where the old ones were set - as cool as possible, as hot as possible, or somewhere in the middle?

If the bias is set conservatively before, and you replace like with like, you have more change of it being correct afterwards, but it's not a certainty.

The reason it used to be more possible to do it was simply because old-production tubes were made to tighter tolerances, to the correct spec - you could be reasonably sure that even tubes from different manufacturers would be fairly similar - and amps were generally (though not always) biased cooler than has now become fashionable.

[Blue Strat]

one last thing i'd heard (and it seems like this was somewhat echoed by john p) that if you are replacing the tubes with the same brand that can help you skip the bias process. ie groove tubes chinese 6L6's for new GT chinese 6L6's = no need to bias.



Also not true. Every brand and type of tube have a range of operating parameters and are grouped into pairs or quads with similar parameters (idle current is one primary parameter). Since Groove Tubes, for instance, rates their power tubes from 1 to 10, replacing a pair of Groove Tubes with another pair of GTs with the same rating would NOT require rebiasing. Any other pair, or an unknown pair, etc, would absolutely necessitate biasing (assuming that you have any intention of biasing and haven't completely written it off as BS).

[greggorypeccary]

I'd agree with this statement....most EL84 cathode biased amps that I've seen are essentially biased wrong. Leads to early tube death....probably more so with new production EL84s vs old stock.

Cheers

Kris

How about 6V6's as in the old standby 5e3? (or clone)

[wixedmords]

I actually inquire about biasing when I am purchasing a used amp. It can harm the sale. Proper maintenance is a good selling point be it an automobile or an amp.

It befuddles me how far people go to to get out of biasing an amp. The funny thing is that it isn't too difficult of a process.

You get the feeling that in a simple post asking about the need to bias an amp, there could be 9 of 10 respondents telling the person the amp needs to be biased, and many people will wait for that one person that posts, "Don't worry about it, plug them in and be done with it", and they seemingly go with that advise. I find this happens regularly because I ask questions after the fact.

[Blue Strat]

How about 6V6's as in the old standby 5e3? (or clone)

5E3s and clones tend to run 6V6s at 120% of max or over. In case you're wondering, 100% is not supposed to be exceeded.

[Old Tele man]

But seriously, this only applies to fixed bias right? Cathode bias will take a pair of matched tubes and be fine?...correct, *IF* the tubes are "approximately" same characteristics as the cathode resistor was calculated for!

...this was true back in the NOS-days when tubes from same manufacturer were almost "matched" because of excellent quality-control production...today's imported tubes, however, are NOT so good, with "matching" actually being more of a "...weeding out the BAD ones" operation that YOU pay for (but really shouldn't HAVE to!).

...thus, "checking" bias and resultant plate currents is in reality YOUR last chance verification that the tubes you just installed aren't *way off* from the amp designers' original specs.

[electron transl]

I actually inquire about biasing when I am purchasing a used amp. It can harm the sale. Proper maintenance is a good selling point be it an automobile or an amp.

It befuddles me how far people go to to get out of biasing an amp. The funny thing is that it isn't too difficult of a process.

You get the feeling that in a simple post asking about the need to bias an amp, there could be 9 of 10 respondents telling the person the amp needs to be biased, and many people will wait for that one person that posts, "Don't worry about it, plug them in and be done with it", and they seemingly go with that advise. I find this happens regularly because I ask questions after the fact.


i'm a busy man. anytime someone can tell me "yeah, it's no biggy. just do it." then i'm all for it. obviously this is a fuzzy topic as several members have chimed in with experiences of NOT biasing and everything turning out just dandy.

i'm just trying to eliminate misinformation that may or may not have been given me.

john p -- my amp is still relatively new/modern. it's the '65 Twin Reverb Reissue. maybe 3 years old? i'd be under the impression that the GT tubes in there are probably produced on the same scale of tolerance/bias level as a set that came out today. i guess i'm wondering if your comments regarding NOS tubes vs. modern tubes really applies here? just curious. and thanks for steering me in the right direction and NOT blowing up my wonderful twin (if that happened, what would my AC30 have to sit next to?). :)

[Blue Strat]

several members have chimed in with experiences of NOT biasing and everything turning out just dandy.



There's no doubt that replacing tubes without rebiasing has been done by many people for decades with no perceived ill effects. However, there have always been amp "mysteries" like "my amp doesn't sound the same as it did with the old tubes", "my amp runs hotter, cooler", "a tube shorted and took out some components inside my amp" or "this pair of tubes lasted half as long as the previous pair". Biasing can eliminate these mysteries.



i'm just trying to eliminate misinformation that may or may not have been given me.


I can guarantee that if you bias new power tubes you'll be fine. No guarantees can be made if you simply plug and play. Tube warranties will often be void by not biasing your tubes. That's my policy.



john p -- my amp is still relatively new/modern. it's the '65 Twin Reverb Reissue. maybe 3 years old? i'd be under the impression that the GT tubes in there are probably produced on the same scale of tolerance/bias level as a set that came out today.

I think you need to go back and read the thread again.

[wixedmords]

i'm a busy man. anytime someone can tell me "yeah, it's no biggy. just do it." then i'm all for it. obviously this is a fuzzy topic as several members have chimed in with experiences of NOT biasing and everything turning out just dandy.

i'm just trying to eliminate misinformation that may or may not have been given me.



My previous posting wasn't pointed at you ET. I was just trying to present a point of view of what happens in a forum and a commentary on the behavior of people. Funny things happen in forums. Like the guy who "bumps" after getting the correct answer to his question. We have all seen that.

Life is a busy place that is for sure. But, there is plenty of information on the internet on how to bias, and some misinformation as you mentioned. One just needs to take the time. It really isn't that hard, or time consuming.

PM coming at you bro. ;)

[greggorypeccary]

Thanks for tolerating my noobie questions here! Been playing forever but I'm really just getting into this technical stuff.
Now about this:
5E3s and clones tend to run 6V6s at 120% of max or over. In case you're wondering, 100% is not supposed to be exceeded.

Isn't the overbiasing part of what makes the sound of those amps?:confused:

[Blue Strat]

Thanks for tolerating my noobie questions here! Been playing forever but I'm really just getting into this technical stuff.
Now about this:


Isn't the overbiasing part of what makes the sound of those amps?:confused:

No doubt it affects the sound but operating at 100% isn't that much different. It also affects tube longevity. If you think running stuff out of spec sounds better, be prepared to replace tubes more often.

[Jef Bardsley]

If you think running stuff out of spec sounds better, be prepared to replace tubes more often.
Amen to that.

And, given the "quality" of modern tubes, if you're running hot, it's all the more important to be sure your plate voltages are where you think they are.

[hoochiecoochie]

Fender has been shipping the RI series (and maybe the rest) with GT tubes that are color coded. Seems like they usually have the white writing/logos on them. According to Fender, you can replace the power tubes with the same colored/logo tubes and NOT have to rebias. Maybe that will somewhat assist you. I would still rebias eventually, as my experience indicates that Fender does bias them a little cold.

[greggorypeccary]

Amen to that.

And, given the "quality" of modern tubes, if you're running hot, it's all the more important to be sure your plate voltages are where you think they are.

Thanks guys! One of these days I'll take my 5E3 to my tech to check out. After I play the hell out of the 6V6's in there and they need to be replaced! Which, as I understand, may be sooner rather than later...

[electron transl]

Fender has been shipping the RI series (and maybe the rest) with GT tubes that are color coded. Seems like they usually have the white writing/logos on them. According to Fender, you can replace the power tubes with the same colored/logo tubes and NOT have to rebias. Maybe that will somewhat assist you. I would still rebias eventually, as my experience indicates that Fender does bias them a little cold.


this is essentially the original advice that was paraphrased to me. if you swap the same, supposedly Fender says it's fine. i agree with everyone here though, bias to be safe -- and get a better sound.

[Dezzy]

I know the feeling... I've just started biasing myself and of the 2 amps I've checked, one was too hot and the other too cold, but both their bias pots were maxed out in the wrong direction. Now I've got to hunt down tubes for them that are at the far end of the spectrum, or get the bias resistors changed... I should have left well enough alone... :puh

Is that becasue it was fixed bias ?

I just stuck some new GT 6l6s in my bassman 100, which is fixed bias, but I never took it to a tech, not sure it will hurt the amp, at the end of the day as long as their is no red plating it should be fine, I am sure the tubes will last a good time anyway,

[zzmoore]

Hello John Phillips.......
You are a real decent human being.

[pfflam]

Biasing is important but....I sure long for the days when I was young and ignorant of such things. Life was soooo much easier back then...slam a new set of tubes in an amp and let er rip!! Nowadays since "I've aquired this knowledge" it's turned into a frickin nightmare...pull the chassis,install the ol biasrite install the tubes into the bias rite let simmer for 10-20 minutes,check bias , agonize over the 3-6MA difference that 1 tube will enivetibly(sp) have between the other 3 (100 watt heads) do the calculations 10 times to get the exact difference between each tube due to the 1 tube variance. But of course....you only start this procedure after checking the plate voltage and yes!! there will be one socket getting a different reading than the other three and it's not the one showing the difference in bias so....you pull the tubes and start all over only to find that everything has shifted and it's all different now!! But....it's what we do cuz in my case..I love High maintenance women too....lol!! ;)
You forgot the ending where you decide to F it just bias till it sounds great, after all it was probably some multimeter problem that kept the readings wandering in the first place

;)

[pfflam]

Hello John Phillips.......
You are a real decent human being.
Wow . . . I just realized this thread is old . . . too bad Mr Phillips isn't around anymore

:(

[gtrnstuff]

Wow . . . I just realized this thread is old . . . too bad Mr Phillips isn't around anymore

:(

Yeah, he left the party. Always informative and helpful.

[cruisemates]

I have a bias-rite on the way. The thing for me is that I want to find the best power tubes for my amps; with so many octal choices from EL34, KT88, 5881, 6L6 etc. I keep thinking there is a combination our there for my Marshalls JCM800s I just haven't found yet - but I also know that when you change tubes you have to bias for them.

You can just throw different tubes in there and hear how they sound, but its like buying different guitars to see what plays better without setting up the action. You don't really know anything unless you do it right.

[whiteop]

1. Probably. The factory bias is quite cool on these, so putting in a new set without rebiasing will probably be OK... but not necessarily (especially if you don't use the same brand of tubes, and some of the modern ones vary significantly from each other), and in any case you won't necessarily get the best performance. IMO biasing is generally over-rated - ie it isn't critical, and if the amp is properly designed to run conservatively for an in-spec set of tubes, it often isn't necessary - but it's still important to at least check it. Especially as many modern tubes are not correctly in spec for real 6L6s.

2. Definitely not. Just because the tubes are matched to each other - which is what a 'matched set' means - there is no guarantee they're matched to the amp's bias setting.

True. If you put the tubes in the adjustment pot may actually be set incorrectly and they won't be sharing the same power load.

[big mike]

Wow. Necro-thread.

We miss you John!

[twangbanger]

Hello John Phillips.......
You are a real decent human being.

I sure miss him, lots of great knowledge in his posts.

[cruisemates]

I just got a Weber Bias-rite (which I bought in the emporium) today and I just tried it - wow, amazingly easy.

I have tried to meter-method before, and while I sort of understand it in principle I never could fine a guide that made it easy for me to bias that way - but the Weber is like magic - just plug it in the amp, plus in the tubes, turn it on and read the bias.

Definitely worth it in my opinon.

I just saw that I have a pair of winged Cs' where one is set at 26 and the other is at 36. That is a pretty serious mismatch I would say. How would that manifest itself? (what would I hear?)

[dazco]

biasing myths are all over the map. I can tell you my experiences. I've built several amps and i have biased each of them hundreds of time with different tubes from hot to cold and no matter how i bias them the tonal difference between the hottest bias you would safely use and what would be considered fairly cold is minimal. The main reason to bias tubes is if the amp is already biased fairly hot theres a chance a new set will be over the "limit" and red plate. The best way IMO to do this is get a pair of tubes you like and have the amp biased to 60%. That way pretty much any set you install will run fine w/o having to rebias. This way you aren't likely to ever throw a set in that will red plate and the worse case scenario is they may not sound quite as good. If you like the sound of a certain bias amount and your ears really hear the difference, then you DO have to bias new tubes every time to be happy. But my experince is that the tonal difference is no where near the myths I've heard forever, which is that if not biased perfectly the tome will suffer. Fact is, theres a wide range within the bias that will make no difference to most ears and subtle to the ones that hear small details. The biggest concern IMO is that they don't redplate. And if the amp is biased to 60% no set is going to draw enough current to redplate, atr least it's extremely unlikely

[TweeDLX]

I've noticed that really cold biasing can sound thin and ratty. Just my experience however...

[RussB]

Is that becasue it was fixed bias ?

I just stuck some new GT 6l6s in my bassman 100, which is fixed bias, but I never took it to a tech, not sure it will hurt the amp, at the end of the day as long as their is no red plating it should be fine, I am sure the tubes will last a good time anyway,


Thanks for bumping a 4 years old thread to tell us that


I miss John :(

[bigboy]

Yesterday I put a duet of Tung Sol el34b's in my Metropoulos 50 watt. I used a dual bias probe and plate voltage probe with two MM's (all from Amp-Head) so I could monitor plate voltage in real time while adjusting bias on either tube with the flick of a switch. It made the job super easy.

The thing was; my plate voltage was hovering around 441v, with an el34 tube, 70% is about 39.7mv. I could only get my bias trim pot to 36mv before I ran out of travel and it wouldn't turn anymore! It's okay because it sounds really cool at that setting and there's no red plate worries and tube life should be extended. I would have liked to hear what a full 70% sounds like though!