scratchy presence pot

[dwhutchens]

I added this to a post over on "Build Your Amp" to one that I started when I first built this amp but haven't gotten a response yet. I don't mean to be impatient but if I can get this worked out I think I'll be done. Long story short, it started life as a "light" 6g3 Brown Deluxe clone (minus one channel and tremolo). After getting the bugs out I changed the tone control to a bass & treble control from the Black Face Deluxe to cut back the gain. Then I added a presence control similar the the 5f6a Bassman. I used a 10k linear pot and a .1uf cap. Not sure if the pot should be audio or linear. It seems to work ok but the pot makes a scratching noise when I turn it. I found about 20vdc on the top side of the pot. What can I do about this?

Thanks,
David

[jrfrond]

If there is DC superimposed with the audio signal on a pot, it will sound scratchy. Your pot is most likely good.

[Twist]

Use the presence circuit on the Fender 6G6-B.

[dwhutchens]

Is there a way to get rid of the noise in the presence circuit or is it normal? If you look at the diagram I replaced the 1k5 resistor on the PI with a 10k pot and tied the wiper to ground through a .1uf cap.

[dwhutchens]

Sorry, here's the diagram

[Chuck H]

As you can tell from the schem, there is no decoupling to keep DC off that pot. All Marshall amps were wired this way for years. And of course, the 5f6a Bassman. The scratchy pot is normal. It isn't scratchy while your playing of course. Unless you like to adjust the presence control while playing. If it bugs you just do as twist said. It's a slightly different circuit that blocks DC from the pot.

Something you should be aware of is that just changing out the 1500 R below the PI tail for the 10k pot has dramatically altered the power amp. The 5k pot in the 5f6a and the Marshall amps is not just a presence control, It's one half of the voltage divider for the negative feedback loop. If you change one resistance in the loop without adjusting the other you alter the feedback ratio. In your case you've increased the amount of negative feedback by over 600%. That could make the amp sound kind of flat and boring.

If you look at the stock circuit you'll see that the 56k feedback resistor enters the circuit at the top (not grounded) side of the 1.5k R. Thats the voltage divider for the feedback loop. Thats a ratio of 37:1. By increasing the 1.5k to 10k you've established a ratio of 5.6:1. To me thats alot. Although it's about the same as a stock 5f6a Bassman. Marshall amps that use the same circuit typically use a 100k feedback resistor and a 5k pot. Thats a ratio of 20:1. Point is, you may want to experiment by putting in a 5k pot. That would be 11:1. Still much more NFB than the stock 6g3, but half what you have now. You may even want to try a 5k pot and a 100k feedback resistor for a 20:1 ratio. It's an area thats fun to experiment with since the amount of NFB in any amp will present a sweet spot if you look for it. I would use a 1k pot and then temporarily replace the feedback R with a 50k pot. Then I would adjust the pot to find the value I like and solder in a fixed resistor of that value.

Cheers

Chuck

[MWJB]

Most 5F6A type amps built today (RI, Kendrick, Victoria) don't use the early 5K presence pot set up for this reason. Use the 5F6A RI/6G12A set up with a 100K (or maybe larger) dropping resistor.

[dwhutchens]

Hey guys, thanks for all the great info. The 6g6 control makes sense but I looked at all the diagrams I could find and they are pretty poor quality. It looks like a 4k7 resistor bypassed by a 25k pot and a .1uf cap. Is that correct? And should the pot be linear? I will probabley take Chucks advice and play with the values but I just wanted to know what was on that 6g6 schematic.

Thanks again,
David

[MWJB]

Yeah, 4.7K load, bypassed with .1 & 25K pot in series with each other. Linear pot is typical, log pot might ramp up to quick in last third of a turn.

[Chuck H]

How about this:

Build the 6g6 presence control, but instead of a 4.7k load R, use a 2.7k. Then you can use the 10k pot you already have in place instead of having to take it out and replace it with a 25k. Not only that, but the 2.7k combined with the stock 56k feedback R will give you a NFB ratio of 20:1. Thats a pretty good place to start, and you won't need to remove or replace the existing feedback R. You can experiment after that of course if you don't like it.

If you do it this way all you'll need to add to the circuit is a 2.7k resistor...Cake. The numbers all jive. The RATIOS are more important than the actual componant values...Within reason.

Chuck