6505+ Screen Grid resistor?? Fixed!

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bubbastain

bubbastain

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I’ve had my 6505+ head for a while now. Got it used and pretty cheap. It sounds great! I normally remove the amp from the headshell, clean , visually inspect and check the bias of almost every amp I get, but I didn’t with this one.
So tonight I finally decided to check the bias of each tube. Looking at the back of the amp from left to right I got 24.9, 22.2, 24.6 aaaannnddd 0.0. I swapped in a good tube and still a zero. Tube lights up.
So I’m assuming screen grid resistor right? 100 ohm 5 watt. Anything else it could be? Where do you guys get your parts from?

thanks
 
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Sounds like a screen grid resistor. Had the same exact thing on a Peters I had a while back. Replaced and good as new.
 
I would say it's a bad screen grid resistor Bubba. Splawner had a similar issue lately here's the thread, it was a bad screen grid resistor as well. The 5150's had lower value screen grid resistor's compared to a Marshall amp, these tend to go bad over time in the 5150's, I'm not sure what value's were used in the VTM's and such.

https://www.rig-talk.com/forum/threads/bias-problem.215903/
 
Thanks guys. So where do you guys order your parts from? In addition to replacing the screen resistor on the 6505+, I think I going to put new caps in my VTM60.
 
Valvestorm, Mouser, Antique electronic supply, all are goo to deal with.
 
Take the daughter board out and measure the resistor. You don’t have to unsolder it as long as no tube is installed of course. That will tell you all that you need. If you’re popping SG resistors, likely time for new tubes.

A long time ago when I recapped my 6505+, I also upped the screen grids to 1k 10W ceramics to help elongate tube life. If you have to replace one, I’d recommend doing the same thing. Also do the bias modification so that you can bias them out of crossover distortion. I set mine to 32mA.
 
glpg80 said:
Take the daughter board out and measure the resistor. You don’t have to unsolder it as long as no tube is installed of course. That will tell you all that you need. If you’re popping SG resistors, likely time for new tubes.

A long time ago when I recapped my 6505+, I also upped the screen grids to 1k 10W ceramics to help elongate tube life. If you have to replace one, I’d recommend doing the same thing. Also do the bias modification so that you can bias them out of crossover distortion. I set mine to 32mA.
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I’ve read up a bit on moving up to a beefier resistors. Are there ANY cons to doing this? I absolutely don’t want to affect the tone of this amp. This one sounds better than the other 6505+ I had years ago. I even did the bias mod on that one. Shit, maybe this one sounds so good cause it’s only running three tubes! :LOL: I am also going to order a new set of power tubes.
 
No cons, tubes won’t be stressing the screen grids as much. It does make the tube run less like a triode strapped pentode and more like a regular pentode (not true pentode). I noticed no negative effect doing it to mine. If you don’t want to bother that’s fine - tons of them out there rocking the regular values. Definitely get some new power tubes soon though.
 
Well I got the board out and it was much easier than I thought it would be. I read that the board was riveted in. Nope. I tested the SG resistors and yup, that one was bad. I’m about to order new ones. And tubes.
22534F50-9B68-4D8D-864A-3B1807A5ACBA.jpeg
 
Keep in mind that it will affect the tone since you will now have more symmetrical output running on 4 tubes
 
Keep in mind changing the value of the screen resistors will change the amps sound. Higher values will sound a bit darker.
Since you have the board out, I’d change all 4 screen resistors.
 
psychodave said:
Keep in mind changing the value of the screen resistors will change the amps sound. Higher values will sound a bit darker.
Since you have the board out, I’d change all 4 screen resistors.
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Yup, I ordered 4 and will replace them all. I ordered the stock values (100 5 watt). Unfortunately Ruby 6L6GCMSTR ended up being harder to come by, so I ordered a quad of JJ’s.
 
scottosan said:
Keep in mind that it will affect the tone since you will now have more symmetrical output running on 4 tubes
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God I hope not too much. This is the best sounding 6505+ I’ve played. For all I know it could have been this way since I’ve had it.
 
Surprisingly I got both my tubes and SG resistors in the mail today. Popped them in and put it all back together and it's rocking again. Sounds killer still.

 
Cool Bubba... glad you got it fixed... did you use the stock values for the screen grid resistor?
 
bubbastain said:
Surprisingly I got both my tubes and SG resistors in the mail today. Popped them in and put it all back together and it's rocking again. Sounds killer still.

Click to expand...

Sounds great! What kind of cab is that? speakers?

Thanks!
Mark
 
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