re-bias questions

[BB]

My Winfield Thomas Elizabeth started making some funny noises a few weeks ago. Until it was good an warm, some of the notes would fuzz out and there was an intermittent bit of noise that would show up out of the blue.

I bought a set of GE 6L6's and a Sylvania GZ34 rectifier tube from Ernie Ball way back in 1979 or so for an old Fender amp I had. By the time the tubes arrived, I had sold the amp. So, I stuck them away in a parts box. Of course, I forgot where I put the box, but after an hour or so of scouring my garage, I found them. I was leary of replacing the power tubes without a prober re-bias, so those will be changed at a later date. I did switch out the Chinese made 5U4 rectifier tube for the Sylvania GZ34 however. MAN! What a difference! The noise went away, no more fuzzy notes and the amp ( which already sounds beautiful ) seemed louder, richer and crisper. Now, I'm anxious to replace the Sovtek 6L6's with the GE's.

Should I attempt to re-bias the amp myself or should I take it to a qualified tech? I'm familier with tube amps and I respect the lethal voltages that reside inside, but I have never attempted a re-bias before. Is this something I could do myself, ( with proper instruction ) or should I leave it to a qualified tech? Also, should I change the two preamp tubes ( a JJ 12AX7 and a JJ 12AT7 ) at the same time, or just leave them alone. They don't seem to be microphonic or anything, just wondering if it would be a good idea. Thanks in advance. Brad

[Stan Martin]

alone if they're not making any noise. Those Chinese rectifiers are horrible and you did yourself a big favor replacing it. If the amp sounds better now, I would leave the 6l6's in until the amp started to sound weak.

Also your bias went up by replacing the rectifier tube. Which probaly isn't a bad thing. Most amp manufacturers bias their amps on the cold side. Make sure and check and see if the plates are glowing cherry red.

[Ben Harmless]

I would say that you are a perfect candidate for biasing your own amp. You know you can hurt yourself if you do the wrong thing, and you have the desire to try different tubes.

All that's left is the tools and the know-how associated with your specific amp. The tools will vary, depending on the method you use. I have a Weber Bias-Rite, and the only other thing I need is a big phillips head screwdriver for taking the chassis out and a little flat head for adjusting the bias pot. I can do the job safely in probably about 15 minutes.

And yes, remember that there's some major shocks waiting in that amp for you. Because it has to be on while you're doing this procedure, dont touch anything other than the bias pot(s) with the end of your screwdriver. Unless you drop the amp on your foot afterward, you'll be just fine, and happier overall.

Enjoy!

[BB]

Originally Posted by Stan Martin

alone if they're not making any noise. Those Chinese rectifiers are horrible and you did yourself a big favor replacing it. If the amp sounds better now, I would leave the 6l6's in until the amp started to sound weak.

Also your bias went up by replacing the rectifier tube. Which probaly isn't a bad thing. Most amp manufacturers bias their amps on the cold side. Make sure and check and see if the plates are glowing cherry red.

Thanks for the info Stan. You're right...that Chinse rectifier is JUNK! The preamp tubes seem fine, so I'll leave them alone along with the power tubes ( also JJ's ) till they need replacing. The JJ's sound pretty good. I did check the tubes several times and everything seemed fine. Thanks again for the info.

[BB]

Originally Posted by Ben Harmless

I would say that you are a perfect candidate for biasing your own amp. You know you can hurt yourself if you do the wrong thing, and you have the desire to try different tubes.

All that's left is the tools and the know-how associated with your specific amp. The tools will vary, depending on the method you use. I have a Weber Bias-Rite, and the only other thing I need is a big phillips head screwdriver for taking the chassis out and a little flat head for adjusting the bias pot. I can do the job safely in probably about 15 minutes.

And yes, remember that there's some major shocks waiting in that amp for you. Because it has to be on while you're doing this procedure, dont touch anything other than the bias pot(s) with the end of your screwdriver. Unless you drop the amp on your foot afterward, you'll be just fine, and happier overall.

Enjoy!

Thanks for the support Ben! Per Stans recommendation, I'll leave the power and pre-amp tubes alone for now. The amp is sounding good so I'll leave well enough alone!

I'm going to do some research on re-biasing though as I would love to be able to work on ( well, at lest bias ) my own amps. I've been doing my own guitar repair and set-up for years, so why not amps! How do settle on the proper plate voltage for your amp? Does setting it colder give more clean headroom? Hotter break up earlier?

[zook]

Brad,

The discussion on the chinese rectifiers is accurate. I stopped using them a long time ago in favor of NOS 5U4's, although the last two JJ 5AR4s I tried are fine.

Being cautious, as you should, get a digital voltmeter. Ground ther black lead (I use an alligator clip on the black lead, squash it with plies so that it has an interference fit) to the chassis.

Measure the plate voltage of the new 6L6s at pin 3 of each tube, Using these tables decide on the bias you desire. I usually bias in the middle at 60%, let your ears decide.

http://members.shaw.ca/house-of-jim/...as_tables.html

There is a 1 Ohm resistor on the cathodes of the 6l6s at pin 8. Measure the voltage at the tube. You will get a reading in Millivolts. That translates into Milliamps with ohm's law (y mv/1ohm = y ma) use the pot that is inside the chassis to reduce or increase the figure to the bias you wish.

I have a new email since I moved to AZ wnthomas AT earthlink DOT net

Winnie