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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. |
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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 |
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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 |
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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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