5150 Bias mod.

robnobcorncob · Post by **robnobcorncob** » Tue Mar 25, 2008 5:03 pm

Anyone done it? or done the bias mod to another amp?

blueguitar.org/new/schem/peavey/modnote ... as_mod.pdf

It more or less inserts a 10k pot and 6.8k resistor in place of a 15k resistor which adjusts the bias of the 4x6L6GC Power tubes. 5150s are biased very cold from the factory (some as cold as 12ma...) and this turns it into quite the beast...

thoughts and opinions? Obviously I'm not going to attempt to do the mod myself.

ant · Post by **ant** » Tue Mar 25, 2008 5:20 pm

If your 6l6s are actually at 12ma than for sure it will help, you can order high current 6l6's.

If you can hear a kind of insect like buzz on the bass notes which dies usually quite quickly.
Set the amp cleanish with the poweramp volume maxed so you are just getting it to clip.
With a bit of dirt usually you hear it the initial attack - in the first second, that means you have crossover distortion from biasing to cold.

robnobcorncob · Post by **robnobcorncob** » Tue Mar 25, 2008 5:38 pm

ant wrote:If your 6l6s are actually at 12ma than for sure it will help, you can order high current 6l6's.

If you can hear a kind of insect like buzz on the bass notes which dies usually quite quickly.
Set the amp cleanish with the poweramp volume maxed so you are just getting it to clip.
With a bit of dirt usually you hear it the initial attack - in the first second, that means you have crossover distortion from biasing to cold.

The distortion from a too cold fixed bias is a problem on the majority of 5150s. Since posting this thread i've read on the net that one user had 'About 5ma' on the power tubes

I'm not sure what the reading on mine are however.

robnobcorncob · Post by **robnobcorncob** » Wed Mar 26, 2008 11:01 am

Well I had a look inside to get a look at what needs to be done (and yes I turned it off before doing so) and the PCB reads November of 1995. Original tubes are still in there

(I thought it was from 2003) so it's definitely time for a retube! gonna get the bias mod done at the same time.

slash-ed · Post by **slash-ed** » Wed Mar 26, 2008 4:30 pm

I thought you sold your 5150??

RobRoyMcCoy · Post by **RobRoyMcCoy** » Wed Mar 26, 2008 7:26 pm

Please make sure you discharge the power supply electrolytic capacitors fully before commencing work on the inside of the amp to avoid death or simply large shock

Most amps have a bleed resistor (not sure on 5150) to make them safe but it pays to check to voltage on the plates before commencing and discharge if they are held high by the caps.

Rob

robnobcorncob · Post by **robnobcorncob** » Wed Mar 26, 2008 7:47 pm

Cheers for that Rob. The 5150 has bleed resistors but it never pays to be too safe.

Ed: Not quite. Not yet anyway.

crushing day · Post by **crushing day** » Thu Mar 27, 2008 10:07 am

RobRoyMcCoy wrote:Please make sure you discharge the power supply electrolytic capacitors fully before commencing work on the inside of the amp to avoid death or simply large shock

Most amps have a bleed resistor (not sure on 5150) to make them safe but it pays to check to voltage on the plates before commencing and discharge if they are held high by the caps.

Rob

The 5150 will kill you if you don't kill it first .. I personally have killed 2 so far.. they just don't seem to like me

robnobcorncob · Post by **robnobcorncob** » Thu Mar 27, 2008 2:06 pm

crushing day wrote:
RobRoyMcCoy wrote:Please make sure you discharge the power supply electrolytic capacitors fully before commencing work on the inside of the amp to avoid death or simply large shock

Most amps have a bleed resistor (not sure on 5150) to make them safe but it pays to check to voltage on the plates before commencing and discharge if they are held high by the caps.

Rob
The 5150 will kill you if you don't kill it first .. I personally have killed 2 so far.. they just don't seem to like me

What were you doing to them CD? I bet the screen grid resistors failed, a common 5150 problem

crushing day · Post by **crushing day** » Thu Mar 27, 2008 2:18 pm

They just dont like me .. The combo i had back in '95 lasted 10 days i think ... blew fuses then cooked itself. The head that i was supposed to use at a festival in Feb lasted for soundcheck, and then shat itself 2 mins before taking the stage .... i now dread powering a 5150 up, and watching all those little lights go out

robnobcorncob · Post by **robnobcorncob** » Thu Mar 27, 2008 2:25 pm

crushing day wrote:They just dont like me .. The combo i had back in '95 lasted 10 days i think ... blew fuses then cooked itself. The head that i was supposed to use at a festival in Feb lasted for soundcheck, and then shat itself 2 mins before taking the stage .... i now dread powering a 5150 up, and watching all those little lights go out

Ouch

mines lasted fine for 13 years so far, here's hoping to another trouble free 13..

NZGuitars.com : Filling you with GAS since 2003

5150 Bias mod.

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