Tube Bias....question

beardog03

It was suggested that I had my bias set too high.
The book say`s 275ma, and I had her running a steady 280ma for quite some time now....Hot yes, but sounded good.

I posed a question on another forum, and was told that there was a mistake in the printing, and that I should be running at 238ma.

I adjusted...

Question....
Is it true that you get more from the tube (sound) if set high...I know that it shortens the life of the tube, but...

Doesn`t seem as clear now..

maybe it`s just me
whadaya think ?

I rolled some tubes yesterday in the pre, and took out the Pope`s and put in some EH`s.

This morning I put he Pope`s back in (buffer stage)....got my soundstage back and it`s...well, kinda like a mullard but more refined...

anyway , I digress

bias....is too high , too high ?

and no I wasn`t when I did it !!!

F1nut

I think you should call Cary.

beardog03

agreed

I put her at 250ma for now...

organ

Hey Beardog,
Try to set it at it's reccomended setting. Usually, it should sond best there because the designer have taken the entire design and circuit into consideration. Keep in mind that as your wall voltage vary, so will your bias.
Some people like to run high bias and some are on the conservative side. Your tube life will also increase with a more conservative bias setting. If you're near the max or over the max for the output tube, you could see fireworks or shorten the tube's lifespan.

beardog03

I put them back @ 275ma a few hours ago

I looked at everything I could find to see what was recommended, and if maybe the book was wrong, but until I can contact Cary Audio, I`m going to run them at the suggested 275ma

They do kind of vary, up and down, but I took my time and got them to stay steady @ 275ma....I will recheck them in a bit to see if they stayed put

RuSsMaN

Lower bias makes the tubes run 'hotter'. A lot of people think of it backwards - Bias is actually the negative voltage applied to a tube's control grid which allows the tube to stay within its operational parameters for your specific amplifier.

Too much bias, the amp will sound like crap. Too little, tubes can overheat and fail, taking output transformers and other parts in the circuit with them.

275-280ma is the correct setting for the V12 (or the earlier Superamp, or latter Rocket 88 - which all essentially share the same circuit) IIRC. Run the amp with no signal playing for a 5 minutes or so, set the bias. Check it again at 45 minutes after playing some tunes - then again at 2 to 3 hours. After that, you should be good to go for quite some time, unless your amp has bias resistors out of spec and drifts over a short time. You have the bias lights, right? They will 'bounce' on and off while playing music, just set your bias with no signal present.

Cheers,
Russ

beardog03

Thanks Russ

Long time no hear, hope all is well with you these days

Yes I have the lights, and yes you are correct...I thought it was the other way around (higher ment hotter)

They stay were I put them, but when I change them, it usually changes (lower) after a hour or two, then when I reset it,it stays where I put it

what tubes do you recommend for this amp ?

RuSsMaN

It comes with EL34's, but you can run any triode in that family, biased properly. KT88s, 6550s, KT90s, KT66s, 6L6, you could even run the tiny 6V6's for about 25 watts per channel.

Tubes are like cars, hard drives, anything subjective. Ford, Chevy, Dodge - Maxtor, Seagate, etc etc. There are stories of praise and failure for each. What you will probably hear from most is KT88's. Some may even get really crazy and suggest spending a small fortune on Gold Lions or some such. If I had enough Gold Lion KT88's to put in a V12, I'd listen to them for a weekend, then sell them and buy a lifetime supply of current issue Tungsol 6550's.

The EL34 and 6550 are my favorites, and current production models would be the JJ Tesla E34L (yes, no type-o), and the Tungsol re-issue 6550. In EVERY application where I've had a KT88, I've always enjoyed the music experience better with a 6550. You can get CRAZY good deals on JAN 6L6's, but you will get a little less power wise out of them. If power isn't a huge issue, play with it - you've got a SUPER amp (pun intended) to really tweak your sound.

Run either one of those for power, and tweak your sound on your front end driver tubes. That's what I would do.

Cheers,
Russ

beardog03

thanks Russ

you are right, it aint cheap retubing a V12 !!

and alot of folks have been pushing the Gold Lions, but I think I will look into the front end as much , if not more for tweeking the sound

I have Pope`s and Hytron`s in the pre, and man are they sweet !!

halo

I agree with Russ - the Tung-Sol 6550 is a great tube. I'd have liked to try the Ei elite KT-90 though.

SEH

RuSsMaN wrote: »

Lower bias makes the tubes run 'hotter'. A lot of people think of it backwards - Bias is actually the negative voltage applied to a tube's control grid which allows the tube to stay within its operational parameters for your specific amplifier.

But he's measuring current. Under bias = idle current too high = tubes hotter. If he was starting with a new set of tubes he would turn the bias trim pot fully counterclockwise (most over bias, least current, coldest, safest) then start turning it clockwise to increase the mA reading until he gets in the bias range, right? If 238 mA was the correct setting, then 280 mA would be under biased - hotter.

The "safe" range is probably 50-75 mA wide. If the plates aren't red, you don't hear any funny noises, it sounds good, then you're not too hot.

reeltrouble1

There can just be so much semantics so lets add some,

We have a grid, so, we apply bias voltage to the grid. This is a negative voltage, more negative than the cathode. Since like charges repel each other , the negative voltage reduces the number of electrons passing through the grid. Otherwise there would be too many electrons.

Now as for tube biasing, given 10 tubes when bias is measured you will see a traditional bell curve, say 2 low, 6 middle and 2 high. The tubes biasing low will break up with distortion early, the high ones will not break up until the gain is pushed higher, the ones in the middle will operate optimally given the steady bias.

Most important is that different tube samples will bias differently.

RT1

janmike

I need to learn how to do this bias thing.

beardog03

janmike wrote: »

I need to learn how to do this bias thing.

does your new amp self - bias ?

probably not........I remember setting the bias on my F1 SLI-80...it was a piece of cake ?

Now when you have six in a row on either side, it can jump up and down a bit


I have decided to run with TroyD, and Russ` suggestion, and go with the 6550 Tung-sol`s...

Of course I will also take the advise and run with it , in regards to other not so commomn tubes , just to see what they sound like


I went all the way down,waited 5min, then slowly went back to 275ma....waited after 45min I checked again...slight adjustment, and she`s stayin steady @ 275ma

sound great, but I am still gonna roll some 6550`s...;)

janmike

No, I need to do it manually. I will pull out the manual for my amp and maybe it has a hand holding procedure for this. I'm real green at this.

beardog03

they usually do...it`s pretty easy

I like to "tune up " my amp, and make sure the numbers are right

I like the"hands on" with tubes, and I learn more every day

Thanks to the good folks here....
Jesse has pointed my to many articles on tubes, and Russman, TroyD, madmax, Mark....the list goes on

Many sources for info

once your hooked , nothing else will quite do it for ya !!!


TUBES RULE !!

janmike

So far Robert the tubes are doing it for me too. I guess we all start somewhere so nothing that cannot be learned, even by an old dog like me.