Just did the 5150 bias mod

[Mr. Brokenamp]

Thought I would post this for all the 5150 fans here.
The amp sounds alot better. The ugly distortion is no more.
Here's b4 and after pics of what I did and an explanation:
After I take the chaasis out of the box I put my resistor across the caps, one probe on each side while set on the highest volt setting, to make sure there is no voltage.
I also check across the resistors near the filter caps (the big ones sticking up not pictured here). Or some people will check from pin one of a preamp tube to ground but that's awkward on these amps.
That lone resistor is the bias resistor (15K if I remember correctly). I snipped it out and I replaced it with a 10K pot a 1.5K and 4.7K resistor end to end (so I have about 15k like the amp was designed with) on one side and on the other side of the pot is a 10 ohm so when I read the voltage across it (one probe on each side of the resistor) I know how many milliamps I'm throwing at it. Before turning it on make sure the new pot, that got glue gunned into place, make sure it is turned all the way up for 10K ohms.
Before:

After:

 

[JeffTD]

Awesome! Mine had a cool version of it when I got it - the pot for adjusting the bias was installed to the left of the powertubes, so I don't have to open the amp up to screw with it.


That said, I think I could be the only one who prefers the 5150 without the bias mod - my pot is turned back to the stock setting. It sounds more... raw, has more grind, more 5150.

 

[KeithRT99]

clips?

 

[Keiffer]

What power level did you bias to?

 

[Mr. Brokenamp]

I think it's at 36mA. From what I've read 5150's are designed with crossover distortion for some reason, which is not good sounding distortion. Very strange someone would design an amp like that.
Good idea maybe I will move the pot so it's accesable w/o opening it up.
Clips...I'm waiting on my new mic pre.
Also I started experimenting with my 30 band eq in the loop that tightens it up.

 

[Keiffer]

to do this correctly, you need to know the Plate Voltage. then bias to a power level

on my 5150 my Plate Voltage is ~475 and I biased to ~ 37ma

P = V * I = .037 * 475 = 17.5 W

(17.5 / 30) * 100 = ~ 60 % of the tubes rating

 

[kaomao]

Before turning it on make sure the new pot, that got glue gunned into place, make sure it is turned all the way up for 10K ohms.
Before:

why this? why do you have to turn it all the way up?
I'm not a tech, so can you explain it?

 

[Mr. Brokenamp]

"why this? why do you have to turn it all the way up?
I'm not a tech, so can you explain it? "

Just to be safe I turned the pot to the maximum resistance so it's like it is from the factory, biase really "cold".
Also to be 100% on Keiffer is right, there are formulas to use to, then follow the safe operating guidelines for the tubes.