Fender_super_champ_mods 电路图 维修原理图.pdf

Fender Super Champ Modifications These details are believed correct but the author cant take any responsibility for what happens if you attempt them. Valve/tube amps store lethal voltages even when switched off and unplugged from the wall, and if you dont know what youre doing, you can be badly hurt. RELIABILITY MODIFICATIONS As l describe the works, Im standing with the chassis front panel nearest to me and the row of valve/tube sockets (were looking at the wires side of those, not the valve side) running along the far edge of the chassis. The output valve sockets are the last 2 on the right. Please note that this writer has only done these mods on a Princeton Reverb II, not a Super Champ, but the 2 amps are very similar. If youre going to do both mods, test the amp after each one. 1 Upgrading the Bias Feed resistor Failure of the bias feed resistor is the single most common reason a Super Champ will die. This mod makes sure it wont happen to your amp. After dismantling, this takes 5 minutes, costs about 35 cents US, and could save a lost gig and a trip to the tech. PHOTO A This photo is clipped from the excellent Super Champ chassis photo on 收音机爱好者资料库 Ra d i o Fa n s .CN (enter Super Champ in the search box at the left of the screen) The little resistor identified by the green arrow I photo A is 47 ohms (yellow, purple, black). 47 ohms is correct, but the wattage isnt big enough on some Super Champs - in other words its always in danger of overheating. The one in this photo is probably OK but on some models its smaller and may be badly discoloured from overheating. If it fails there will be no bias supply to the output valves (tubes), and if that happens, theres nothing to stop the valves from running at a very high idling current, which they probably wont take for very long. The danger of this component overheating is the same at high or low volume. The solution is to replace it with a 47 ohm resistor, one watt or bigger, wirewound. More than one watt is OK too, but much more might be too big physically. It will take you 5 minutes, not counting the trip to your local component shop, and dismantling time. Careful not to overheat the diode next door while soldering. PHOTO B NB this is a photo inside my Princeton Reverb II, NOT a Super Champ In the photo above, the new bias feed resistor is the white rectangular component circled in green thats one type of higher-wattage 47ohm resistor you could use. On your Super Champ the resistor will be mounted on the tag strip where the original resistor was (arrowed in green in photo A). In the Princeton Reverb II its on the 收音机爱好者资料库 Ra d i o Fa n s .CN circuit board as shown in this photo. The new screen-grid resistors (see mod below) are the white 3W470ohmJ, one on each power valve socket, circled in blue. Photo A. Waugh. 2 Fitting Screen-Grid Resistors An important mod especially if youre using your Super Champ at high volume most of the time. This takes 10 minutes once youre inside. And two resistors, each costing about 50 cents US. Photo B shows the finished mod in a Princeton Reverb II. The idea is to insert a 470-ohm 2 watt wirewound resistor in the wire running to pin 4 on each output valve (V5 and V6, 6V6GTA). These new resistors will prevent the tube grids drawing as much current under heavy load, and also make sure they remain at lower voltage than the plates. At very high volume, the plate voltage can actually drop below the screen grid if no resistors are fitted, which results in a massive increase in screen grid current (because it begins to act as a secondary plate) and very shortened tube life, or even sudden failure. It is strange that Fender didnt fit them to all their amps - the larger amps all have them. see for a drawing of the tags, with the tags numbered. PHOTO C 收音机爱好者资料库 Ra d i o Fa n s .CN Photo C shows the power valve sockets inside a Super Champ. Pin 4 is shown on each power valve/tube socket. The pin numbers go up clockwise. Pin 8 has the short bare wire soldered to the chassis. As you can see in photo C, more clearly on the right-hand socket, pin 6 on each power valve socket is unused (pin 6 goes nowhere in the valve) so its available as a spare solder mounting tag. Remove both power valves from sockets. On the left socket, pin 4 has a wire, probably green, going to pin 4 on the right socket, which also has another wire going away to power supply capacitors. On the left socket, unsolder the wire from pin 4 and resolder it to pin 6. On the right socket, unsolder both wires from pin 4 and resolder both of them to pin 6. On each socket, solder the new resistor to pin 4 and pin 6. Keep the resistor leads short so the resistor cant vibrate too much. It will now look like the power valve sockets in Princeton Reverb II photo B, circled in blue. Replace the valves. Job done After doing this mod on my PRII, Ive noticed that the amps bass response seems better in a way thats hard to describe. Its like theres less wasted effort, or more energy focussed in the right place. The bottom end cuts through the band more clearly and theres no added boom. This didnt show up when playing quietly at home. Your experience may be different. Good luck!