Getting the Bias Right

squid · Post by **squid** » Wed Mar 07, 2007 5:32 am

Hi, Charlie. I bought a re-issue Fender Twin Reverb about eight months ago. I play it nearly every day and as a result some of the stock tubes are starting to go. I haven't done any diagnostics on which ones are starting to squeal and/or underperform -- I'm just going to do a complete replacement for the whole set.

My other amp does not need any bias adjustments, but I understand that this is essential to the proper functioning of the Twin Reverb and the longevity of the tubes themselves. (If the bias isn't within tolerance, one tube in the pair will give out prematurely, right?) I'll be buying the tubes from the Tubestore and of course they sell most of their stock in matching pairs. Is this the only thing I have to do (order matching pairs) or are there further steps that I need to take? What sort of equipment do I need to take the required measurements and what do I do if the bias is completely whacked out? New tubes? Switch positions?

Also, do you have any recommendations for specific tubes in specific positions? I read in some earlier posts that you prefer Tung-Sol's in Fender amps, but if you have any further hints, I'm all ears. I prefer the "fatter" Fender sound, if you know what I mean (I only use the bright switch as an absolute last resort if I'm not cutting through) but I do like good string to string definition and a not-too-flabby bass response.

Thanks for the help!

soundmasterg · Post by **soundmasterg** » Wed Mar 07, 2007 3:57 pm

Any amp that is fixed bias will have to be adjusted when the power tubes are replaced unless it is like most Mesa Boogies where the pot has been replaced with a pre-selected resistor. Your Twin Reverb uses a pot to adjust the bias voltage for all four power tubes, so matching the tubes is a good idea. If the tubes aren't closely matched enough, then the tone could suffer, with it being more distorted that normal, and you could also get more hum. If the tubes used are very far out of match with each other, then some could wear out prematurely since you would have to compromise on the bias adjustment.

The easiest way to set the bias in an amp like yours is to get a bias meter like the Weber Bias Rite. (www.tedweber.com) As long as you get the right model, these devices measure the cathode current of the power tubes, in addition to the plate voltage. This allows you to use some easy math to figure out the plate dissipation and set the tubes to a proper bias level so they will get you the best tone and power. Without one of these devices, you need to do a lot more invasive surgery on your amp to be able to measure the cathode current, since current can not be measured in parallel. This would require you to open the amp chassis up, solder in some 1 ohm resistors between the cathode and ground of the power tubes, drill some holes to put in some tip pin jacks, and have a digigtal multimeter to measure the plate voltage. The current would be measured by the DMM probe going into the tip pin jack which measures the voltage across the resistors. Using ohms law the current coverts to a voltage, and then you can use the same math I talked about above. If you open the chassis of the amp, you expose yourself to the high voltage in there usually around 460v, which is enough to kill you if you don't know what you are doing. So for safety's sake, either get a Weber Bias Rite meter, or take your amp to a tech to do the work. If you need to know more about how to figure out the math I can elablorate in another post.

For tube types to use:

Your preamp tubes probably do not need to be replaced unless they are microphonic or otherwise noisy. Preamp tubes last a long time. The driver/phase inverter tube which is the 12AT7 closest to the power tubes should be replaced when the power tubes are. The reverb driver tube which is the other 12AT7 in the amp should be replaced at that time also since it works very hard. If you want to replace all the tubes, go ahead and do so. Then you can keep the old preamp tubes around as extras. The new Tung-Sol 12AX7's and 5881's are excellent and you could use those if you want. JJ and Winged C also make some good things. Some people like the EH and Sovtek lines too. I'd probably suggest JJ 12AT7's, either Tung-Sol 12AX7's or JJ ECC83's or ECC803's, and either JJ 6L6GC's, Winged C 6L6GC's, or Tung-Sol 5881's.

ken_j · Post by **ken_j** » Wed Mar 07, 2007 4:10 pm

I believe that the current production of the Fender Twin Reverb uses the GT-6l6GE for power tubes.

jingle_jangle · Post by **jingle_jangle** » Wed Mar 07, 2007 4:58 pm

My own bias was just right last week. I was excused from jury duty because of it...

squid · Post by **squid** » Wed Mar 07, 2007 5:22 pm

Paul! I'm shocked! There is no greater opportunity for the layman to contribute to the proper functioning of his community -- nay, to the proper functioning of democracy itself -- than through his contribution to a jury. Or so the judges keep telling me. I'm a little skeptical myself, on account of the fact that "panels" (about 100 prospective jurors who are sitting there waiting to be picked) keep cheering everytime they're told that they're not needed. Just before Christmas a local judge got a standing ovation for letting the panel go. Hilarious.

Greg, thanks very much for the detailed reply. I'll be tracking down a Bias Rite very shortly. And my old physics textbook. That reference to Ohm's law shook some cobwebs loose.

jingle_jangle · Post by **jingle_jangle** » Wed Mar 07, 2007 5:30 pm

I was prepared for a long one...they would not let me off on hardship grounds. (Hardship being on the part of all my waiting customers!) There were 70 jurors in each pool, 2 pools for a total of 140. One case went to trial, the other decided to plea bargain, so half of us were dismissed the same afternoon.

That's the troof, your honor.

soundmasterg · Post by **soundmasterg** » Thu Mar 08, 2007 3:20 pm

Ken those Groove Tubes GT-6L6GE are great tubes, and are a replica of the old GE 6L6GC, so they will have mostly the same specs. The only problem with them that I have is the price. Most of the other Groove Tubes products are just rebranded and graded/tested stock from other manufacturers like JJ, Winged C, EH, Sovtek, etc.

John, when you get the bias rite, if you need help with how to use it to set the bias properly, give me a shout...I'll be glad to help. You'll need to know the max dissipation of the power tube you plan to use, and you'll need to know the plate voltage, the cathode current, and multiply them together which gives you the dissipation. Generally aim for between 60% to 70% of the max value for a given tube. A 6L6GC has a 30 watt dissipation, and 70% of that is 21 watts, so your multiplied number should be somewhere in that ballpark. Make sure to remeasure the plate voltage and recalculate if you adjust the bias current because it will change with every adjustment.