| Author | Message | ||
| david manzullo |
I have two MM amps,..the 212 sixty five is the first issue...My amperex EL34 tubes just went microphonic on me. I have a pair of 6CA7EH tubes, do I need to bias the tubes before I can use the amp again?   | ||
| michael kaus |
If you have a 12ax7 Phase inverter tube in your amp, yes you need to rebias. If no tube, they are pretty much plug and play ALTHOUGH I WOULD STILL CHECK THEM. If you have 12ax7 PI tube, bias tubes to approx 21-23ma idle current PERE tube and you will be fine. Mike. | ||
| david manzullo |
Yes...I have the 12ax7 PI tube, so can I bias the new tubes myself, never done it before. Is it diffucult..? what do I need? | ||
| michael kaus |
Well, if you are a gambling man, you can bias that amp with just a digital multimeter. You would just have to adjust to get .5 volts as measured at pin 8 on the octal tube socket(power tube). I say gambling man because I don't like biasing by taking the total bias current of a pair of tubes. I prefer to check bias current PER TUBE so that I know the tubes are equal. If you are sure they are balanced(and yet, how would you know?), you can do it this way. I would prefer that you use a bias adapter and check out the current per tube. This way you can see if you have one hot tube or not. Mike. | ||
| Jay Hammond (jead) Username: jead Registered: 01-2011 |
I'm new to the board and loving the info you people are kind enough to share. I just got a 212-65 and plan to swap out the groove tube El34's with JJ EL34L's. I have the 12ax7 version (Phase Inverter?). Just to confirm, If I've been reading correctly, the Bias trim is the blue pot nearest the power tranny (towards the outside of the board) and I should be aiming for 21 to 23ma per tube with octal probe sockets, Have I got that right?? I'm wondering about the other trim pot. Is that for trem? Is there any point in tweaking that? Thanks in advance. |
Tue, 03/25/2014 - 09:25
#1
Tube change
I have a 212HD one thirty with the solid state pre amp...I changed the power tubes to a matched quad of JJ E34L's and use a mesa boogie V1 bottle rocket pedal between the guitar and the clean channel and that gives me a lot of room to try different pre amp tubes....since the pedal takes two pre amp tubes...just don't use the original chinese 12ax7's that come in it ..they are too grain driven for me. Now I have two JJ 12ax7a's and it's pretty sweet...smooth overdrive...but you have to do a lot of knob turning to get a sweet spot. The rca 12ax7 blackplate sounds good also in the left socket....but lower volume.