Just had a guy bring me a 6505/5150, brand new with less than 10 hours on it.Umm are you asking if I'm Steve? Nope sorry.
I really like my amp tech I go to but as far as modifying an amp thats where he hasn't ventured. Well at least with 5150's. He was the one that installed my transformer and choke; he did a great job I might add. Also helped me correct a supid problem regarding a speaker cab miswiring. Really should have taken my time at that project.
That seems to be a big problem with the 5150's/6505's. A bad tube coming from factory resulting in a blown or wide open SGR. I think thats why a lot of the 5150's have gotten a bad rap for being too buzzy of "sounding like a can of bees". I might have noticed that problem when playing at a volume of .5-1.5, but a nice sweet spot and still tolerable for the ears is a volume of 3. It only gets better the louder you go.Just had a guy bring me a 6505/5150, brand new with less than 10 hours on it.
Among other mods, he wanted me to install an adjustable bias control and bias the tubes warmer. When I went to do the bias adjusting I immediately found that one output tube wasn't working!
This unbalances everything. This kind of amp works with an even number of tubes on each side of the output tranny wiring. A dead tube means more distortion (harsh, not nice) and big current swings in the now unbalanced transformer, which is NOT good for it!
Turned out to be an open screen resistor. Looked just as pretty as the day it was made. No signs of burning - just wide open inside! This would leave the tube lit up but nobody at work.
There's no way the tube could have pulled enough current through that resistor to burn it out. Peavey must have gotten a bad batch on the production line.
This is easy to miss! This amp when it leaves the factory has no bias adjustment so they'd just stick the tubes in and try it out! They wouldn't have a meter on it to see the lack of current from one tube. With 3 tubes working the amp would still be loud and the extra distortion would not necessarily be obvious to the ear of the "tester".
It's a good thing he got me to do the bias-adjust mod. Makes me think that the idea of not having one at the factory may not always be a good idea...
Blowing the resistors? That's hard for me to believe, NR!5150's IMO are notoriuos for blowing screen resistors. I've had a few over the past year with this problem...they also wind up blowing the neg. bias feed resistors on occasion too.