5150 Mod Thread

[Wolfeman]

Figured since about 50% of us on here own one, or one of it's equivlents, this thread might be useful.

I just picked up my 5150 this weekend(60watt combo fyi). I've played through one many times but have never owned one. Well, after giving it the run through for the past few days I can say there are a lot of things I don't like about it, so I decided to pull it out of it's home and start doing some mods.

I did 3 mods so far today- Bias mod, tone stack mod, and power amp mod.

For the bias mod, I added 3 test points to the back(one for each tube, and a common point). I added a pot for each tube to set the bias. I had a hard time getting a good working range, but it worked out in the end. I can now bias each tube individually, and use other tubes if need be. I don't really know the specs on the filament winding, so I'm afraid to try anything other than 6L6's. I suppose I can throw some KT88's in it, and just watch the temp on the transformer. If it starts getting hot, or if the voltage sways, I'm probably pushing it. I may even add a separate transformer, since I have a ton of 12amp ones laying around for my amps I'm building. That way I could run anything under the sun. Anyway, adding adjustable bias to this thing really helps out the power section.

For the tone stack mod I simply cut out the 100pf cap that parallels the 470pf cap. I also added my mid shift knob that I have on my Hellfire build. I also removed the 47k resistor across the mid pot, and also changed it from a 50k to a 25k pot. So it's back to a more 'known' tone stack.

For the power amp, I just changed the caps coming of the PI to .1uF as I prefer a little more low end to pass to the power tubes.


I like what I have so far, but I'm going to start screwing with the lead channel to get rid of the fizz... I can't believe people actually like this amp like that. A few little changes and it should be sounding a bit more 'SLO'ish.

 

[JeffTD]

FWIW the 5150 combo is voiced closer to the 5150II, the less liked younger brother of the 5150's. Marked by a twangy top end, more fizz up top, not as much of a raw growl to it, and a more processed feel/sound overall.

 

[Wolfeman]

FWIW the 5150 combo is voiced closer to the 5150II, the less liked younger brother of the 5150's. Marked by a twangy top end, more fizz up top, not as much of a raw growl to it, and a more processed feel/sound overall.

Correct. I'm actually in the process of converting a few parts over from the head schematic. There really aren't that many differences and it's easy to do, so I might as well. Pretty much just coupling cap changes for the most part.

 

[Josh Burgess]

Also, being only 60 watts, it has less headroom and gets compressed much more quickly.

 

[Wolfeman]

Just an FYI, before adding the adjustable bias, one tube was sitting at 20mA and the other was at 72mA! Bad tube for sure, the 20mA sounds more like what a stock 5150 would be. I put in some new 6L6s and the bias was sitting right around 21mA on both. After the mod, I now have them at 42mA, and boy did that calm the power amp down. All the buzz and fizz just gone. Crossover distortion is a horrible thing.

And damn I love my mid sweep. I'll post a clip in a bit of what it does.

 

[Chris_P_Critter]

You're a god damned genius dude.

The slogan for your amp company should be "Here at Wolfe amplification, we make amps that sound bad ass, and take other shitty amps and make them sound bad ass too".

 

[JeffTD]

Wolfey how secretive are you about these mods? I'm already retolexing mine, might as well go apeshit with bias and mid sweep too!

 

[AudioPhile777]

The slogan for your amp company should be "Here at Wolfe amplification, we make amps that sound bad ass, and take other shitty amps and make them sound bad ass too".

+1

 

[Wolfeman]

Wolfey how secretive are you about these mods? I'm already retolexing mine, might as well go apeshit with bias and mid sweep too!

The mid sweep is easy to do. You can even mess around with values to get it more to your liking.

Just need to pull out one resistor, and in place of it add- .1uF cap, wire into a 250k pot, wire out of pots wiper to a 10k resistor.

To my ears, this sweeps the mid center point from around 120hz-4000hz. Yeah it's a wide range, but I like the option of putting it anywhere. You can change the pot value and resistor to tighten the range and change the stopping point. I use the .1uF cap to keep AC off the pot. Most manufacturers(Krank for example) just throw in a pot and call it a day. I just went a little further.


Edit- If you are really interested in the mid sweep, I can write up a how to and take some pics next time I pull the board out.

 

[Wolfeman]

And for sure add a bias pot. No reason not to. It really does make a drastic change.

 

[Chris_P_Critter]

Dude - can you explain Bias to me - I am not certain I understand the concept?

 

[Wolfeman]

Dude - can you explain Bias to me - I am not certain I understand the concept?

Great explanation on what it does here-
http://www.lynx.bc.ca/~jc/bias.html

In laments terms it's just a negative voltage fed to the grid of the tube. It's a necessary voltage that the tube needs in order to function.

Too low of a setting, and you will get crossover distortion and possibly meet class B conditions. Too high of a setting, and the tube basically will work it self to death. Higher settings usually sound way better, but push the tubes to its operating limits. Kinda like overclocking your computer. I like high settings, but only on good tubes. I have run SED 6L6's for ever with settings around 40mA, and they still last a very long time. Try that with cheaper tubes and they probably won't last long at all.

 

[MaxStatic]

The slogan for your amp company should be "Here at Wolfe amplification, we make amps that sound bad ass, and take other shitty amps and make them sound bad ass too".

Add to it "Fuck it, if you think you won't get shocked, I'll even tell ya how to do it."

Kick ass man.

 

[Josh Burgess]

Just an FYI, before adding the adjustable bias, one tube was sitting at 20mA and the other was at 72mA! Bad tube for sure, the 20mA sounds more like what a stock 5150 would be. I put in some new 6L6s and the bias was sitting right around 21mA on both. After the mod, I now have them at 42mA, and boy did that calm the power amp down. All the buzz and fizz just gone. Crossover distortion is a horrible thing.

And damn I love my mid sweep. I'll post a clip in a bit of what it does.

So you think all of the awesome sounding albums that were recorded with stock 5150s sound buzzy and fizzy?

 

[Wolfeman]

So you think all of the awesome sounding albums that were recorded with stock 5150s sound buzzy and fizzy?

Nah, but mine sure sounded shitty.

 

[LosingReality]

I would like to mod mine so it doesnt drop volume all the time.

 

[greyskull]

I would like to mod mine so it doesnt drop volume all the time.

then replace your power valves - they're fucked

 

[ttrentt]

Great thread. I love reading about this kinda stuff. I want to do the mid-sweep on my SLO (finally got it working 100% and it is quiet, even without heater mods).

 

[JeffTD]

Edit- If you are really interested in the mid sweep, I can write up a how to and take some pics next time I pull the board out.

This would be MOST appreciated, as well as a pic of what/where the bias mod went would be epic!

 

[Wolfeman]

This would be MOST appreciated, as well as a pic of what/where the bias mod went would be epic!

Will do.

As for the bias, you are better off using this guide, since the combo has a different bias circuit from the head- http://www.blueguitar.org/new/schem/peavey/modnotes/fja_5150_bias_mod.pdf

If you want bias test points, you'll need some banana jacks, and a few 1ohm 2watt resistors. This involves cutting a few traces on the power amp board and adding some jumper wire to the testpoints/resistors.