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Inside the amp |
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| Find the 6 - 25uf/25volt caps in the layout diagram above. I have cleaned up the layout diagram section above so it is easier to see the 25uf/25volt caps. |
| Inside the amp on the main circuit board you will find some small electrolytic capacitors.
They can be different values and different shapes but most of the time the value is 25uf at 25 volt. Sometimes these caps
are double caps, there will be two 25uf caps inside one package and they will share a common negative lead. Older Fenders
had double paper style 25uf/25uf at 25 volts. Later they were doubles with a black body and a burgundy end. Later still
they were a single 25uf/25 volt white waxy looking Mallory cap. All of them are electrolytic caps with a positive end and a negative end. The negative end points towards the pots and the positive end points towards the tube sockets. You will have to look around inside the amp to find them all. Most Fender combo amps have seven caps total. Either 7 single caps or 3 double caps and one single. Smaller amps like Princeton's may have only 6 of these caps. Tweed Fender amps will vary also, you will have to look at the amp to find them all. Some Fenders have one 25uf25v cap along the back of the amp near the reverb jacks. They all need to be changed to new caps regardless of what amp you are working on. These caps make a huge difference
in how the amps sounds and if they are over 10 years old they need to be changed. Sometimes the tremolo section has a
4.7uf cap instead of a 25uf cap. I replace these with a 25uf/25v cap or a 22uf/50v cap. Either cap works fine for a replacement.
The first thing to do is remove the 25uf/25v caps from the circuit board and replace all of them with a new 22/50v or 25/25v cap. Unsolder the lead closest to the tube sockets and pull the cap up. The negative lead of some old caps continues through the eyelet and is soldered at the brass plate under the pots. Snip the lead flush at the eyelet, you will leave the section that goes from the eyelet to the brass plate. It's a ground wire that you need to leave in place. |
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| 7th 25uf/25v cap in the tremolo section | Bias cap - 47/100v |
| You can pre bend the leads of your replacement caps so that the bends are
directly over the eyelet holes. Snip the new cap leads to the proper length. Heat up the positive side first and push
the lead down into the hot solder. I do all the caps positive ends first and then push any shared negative wires into
the hole at one time. The positive end usually is black rubber and the negative end is usually aluminum. There is one more electrolytic cap inside the amp on the bias board. It is usually a 25uf/50 volt or a 50uf/50 volt. It needs to be replaced also. I use a 47uf/100 volt cap. A 50 volt rating is not high enough for this cap. There is more than 50 volts across this cap, so it is a good idea to raise the voltage rating of this cap to 100 volts. Notice the polarity of this cap. The positive end goes to ground because this cap is smoothing out the negative voltage for the bias circuit. After all the caps are in place and situated properly, go back and add a tiny bit of fresh solder to all the eyelets where you just installed the new caps. |
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| Power tube sockets |
| There are two large 470 ohm resistors soldered on the back of the power tube sockets.
If you do not have them, you should add them to your amp. They are usually a one watt 470 ohm carbon resistor. Most of
the time they are not 470 ohms any more because they were under rated and should be a higher wattage. Even if they do
read close to 470 ohms I would replace them because I have seen them fail so many times. Replace them with a 470 ohm 3
watt metal film resistor and they will be good to go for a long time. Remove them from the tube socket by carefully
heating up the tube socket pins and gently pull each end free. Notice that Fender soldered them over the top of the tube
socket. This is not a good idea because the resistor itself and the tube socket generates heat. The small 1.5K 1/2 watt resistors are ok sometimes and sometimes they are burned up from the heat. Replace them also if necessary. They do not generate heat themselves so it seems to be ok to mount them across the back of the tube socket. Solder them high up away from the socket just to keep the heat from the tube socket to a minimum. |
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The only other components that I may or may not change on the main circuit board are the 100K plate load resistors. I only change these if the amp has an excessive amount of hiss when operating. These resistors are in a V shaped pattern usually but you may have to trace them from the tube socket to find them. Pins 1 and 6 are the plates on the pre amp tubes. You can follow those pins back to the board to find the 100K plate load resistors. On most Fender combo amps, the phase inverter tube has an 82K and a 100K plate load resistor. |
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