A few weeks ago I posted about the new Champion 600 amp that I had picked up and the mods plans I had for it:viewtopic.php?f=13&t=62533
Well, all of the mods are complete and the little bugger sounds fantastic. I'll try to summarize the mods here (with photos and schematics), so if you bore easily you should leave now as this will be a bit lengthy.
The Champion 600 schematic is basically the same as the Silverface Champ amp, except the tone stack in the Champ 600 is fixed.
Champion 600 schematic:http://support.fender.com/schematics/gu ... ematic.pdf
Silverface Champ schematic (AA764):http://ampwares.com/schematics/champ_aa764.pdf
I did a fair amount of research on the Web and found a lot of interesting mods for the Champion 600. In the end, I basically converted my amp to the schematic of 1956 Tweed Champ.
1956 Tweed Champ schematic (5E1):http://ampwares.com/schematics/champ_5e1.pdf
The basic Champion 600 is a pretty well built little amp, with a decent tone right out of the box. But there is room for improvement. On the plus side, the cabinet and chassis/PCB are well built. It just exudes "cool" in its looks. On the negative side, the amp is stuck with cheap Chinese tubes, a cheap Chinese speaker, and a thick, opaque grille cloth that muffles whatever sound the speaker is able to generate. To refresh everyone's memory, here is the stock Champion 600:
The first thing I did was rip out the stock grille cloth and speaker. The grille cloth was replaced with a dark brown, acoustically transparent cloth:
The stock speaker was replaced with a 6" Jensen Mod speaker:
That was the easy stuff. On to the harder stuff (the electronics). I should note that it was not necessary to remove the main PCB from the chassis to do any of the circuit mods. The component mounting holes are plated through, so it was easy to unsolder/solder components from the top side of the board. The input PCB did have to be removed to replace the volume pot and add the revoicing capacitor to the low level input, though.
The first mod to the main PCB was the removal of all components that make up the tone stack (C1, C8, C9, and R18, R19, R20, R21, R22). Once those components were removed the two halves of V2 were coupled with a 0.022mF DC blocking capacitor. (I used a Mallory 150M type cap. I also used the same type cap to replace the cheap coupling cap used in C2).
Next, I removed the cathode bypass cap (C10) from the second half of the preamp tube. I also removed R3 and R23 from that circuit. A jumper wire was installed where R3 was, and a 1.5K 1/2 watt resistor was installed where R23 was. This simplifies the cathode circuit and brings the feedback resistor into the proper point in the circuit. The feedback resistor (R7) was replaced with a 22K ohm 1/2 watt resistor. Here is a photo of the modded main PCB:
Then I moved on to the input PCB. The input PCB was removed and a Dremel tool was used to slice off the portion of the board that held the cheap plastic volume control. A 0.022 mF Orange Drop cap was soldered across R5. This rolls off the bass a bit on the low level input and keeps humbuckers from sounding too muddy through the amp. To mount the cap I just bent up the legs of the cap and slipped them under the leads of R5. I then squeezed them shut and soldered the cap to R5. Here is a photo of the modded board:
Nothing was done to the chassis:
Finally, the chassis was installed in the cabinet for a trial run:
After the initial checkout, I experimented with preamp tubes. For the power tube, I used a JJ 6V6S. I had a choice of tubes for the preamp:
1. NOS JAN GE 12AY7
2. NOS JAN Philips 5751
3. JJ 12AX7
4. JJ high-gain 12AX7
One thing to note is that the removal of the tone stack results in a significant increase in gain through the amp, so it is a lot easier to send the tubes into overdrive. To steal a line from "Goldilocks and the Three Bears", I found the two JJ 12AX7s to be too hot (the amp broke up way to early which made it hard to get a clean tone), the 12AY7 was too cold (the amp stayed clean with the amp volume dimed and the guitar volume almost so), but the 5751 was "just right". I was able to dime the amp, turn the guitar down and get a nice loud clean tone, or turn the guitar up all the way up for some sweet overdrive.
Before buttoning everything up, I measured the bias setting of the 6V6S. Using my bias probe and DVM I measured the cathode current at 43 mA and plate voltage at 324VDC. Using the Weber bias calculator, the plate current was approximated at 41 mA. Taking into consideration the tolerance of all the measurements, the % of max plate dissipation for the JJ 6V6S is about 93% - 95%. A bit hot, but there is no sign of redplating and the tube sounds really good. I'll probably spend some time looking into ways to reduce the bias levels down to around 90%.
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