50 Watt Plexi Reissue Bias Questions

[Cjsinla]

I need some advice from some of the amp gurus who frequent this forum. My Marshall Plexi is about 10 or 15 years old, made in England, has a PCB. I recently bought some 1 ohm resistors to solder between terminal 1 and 8 and ground so that I could easily check the bias of the amp. But when I looked inside the construction was different than I expected. The EL34 power tubes were joined together at terminal 8, sharing a common ground with one of the cap cans. Is there some type of advantage to that? I was thinking that I could just disconnect that common ground and just ground each tube to its own ground through a 1 ohm resistor. But, I didn’t do it as I didn’t want to mess anything up.

So I decided to check the bias by measuring the voltage at the plate and subtracting that from the voltage at the center tap of the OT and dividing the difference in DC voltage by the resistance in the windings of the OT. The math looked like this:

(446.0 - 445.0) / 40.1 = 0.0249. I assume that meant about 25ma. This is where things started to get tricky. I started to adjust the trim put right next to the diodes in the power section, the pot labeled VR2, but that didn’t do much. There was a second pot labeled VR1 between two small electrolytic caps. When I hooked up my meter to the OT center tap and manipulated that pot the center tap voltage would go up or down by 3 or 4 volts. Turning up the voltage seemed to be the logical step here but the plate voltage kept going up right along with it, almost in lockstep. According to my calculations I needed a little more voltage difference between the center tap and the plate to get a hotter bias. I finally got the bias that was originally 25ma at each tube to go up by turning the pot just a hair. It’s now reading 39ma at one tube and 47ma at the other. I was shooting for 38 to 39 ma. Is this reading ok? Do I need to buy a matched set of EL34’s and re-bias?

 

[Cjsinla]

Did some research, the VR1 is the trim pot and VR2 is “hum balance”. Got some matched tubes but still having trouble getting consistent measurements to calculate bias as voltage goes up and down by a volt or two each minute.

 

[Cjsinla]

So, I was able to figure out my problem with a little help from @Leña_Costoso. I got matched tubes but I didn’t have to use them as the old tubes worked perfectly well. I ended up pulling the common ground that connected both power tubes’ pin 8 and placed 1 ohm 1 watt resistors between pin 8 and ground. I started working the VR1 trim pot while watching the multimeter. The reading was 25ma and by my calculations that reading should have been closer to 39ma. It turns out that I was originally turning it clockwise when I should have been turning it counterclockwise. I believe that my tech put a drop of red nail polish on that pot when he last set the bias. You can see how far I had to turn the shaft on the VR1 pot to go from 25ma to 38ma.