The reason it is important to rebias a push-pull amp when changing the tubes is that when changing the tubes, the new tubes can then be improperly biased either hot or cold.
If your amp has a phase inverter, it is a push-pull configuration. It is also important to note that cathode bias is very common in Single ended (one power tube, or multiple in parallel) amps, and very uncommon in Push-pull amps. This configuration is commonly referred to as "Self Biasing", and so long as the amp was biased well initially, it does not need to be rebiased when changing tubes. On a side note, a cathode bias amp (traditionally biased via a cathode resistor or pot), controls the mA to ground via limiting the current to ground, and not by applying negative voltage to the control grid. While old Fender amps determine the bias via the value of the negative voltage supplied by the bias circuit, the accurate way to bias an amp is by measuring the mA DC that flows through the cathode to ground. This negative voltage will control how much current flows through the cathode. That seems like a shit ton of money for a new PCB board, and biasing but at this point I'm just happy to hear that I should be getting it back in working condition.īiasing an amplifier (In a fixed bias design, which is by far the most used) applies a negative DC voltage on the control grid of the tube. He just ordered a replacement but he's charging me $300 because he spent so much time working on it. Apparently it was a bad PCB board which is known problem with these amps.
Apparently the tubes were fine and he was able to bias them correctly, but I instructed him to bias the tubes to 70mV instead of 80-90mV (wanted to get more life out of them to avoid this again!) he told me he couldn't get a reading from one side. UPDATE!: So I finally heard back from the technician after a couple days of panic. The closest I've gotten them is about 100mV each, and then from there things go all out of whack.Ĭan anyone give me some advice? Luckily I have a month off from band practice but I don't really know any reputable amp repair places to bring this to, and would really like to be able to fix it on my own. Every attempt I've made to bias the amp has been a failure. I don't know if the tubes are bad, but the amp works, it just doesn't sound great. By the time I think I'm where I'm supposed to be, both sides end up over 150mV. Marshall recommends between 80-90mV per pair, and the best I can do is get one side around that area, while the other is way hot over 150mV. But attempting to bias them has been a nightmare thus far.
#HOW TO BIAS GUITAR AMP HOW TO#
It's my first tube amp that was recently purchased and I figured I'd learn how to bias the tubes to save money and time having it done for me. So I bought a new set of tubes for my Marshall DSL 100.