Screen Grid Resistor question.

[hunter1970]

Okay my 6505+ had a mismatched power tube in it when I first got it a couple of months ago,I haven't had time to try and learn how to bias the tubes and decided to let an amp tech that I trust do it,well he gets into it and put a set of Groove Tubes in it but tells me by the old tubes not being a matched set he would have to also replace the screen grid resistor.
Well I got it back today and now it just rips, I mean it gets loud but stays really clear and how I expected it should have sounded all along.I mean it sounded good before but I shouldn't have ever been able to turn it up to 4 or 5 without shaking the earwax out of my ears.
Now I can barely play it at 2,2.5 and it's loud but incredibly clear.I'm glad I took it to the tech.
My question is can somebody explain the screen grid resistor in more detail.

[Twitch]

If you say it sounds good, cool, but I have no idea why he would say that. Maybe he was under the assumption the 6505+ was fixed bias, even then, its not a "screen grid" resistor that needs replaced in that instance.

[hunter1970]

He said it was bad or blown due to the fact that it had a different 6L6 tube,and it was somehow restricting current causing the screen grid resistor to go bad.
But I now have a matched set of Groove Tubes and it sounds really good.

[Twitch]

Never heard of an unmatched set doing damage to anything but your tone, but I could happen I guess. Glad its workin for you now. Im not gonna praise mine too much, but Im having fun with it for now. One thing that is a definite up side to my 6505+ is that I dont have to tweak the knobs away from my live tone to get a decent recorded tone. Every other amp Ive had, Ive had to change my settings to get a good recorded tone. Do you have the matching cab too??

[Twitch]

Did some research, seems your amp had a tube blow at some point. It wasnt the unmatched set that did it most likely, but the reason that set was unmatched was because the original tube that was in that spot blew and was replaced with a tube of a different rating. At any rate, it got fixed. Heres an article, tip #5.

http://www.kbapps.com/tubeamptips.html

Incidentally, I do now recognize the problem, just never knew the proper name of the resistor that usually takes the beating when a tube fails. I just knew to look for it and replace it. Some manufacturers actually do this by design to save the OP tranny in the case of a tube fault, others like Marshall and ENGL, use fuses and breakers.

[hunter1970]

No I don't have the matching cab, I have a Mesa 4x12 standard size slant cab with V30's in it.

[Twitch]

No I don't have the matching cab, I have a Mesa 4x12 standard size slant cab with V30's in it.

Cool, youre not missing anything then. Thought maybe you still had the VS cab.

[DonP]

Your tech did his job, but I think there's some confusion here.

A mis-matched set of tubes won't take out a screen resistor.

The screen grid resistor does two functions. It lowers the voltage so that the screen voltage is lower than the plate and it protects the amp by burning up if there is a tube short.

Without this resistor (or if it's blown), you amp won't put out full power.

My guess is that the tube socket that had the mismatched tube had a prior tube that went bad and took out the screen resistor, and it just needed to be replaced. That, and the new set of tubes is why your amp is sounding a lot louder.

Either way, it's good to know you amp is running correctly.

[Twitch]

Read the top part of my 3rd post Don.

[DonP]

Read the top part of my 3rd post Don.

LOL! Sorry about that Twitch. I must have skipped over it. Actually all of your 3rd post is pretty much what I repeated.

[Twitch]

No prob sir, its nice to have confimation of my facts, theyre wrong sometimes.