.68uf Cap In Place of .1uf Presence Cap - Swap Help Needed

Hazy · Post by **Hazy** » Wed Nov 30, 2005 9:48 am

I want to try a .68uf cap at the Presence pot with my 50W JMP, which means replacing the existing and original .1uf cap. The feedback resistor is a 100K that I installed with my Metro board. My questions are:

1. Do I need to swap out anything else? Feedback resistor? Pot?
2. What's the difference in tone with the swap?

Any other things that I need to know?

Thanks

Post by **VelvetGeorge** » Wed Nov 30, 2005 11:32 am

You can just swap the cap. But if you're looking for that '73 era high mid cut, there was also a 4.7k resistor added to the circuit.

George

Brentsp · Post by **Brentsp** » Wed Nov 30, 2005 3:19 pm

VelvetGeorge wrote:You can just swap the cap. But if you're looking for that '73 era high mid cut, there was also a 4.7k resistor added to the circuit.

George

Does the 4.7k change the tone in addition to eliminating the scratch noise?

Hazy · Post by **Hazy** » Thu Dec 01, 2005 6:44 pm

VelvetGeorge wrote:You can just swap the cap. But if you're looking for that '73 era high mid cut, there was also a 4.7k resistor added to the circuit.

George

George,

I have an original resistor in the circuit (measured 4.1k) running from the end lug of the Presence pot to ground. Is that the resistor?

I'm sure that this is a stupid question, but, how were the Plexi's set up as far as the presence pot, resistor and cap, did they ever use the .68ufcap/4.7k resistor combination?

Brentsp · Post by **Brentsp** » Thu Dec 08, 2005 12:20 pm

Hazy wrote:
VelvetGeorge wrote:You can just swap the cap. But if you're looking for that '73 era high mid cut, there was also a 4.7k resistor added to the circuit.

George
George,

I have an original resistor in the circuit (measured 4.1k) running from the end lug of the Presence pot to ground. Is that the resistor?

I'm sure that this is a stupid question, but, how were the Plexi's set up as far as the presence pot, resistor and cap, did they ever use the .68ufcap/4.7k resistor combination?

Yes thats the resistor George is talking about. As far as the plexis they used a .1uf without the resistor. Without the resistor there the pot is scratchy sounding due to a bit of DC voltage leaking thru.

Hazy · Post by **Hazy** » Thu Dec 08, 2005 7:02 pm

Brentsp wrote:Yes thats the resistor George is talking about. As far as the plexis they used a .1uf without the resistor. Without the resistor there the pot is scratchy sounding due to a bit of DC voltage leaking thru.

Thanks. I added the .68uf cap at the Presence pot. I'm still confused a bit about the whole presence control...so I set it at 12 o'clock and let it go.

It still sounds good; but, there is a bit more idle hum than there was before, which I can only attribute to the cap. Ya know, I have a '76 JMP 50w that also has the .68uf cap and it has a bit of idle hum. I'm beginning to see a pattern.

sub · Post by **sub** » Thu Dec 08, 2005 9:45 pm

I don't like 0.68 on presence pot..... too much IMHO

novosibir · Post by **novosibir** » Thu Dec 08, 2005 9:47 pm

sub wrote:I don't like 0.68 on presence pot..... too much IMHO

I also don't like it! Too much upper mids instead of "real" presences.

Larry G