5150 Bias Mod

SQUAREHEAD

SQUAREHEAD

Well-known member
Just got this head and I want to do the bias mod… The tubes are running around 12 mA each… Cold.
Where do most people put the trim pot? On top of the chassis near the power tubes or on the back of the chassis where it says ‘preamp out’?
I could use that jack hole without having to drill…
Just wondering what is most popular…
Thanks guys
 

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I would use some Loctite power grab and glue the pot to the PCB near where you remove the fixed bias resistor and install your leads from there to the pot. I did 4 6505's with bias pots and added choke. . Not sure if they are similar.
 
Thanks brother, what affect did the choke have on the tone?
Does it tighten the tone up? The rhythm channel with crunch engaged gets very loose in the bottom end…
I sent you an email, bro :thumbsup:
 
SQUAREHEAD":da9r2hpf said:
Thanks brother, what affect did the choke have on the tone?
Does it tighten the tone up? The rhythm channel with crunch engaged gets very loose in the bottom end…
I sent you an email, bro :thumbsup:
Click to expand...
Less sag/tighter. It should give better dynamics. I did not have a stock amp to compare when I did the mods.
 
psychodave":gpoe4npc said:
I believe the choke mod causes the amp to make an audible pop when switching off standby?
Click to expand...

Really?
I guess I’m stupid but why wouldn’t James Brown have built / designed the 5150 with a choke?
All my old marshals have a choke, All four of my Bogners have a choke…
 
psychodave":2mblkw8q said:
I believe the choke mod causes the amp to make an audible pop when switching off standby?
Click to expand...
It does Dave. Thats why it needs to be relocated before the choke.
 
I drilled a hole in the top of the chassis near the power tubes. For the pot, I bought one of those Fender bias pots that they use on a lot of their newer amps; it’s the kind where you have to insert a small flathead screwdriver to actually turn it.
 
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