Repaired Plexi died in 10 mins

[LoudStroud]

The other thing to check is the AC voltage out of your wall. That '67 Plexi will likely want to see 115 VAC tops. Wall voltages these days can be 123-126 VAC. If the power transformer in your Plexi has a high voltage on the secondary side, which is very likely, it can smoke new production tubes if running the amp off of a hotter AC wall power. EHX brand EL34's cannot handle higher plate voltages and current. JJ's can handle the higher voltages.

'70's 100 watt Marshall's had a secondary plate voltage around 475 VDC. Plexi era around 520, some as high as 600 VDC!

Your tech can check it by measuring the filament voltage with the amp on a Variac. When the filament voltage sits at 6.3 VAC, the primary AC voltage will likely be around 115 VAC, possibly even 110 VAC. If your wall AC voltage is much higher, get a step down transformer like the BrownBox, specifically designed for this purpose.

 

[Matthews Guitars]

Pete Farrington, thanks for posting what you did about fuse ratings, IEC vs UL. This is good news for me because I found that one of my 1959s would routinely eat a 1A HT fuse but there's nothing wrong with the amp that I can detect. I've gone up to a 1.25A and that has done the trick. But I'm using the fuses I can get in the USA, which will be UL rated rather than IEC rated. So that explains that.

 

[EADGBE]

...My first choice was the great vintage guitar/amp shop here. But all the techs they work with — people with true expertise and talent with vintage Marshalls — have at least a 3-month waiting list to even look at an amp.

The wait is worth it to have a good amp repairer work on your amp.