In Reply to: RE: blown gz34 st70 posted by FenderLover on February 8, 2009 at 17:50:07:
The ST70 is a worse case design for the rectifier and filter caps.
Most 5Y3's won't survive, I wouldn't chance it. A 5U4GB is just fine short term if you don't have a 5AR4.
Also running the ST70 with no output tubes is a recipe for disaster. The B+ will rise above the ratings of the marginal 525v filter cap with no load. Leave output tubes in at least one channel. If you don't have good EL34's you can substitute KT66's or 5881's in a pinch (just rebias).
The best trouble shooting is to lower the AC voltage by 20% with a variac.
If the fuse blows instantly suspect the rectifier. If it doesn't blow until it begins conducting (say 15-20 seconds) then you probably have a bad filter cap. Thats probably 95% of ST70 failures. The other 5% are a shorted output tube, bias failure or other wiring error.
- Gary
http://www.the-planet.org
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Follow Ups
- RE: blown gz34 st70 - Gary Kaufman 02/9/0914:24:40 02/9/09 (2)
- are ST70 this sensitive? - FenderLover 19:08:51 02/12/09 (1)
- RE: are ST70 this sensitive? - Gary Kaufman 19:42:40 02/12/09 (0)