Guitar Gear Forum

Hi,

I've got a Fender Red Knob Twin that Im going to be working on, I was wondering about some of the resistors in the PSU.

As far as I can tell, all of the really high voltage caps in the PSU have resistors across them (though the rightmost two on the picture have other resistors in series with the cap also). Most of them are 220K, but one is 30K 10W across 400V unless I have misread the schematic (its a little hard to read).

The schematic of the PSU is here

http://www.christianguitar.com.au/image ... in-PSU.gif

What are these resistors for other than to discharge the caps for safety when working on the amp? According to P = V squared / R the 30K one is using 5W of power all the time the amp is on? Which made me wonder what it is for....

About how long would you expect these caps take to discharge, with those resistors there?

The 75V winding doesnt have any resistors across the caps (bottom of the image), is that because 75V isnt dangerous?

Unless I've missed anything, is it correct that I don't really need to worry about making up a jumper with a resistor (like 10K 10W) to bleed the caps? Or maybe I should do that anyway for the 75V caps?

Of course I can check the voltages with a multimeter before doing anything, but I was interested in what some more experienced amp techs thought.....

I am not an experienced tech like some of the guys here, but I'd make a jumper with the resistor and bleed the caps just in case. It literally takes less than 5 minutes to make one and there is absolutely no risk of damaging the amp if it's not necessary.

I bleed the caps every time I have to do anything inside a tube amp just by force of habit, you should too. It saves you a lot of potential trouble.

A few thoughts in no real order as I'm about to leave for work:

This amp uses a voltage doubler - this is why the center tap of the PT is connected to the middle of the two reservoir caps (C301 & 302).

The caps in the arrangement need balancing. That is what the 220K resistors (R301 & 305?) are for. These form a path to ground and will discharge the filter caps. always always always check that these have done their job my measuring the voltage from the positive terminal to ground every time you go in there. Paranoia saves lives in tube amps. Did I mention that you should always check? They should discharge fairly quickly but measure first!

The hi lo switch is connecting the LC filter which feeds the screens either to the full B+ from the top of the filter caps, or half of it by connecting to the middle of the filter caps. This is a cool trick that can be used with amps using a voltage doubled arrangement.

The 30k 10W resistor is forming a voltage divider with the 2k7 10W resistor to reduce the supply voltage to the preamp to sensible levels. From a guess at the numbers on the schem the plate voltage is about 435VDC? Thats what you'd need to put into that divider to get ~400VDC at node A.

Hope that helps. Any other questions just ask.