biasing...how crucial?

[krog]

Greetings, I have had tube amps since the 70's and I usually just replace power tubes without rebiasing. My SF Princeton Reverb recently blew a power tube and the closest amp tech is 180 miles from here. Any harm in just throwing in a matched set of tubes and calling it good? Never really worried about it in the past, but I really love this amp! The amp was fully serviced a couple of years ago and all of the components are sound. Thanks!

[TweeDLX]

I've notified the bias police, and they're on the way... Hell no, you don't HAVE to bias...b u t...it's much better on your tubes if you do. Every set is different in the amount of current they draw. What works great on one set might cook another (and take out something more expensive with it). The opposite end of the spectrum is having "anemic" tone from using UNDER-biased tubes. You might luck out and get a set that's almost an exact match for your old set. You might not... If you have any soldering skills, it's incredibly easy to add a bias pot and test jack to a Princeton. Then you can do it yourself in a few minutes. http://www.el34world.com/charts/Biascircuits.htm

Mike

[mbratch]

Or put a matched pair of JJ 6V6S in there and not worry. They're tough. :)

Although to Mike's point, a bias check/set might ensure that you're getting the best sound out of your amp.

[jcs]

if nothing else at least observe the plates of the tubes in a dark room, if they get any red stripe on them after a few minutes to 15 minutes they will soon be toast (and might take a tranny along with them).

installing power tubes without checking cathode current is risky.

[skhan007]

This is what happened to my EL34's in my improperly biased Marshall amp. Melting & sparking are not good things. Bias you amp properly.

http://i69.photobucket.com/albums/i57/skhan007/DSC_0004-5.jpg

[skipm45]

This is what happened to my EL34's in my improperly biased Marshall amp. Melting & sparking are not good things. Bias you amp properly.

http://i69.photobucket.com/albums/i57/skhan007/DSC_0004-5.jpg

+100
Save the gas money on the 180 mile drive.
Here's some cheap insurance. https://taweber.powweb.com/biasrite/br_page.htm

Use this calculator in conjunction with the bias-rite to find the correct bias point.
http://www.ionpool.net/arcade/tech/Weber_Bias_Calculator.html


HTH

Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)

[Tubebender]

Or put a matched pair of JJ 6V6S in there and not worry. They're tough. :)

Although to Mike's point, a bias check/set might ensure that you're getting the best sound out of your amp.

+1- There is a video on this site where Bob tries to overheat some JJs. Didnt happen:
http://www.eurotubes.com/

I'm actually having a simmilar issue on my Fender prosonic. I dunno how to bias it (new thread posted), and I figured if I didnt, I'm goin with JJs since they are solid.

Explore this site, lots of good info, and good deals on full tubes sets. See "shop online", "Fender tube kits".

[Rosewood]

This is what happened to my EL34's in my improperly biased Marshall amp. Melting & sparking are not good things. Bias you amp properly.

http://i69.photobucket.com/albums/i57/skhan007/DSC_0004-5.jpg
Ahhh, mig welding, zzzzzzzitt.......zzzzzzzittttt

[TED STEVENSON]

Checking the tube bias is very important in my opinion. It can reveal other problems with the amp.

[skhan007]

After my tube melting incident, I spent $75 on the Amp-Head MPD Bias Tool and learned how to bias my amps myself. Piece of cake. Use the simple formula: (tube watts) x (70%) Divided By your plate voltage = milliamp value you should set as your max. I got that formula from the Metroamps site.

[jcs]

it doesnt matter if the tube will survive way too much current draw because you are putting major stress on other CRUCIAL components if the amp IS indeed pulling too much cathode current.

this is what makes me so reluctant to sell off any of my power tube collection,the fact that some choose to completely ignore bias parameters.

you may get lucky half the time and the power tubes will idle in a safe range,you may get lucky even more than half the time with cathode bias amps.

keep in mind the dang wall volts vary a LOT at times and when they swing upwards to 123-128 (as is often the case during the summer) you are really taking a chance by not at least checking for redplating in a darkened room as the cathode current and the plate volts both rise when the wall volts rise.

[rockon1]

After my tube melting incident, I spent $75 on the Amp-Head MPD Bias Tool and learned how to bias my amps myself. Piece of cake. Use the simple formula: (tube watts) x (70%) Divided By your plate voltage = milliamp value you should set as your max. I got that formula from the Metroamps site.

Thats most unusual even for just not biasing an amp. Usually for a tube to redplate or get so hot something like that occurs something else is going on. Thats just from swapping out tubes without biasing? In many years of not biasing I never had anything like that happen. Dont get me wrong I bias my amps now to get the most out of my tubes I just surprised by that. Bob

[rockstarjay]

Silverfaces don't even have a real bias adjustment. Get anything in the middle range and you'll be fine. Tell whoever your ordering from you're replacing #5 groove tubes.

Amps really only get whacked when someone biases them on one extreme end of the spectrum and then pops in a tube rated at the other extreme. If you haven't modded your SF you should be fine. I do it all the time with my bassmen.

[krog]

Thanks for all of your suggestions, I ordered a set of JJ's and will get it biased soon.