music, guitars & other obsessions


Spent the whole day on the VVR for the 18 Watt amp, what should be a simple and straight forward installation turned into a time-sucking project… First I mis-wired the MOSFET (even with the darn pinout in front me!), and blew one up immediately. Then I discovered the IRF840 got pretty hot with the small heatsink, so I had to bolt it directly to the chassis, which wasn’t so bad if I had a plastic screw to hold in place (as it turned out it wasn’t necessary the case), of course, I did not, so I ended up using a cable-tie, ugly!

When I plug in the guitar, the noise was un-bearable, a quick search pointed to mistake number 2 – I mis-wired the VVR (even with the schematic right on the computer screen!), it should come after the first filter capacitor, so it did not get bombarded with pulsating DC right off the rectifier, duh! So I had to rewire a bunch of wires and parts, in the process of moving the wires around, led to the next disaster… when I fired up the amp after the re-wiring job, I heard something frying, shut it down, look over everything, and couldn’t find anything obvious… so I fired it up again, this time, I saw a quick flash on the EZ81, I hit the power switch to kill it, and dreaded that I blew up the rectifier, that would be BAD.

It turned out that the fuse was blown, probably about the same time that I saw the flash, after taking things apart and measuring lots of parts, it turned out the first half of the dual capacitor shorted out, how, I had no frigging clue – may be the voltage surge melted the insulation inside it turned out that a piece of the solder was lodged against the metal can under the plastic cover, after I dis-lodged the solder, the cap was not shorted anymore… Anyway, it was shot, so I had to replace it.

Since I already took out the dual can capacitor, I decided just to keep the replacement cap in place, since it offered more filtering, 330uF vs. the old 32uF and it provided a separate ground for the rectifier to tied to. As it is usually a good idea NOT to use the exact same point for the B+ ground and the rest of the HV ground. Now the chassis looked somewhat un-tidy with the VVR circuit installed. Oh well, at least it finally worked.

With the VVR set between 40-80%, the amp put out decent level without making my ears bleed, at below 40%, the tone suffers a bit, but could be brought back somewhat with some EQing. I think the amp now is much more usable than before, and I will install the mod on all the high-power amps I build in the future.


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