Peavey 5150 Adjustable Bias Mod

JD · Post by JD » Tue Jan 10, 2006 2:50 pm

Can someone send me the details or a link to install this mod properly? Thanks.

Necrovore · Post by **Necrovore** » Tue Jan 10, 2006 8:39 pm

Taken from: http://www.storm.ca/~rheslip/ampmods.htm

Another amp I recently modded. The customer wasn't happy with the tone of the amp which was quite thin and buzzy, and it lacked punch. This amp is fixed bias and its four 6L6's were idling at an ice cold 12 ma. A very popular mod for the 5150 is to add an adjustable bias circuit which I did on this amp. The mod is pretty simple - replace the 15k resistor in the last stage of the bias filter with a 10k pot in series with a 4.7k to ground. I used a 10 turn pot which allows quite fine adjustments. I rebiased the tubes to 32 ma. What a difference ! The fizz was gone and it had a lot more warmth and punch. I'd heard the 5150 can be a very noisy amp with six preamp stages but this one was very quiet - had Sovtek 12AX7 LPS low noise tubes and Electro-Harmonix 12AX7's in it. The owner was extremely happy with the modification.

Amp Biasing Tip: There are no resistors in the cathodes of the power tubes of the 5150 to measure the cathode current. The usual solution for this is to use a bias probe, which basically inserts the resistor temporarily. There is another way to measure bias current - measure the resistance of the output transformer secondary windings and measure the voltage drop across the windings with the amp powered up. Don't assume both halves of a push pull transformer have the same resistance because they generally don't due to the way the transformer is wound. Ohms law i=v/r tells you what the plate current is. In this amp there are two tubes per side so don't forget to divide by two. Measuring the drop across the transformer secondary only measures the plate current. When you measure with a bias probe you are actually measuring the tube's plate current PLUS it's screen current, so you should account for that. Screen currents vary by tube and voltage but if you assume 5ma you'll be in the ball park, so set the plate current a few ma lower than with the bias probe method.

CAUTION - this is a dangerous method of measuring bias current because your voltmeter is attached to the highest voltages in the amp. Best not to touch the meter or probes during the measurement -be VERY CAREFUL !

Here is an even better example. http://www.gearo.com/Files/5150biasmod.pdf

I hope others can chime in more about this.

JerryP · Post by **JerryP** » Wed Jan 11, 2006 9:37 am

I have some info on my forum FJA Forum
Jerry

JD · Post by JD » Wed Jan 11, 2006 10:11 am

Killer, thank you both

JD · Post by JD » Sat Jan 14, 2006 1:56 pm

I picked up the 5150 half stack and had a chance to give it a good evaluation. I was suprised to find this amp had very few hours on it and still had all of the original tubes (all 5 12AX7 and all 4 6L6GC)! All of the tubes are rebranded Peavey and the power tubes are etched 6L6CG U.S.A. I am quite certain that the amp had never been removed from the chassis. The pots were all very scratchy and it was so covered in dust it didn't appear to be black. Quite honestly I was disappointed in the sound.

I decided to install the adjustable bias mod. Before the mod the high voltage read 462.2V and the tubes were idling at 32.1mA, which is 14.8W idle. 14.8W on 6L6GCs is very low indeed! I decided to remove the chassis from the box and went further removing the main circuit board to thourally clean all the pots.

I did a variation on the mod, using (2) 3.3K 1/2W resistors (one soldered to each PCB lug of the original 15K resistor, with a 15-turn fine adjust 1.25W "cermet" 10K PCB pot in series between them. I was able to find the exact same type of resistor as used on the PCB which looks physically identical to the others. This allowed a very clean and professional installation with 2 points to solder in the pot. When I finished I put down some hot glue to hold it securely, which oddly enough, looks like the exact same type of glue used all over the PCB. I carefully laid out everything so it honestly looks like it came this way from the factory. I mean you wouldn't know it was done if you didn't know this amp. All for $3.58 in parts!!!

After the mod I dialed in the bias to 457.0V and 40.0mA, which is now 18.3W idle on the power tubes. HOLY SHIT! I was shocked by the sound of this amp! The piercing treble screech is completely gone and the farty bottom end has smoothed out nice and fat yet tight. I think these amps got a bad rap. This thing sounds incredible! It no longer sounds solid state or death metal. Pots don't make a single crackle either.

Necrovore · Post by **Necrovore** » Sat Jan 14, 2006 3:52 pm

Cool, it would have been neat to get a couple of clips to do a before/after comaprison. I still would like to pick one of these up sometime. The only reason that I have not was always the Peavey stigma. Kind of foolish in retrospect as these are supposed to be really nice with a couple of mods are done to them.

JD · Post by JD » Sat Jan 14, 2006 4:29 pm

Unfortunately my computer is not equipped to record sound clips (or at least I'm not aware how) but reverting back to the original 15K (15.1K exactly) on the bias to get the same stock sound is no problem with the mod.

These amps are US made and on first impression are not bad at all. Damn this is one loud amp turned up that's for sure. It does have tons of gain to dial up, but around 2-3 only on the preamp and about 5+ on the power gives a pretty warm OD with lots of sustain and harmonics. Dialing up death metal is still no problem.

This is my first 6L6 powered amp. They sound completely different than EL-34s. Much tighter and punchier if that makes sense, especially at high volume, lots of headroom. Cool looking tubes too, they all glow in the dark with this faint blue halo around the plates surrounding the orange glowing heaters (no, no red spots on the plates). Seems like this amp can cover alot of ground between the 2 channels and various setting combinations. Small changes on the controls have significant changes to the sound so that will take a bit of time to get the hang of I guess.

Necrovore · Post by **Necrovore** » Sat Jan 14, 2006 4:34 pm

Sounds like a pretty cool amp. I ahve to ask though, does it have actual chassis mounted tube sockets? I used to own a Peavey Butcher about 15 years ago and it had no sockets perse but just the metal pieces that are in the holes in a socket mounted directly to the pcb.

JD · Post by JD » Sat Jan 14, 2006 5:01 pm

Good question, I didn't even look. From the outside of the chassis, the tube sockets look just like any Marshall. They may be on a small PCB that is located behind the main PCB (the one I removed to do the mod).

Jim SS · Post by **Jim SS** » Sat Jan 21, 2006 7:22 pm

I did this mod. I think it cost less than $4. It was worth every penny. It really opened up the amp by having a proper bias.

- Jim

Necrovore · Post by **Necrovore** » Sat Jan 21, 2006 10:31 pm

I have heard doing the same type mod to a Mesa rectifier series amp actually makes it sound good too.

JD · Post by JD » Sun Jan 22, 2006 7:23 pm

Necrovore wrote:Sounds like a pretty cool amp. I ahve to ask though, does it have actual chassis mounted tube sockets? I used to own a Peavey Butcher about 15 years ago and it had no sockets perse but just the metal pieces that are in the holes in a socket mounted directly to the pcb.

I took another look. Yep, it has 4 power tube sockets, each riveted directly to the chassis. All 4 sockets are soldered on the bottom side directly to the small PCB which is just suspended by the sockets. So it actually has sockets that are serviceable and not just pins on a PCB.

I upped the bias to ~45mA, or about 20.6W, and its even better sounding. Haven't seen any red on the plates but I'm going to keep an eye on them.