My two 6L6 required equal bias adjustment when I put them in a few moths ago (I have two bias pots). But suddenly one of the tubes requires the bias pot to be turned all the way down to get a bias of about 39mA. The other tube is ok, and the bias starts around 15mA when it's turned down, and I reach about 39mA half way or so.
What could be wrong with the tube?
Tommy
strange bias problem
Moderators: pompeiisneaks, Colossal
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: strange bias problem
Hi Tommy: Long time no see. Anyway this is a wild a**d guess but I sometimes wonder if we exceed the screen power dissipation ratings on these new manufacture tubes with just 470 ohms between anode and screen, + whatever minimal resistance is provided by the OT primary. It doesn't take long to destroy a tube when we exceed screen power ratings.
Just a thought
Dan
Just a thought
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: strange bias problem
Hmm. I don't know too much about this, so your answer doesn't make all that much sense to me 
Could you try to explain?
Could you try to explain?
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: strange bias problem
When the plate voltage and screen voltage are equal (or thereabouts), which they definitely are when these amps are off standby without signal applied, the screen can begin to take on the role of the plate. When this happens screen current will go above the couple of MA that it's supposed to be when at 0 signal. When you multiply screen current times screen voltage you get screen power dissipation. I'm not looking at a tube spec sheet right now but IIRC a 6L6GC is rated at a couple of watts screen dissipation. When you go over that, you destroy the screen and it takes on even more "plate" characteristics and pretty soon the screen current will run away. If it's necessary for you to raise (more negative) the grid voltage more and more to control tube current flow, then I suspect you are dealing with a fried screen.Tdale wrote:Hmm. I don't know too much about this, so your answer doesn't make all that much sense to me
Could you try to explain?
But I could be full of balogna too!
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: strange bias problem
Ok, that makes more sense.
Maybe a stupid question, but why does tube draw more current when the screen acts like a plate, than when the plate does? Does it have something to do with the distance between the grid and the screen?
What should I do to prevent the grid from being damaged?
Tommy
Maybe a stupid question, but why does tube draw more current when the screen acts like a plate, than when the plate does? Does it have something to do with the distance between the grid and the screen?
What should I do to prevent the grid from being damaged?
Tommy
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: strange bias problem
#1Yes and closer to the cathode too. #2 use higher value screen resistors. The object is to get a few more volts between plate and screen values. But before we break out the soldering iron, let's wait for some consensus here. It's just a guess so far. Hey, Larry? Dana? Heisthl? Firestorm? Mr. Thud? Waaasup?Tdale wrote:Ok, that makes more sense.
Maybe a stupid question, but why does tube draw more current when the screen acts like a plate, than when the plate does? Does it have something to do with the distance between the grid and the screen?
What should I do to prevent the grid <screen> from being damaged?
Tommy
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: strange bias problem
It's a Hybrid-a build, and the schematics say 1k for the screens, but I used 470...because someone said it was ok, and it was the only high watt resistors I had....
tommy
tommy
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: strange bias problem
I'd say change to the one k then Tommy.Tdale wrote:It's a Hybrid-a build, and the schematics say 1k for the screens, but I used 470...because someone said it was ok, and it was the only high watt resistors I had....
tommy
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: strange bias problem
will I have to change the bias circuit if I do that?
Re: strange bias problem
I assume you've tried swapping tubes in their sockets to make sure the problem follows the tube...
Kevin O'Connor always recommends screen resistors of at least 1K. It's probably not a bad idea, but a 6L6 can be connected as a pseudo-triode with the screen connected to the plate, or in an ultralinear circuit where it's connected to a transformer tap. And in the tweed days, Fender used to run the screens at higher potential than the plates. Nowadays, it looks like a mistake, but Fender actually specified their transformers that way.
The screen in a 6L6 is rated for 5 watts dissipation and it seems like it would be hard to push it that far unless the tube was being run way hot.
So what I'm thinking is that 1K screen resistors ain't a bad idea, but I don't think the screens are necessarily related to the tube's wacky bias.
One possibility: if the coupling cap has "pinholed" just a tiny bit, it could leak just enough of the bias voltage that you'd have to turn the pot way up. More likely though, the tube has gone "gassy." Gas in a tube causes the grid to draw current which reduces the effective bias, which causes everything run hotter, which releases more gas, etc. etc.
Screen geometry (in fact the alignment of all the electrodes) is pretty inconsistent in most new-manufacture tubes, but I think that the vacuum pumping and gettering are even more suspect.
A lot of tubes won't really settle into their actual operating points until they've been burned in for at least 24 hours. Some of them never settle down and wind up dying prematurely.
Kevin O'Connor always recommends screen resistors of at least 1K. It's probably not a bad idea, but a 6L6 can be connected as a pseudo-triode with the screen connected to the plate, or in an ultralinear circuit where it's connected to a transformer tap. And in the tweed days, Fender used to run the screens at higher potential than the plates. Nowadays, it looks like a mistake, but Fender actually specified their transformers that way.
The screen in a 6L6 is rated for 5 watts dissipation and it seems like it would be hard to push it that far unless the tube was being run way hot.
So what I'm thinking is that 1K screen resistors ain't a bad idea, but I don't think the screens are necessarily related to the tube's wacky bias.
One possibility: if the coupling cap has "pinholed" just a tiny bit, it could leak just enough of the bias voltage that you'd have to turn the pot way up. More likely though, the tube has gone "gassy." Gas in a tube causes the grid to draw current which reduces the effective bias, which causes everything run hotter, which releases more gas, etc. etc.
Screen geometry (in fact the alignment of all the electrodes) is pretty inconsistent in most new-manufacture tubes, but I think that the vacuum pumping and gettering are even more suspect.
A lot of tubes won't really settle into their actual operating points until they've been burned in for at least 24 hours. Some of them never settle down and wind up dying prematurely.
Re: strange bias problem
Yes, I did swap them, and the problem follows the tube..
tommy
tommy
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: strange bias problem
Not at 39MA though right?Firestorm wrote: It's probably not a bad idea, but a 6L6 can be connected as a pseudo-triode with the screen connected to the plate,
Probably might just be a gassy tube but I think Tommy plays quite a bit so you'ld think it would give up sooner. Either way ticks you off when money goes out the window. I had a Valve Arts KT-66 bad out the box one time.
Get some more tubes and better luck next time Tommy.
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: strange bias problem
I actually don't play much. The tubes have been played for a few hours, so that's probably why. A friend borrowed it at a gig, and he played pretty lound for a couple of hours straight...maybe that was when things happen.
I'll just change the tubes. But when we're on the subject. Can I use KT66 tubes in my circuit? Or does the bias circuit have to be changed?
Also: If I put in 1k screen resistors, will the bias circuit have to be changed?
I'll just change the tubes. But when we're on the subject. Can I use KT66 tubes in my circuit? Or does the bias circuit have to be changed?
Also: If I put in 1k screen resistors, will the bias circuit have to be changed?
Re: strange bias problem
KT66s will work fine (if they'll fit physically 
). They are pretty similar to 6L6GCs in characteristics.
Changing the screen resistors will have only a small effect on the bias, as long as you're not at the end stop already, you should plenty enough room to readjust.
--mark
). They are pretty similar to 6L6GCs in characteristics.Changing the screen resistors will have only a small effect on the bias, as long as you're not at the end stop already, you should plenty enough room to readjust.
--mark
Re: strange bias problem
Ok, thanks!
Using 1k screen resistors won't affect sound much, will it?
Using 1k screen resistors won't affect sound much, will it?