Years ago when I purchased my first Peavey 5150 I researched doing a bias mod on the amp and then on an old band web site I wrote up an instructional post. In looking back I must say that I like the way the stock amp sounds better considering the amount of work that went into modding the amp and the possibility of DEATH associated with opening up an amp head without proper training. There was a difference but maybe not as big of a difference as I once thought. Of course each amp will react differently so this may be a good choice for you and your 5150/6505. Here is what I wrote:
So I purchased a Peavey 5150 and heard about doing a bias mod since the stock one has a fixed bias. Being able to bias the 5150 I have found there is more clarity as you are out of the “crossover distortion”. In researching how to do a bias mod I came across fjamods.com with their 5150 bias mod (Here is the link: http://www.fjamods.com/5150BiasMod.html), but I needed more detailed instructions and I wanted an out-board bias so I wouldn’t have to open up the chassis every time I needed to bias.
I then contacted Bob at www.eurotubes.com, which is where I buy my tubes, and he was a big help giving me pointers and making the mod procedure more clear.
To be honest, I was scared to open up the 5150 head, or any head for that mater, because I don’t want 400+ volts running through my body and KILLING ME! So I was very cautious.
IF YOU ARE PLANNING ON DOING THIS MOD BE VERY CAREFUL. AMPS HAVE HIGH VOLTAGE RUNNING TROUGH THEM AND CAN KILL YOU! IF IN DOUBT TAKE IT TO A PROFESSIONAL.
Also, I am NOT a professional tube amp repairman. The mod done here was done by a half witted crazy guitar player, me. PLEASE BE WARNED: You are responsible for either your own death and or blown head, NOT ME. If you want to do this mod you do so at your own risk!!!
Now lets get on with it.
What you need:
- Decent soldering skills (if you don’t have experience here, get some before doing this or have a pro do it)
- Philips head screw driver
- 10k pot
- Knob for your pot
- Between a 4.7k and 6k resistor
- Soldering iron rated between 15 and 30 watts
- Electric or cordless Drill and drill bits
You can readily get these supplies at a Radio-Shack.
Oh yeah, and Patience
STEP ONE: Disassemble Chassis
I did this mod to an original 5150 head, I am pretty sure the design stayed the same for the 5150 amps, so the info here should be accurate for all 5150 models, and possibly the new 6505. However, this is NOT for the 5150 II or 6505 plus heads.
Take off the back “grill plate” and take of the four rubber feet. Unscrewing the feet will unfasten the whole metal box from the wooden head enclosure.
VERY carefully remove the metal box from the wooden box. You might want to take out the power tubes, I did not.
BEFORE GOING ON: DRAIN YOUR CAPS OR YOU MAY DIE!!!!! If you don’t know how find out…..if you can’t find out have a pro do it! 😉
STEP TWO: Drill a hole
This hole is for your potentiometer (pot). This will give you access to biasing without having to take the head apart. I drilled my hole next to the first power tube, it seemed to be the most logical place. Make sure that you drill a hole large enough for the shaft of your pot. However, make sure it is not too big.
STEP THREE: Clip the 15k resistor
The next thing to do is to clip and remove the 15k “fixed bias” resistor located next to the three blue caps.
STEP FOUR: Assemble and Solder the Bias Pot
First solder a 4.7k resistor to terminal 1 on the 10k pot. (The suggested resistor was between 5k and 6k but my local Radio Shack only had a 4.7k so I used that.) Second, solder two different colored wires, I soldered the red from the 4.7k resistor and the yellow was wired from terminal 2. Note: Terminal 3 is unused. Now put the 10k pot into your drilled hole in the chassis with a lock washer and fasten it securely. Lastly, solder the red wire to the NORTH side of the third blue cap and the yellow wire to the south side of that same blue cap. The NORTH side of the cap is nearest the tube PC board.
Put a knob on the shaft of the pot.
STEP FIVE: Put the head back together
Do a reversal of step one. POOF!!! You’re done! Now you can properly bias your 5150 head with a bias probe or other method, whatever they may be.
A good bias setting is between 35mA and 40mA. I set mine at about 35mA and it screams!!!
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