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This is a fun little amp to modify and a great place to start if you’ve never done anything like this. There are many mods you can find for this and here are some of my favourite.<\/p>\n
This first mod is rather interesting. Try this: remove the valves from the amp and turn it on.\u00a0 It still hums.\u00a0 Amazing!\u00a0 This is because the magnetic field of the power transformer couples to the nearby output transformer.\u00a0 So our first mod is to relocate the (smaller) output transformer.\u00a0 Here is a picture showing the new location.\u00a0 You have to unplug the blue and red leads and undo the screws of the transformer. You need to extend the red lead by about 2-3 inches (5-8cm). Remember to properly and carefully insulate the joins.\u00a0 I use a soldered hook joint and use a double layer of heat shrink sleeving to insulate them.\u00a0 Mark and drill holes at the new location and screw into place.\u00a0 Hook the leads back up and power it up again.\u00a0 Notice the difference?<\/p>\n
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This is a big problem with these.\u00a0 In the samples I’ve seen the 6V6 valve is dissipating over 16 watts and is only rated for 12.\u00a0 The result is a much reduced life.\u00a0 The way to fix this is to change the cathode resistor R10 from 470 ohms to 680 ohm 2 watt.\u00a0 On the sample in the pics, much of the top of the board was covered in sticky clear gloop to keep the electrolytic capacitors in place.\u00a0 This needs to be removed from around the resistor by gentle warming with a heat gun on low while scraping away the excess glue. Or, if you have it,\u00a0 apply some freezer spray and chip it off.<\/p>\n
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The power supply is not especially well filtered. Also the B+ supply voltage can be a bit on the high side at over 380V for the poor 6V6 here in the UK where the 240V supply is at the high end of the range. We can lose some of these volts by adding a 220 ohm 2W cement (flame proof) resistor in the B+ line.\u00a0 By the way, this has nothing to do with cathode stripping as that only affects valves with very high voltages, think 10KV or more.\u00a0 It’s just that the voltage is higher than intended and that can have a negative impact on reliability.\u00a0 To do this you need to cut a track on the bottom of the the circuit board and then bridge it with the 220 ohm resistor. See here:<\/p>\n
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Since we have the extra resistor in the power rail we can add a smoothing capacitor after it and get a big reduction in the power supply ripple to about 1\/10th of the original value.\u00a0 Put the board back and then add a 100uF 450V electrolytic on the top as shown here.\u00a0\u00a0 Note the negative end joins on to the ground end of the 6V6 R10 cathode resistor.<\/p>\n
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Having only two valves means this amp does not have very much gain.\u00a0 We can reclaim a little and allow more tubey harmonics through by removing the negative feedback.\u00a0 If you don’t mind drilling the front panel,\u00a0 you can make it switchable.\u00a0\u00a0 Maybe call it the Jazz\/Blues switch?\u00a0\u00a0 You do this my adding a SPST toggle switch in series with the 2.2K resistor R7.\u00a0\u00a0 Relocate R7 and solder one end to one of the terminals of the switch.\u00a0 You can wire the remaining terminals of the resistor and the switch into the holes on the PCB where R7 used to be.\u00a0 Cover the resistor and joins with heat shrink to insulate and support it.<\/p>\n
![\"\"](\"https:\/\/bmamps.com\/v01\/wp-content\/uploads\/2017\/10\/Gretch_5222_nfb.jpg\")
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The last hum fix is to add change the two 100 ohm resistors R8 and R9 that balance the heater supply, sometimes called an artificial centre tap.\u00a0 Replace these with a 220ohm 5W potentiometer with the wiper to ground.\u00a0 The track ends go to the two heater wires. Adjust the pot for minimum hum.\u00a0 I found this to be more successful then a DC heater supply and a lot less effort.\u00a0 In the picture above you can see fixed resistors used here.\u00a0 If you measure the setting of the pot you can replace R8 and R9 with these values rather than try to find a place to locate the big pot.\u00a0 Use 2W flameproof types for these in case the tube shorts out.<\/p>\n
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Lastly, enjoy your new amp!<\/p>\n
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Fender Champion 600 \/ Gretch 5222 This is a fun little amp to modify and a great place to start if you’ve never done anything like this. There are many…<\/p>\n