What is the correct impedance for a 50 watt OT? HELP!!

Zoso · Post by **Zoso** » Sat Oct 16, 2004 5:48 pm

I was checking my 50 watt non-mv OT specs. I measured about 41 ohms from the center tap to one plate and about 44 ohms on the other, for a total of about 85 ohms. Does this sound about right for a 50 watter? The main reason I'm concerned is that I'm getting a slight glow on my plate on V4. I read the other thread on the Marshall forum about a similar problem that Aaron was having. I plan on trying out a new set of tubes this weekend, but I also went ahead and checked some of the parts that were mentioned in the other thread. Everything seemed to be OK, this OT measurement was the only thing I wasn't sure of. I just retubed about three weeks ago, so I'm worried it may be something besides the tubes, although I could have just gotten a bad tube I suppose. Anyway, i just wanted to check that spec and I'll know more after I try the new tubes. If they redplate as well, I will probably try replacing the 82k plate resistor on V3 with a 68k as recommended.

Post by **VelvetGeorge** » Sat Oct 16, 2004 6:39 pm

Those readings are right. You'll find that the two sides of the primary are never exactly the same. A difference of several ohms is fine.

Let us know if the 68k solves your red plating.

George

Zoso · Post by **Zoso** » Sun Oct 17, 2004 10:48 pm

Thanks George!

I didn't get a chance to mess with it this weekend. But I am going to get a new set of JJ's on Monday. Like I said, if they do the same thing, I'll try the 68k. If that still doesn't work, then I need some serious help.

I'll keep you posted.