The load on the rectifier is to heavy on some models and causes the 5Y3 to fail during switching between the SS and tube mode.
I have had mine for 4 or 5 years and never had any failures, but the 5Y3 failure is pretty well known. You very rarely feel like you have to fight with it. On the plus side, it is an easy amp to play and is very forgiving with regard to less than stellar technique. There are few guys in my area that use them and they sound like dogshit, but that is an operator problem. The choice of rectifier playes a big part here. It is often difficult to get the highs right, at least for me. It doesn't have alot of headroom so it can be a bit on the mushy and/or dark side. It can be difficult to tweak, but there are alot of good sounds available. Which makes the whole process a big piece of legal/political shit. Of course, all this involves lawyers and money. The current patents are not challengable. This is potentially where Mesa may have problems with future patents. A patent can be challenged up to 9 months after it is granted and potentially be disallowed. This introduces a challenge system into the process. The post grant review process went into effect Sept 16, 2012. Yes, alot of changes were recently approved, but some of those don't go into effect until March 16, 2013. It's first to file, not first to invent anymore. You guys are aware that US patent law has recently been changed? Rectifier tube sag (occurs when the amp is dimed and the power supply can't keep up) wouldn't be audible in a 100Watt amp unless heard from a some distance. PS The concept of a dual rectifier isn't exactly original either, saw concept in a book copyrighted 1997 and still widely distributed. It has been successfully prosecuted against infringers who have paid very large fines and/or been forced out of business!" This design and circuit is protected under U. His schematic for the Tri-mode programmable preamplifier contains the following Warning: Smith cloned the Soldano's SLO pre-amp exactly component value for component value when he released Dual Rectifier (later versions slightly tweaked these values). His Push-Pull to SE patent uses a switch instead of a pot to switch between the two the concept of PP to SE was incorporated and sold in guitar amps in Canada a decade previously, and published in more than one book prior to Mesa's patent application. Then again the USPTO isn't populated with electrical engineers, just lawyers, and the judges are loose cannons on this and any other subject. Randall Smith patented quite a few circuits which had fallen into public domain from the RCA manual, so I wouldn't be surprised if he hadn't attempted to patent the wheel. LOL, I wouldn't be surprized if he tried.