I got an Express today and I have a pair of GTE34Ls that was sent with it. I need to bias them and am not sure where to hook my meter probes or what setting to put the meter on and what values these tubes should run at . I am aware of the dangers and voltages and have actually biased amps before but never dealt with one of these amps or these tubes.
Can I get some advice Please!
Honest Honey! I had to buy the guitar to help out a friend in trouble.
Duncan has a great site. I suggest you read carefully and use caution. I use the shunt method, listed below the cathode resistor method. If your amp has the cathode resistors installed, then the cathode resistor method is easier and much safer.
Good luck, and BE CAREFUL!
HTH, D
KF stated in the Trainwreck pages that you should put -30VDC on the output grids. Now of course, I wouldn't just leave it there but hey, what do I know. As a side note, the two TW Expresses I measured came from Ken's shop biased at 49mA
I like using the 1 ohm cathode resistor method, the advantage of this method is there are no high voltages involved, , On both power tubes remove the ground from pin # 8, from pin 8 to ground add a 1 ohm 1 watt resistor. with the amp on & your multi meter set to dc millivolts ground the negative lead & the positive lead goes to pin # 8 (cathode) the voltage drop across 1 ohm converts directly to milliamps - 40 millivolts=40 milliamps. for EL34's 40 millamps will get you in a good range. depending on what type of tubes you have you can go up to 50 or 60 ma. If you want to get tecnical use this formula (.75x25/pv) & set your bias milliamps there. . 25 volts is the maximum plate dissipation for an EL34, .75 gives you 75% of this value, pv is your plate voltage. .75x25/400 = 46ma. , Keep in mind that the (stock) Express does not have dual bias therefor you are biasing both tubes at the same time so you need a closely matched set of tubes, I have had 2 new tubes of the same brand be 20 ma off from each other. Here is a very good artical on tube biasing - http://www.aikenamps.com/Biasing.html
. JOE
I just biased my Ceriatone Express for the first time. I used one of the SRS bias tools. It is one of the bias tools that plugs into one of the power tube sockets, with a pair of test leads to connect to a mA meter. Following the instructions with the bias tool, I ended up biasing the tubes at 31.8 mA. That seemed to be very conservative. I wasn't very happy with the sound. Turns out that the SRS calculation appears to be specified for 50% plate dissipation. I adjusted the bias to 46.8 mA, about 75% dissipation at 393 volts on the plates, and the sound came alive.
Incidentally, I like the SRS tool. It's great for those of us who didn't build in bias test points and don't like to get our hands in a live amp to set the bias.
I got an Express today and I have a pair of GTE34Ls that was sent with it. I need to bias them and am not sure where to hook my meter probes or what setting to put the meter on and what values these tubes should run at . I am aware of the dangers and voltages and have actually biased amps before but never dealt with one of these amps or these tubes.
Kborg, I too use an SRS tool, but use the dual socket version. I bought it after trying to make my own and becoming frustrated with the limited space available in a tube base and socket (besides wasting a perfectly good socket that I could have used to build amps with!). The dual socket one is great because you plug 'em in with tubes in 'em and the mini toggle allows you to go back and forth to fine tune each tube's bias. As stated, some tube pairs out of the box suck and you will find them unusable as a pair, since they wont bias anywhere near each other. I try not to use any pair that are out of "sync" by 5mv or more. I will offer my opinion that EL34's tend to be best around 50, 6V6's around 20, 6L6's about 32-35, depending on the amp. As far as tubes go, US 6CA7's are my faves for EL34 amps and 7408's for 6V6 amps. There are a shitload of great 6L6 tubes and related brethren (KT66, 5881, etc.) which are worth trying in appropriate circuits to see what you like. That being said, my personal preferences are arrived at using quality built circuits and neutral speaker cabs as much as possible, w/ JBL, EV, Altecs, and other similar types of speaker installed that allow the amp sound to be showcased without a large amount of speaker quirks coloring the sound. Just my 2 cents...
Kborg, I too use an SRS tool, but use the dual socket version. I bought it after trying to make my own and becoming frustrated with the limited space available in a tube base and socket (besides wasting a perfectly good socket that I could have used to build amps with!). The dual socket one is great because you plug 'em in with tubes in 'em and the mini toggle allows you to go back and forth to fine tune each tube's bias. As stated, some tube pairs out of the box suck and you will find them unusable as a pair, since they wont bias anywhere near each other. I try not to use any pair that are out of "sync" by 5mv or more. I will offer my opinion that EL34's tend to be best around 50, 6V6's around 20, 6L6's about 32-35, depending on the amp. As far as tubes go, US 6CA7's are my faves for EL34 amps and 7408's for 6V6 amps. There are a shitload of great 6L6 tubes and related brethren (KT66, 5881, etc.) which are worth trying in appropriate circuits to see what you like. That being said, my personal preferences are arrived at using quality built circuits and neutral speaker cabs as much as possible, w/ JBL, EV, Altecs, and other similar types of speaker installed that allow the amp sound to be showcased without a large amount of speaker quirks coloring the sound. Just my 2 cents...
and I have a pair of GTE34Ls that was sent with it. I need to bias them and am not sure where to hook my meter probes or what setting to put the meter on and what values these tubes should run at 
. I am aware of the dangers and voltages 
and have actually biased amps before but never dealt with one of these amps or these tubes.
