Was wondering if I could get so help biasing my amp. I have been reading all the post here for weeks and I am still having issues getting it right. First thing you probably need to know is My amp is a 210-65 ss driver, chassis 2275-65. It looks to be all original. I have ordered new capacitors to replace the big silver bullets under the hood, but do not have them yet. I was trying to get familiar with the workings inside before I start the rebuild or tune-up. I have taken some measurements with a mm and this is what I'm getting. I measured resistance from rectifier board as I call it (red wire) to pin 3 (blue wire) of inner tube socket with a reading of 163.5 and outer (brown wire) was 167. This was with mm dial set to 200ohms. Then tested voltage drop (red wire to Pin 3) with power on low setting and only got 0.1vdc on inner and 0.2vdc on outer (red to pin 3) with mm set on 200vdc. Voltage drop seems way out, wouldn't this give me an extremely hi plate current. v / r = pc. I don't get it. Am I doing something wrong. Few question at this point. Do I take readings with tubes in or out (el34's were in), power high or low. speakers plugged in or not, and if speakers are plugged in do I need volume set to high or low? Also measured plate voltage at 510 inner and 495 outer on low power setting. My amp has three boards inside, main board, what I call the rectifier board (with the red wire going to it), and a third board I assume is the ss board ( has the two 3.9 resistors and some sort of trim pot in the center of the board). From what I have been reading the bias pot is on the main board to the left alittle while looking from the front of the amp. If so, what is the other trim pot on the smaller board? All I was trying to do was get the plate dissipation. With formula: Volt drop / by resistance = Plate current, plate volts x plate current = plate dissipation. Amp doesn't sound bad at this point, I would like to get it to a point where it sounds it's best. Thanks in advance for answering my questions and helping me to solve this puzzle.
Mon, 11/24/2014 - 16:43
#1
Leave the tubes in!
No need to measure resistance values of everything unless you have a bias matching problem with a matched set of output tubes. You can measure the quiescent bias of the output tubes best (directly) with a Bias Probe adapter for your meter. Otherwise, read the voltage drop across the resistors as indicated in the Factory Service Bulletin relating to Biasing (posted on the web site).
Measure all the DC voltage values written on the schematic to verify that the power supplies are functioning properly. You can determine the proper bias for desired plate dissipation by measuring the B+ on the tube plates and the bias and plugging these into the Bias calculator tool found on the WeberVST web site:
http://www.webervst.com/tubes1/calcbias.htm
You can play what-if games to get what you want without doing direct calculations that are more prone to be screwed up.
The pot on the small board that has the driver transistors is the bias pot. The pot on the main board is the tremelo adjustment pot (also covered in the Factory Service Bulletins).
Thanks so much. Once I get
Thanks so much. Once I get the caps and new tubes I should be able to check and adjust bias with no problems now.