| Author | Message | ||
     Doug Elick |
In reading the howto posts, there seems to be a bit of ambiguity in regards to the bias voltage. I keep seeing 15-25mv mentioned; is this for solid state phase inverter version? The tech paper lists .5V bias for the 12AX7 version (mine). I know the E34Ls need to be run a little cooler than the original 6CA7s, but the difference I'm seeing in previous posts seems huge. Am I missing something? Don't want to fry a brand new quad of JJ E34L's Doug | ||
| JaySi |
http://www.pacair.com/discus/messages/6/ 35.html?1103401318 here is what Steve has to say on the procedure | ||
| JaySi |
Im sorry, I think that was for the original 6CA7, They are talking voltage drops across the cathode resistor to calculate cathode tube current, at idle, using ohms law... I=E/R where each amp has different value cathode resistor. for example, an 6l6gc requires around 30-35 mA idle current at the cathode | ||
| JaySi |
Im sorry, I think that was for the original 6CA7, They are talking voltage drop across the cathode resistor to calculate cathode current at idle, using ohms law... I=E/R where each amp have different value cathode resistors. for example, an 6l6gc requires around 30-35 mA idle current at the cathode as for the JJ E84L, it should have something like PC=20ma...or cc 20 ma on the box or spec sheet....I just pulled numbers out of the air not knowing the actual current of your tubes... | ||
| michael kaus |
Actually, there is no "requires" figure out there. If you do the math, at 700v times .025 amps, you get 17.50 watts. 17.50 watts divided by 30 watts= .58 or 58%. This is within the range of safe usage for an E34L type tube. I do not advise going any higher though as the track record of Music Man amps is to eat GOOD tubes when set by the "1/2 volt at point Y" method. Will have to do a comparison some day as to what that equates to. Do you have the SS driver version or the 12x7 version? Mike. | ||
| Doug Elick |
Doesn't E=IR say that over a 10 ohm resistor, 25ma is .25 volts? The specified .5 volts across the 10 ohm would be 50ma, or 35 watts!! Idle dissapation greater than the tube's total safe limits. Somehow I don't think MusicMan biased their amps to dissapate 140 watts at idle, yet this is what "Adjust the bias trimpot located farthest to the right (looking from the front) on the GB-1 and BB-1 circuit boards until you read .5 volts DC across the 10 OHM cathode resistor." seems to suggest! So the question is, "For my 12AX7 HD130, what voltage across the 10 ohm resistor would you like to see, for a safe bias setting for JJ E34Ls"? About .25v, right? | ||
| Doug Elick |
I just realized... the 10 ohm resistor is "driving" two tubes for a TOTAL of 35 watts a pair, or 17.5 each, isn't it? I'm tired and can't think straight! .5 or .25 volts? Doug | ||
| michael kaus |
The .5 volts thing is only for the 12ax7 amps. The adjusting procedure for SS driver amps is not the same. The cathode current should be measured with an interupter device and read direct current to groung or with 1 ohm resistors and read DC voltage to ground. The 10 ohm resistors will work also. You are right though, the 10 ohms are for TWO tubes so 17.50v is about .58% of idle disp. Just make sure that the 10 ohm resistors are REALLY 10 ohms. They will tend to be drifty. I've replace mine with 5 watt versions for safety's sake. Mike. | ||
| Doug Elick |
I suppose then for my 12AX7 equipped HD-130 Reverb, .5v across the resistors (using the higher of the two for the measurment) is a reasonable starting point. If the tubes seem to be glow free, at .5 (or a hair colder), I'll consider all well. Any idea where I can find the data sheet for JJ E34Ls? My DMM has temp. function/probe that's useful for measuring tube envelope temps. I currently have a rogue tube in the quad of Jolidas(regardless of socket position) that'll get up to 395F if I run the amp on high. Yeah!!! Am I in the minority on this board in having an older 12AX7 phase inverter amp? Doug | ||
| JaySi |
I apoligize on this one, I was putting my ideas from another amp into this as the info I gave is completly wrong in this case...I guess it teaches me not to second guess the MM amp repair techs.... | ||
| michael kaus |
This is why I like to read each tube separately. You can have two out of balance tube that give you a TOTAL of that voltage and one too hot and the other's too cold(sounds like like a fairy tale somewhere.............). You can either get a pre made adapter or make one yourself. Either way, measure each tube separately and you can also tell how well matched your tubes are, if that's important to you. I just want to make sure they are not grossly out. Mike. | ||
| Doug Elick |
I'm going to have to trust "The Tube Store" in that they're selling a matched quad honestly as such. I know I can get the bias job done safely, but that doesn't mean I'm particularly comfortable working with an energized circuit at 700 volts potential. To that end, I'll be engaging in minimum dickering. Warm it up, test the voltages, adjust (with ONE hand), wait, adjust, wait (ad nauseum), watch the tubes in a dark room, measure their temp and then see how everything sounds. If there's a gross imbalance that can't be minimized by adjustment or swapping tube orders, I'll take it to a shop for further diagnosis. The way I see it, my "high tension" paranoia keeps me safe and alive... Doug | ||
| michael kaus |
Paranoia keeps a lot of people alive. By the way, temperature is an interesting thought. I may have to do a little investigating. MIke. | ||
| Doug Elick |
I purchased the temp probe for my DMM and wanted to play with it. Since I had everything out to fix the reverb on the amp, I decided to measure the tube temp. for the heck of it. I already knew one or two tubes were exhibiting the blue glow around the plates, but when I put my probe on the envelope of the far right valve, the temp quickly shot up and settled around 380 degrees (the others were barely cracking 190). It turned out that the hot tube had a wicked plate glow that was hidden because it was in the far right position. Swapping revealed that the problem followed the tube. I also realized that the hot tube was also responsible for an odd low frequency hum (regardless of volume selection); it was so microphonic it set up a feedback loop between itself and the cab the amp was sitting on. I'll probably tool with the bias this weekend. Wish me luck. Doug |