can bias on amp just change??
- Thread starter BloodRose
- Start date
Gearjoneser
Gear Ho
Re: can bias on amp just change??
Sometimes if you change tubes without a bias, the two brands are far from each other. That's the only way I could understand how the mini pot is way off. But bias drift just from tubes wearing out is more common.
This is why it's good to get familiar with bias pots in your amps. Even if you don't know what you're doing, there's a good chance you'll find the sweet spot by ear. Just don't electrocute yourself by touching anything.
Plastic handled screwdriver and gloves are recommended.
At least you took it to a guy who could confirm the problem. What did he charge you to bias it?
Sometimes if you change tubes without a bias, the two brands are far from each other. That's the only way I could understand how the mini pot is way off. But bias drift just from tubes wearing out is more common.
This is why it's good to get familiar with bias pots in your amps. Even if you don't know what you're doing, there's a good chance you'll find the sweet spot by ear. Just don't electrocute yourself by touching anything.
Plastic handled screwdriver and gloves are recommended.
At least you took it to a guy who could confirm the problem. What did he charge you to bias it?
Re: can bias on amp just change??
Can be kind of confusing.....it is probably not the actual DC bias that changed...more likely the old tubes themselves drifted...which they do quite a bit.
If you are saying the tech installed the new tubes, and they drew cold.....well yeah... of course, as pointed out in the post above.
best
Can be kind of confusing.....it is probably not the actual DC bias that changed...more likely the old tubes themselves drifted...which they do quite a bit.
If you are saying the tech installed the new tubes, and they drew cold.....well yeah... of course, as pointed out in the post above.
best
Re: can bias on amp just change??
The amp doesnt have much time on it.. maybe 30 hours at in home level, no gigs. Was making a real flubby tone on the low e and the high notes sounded really metallic. I bought new tubes and replaced the pre's myself.. Took the power tubes to the tech for install and trouble shoot. Costing me about $80
The amp doesnt have much time on it.. maybe 30 hours at in home level, no gigs. Was making a real flubby tone on the low e and the high notes sounded really metallic. I bought new tubes and replaced the pre's myself.. Took the power tubes to the tech for install and trouble shoot. Costing me about $80
RealtyViking
New member
Re: can bias on amp just change??
Did the amp get biased with the new tubes or the old tubes?
What else was listed in the tech's report as a bias for $80 seems a bit steep.
Did the amp get biased with the new tubes or the old tubes?
What else was listed in the tech's report as a bias for $80 seems a bit steep.
Re: can bias on amp just change??
Marshall should have been taken to court in a Class Action law suit.
THEIR Circuit Boards were bad, and they were (eventually) Very Happy to sell you a new one for 183 bux. All this to fix a problem that they NEVER admitted any fault to. How can that be.?
If the TSL was a car, they would have recalled every one of those sacks of crap.
.....Guess I am one of those guys that thinks Marshall stopped making good amps in 1983.
Whatever Marshall does today, there are at least 2 companies that do it better.
best
Those TSL were a Night-Mare.
Marshall should have been taken to court in a Class Action law suit.
THEIR Circuit Boards were bad, and they were (eventually) Very Happy to sell you a new one for 183 bux. All this to fix a problem that they NEVER admitted any fault to. How can that be.?
If the TSL was a car, they would have recalled every one of those sacks of crap.
.....Guess I am one of those guys that thinks Marshall stopped making good amps in 1983.
Whatever Marshall does today, there are at least 2 companies that do it better.
best
Lake Placid Blues
New member
Re: can bias on amp just change??
Being able to stay on top of bias your self is a big plus. Biasing your amp is easy to do with a multi meter. But first you need to add (or have a tech add) a precision 1 ohm resistor between the cathode (pin number 8) and ground- if the amp doesn't come stock with one already there (many marshall types do). This way amps = volts. You can then measure the current flowing through the tube then from cathode to ground.
The correct reading then requires a little bit of math. For example EL34's have dissapation rating of 25 watts. Measure the plate voltage and divide 25 by the plate voltage. If the plate voltage is 450 then 100% current flow would be 0.0555 or 55. This is way too hot and you want about 70% of that, or about 38 measured cathode to ground through the resistor. You can fine tune from there by ear by going up or down a couple values.
Being able to stay on top of bias your self is a big plus. Biasing your amp is easy to do with a multi meter. But first you need to add (or have a tech add) a precision 1 ohm resistor between the cathode (pin number 8) and ground- if the amp doesn't come stock with one already there (many marshall types do). This way amps = volts. You can then measure the current flowing through the tube then from cathode to ground.
The correct reading then requires a little bit of math. For example EL34's have dissapation rating of 25 watts. Measure the plate voltage and divide 25 by the plate voltage. If the plate voltage is 450 then 100% current flow would be 0.0555 or 55. This is way too hot and you want about 70% of that, or about 38 measured cathode to ground through the resistor. You can fine tune from there by ear by going up or down a couple values.
Re: can bias on amp just change??
Well I need to agree with you on the TSL. The only channel I liked was the crunch channel. The amp didn't sound very good till it was really cranked, but also the quality was pretty bad. One day the output dropped to about 25% of normal. Wasn't the tubes, or any of the usual suspects. I had to take it to 2 techs and blow 250 bucks getting it "fixed" just so I could sell it. Techs hate these amps so much it's hard to find a good one that will work on them. Glad to see that one go bye bye.
Well I need to agree with you on the TSL. The only channel I liked was the crunch channel. The amp didn't sound very good till it was really cranked, but also the quality was pretty bad. One day the output dropped to about 25% of normal. Wasn't the tubes, or any of the usual suspects. I had to take it to 2 techs and blow 250 bucks getting it "fixed" just so I could sell it. Techs hate these amps so much it's hard to find a good one that will work on them. Glad to see that one go bye bye.
UberMetalDood
New member
Re: can bias on amp just change??
It's called "bias drift" and I have read about it on other forums. It seems to be a controversial issue. Some people say that it happens in all amps that are not fixed bias, and other people say it doesn't. There are probably extensive technical discussions if you do an Internet search. One thing that I noticed is that if I run an amp at low volume for many days, it starts sounding kind of lousy. However, if I crank the amp every now and then, it's like it adjusts itself. I don't know if there is any merit to that but it seems to do the trick. I wonder if anyone else has noticed the same thing.
It's called "bias drift" and I have read about it on other forums. It seems to be a controversial issue. Some people say that it happens in all amps that are not fixed bias, and other people say it doesn't. There are probably extensive technical discussions if you do an Internet search. One thing that I noticed is that if I run an amp at low volume for many days, it starts sounding kind of lousy. However, if I crank the amp every now and then, it's like it adjusts itself. I don't know if there is any merit to that but it seems to do the trick. I wonder if anyone else has noticed the same thing.
FuseG4
Our Neighbor Totoro
Re: can bias on amp just change??
when I had a DSL I biased some new chinese tubes and a month later they needed an adjustment by about 3ma.
In my DRRI I noticed that when I first got my tung-sol 6v6 they matched pretty well, <1 ma apart, but about 8 months or so later they're off a good 2ma from each other.
I have no idea if it was the amp or the tubes in either case and don't want to speculate but these days I just say it couldn't hurt to check the bias again after a month or so after installing but I'm not gonna worry too much about it after that.
when I had a DSL I biased some new chinese tubes and a month later they needed an adjustment by about 3ma.
In my DRRI I noticed that when I first got my tung-sol 6v6 they matched pretty well, <1 ma apart, but about 8 months or so later they're off a good 2ma from each other.
I have no idea if it was the amp or the tubes in either case and don't want to speculate but these days I just say it couldn't hurt to check the bias again after a month or so after installing but I'm not gonna worry too much about it after that.
