Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: supro66 on February 08, 2015, 10:51:52 am

Title: 1 meg resistor on input jack
Post by: supro66 on February 08, 2015, 10:51:52 am

Is the 1 Meg resistor required on the input jacks
I built two Supro 6420 Thunderbolt and no 1 meg resistors were on the original
what do they do

Title: Re: 1 meg resistor on input jack
Post by: sluckey on February 08, 2015, 11:11:15 am

They ensure the grid has a return path to ground. Otherwise, you are relying on the resistance inside your guitar to provide that ground path. They are not strictly required and many cheap amps omitted them as a cost saver.

I would not build an amp without them.

Title: Re: 1 meg resistor on input jack
Post by: tubeswell on February 08, 2015, 11:53:08 am

What Steve said.

Title: Re: 1 meg resistor on input jack
Post by: supro66 on February 08, 2015, 12:01:22 pm

Ok I will do a remod on all three


I did remove the death caps from the original

Title: Re: 1 meg resistor on input jack
Post by: HotBluePlates on February 08, 2015, 02:12:11 pm

According to the 6420 schematic in the Hoffman library, the Supro already has a 270kΩ resistor to ground, after a coupling cap from the jacks.

So you don't need to add 1MΩ at the jacks. You could replace the 270kΩ with 1MΩ to see if you like the slight difference...

Title: Re: 1 meg resistor on input jack
Post by: supro66 on February 09, 2015, 08:28:25 am

you are right
and I made up one of those schematics


http://el34world.com/charts/Schematics/files/supro/Supro_6420.pdf (http://el34world.com/charts/Schematics/files/supro/Supro_6420.pdf)


also if you look at weatherford schematic he thanks me for seeing a mistake [ Jim Mitchell ]