5150 Mod Thread

[Lasse Lammert]

Here's Mark Cameron's Mod: (taked from RigTalk):

V5B(middle preamp tube): add 10pf silver mica cap to pins 7 and 8. This is the main mod!!!
R9: piggyback a resistor on top of the 1 meg resistor. I used a 120k based on feel. You can try out 68k, 100k, 120k, 150k, etc. This tightened up the map a lot!
C26: cut one leg for less bottom end on the rhythm channel
C15: bypass cap for better "feel"

I tried tinkering with R15 and using Marshall vlaues, but it took too much away from the original sound. The 39k is the SLO value as well.

 

[endofall]

V5B(middle preamp tube): add 10pf silver mica cap to pins 7 and 8. This is the main mod!!!
R9: piggyback a resistor on top of the 1 meg resistor. I used a 120k based on feel. You can try out 68k, 100k, 120k, 150k, etc. This tightened up the map a lot!
C26: cut one leg for less bottom end on the rhythm channel
C15: bypass cap for better "feel"

Can someone tell me how to do this in the 5150 II

 

[RichMinerva]

I know I'm flogging a deadhorse but I couldn't find what I was looking for in any great detail,
Since I am an amp tard and don't have the foggiest I wondered what it would take to make the 5150II
voiced more similar to the original?

 

[RichMinerva]

nobody?

 

[Wolfeman]

I know I'm flogging a deadhorse but I couldn't find what I was looking for in any great detail,
Since I am an amp tard and don't have the foggiest I wondered what it would take to make the 5150II
voiced more similar to the original?

Lots of component value changes. I did it to mine. And you still won't get it to be exactly like the 5150, since the circuit is still slightly different. But you could get the voicing a tad closer.

 

[RichMinerva]

so is it a massive job wolfeman?

is there a guide or list of things to do that i can follow?

 

[Wolfeman]

so is it a massive job wolfeman?

is there a guide or list of things to do that i can follow?

Well the worst part about it is you have to get the board fully out to do it properly.

As for a guide, just go by the schematics. Open up the 5150 and 5150 II schems, and start comparing. Write down the differences on the 5150II schem, then go in the amp and change them.

It's mostly cathode resistor and bypass cap changes.

 

[RichMinerva]

You know where I can get the schematics mate?

 

[Wolfeman]

You know where I can get the schematics mate?

http://dl.dropbox.com/u/2033066/5150schems.zip

 

[RichMinerva]

is it as hard as it looks

 

[TheWinterSnow]

I am having a bit of a dilemma here, evn though I am sure of the answer, I really don't want to assume anything.

I am doing a mod in the preamp that tightens up the preamp. Now I am modifying a 1M resistor that is tied to the grid. All of the 12ax7 pinout charts are showing that pin 3 is the cathode and pin 2 is the grid, but the 1M resistor is connected to pin 3 (with the 39k cathode resistor connected to pin 2) and the pictures from similar mods show the modification happening to pin 3. Now I would assume that pin 3 is actually the grid, but for whatever reason I don't want to risk anything. It would make no sense though to have a 1M Cathode and a 39K Grid when this is the famous Soldano stage with the 39K cathode resistor.

EDIT: scratch that, I am an idiot and forgot which way the the pin count was orientated, its pin 7 that is connected to R9 which is indeed, the grid.

 

[Wolfeman]

Don't feel bad, I get all cross-eyed when staring at the ass of a tube socket trying to wire them up. All those pins and differentiating between the 2 half's gets confusing sometimes.

 

[JeffTD]

I'm definitely doing this Cameron mods. I already have my chassis out because I'm retolexing this week, and these don't even require removal of the board. Fucking win.

 

[TheWinterSnow]

Hey Jeff if you are doing the Cameron mods, try to add the 10pF cap to pins 6 and 7. The guy that posted that had a few people disagree with going from pins 7 and 8 because going from pins 6 and 7 increases the miller capacitance (plate to grid capacitance) which has a larger effect on reducing Highs. Increasing Miller capacitance is the correct way to get rid of highs in a stage.

 

[TheWinterSnow]

I am modding right now, just waiting for my solder to cool down so I can replace the tip because I didn't tin it correctly. I just beefed up the power section by replacing the screen grid resistors with two 2K carbon film resistors (equivalent to 1K) from the 100 wirewound resistors that where stock. I am about to do the Cameron mod that drops the input impedance of V2a to a little less than 220K (220K was the nearest value I could find, I rather prefer 120k, but I still worry about blocking distortion). I will have to order the 10pF cap for the fizz mod and the 250K audio taper pot for the mid sweep mod, as I could not find them, but I have the resistor for the sweep mod and the SPDT switch to revert back to stock settings. Here is a pic of the cob job mod that I did on the power section because I am d little drunk and was trying to deal with a shit soldering iron (I actually got better at it the more I drank ).

EDIT: Part I of the Cameron Mod:

I like how this mod is easily removable if it absolutely fails hard.

More pictures when I finish up.

 

[JeffTD]

Ahhh cheers, fair call! Curious to see how you get on with it, and your pictures will probably help more than his!

http://www.rig-talk.com/forum/viewtopic.php?f=3&t=80183

 

[TheWinterSnow]

Sorry for the blurry pics. I will have to get my hands on a better camera and take good pictures showing where to put what for each mod. Here is an improved pic though

the top of the resistor can be connected to anything touching the top bar indicated by the top red arrow, you see the connection better here:

 

[Wolfeman]

Hey Jeff if you are doing the Cameron mods, try to add the 10pF cap to pins 6 and 7. The guy that posted that had a few people disagree with going from pins 7 and 8 because going from pins 6 and 7 increases the miller capacitance (plate to grid capacitance) which has a larger effect on reducing Highs. Increasing Miller capacitance is the correct way to get rid of highs in a stage.

Well both ways can be correct, but it depends on what value you want to use and how you want it to react. I would do it the way you have mentioned, since the capacitance from grid to anode will get multiplied by the voltage gain of the stage. So a lower value cap can be used. You could still put a cap from grid to cathode, but you would need a much higher value cap to get any reasonable difference.


FYI for everyone going to try this, I had a thread about this over on a builders forum a while back. There was a big discussion on putting caps in these positions, and after messing with it on my breadboard, the consensus was that it's better to achieve the effects in other ways throughout the preamp. It just didn't sound right/do enough. In fact, I think a lot of the guys over on the 'other forum' that did this mod are just 'thinking' they are hearing a difference. Because that one cap, in that spot, at that value, shouldn't make to crazy of a change.

 

[Wolfeman]

Also, I think adding a resistor between pin 7 and the cap would be even more beneficial.

 

[Osse]

Lots of component value changes. I did it to mine. And you still won't get it to be exactly like the 5150, since the circuit is still slightly different. But you could get the voicing a tad closer.

Could I translate this into that people actually prefer the sound of the original 5150(which, correct me if Im wrong, is the same as the 6505) over the 5150II(6505+)?

If thats the case Im a happy camper