JTM50 Phase Inverter resistor options...

Roadrunner · Post by **Roadrunner** » Mon Aug 04, 2008 9:34 am

Oops! That should read, "voltage dropping resistor" options... sorry!

I noticed on Brian's site, the pics of the '67 JTM50's, that all of them have a single 8k2 resistor, like the earlier JTM45's.

... then I checked some build pics of Germino's "Club 40" and he's got two 8k2's in series on his board.
I know the later JMP amps used a pair of resistors, 10k and 8k2 in series and the earlier JTM45's used a single 8k2.

What's the advantages/disadvantages of each setup?

The JTM50 I'm building right now has an extra turret on the board for running a couple of resisitors in series, like for a Plexi. I could just run a wire across two of the turrets and use a single resistor but I'm kinda intrigued by Germino's setup.

Any insight would be appreciated... thanks!

Roe · Post by **Roe** » Mon Aug 04, 2008 10:14 am

higher resistor values will lower the voltages in the preamp, giving a browner, middier tone with much more breakup. may work well for overdriven solos, does not work so well if you have low voltages in the first place or if you prefer alot of clarity and definition

Roadrunner · Post by **Roadrunner** » Mon Aug 04, 2008 2:11 pm

Thanks Roe! I was wondering about all that and your answer really helps.

Thanks again man...

thousandshirts · Post by **thousandshirts** » Mon Aug 04, 2008 2:32 pm

Yeah. For people who want better clean, chime, and jangle from a plexi, taking it down to a single 8.2k, or a single 10k, is a good way to go. Preamp voltages will then be up.

Roe · Post by **Roe** » Mon Aug 04, 2008 5:08 pm

BTW it is called the voltage drop resistor

Roadrunner · Post by **Roadrunner** » Tue Aug 05, 2008 6:34 am

Roe wrote:BTW it is called the voltage drop resistor

There ya go... I did not know that!

Thanks Roe.

Nobody says much about it, mentions it or calls it by name much... I just figured it had something to do with the tail end of the phase inverter.

Roadrunner · Post by **Roadrunner** » Tue Aug 05, 2008 6:37 am

thousandshirts wrote:Yeah. For people who want better clean, chime, and jangle from a plexi, taking it down to a single 8.2k, or a single 10k, is a good way to go. Preamp voltages will then be up.

Thanks T'shirts, excellent! I love experimenting with stuff. I've yanked so many resistors/caps and what have you into and out of my first build... just learning what every little change does.

Now I have another area of the amp to experiment with... cool!

I really love this place...