Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on March 28, 2012, 05:57:40 pm

Title: deciphering fender 75 channel switching
Post by: phsyconoodler on March 28, 2012, 05:57:40 pm

I have a Fender 75 on the bench and it has burned up a bunch of the carbon comp resistors in the power supply area.Carbon comp and power supplies don't mix in my book.
 Anyway,one leg of the LDR's go to the power supply and they have a zener diode to ground.I want to eliminate the low power switch and wire up the power supply and bias supply in a normal fashion to get rid of those ridiculous carbon comp resistors,but I need someone who can decipher what the one leg of the LDR needs to see for voltage.I can make a separate supply for just that if I can.
  Anyone understand that nonsense?

Title: Re: deciphering fender 75 channel switching
Post by: Willabe on March 28, 2012, 07:12:08 pm

Is this the right schemo? I don't see any LDR?

http://www.el34world.com/charts/Schematics/files/fender/fender_75_schem.pdf (http://www.el34world.com/charts/Schematics/files/fender/fender_75_schem.pdf)

Title: Re: deciphering fender 75 channel switching
Post by: PRR on March 28, 2012, 09:16:08 pm

Don't mess with stuff you don't understand.

And IMHO nobody can understand the way the '75 is drawn. What a mess. Looks like the streets in the old-old town I used to live in.

You want 35mA current. This flows to the LEDs; or is shorted to kill the LEDs.

A real obvious way is a 1210 ohm resistor from -58V, just like before.

Dissipation in the resistor is 1.5W. 1W+2W of comp is fine for the first decade. Since you didn't trash the beast in 1990, yes, invest in several watts of good resistor.

Title: Re: deciphering fender 75 channel switching
Post by: phsyconoodler on March 28, 2012, 10:13:58 pm

Many thanks PRR.
  Yes it would fit nicely in the trash bin at the back of my shop.

So dump the two resistors and use a 1210 sand resistor and lose the zener or keep it there?

Title: Re: deciphering fender 75 channel switching
Post by: sluckey on March 29, 2012, 07:26:21 am

Just use a 1200Ω (easier to find) and you must keep the zener.

Title: Re: deciphering fender 75 channel switching
Post by: phsyconoodler on March 29, 2012, 10:24:56 am

Thanks guys! I really need to get into solid state more.It can be confusing.

 I do understand whats going on now,believe it or not,thanks to great guys like PRR and Sluckey.

You guys rock! :worthy1:

Title: Re: deciphering fender 75 channel switching
Post by: PRR on April 01, 2012, 07:22:52 pm

> you must keep the zener.

Debatable. Use it, but ideally it never does much. LEDs off, there's zero voltage there. LEDs on, there's 1.7V plus ~~4.7V in the balancing resistors, 6.4V. Zener barely conducts.