10. October, 2012
Peavey 5150 / 6505 series mods
About the different models:
The Peavey 5150 should need no introduction.
The 5150II is a 5150 with an improved clean channel, and a modified distortion channel. It also has a different power supply and a few changes in the power section. You can’t make a 5150 sound like a 5150II or vice versa without severe modification, but you can get them a bit closer by some simple mods.
Later, the 5150 was relaunched as the 6505, and the 5150II as the 6505+. They made no changes to the circuit at this point. If a 5150 sounds different from a 6505 it’s due to different tubes, or the age of the components, not a change in design.
The 5150 2×12 combo is different from both the 5150 and the 5150II. Tone-wise it’s said to be somewhere between the two, but I don’t know this circuit.
The 6505+ 112 combo has a preamp nearly identical to the 5150II, but a new fx loop and power amp.
Make your 5150II sound more like the original 5105
 This mod will make changes to the preamp section to make the ultra channel of your 5150II match the specs of the original 5150. This without affecting the clean/crunch channels. Your amp will be be growlier, more organic, more gainy and get a chunkier attack, but it will also become less tight. It will not sound 100% like the 5150, as there are some differences in the fx loop / power amp stage as well, but it will take it in the right direction. |
 5150 Pre |
| This mod is also applicable to the 6505+ 112 combo, but the component names differ.
5150II head: 6505+ 112 combo: |
 6505+ 112 PRE |
Mod the 5150/6505 lead channel to match the specs of the 5150II/6505+
You will get a more twangy top end, more fizz, not as much of a raw growl to it, and a more processed feel/sound overall. If this is what you want, do the opposite of what’s stated above.
Bias Mods:
| The 5150 is biased very cold. Cold bias gives higher output power and adds crossover distortion.Crossover distortion sounds scratchy/buzzy. There’s too much, it will sound as if something is broken. Also, crossover distortion is most noticeable at low volumes. Hence, if you play loud your bias current can be set low to extend tube life.
Some like the crossover distortion in the 5150, but a lot of people think it sounds better with a cleaner power section. Higher bias current will also reduce the max output power, thereby making it possible to attain power tube saturation/distortion/compression at a slightly lower volume level, which may be something you want.This mod will make the bias voltage adjustable: |
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| 5150: R68: Replace this with a 5k to 7k resistor and a 10k pot in series. Start out by setting the combined resistance to 15k (stock value) 5150 II: |
 5150 bias mod |
6505+ 112:
it is possible. I recently got an email from a man called Brian Mangan, saying that you can adjust the bias of this amplifier by altering the value of R14. I have not taken the time to check the schematic or try it myself yet.
EQ Mods:
| The EQ on the 5150 isn’t great. The main problem is that the mid control doesn’t work in the right frequency range. In my opinion, its’ focus point should be at a lower frequency.
The tone stack on a guitar amplifier is usually a very simplistic design. Leaving every knob in the middle position rarely yields a “neutral” frequency response, and changing the value of one component will usually alter the response of more than one frequency band. Here are 2 ways to improve the 5150 Eq; It’s probably best to do only one of these 2 mods: |
 |
| OPTION1: Shift the mid frequency downward This is a very simple mod which will move the focus point of the MID band to a lower frequency. 5150/5150II: 6505+ 112: OPTION2: Make the mid frequency sweepable 5150: 6505+ 112: |
 5150 tone stack |
Other preamp mods:
I found these mods in the Rig-Talk forum, and they were originally posted by Mark Cameron. They are designed for the original 5150, and may or may not work well with the 5150II or combo versions:
V5B(middle preamp tube): add 10pf silver mica cap to pins 7 and 8. This will remove hiss, make the gain less fizzy, make the high mids clearer and improve clarity in chords. between pins 6 and 7 it’s even more efficient, but will also darken the amp a lot.
R9: piggyback a resistor on top of the 1 meg resistor. 120k is a good starting point. You can try out 68k, 100k, 120k, 150k, etc. Lower value means tighter amp, but if you go too low, the amp will be choked out.
C26: cut one leg for less bottom end on the rhythm channel
C15: bypass cap for better “feel”
R22: bypass for better dynamics and clearer picking response
I tried tinkering with R15 and using Marshall values, but it took too much away from the original sound. The 39k is the SLO value as well.
Power amp mods:
replace screen resistors (with 1k 5W)
add choke somewhere. I have to study these further. To be continued.
Credits:
I didn’t figure all of this out by myself. I’ve gathered some of this information from forum threads, and here are some of the sources:
http://www.mrbobo.com/music_5150_mods.htm (5150 bias mod and tone stack mod1)
http://www.ultimatemetal.com/forum/backline/596150-5150-mod-thread.html (EQ mod 2 + etc)
combo has a preamp nearly identical to the 5150II, but a different fx loop and power amp.
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