5150 mods? - Enzo?

[xshredx]

I have one more question about a 5150 bias mod: what is the minimum power rating for both the resistor and especially the pot??
I'm asking because the trimpots I can find have a power rating of 0,25W... is that enough?

[Amp Kat]

I have one more question about a 5150 bias mod: what is the minimum power rating for both the resistor and especially the pot??
I'm asking because the trimpots I can find have a power rating of 0,25W... is that enough?

That's not a problem. Bias supplies have so little current that power is not a concern.

[Enzo]

If you know the resistance and the voltage across it, use Ohm's Law. You can calculate simply the power dissipated. Double or triple that amount.

If you read that mod carefully, it says remove the 15k resistor, not lift one end and add hte other parts. The other parts are wired in PLACE of the 15k resistor.

[drewl]

I was joking of course. It baffles me why Marshall mounts the little bias pots with the three pins for test points so that the left pot adjusts the right tubes and vice versa.

Drove me nuts fixing one until I figured this out!

[Enzo]

And their other trick, the MUTE button shorts the grids together. OK, it mutes the amp, but now the two bias controls are also hooked together, so they can't be set. That took me a while to fathom the first time I encountered it.

[G-Spot]

I've done one on a 5150 and it didn't really change very much at all. To each his own just my experience. The mod I really liked was adding two bias pots one for each side.

Hi, here's my first post in this forum.

I have seen mods where 1 bias pot was added, but how/where do you add the second bias pot in a 5150?

What do you want from your mods? WHat do you expect a choke to do? It wouldn't be difficult to add one, the standby switch is between the plate and screen nodes. Break into the wires there. Ditch the 400 ohm resistor.

I Enzo, I'm new to amp mods, so sorry if I make silly or redundant questions...

I have read "chokes explained" in Aikenamps.com, and I've also been to Duncan's Amp page... but I can't really figure out how/where this mod is made. (Sorry I know you are no "modster")

Which inductor should one buy? I guess that the big blue resistor is the 400 ohm resistor to be replaced by the inductor, right?

(Meanwhile) I found this example:
MetroAmp.com Forum • View topic - 5150 - Choke and Bias Mod

Was it done correctly? As it is said there, do you need also to change the grid resistors?

I'm probably not going to make this mod, but I would like to know how it is done properly.

Thanks in advance.

[Enzo]

The big blue resistor is electrically between the plate power supply and the screen power supply. We would remove it, and wire the choke leads to where the two ends of it were.

I have no idea what choke to buy. Doesn't Duncan make recommendations? Try searching this forum for choke discussions. very few choke applpications in guitar amps other than this exact one - power supply. A few will focus on tone controls.

[G-Spot]

Thanks Enzo!
Yes, Duncan makes recommendations, but I wanted to see if I could find "short-cuts" . Anyway, I'm not going to do the mod because, as you said it's not really a mod for tone. I just wanted to understand what was going on and where it would be made. Thanks once again.

[Timstonepizza]

Bias Mod

Hi I'm new to the forum and just thought I would throw in my 2 cents. I just did the bias mod and couldn't be happier, and it was simple (if you know how to use a soldering iron). Here is a pic of the finnished product, I did it just like all the posts say, that is a 5k resistor and 10k trim pot. I used a miniture pot and drilled a hole so that I can access it without pulling out the chassis it worked out perfectly! Good luck!
Tim