Fender Champion 600 with Power Scaling

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DaveWW

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Hi folks,

As promised I'm posting details of fitting the SB-2 kit from London Power to my Fender Champion 600.

It didn't go as smoothly as I'd hoped mainly because although there are few parts to actually install the instructions are purely schematic so you have to have some knowledge of schematic circuit diagrams to figure out the installation. I did manage to figure out most of it but my extremely limited knowledge was not quite enough so I sought help from Alnicomagnet who supplied the stuff for my other mod's. The other problem was the very limited space available in the amplifier.

Basically, turning down the "power scaling" pot reduces both the voltage going to the anode, and screen in equal measure.

The "drive compensation" pot is a dual gang 1 meg audio pot wired in parallel. It acts just like a normal master volume control.

After shorting out a few resistors the amp now works fine :D It does exactly what it says on the tin and I can now get nice tube distortion at volumes you can talk over although ther amp was never a gain monster to start with.

Pic's follow

Kit Contents
sb2kit.jpg


Assembled PCB
IMG_0511.jpg


Other Components
IMG_0516.jpg


Power Scale Pot Assembly
IMG_0518.jpg


All Parts Assembled
IMG_0623.jpg


Ready To Go Back In the Amp Cab
IMG_0625.jpg


Front Panel Layout (need some labels and a new knob :lol: )
IMG_0628.jpg


Needs some tidying up now which I'lll do later. It would have helped quite a bit if I'd used different couloured wires from the PCB and some form of plug and socket would be of great help when taking out the main amp circuit board.

Anyway after a bit of trouble I'm now very pleased with the result. It works even better with pedals and the sensitivity seem increased as well :D

Dave
 
Thanks for your kind words. I've learned quite a lot from this forum so it's nice to try and put something back.

It's really worrying if I'm starting to sound like I know what I'm talking about (in this case I assure you I don't). Reminds me of a joke I first heard at work after giving a presentation, "if I didn't know you better I could almost swear you understood every word that you said". :lol:

Anyway after posting this I was worried I'd found a fault, which I have - sort of. After playing for a couple of hours only the master volume worked and I had to turn up the power knob more and more otherwise the amp just cut out, sort of like when you put it on standby and theres sound while the cap's drain. This continued until I had no power scale function at all just the master volume.

So I decided to try it again this morning. After swapping the tubes I just left it on with the master volume down low and the power knob well reduced. Sure enough after about an hour and a half the same thing happened.

I removed the screws and partially slid out the front panel and had a general poke about. After a while I noticed that the power scale was starting to work again and started to wonder if it was to do with heat. So I blew gently on the small pcb/mosfet mounted against the chassis and sure enough everything sparkled back into life.

The instructions with the kit say to mount the mosfets metal side down on top of what seem to be conductive pads which I've done. I could also try removing the assembly and scraping the paint off plus adding some big washers on the underside to help act as a heat sink.

I'll give Kevin O'Conner an email at London Power to see what he recommends and also Alnicomagnet (Nigel Briggs), who has been very helpful. As I previouisly mentioned the space is very limited in this amp and there is not much in the way of ventilation. I should also say here that Kevin did offer to talk with me if I phoned him during office hours. I didn't take this up mainly due to the time difference etc but he does seem a very helpful guy.

I guess this problem may not translate to other amps but very careful consideration should obviously be given to where you mount the PCB.

Hope my drivel still makes sense!

Still sounds great so I'll stick with it :D


Dave
 
Maybe a little PC fan mounted inside might help, or mount the pcb in a seperate ventilated box inside the cab chassis rather than in with the amp??
 
I would first use some thermal paste on both sides of the plastic insulator to help with heat transfer. You should really put those FET's on a heat sink and again apply the thermal paste.
 
Hi, thanks for your advice. I thought about trying to use use a heat sink but if it's in a hot area it will just delay the warm up, same with the thermal paste. I tend to think moving to a cooler location and/or active cooling will be necessary. They generate heat themselves - tried them just hanging out of the cab. So they do need a heat sink or cooling but the heat-sink itself needs to stay cool. Where they're mounted I'm pretty sure it wouldn't.

Dave
 
Hi folks,

Just an update. It seems the considered opinion of the experts is that in a tiny upside down class a tube amp cooling is going to be difficult.

So I think I'm going to turn it the right way up, valves up, mount it in a bigger chassis and mount the mosfets seperately on a big heat-sink.

It'll take me a while to sort out what bits and pieces I need but I'll keep you posted. Meanwhile if anyones got any ideas how to do it, what to use etc I'd be very interested.

PS It's sounding great through the home made 1*12 cab kindly loaned by JohnA. Perhaps he'll forget I've got it :lol:

Dave
 
Saw this and thought of you!


http://cgi.ebay.co.uk/Harley-Benton-Epiphone-Valve-Jr-Amplifier-Head-Box_W0QQitemZ260280612148QQcmdZViewItem?hash=item260280612148&_trkparms=72%3A12%7C39%3A1%7C66%3A2%7C65%3A12%7C240%3A1318&_trksid=p3286.c0.m14
 
Just to finish off.

Before re-housing I tried a very large heatsink from Maplins and placed it at the bottom of the amp cab. Tested it yesterday for over 6 hours and no overheating problems with the mosfets :D

Heatsink.jpg


AmpSpeaker.jpg


So I added some mesh to the rear aperture for safety. Now the heatsink just needs securing to the cab and job done - sounds great :D

Dave
 

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