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Recently one of the tube fuses blew on my JCM900.I'm planning on re-tubing it myself, as it seems a rather simple operation, but recently someone mentioned that you have to have your amp re-biased when you change power tubes.Is this true? I'm also concerned about the safety factor of cracking open my amp. I removed the tubes without any issue, but my amp has been unplugged for over a week. Do you usually have to hook up to another machine (for gounding) when you change them, or is it safe to just pop 'em out, and pop the new tubes in? Any help would be vastly appreciated.

You are supposed to re bias your amp every time you change tubes. Not sure about the other question.

If you go back to the same tubes you might be able to get away from a rebias...I always rebias my tube amps regardless of what power tubes I Put in it though..It gives me a chance to look around in there and make sure everything is happy before I reset the bias..

John

how does one go about resetting the bias?


Originally Posted by Tim 5mphow does one go about resetting the bias?

Are you good with a soldering iron and are you capable of being inside your amp with some electronic experience? Have a multimeter?

The safest way and providing you have an actual bias pot inside your amp?,is to get an actual bias meter...Allessandro makes one,and there's another product called quot;The Bias Ritequot;...Check E Bay or maybe a forum member could let you borrow one etc....

I solder 1 ohm 1% 2 watt or higher resistors onto the quot;cathodesquot; pin 8 of each of my output tube's sockets(This becomes my internal bias meter location)...This is where you put your meter...The meter leads go onto each side of the resistor...From there you can set your bias..There are some guidelines to follow on what to set your tubes to that are based on what tubes you're biasing,plate voltages,etc...Typical readings would be from 30 to 40 millivolts per tube,but these are just round about figures...Again,depends on the tubes...And this is specific to EL34(6CA7),6L6(5881),or 6550 tubes only...

from : localhost/any of this stuff sounds way out or makes no sense at all to you...Take your amp to a tech so you don't kill yourself or your amp!
from : localhost/www.alessandro-products.com/accessories.html

as i have little to no electronics experience, i'll likely take it to a tech. thanks for all the help!!

It's usually around $35, but I'd purchase the tubes from revolutionguitar.com first, then pay someone to bias the amp with them. Then, they won't charge you full-pop on tubes that may not even be as good.

where can i see a chart to see what's considered cold for certain power tubes, and what's considered quot;hotquot;

[Quote/]I solder 1 ohm 1% 2 watt or higher resistors onto the quot;cathodesquot; pin 8 of each of my output tube's sockets(This becomes my internal bias meter location)...This is where you put your meter...The meter leads go onto each side of the resistor...From there you can set your bias..There are some guidelines to follow on what to set your tubes to that are based on what tubes you're biasing,plate voltages,etc...Typical readings would be from 30 to 40 millivolts per tube,but these are just round about figures...Again,depends on the tubes...And this is specific to EL34(6CA7),6L6(5881),or 6550 tubes only...[/Quote]

Can you explain more in depth on where to solder the resistor and test the points?
Don't worry about me getting electrocuted or anything, I've been poking around in my amps for a couple of years already, but never got around to doing a good biasing.


Originally Posted by Quencho092Can you explain more in depth on where to solder the resistor and test the points?
Don't worry about me getting electrocuted or anything, I've been poking around in my amps for a couple of years already, but never got around to doing a good biasing.

Sure...let's use a BF Fender Pro Reverb(I own one).....it has 2 output tubes(6L6s)....

On each cathode of the output tubes(Pin 8) is a little braided ground wire that currently goes from the tube socket pin to chassis ground...

You want to unsolder the end that connects to the socket and solder 1 end of your 1 ohm cathode resistor in place of the braided wire...The loose hanging end of the braided wire now connects to the other end of the resistor....The resistor is now in quot;seriesquot; between the cathode pin 8 and chassis ground...Do this for both tubes because you'll also be able to check how well your tubes are matched...

All you do now is quot;carefullyquot; connect a meter to both sides of that resistor and set your meter to millivolts...Before you remove your output tubes,take a reading and that way you can recreate it with the new tubes if you like? The 1 ohm cathode resistor converts milliamps to millivolts...Millivolts is also safer on your meter than reading amperage and safer for you...If you touch across the plates(Pin 3)then you still get to watch your hair stand up...You also won't need to drink any coffee for awhile....LOL

Let's just say a ballpark reading for 6L6s in a BF Fender amp is 25 to 35 millivolts...Again,this is a ballpark figure only...

If you put the cathode resistors in a Marshall or other EL34 type amp,you now have to connect pins 1 and 8 together and again put the resistor in series between 1 and 8 and ground....That's it..There is an actual calculation that is based on plate dissipation of the tubes and your amp's plate voltages,but I'd have to re-read that info again?