Can't turn bias down any lower

MadAsAHatter

MadAsAHatter

Well-known member
I changed the tubes in my Crate Blue Voodoo 150 this afternoon. It uses 6L6GC tubes. Getting my numbers I calculated the max plate current to be about 52 mA. With the adjustment all the way down the lowest it will go is 36 mA; right at 70% max. I have it set at 39-40 mA; about 75% max. I know 75% max is within tolerance to not be biased too hot and it sounds good as is. My question though is, being that it can't be adjusted lower than 70% max, is this something that I should worry about? like would this be an indicator that it may need to be serviced or the bias trim pot needs replacement?
 
Thanks, I could probably swap that myself, but I'll bring it to the tech. I'm comfortable setting the bias, but still learning and not quite comfortable doing much past that yet. At least not on an amp that's harder to come by.
 
MadAsAHatter said:
Thanks, I could probably swap that myself, but I'll bring it to the tech. I'm comfortable setting the bias, but still learning and not quite comfortable doing much past that yet. At least not on an amp that's harder to come by.
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It's all good.

Not familiar with that particular amp, but I've seen this happen before on a JCM800 where I had to parallel a 100KΩ across the stock 27KΩ bias dropping resistor to solve the same sort of issue.

It's actually an easy fix.
 
You could also just order a set of tubes rated lower. Kind of a workaround. But yeah, replacing the bias resistor is the right way to do it.
 
Gotta increase the resistance to ground so that a more negative voltage can bias the grids to a lower voltage which will lower your plate current.

I bias around 50 to 60% max plate dissipation to save tube life and improve low end and tightness.
 
Critical knowledge. If you check your own bias you need to be aware of this. Bias range resistors. Have multiple values on hand . Much cheaper than different rating tube set. Learn how to solder. Investment in your future.
 
Rocker68 said:
Critical knowledge. If you check your own bias you need to be aware of this. Bias range resistors. Have multiple values on hand . Much cheaper than different rating tube set. Learn how to solder. Investment in your future.
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Although I can't disagree with you, I will say that learning how to solder to tackle a swap like this is not always the answer. Time is money too and a fucked up lead or circuit board due to over soldering could cause more of a headache than it is worth. Plus tubes can be used in other amps. Sounds like MadAsAHatter feels comfortable with it though so it is all good. Not picking a fight here. I have enough tubes laying around so I may have a different opinion. That, and when I order tubes I order them as close to spec as possible. The last time I ordered some JJ's from Eurotubes I asked for 34.5 mA (almost as a joke or a test) and guess what came in the mail? 34.5 mA.
 
Agree with all suggestions. My post is a generalization. Hopefully ones soldering skills are not put to the test in a critical area until the ability is proven. I practiced on junk circuit boards. Auto gauge clusters, video games etc. It all adds up. But it's not a light switch skill either. Enjoy the challenge.
 
Rocker68 said:
Critical knowledge. If you check your own bias you need to be aware of this. Bias range resistors. Have multiple values on hand . Much cheaper than different rating tube set. Learn how to solder. Investment in your future.
Click to expand...
Why do some people assume others don't possess certain basic skills and feel the need to condescendingly suggest to learn said skill? It's like someone asking about an aspect of an internal combustion engine and then being told to learn how to pump gas.

I'm perfectly adequate with soldering, been doing it regularly for over half my life. I'm learning amp circuitry to do some basic work on my own stuff when needed; or at least know what the tech is talking about when explaining what he's doing. I'm aware of components like the bias resistor, but I'm not exactly an expert diagnostician when it comes to this. I'm also still a little unsure of how different values affect things. If it were a Peavey Bandit, sure I'll take a shot to see what I can do on my own. On an expensive or hard to come by amp I'm not going to poke around balls deep in there unless I was 1000% certain of what I was doing.
 
MadAsAHatter said:
Why do some people assume others don't possess certain basic skills and feel the need to condescendingly suggest to learn said skill? It's like someone asking about an aspect of an internal combustion engine and then being told to learn how to pump gas.

I'm perfectly adequate with soldering, been doing it regularly for over half my life. I'm learning amp circuitry to do some basic work on my own stuff when needed; or at least know what the tech is talking about when explaining what he's doing. I'm aware of components like the bias resistor, but I'm not exactly an expert diagnostician when it comes to this. I'm also still a little unsure of how different values affect things. If it were a Peavey Bandit, sure I'll take a shot to see what I can do on my own. On an expensive or hard to come by amp I'm not going to poke around balls deep in there unless I was 1000% certain of what I was doing.
Click to expand...
Just out of curiosity, what Ω value bias dropping resistor is in there now ?
 
MadAsAHatter said:
Why do some people assume others don't possess certain basic skills and feel the need to condescendingly suggest to learn said skill? It's like someone asking about an aspect of an internal combustion engine and then being told to learn how to pump gas.

I'm perfectly adequate with soldering, been doing it regularly for over half my life. I'm learning amp circuitry to do some basic work on my own stuff when needed; or at least know what the tech is talking about when explaining what he's doing. I'm aware of components like the bias resistor, but I'm not exactly an expert diagnostician when it comes to this. I'm also still a little unsure of how different values affect things. If it were a Peavey Bandit, sure I'll take a shot to see what I can do on my own. On an expensive or hard to come by amp I'm not going to poke around balls deep in there unless I was 1000% certain of what I was doing.
Click to expand...

That’s what experts are here for. To help you and the greater community. I’m balls deep in this stuff for a living so I eat breathe and sleep the theory day and night.
 
glpg80 said:
Gotta increase the resistance to ground so that a more negative voltage can bias the grids to a lower voltage which will lower your plate current.

I bias around 50 to 60% max plate dissipation to save tube life and improve low end and tightness.
Click to expand...
This is the way I adjust the bias circuit. Instead of adjusting the dropping resistor from the bias supply. Just raise the value of the resistor before or after the trim pot to ground.
 
PLX said:
Just out of curiosity, what Ω value bias dropping resistor is in there now ?
Click to expand...
I'm not sure off hand, I'd need to go look it up.

glpg80 said:
That’s what experts are here for. To help you and the greater community. I’m balls deep in this stuff for a living so I eat breathe and sleep the theory day and night.
Click to expand...
We all have to start somewhere and are lucky enough to have people that can guide us through the learning experience. I for one appreciate all the help experts here have kindly given.
 
Some tubes have higher bias points then others.
If possible, try requesting 6L6’s with lower bias points.
Another option would be to change the bias resistor to a different value.
 
MadAsAHatter said:
I changed the tubes in my Crate Blue Voodoo 150 this afternoon. It uses 6L6GC tubes. Getting my numbers I calculated the max plate current to be about 52 mA. With the adjustment all the way down the lowest it will go is 36 mA; right at 70% max. I have it set at 39-40 mA; about 75% max. I know 75% max is within tolerance to not be biased too hot and it sounds good as is. My question though is, being that it can't be adjusted lower than 70% max, is this something that I should worry about? like would this be an indicator that it may need to be serviced or the bias trim pot needs replacement?
Click to expand...
…developing a taste for hot bias and the costs of reduced tube life.
 
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