Input Resistors

dynaman · Post by **dynaman** » Tue Nov 16, 2004 5:54 am

I'm making a JTM 45 clone and have decided not to use the usual four inputs. Instead, I'll have one input jack and a rotary switch to combine or switch between channels. I'm also doing away with the board mounted 68K resistors (solder them directly to tube or jack). My question is: Can I use just one 68K resistor between the input jack and rotary switch? Or do bothe halves of the first preamp tube need to have their own 68K resistor from the input? Also, should my input jack have a 100K resistor like the non-attenuated inputs of regular Marshalls? Thanks for any help.

flemingmras · Post by **flemingmras** » Tue Nov 16, 2004 9:26 am

I would install a resistor set for each channel to eliminate any possible popping when switching channels. However, you'll need to use a 1Meg resistor just like on the standard input circuit, but instead of using a 68K input resistor, you need to use a 33K input resistor. This is because when plugged into the High sensitivity input, the two 68K resistors are placed in paralled via the Low sens input's switch contact, making the total input resistance between jack and grid 34K. 33K is close enough to this.

Hope this helps.

Jon

dynaman · Post by **dynaman** » Tue Nov 16, 2004 8:59 pm

That makes sense. I shouldn't have to worry about popping when switching between channels as I'll be using a make-before-break switch. Any problems with me just installing a single 33K resistor from the single input jack to the rotary switch? Thanks for your reply.