5150 Mod Thread

[TheWinterSnow]

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.

I agree. In the rig talk thread they where eventually talking about using a 100pF cap from the cathode to grid or a 10pF from the plate to grid. Since the Plate to grid capacitance will be more extreme, you wouldn't want to use such a high value.

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.

I was looking at the datasheet on my tubes and the capacitance from the plate to grid is 10pF, so adding a 10pF cap would increase the total to 20pF. Now unfortunately I forgot how to calculate the bandwidth of a gain stage from eddy currents and miller capacitance, but I can imagine doubling the capacitance would provide at lease something.

The only other means of decreasing the bandwidth without dramatically effecting the gain of that stage would be to increase the grid resistor. You have any other recommendations Wolfe?

 

[Wolfeman]

Do you have Merlin Blencowe's preamp book?

 

[TheWinterSnow]

I do not. I am looking at getting some books on tube gutiar amp design, so if you have any recommendations, I would love to here it. I am almost done with my degree and feel comfortable enough to build an amp, I just would like to know more about design so that maybe within the next year or so can build an amp from the ground up.

 

[TheWinterSnow]

So I finished up the mods and put everything back together. The piggyback resistor on pin 9 does more than I expected. It drops the gain of the amp massively, but makes the gain much more controlled. I have the gain now on 6-7 and it still sounds a bit under gained for my tastes. Changing that stage also messes with the final output impedance going to the tone stack stage which greatly reduces the mids. I am finding that I am bringing my mids up much higher without the amp being honk city. Overall the amp has a little bit more highs to it as a result and sounds slightly hairy, which I now need to deal with my reducing the highs with some other preamp mods. Now for what the mods was supposed to do...holy fuck does it tighten shit up, doing extremely fast picking, every note is clean and articulate. Its worth the effort to try it out to see if you like it, and even if you absolutely hate it, its really quick to remove the resistor and get your stock settings back.

 

[endofall]

TheWinterSnow, are you talking about a resistor on R9?

 

[TheWinterSnow]

yeah I meant R9.

 

[TheWinterSnow]

Here are some better pictures of my mods:

and here is a shot of the resistor:

Wow my amp is old

 

[endofall]

So I am trying to learn about other mods for the 5150 II, I have a Splawn Quickrod, and one of the big things I notice about it is how 3D it sounds in the low end, it's huge sounding but not bassy, I was wondering if there was something that could be done on the 5150 that would increase the depth of the lowend? Thanks for any help

 

[endofall]

Also TheWinterSnow, are you just piggybacking the resistor across R9? Thanks!

 

[Wolfeman]

Remember, the 5150 II is totally different from the 5150 component wise, so make sure you don't do any of the mods listed for a 5150 to a 5150II.

 

[TheWinterSnow]

Also TheWinterSnow, are you just piggybacking the resistor across R9? Thanks!

Yes, just piggybacking. The new resistor goes from ground to the grid pin.

And like Wolfe said, if you have a 5150 II, you really shouldn't do any of these mods.

 

[Wolfeman]

There's no DC on the tone stack in a 5150, C58 has already done that job.
It's necessary to use the cap on a Marshall style tone stack, but not with the 5150 style. Won't do any harm using it, just not necessary.

Oh crap just noticed this, yes on the 5150's the cap isn't necessary.

That's what happens when you jump the gun and don't look at the schematic carefully.

 

[endofall]

Thanks for the warning about using these mods for a 5150 II, could they not be done to the 5150 II, comparing the 5150II schematic, and the 5150 schematic together to make sure that they are still being applied to the correct part. I know that both of these amps are different (and I have learned in this thread that they are quite a bit different), but as a whole, they seem pretty similar in their layout and design. That being said I assume that this is completely incorrect

"I can only assume that the 6505+ is identical to the 5150 II since I was gone before Ed left Peavey and they changed the name.....if it is, you can take your amp to a qualified service center, and they can remove C17(470pF) and replace it with a 470K and 0.0022uF capacitor in parallel (or they can just tie a 470K and an additional 0.0015 uF cap across C17, without removing C17). Then they would change C2 from a 0.001uF capacitor to a 0.022uF capacitor. The result is that the attack is a bit floppier and thicker sounding, and the gain is much heavier sounding and not as tight/chunky."

I found that on the peavey forms a long time ago, and seems like it has been posted here as well.

 

[endofall]

Also could someone recommend some books about tube amp design. I am currently reading "The Guitar Amp Handbook" by Dave Hunter, and it is pretty good at explaining somethings at a really basic level.

 

[Wolfeman]

Also could someone recommend some books about tube amp design. I am currently reading "The Guitar Amp Handbook" by Dave Hunter, and it is pretty good at explaining somethings at a really basic level.

The Kevin O'Connor books(TuT series)

Richard Kuehnel's preamp and poweramp book(also worth checking out the book about the Bassman disection, if you like building 100 watters)

Merlin Blencowe has a great preamp book and a new poweramp book(very good books for beginners).

And last but not least, The Radiotron Designer Handbook is a MUST READ.

 

[Wolfeman]

That being said I assume that this is completely incorrect

"I can only assume that the 6505+ is identical to the 5150 II since I was gone before Ed left Peavey and they changed the name.....if it is, you can take your amp to a qualified service center, and they can remove C17(470pF) and replace it with a 470K and 0.0022uF capacitor in parallel (or they can just tie a 470K and an additional 0.0015 uF cap across C17, without removing C17). Then they would change C2 from a 0.001uF capacitor to a 0.022uF capacitor. The result is that the attack is a bit floppier and thicker sounding, and the gain is much heavier sounding and not as tight/chunky."

I found that on the peavey forms a long time ago, and seems like it has been posted here as well.

No that's correct, as the 6505+ and 5150II are exactly the same. That particular mod is just messing with interstate filtering.

 

[endofall]

Wolfeman, thanks for the reply, I have been looking at the Kevin O'Connor books, and really want to get those. So the mod that I posted, doesn't really bring the 5150II back to the 5150 Specs, I had tried this mod before, but it seemed to be lacking the aggression of the original 5150. Thanks

 

[TheWinterSnow]

Wolfeman, thanks for the reply, I have been looking at the Kevin O'Connor books, and really want to get those. So the mod that I posted, doesn't really bring the 5150II back to the 5150 Specs, I had tried this mod before, but it seemed to be lacking the aggression of the original 5150. Thanks

You will never be able to get back to original specs. Considering the 6 tubes and the subsequent switching system, everything was redone with the 5150 II including the power supply which will also affect the tone. It would jsut be easier and cheaper to buy an used 5150/6505 MkI.

 

[silence2-38554]

Hey all, I've just read through this entire thread & found a lot of really great stuff, but the one thing I've been looking for was not mentioned. I recently installed a Mercury Magnetics choke in my 5150 block letter head. The amp sounds fantastic, but now I get the expected POP out of the speaker cabinet when I flip the standby switch off.

There seem to be two methods to rectify this, one being to run a freewheel diode in parallel with the choke, the other being to re-wire the standby switch's position in the circuit. I've been emailing MM & they seem to prefer the rewire, plus the implementation of a .22uf cap to ground off the standby switch.

I've been trying to troubleshoot this with a few guys over on the SS forum but we haven't really landed on anything solid. I just stumbled across this thread & you guys seem to have much more in depth knowledge of the 5150 circuit. Here's the LINK to the SS thread. Any help on this would be greatly appreciated!

 

[TheWinterSnow]

You said it. Your best bet is to move the standby switch the the other side of the HT rectifier and place in parallel a resistor. The resistor value is determined by the HT voltage and the maximum power rating of the resistor, derived from the formula:

P = V^2 / R
R = V^2 / P

Where:
P=Power (in Watts)
R=Resistance

You actually want the power that is being dissipated through the resistor to behalf of what it is rated for, so if you have a 5 watt resistor, the number you are using for in your equations is 2.5w. For a 5150 and a 2.5W resistor, the value would be 100k, for example.