Brian, I have one clip of the amp here:Stanz wrote:Dave - I had seen your video before, too bad there are no sound clips of it.
http://ppwatt.com/node/16045
Thanks man. Interesting that you are having some similar microphony issues too -- I think Fredaxis ran into some issues with his as well. The mixing resistor region was definitely the source of some mojo-killing oscillation. You could tap tap tap on the cap (I added for the VVR) just after the 470k/470k pair and between the V2 grid and hear a loud "clunk clunk clunk" in reply. I added a piece of shielded cable between the cap and grid and that not only tamed the oscillation but just let loose the beast. The harmonic content really improved. I think Fredaxis may have put the mixing resistors right on the V2 grid instead of the board. I may do that next time, skip the VVR, and use an Airbrake to manage volume instead. The VVR does work great though.Stanz wrote:Looks fantastic though. The microphonic thing is one of the symptoms. It behaves as if there is a microphonic tube, especially if I tap on the top of the chassis. The 470K mixing resistor is within the area of where it seems to be coming from. This amp did fire up and played. Sounded great and can't wait to get it finished. There is a bit of a run, maybe a couple inches of wire, from the 2470K resistors to the grid input. I also had a dpdt relay at that point for the "JCM800" switch, but have since removed it see if that was the issue. This might be an issue.
I too have a switch to change the amp from parallel channels to cascade like the JCM800. It does the Def Leppard/ACDC sound very well but definitely increased the noise floor of the amp. It's pretty cool sounding but I opted to disconnect it and just use the amp as a good 'ol '68 sounding plexi. I also think the added wire lengths routing signal from V1, to the switch, to Vol 1 and out to the mixing resistors added some instability (so did away with it). Additionally, I found the silver mica bright cap across Channel 1 to be like an antenna. I had 3 mica caps on a switch (100, 250, 500) but ditched that too as it definitely made the amp more noisy and it was bright enough with 500pF in the tone stack. I may reduce that 500pF to 250pF. There is a lot of mid/highs in the amp but this may be also attributed to the guitar (a gibson style PRS).
I learned a lot from that build and one of the things I think is important with the plexi is giving the components enough room to breathe. I think I packed too much under one hood. I was warned by the Amp Elders (Mark Huss et al) about keeping it simple and while I don't want to say I learned this the hard way, I did learn the value of simplicity! To do it again (which I may do), I would voice the amp one way, no mods, ample space, and call it done
It sounds great when you get it dialed in though! I hope you get yours sorted out. Can you post a gut shot?