In Reply to: are ST70 this sensitive? posted by FenderLover on February 12, 2009 at 19:08:51:
Your advise was great, a very nice summary.
The ST70 (and Mark III) ran filter caps pretty close to their ratings - too close. Heathkit was better in this regard and often stacked two caps for a higher rating. Now that line voltages are up and filter caps are old they are dangerously close to their limits.
Put a voltmeter on a ST70 and watch the start up. The timings work well - just as the B+ comes up the EL34's start conducting. Without the load of the EL34's the B+ will exceed the cap ratings in my experience.
If you don't have a spare 5AR4 the 5U4GB is a pretty good temporary substitute in the ST70 (Dynaco even mentions it in the manual). The 3A filament isn't a problem and the 5U4GB has a larger drop in B+ than the 5AR4.
I know that you can push 5Y3's pretty hard, but I've seen some spectacular disasters. I guess I wouldn't trust it (even short term) in a ST70.
I usually reform caps by clip leading them onto an older Heathkit cap tester (IT-11). If I have to reform them in an amp I'll use a solid state rectifier and slowly ramp up the B+ with a variac.
I use a TV7D/U most of the time as a first screen for rectifiers and power tubes. Occasionally rectifiers will flash over in an amp and still test fine and the very modest B+ in a TV7. At least you can eliminate some totally shorted tubes without risking a vintage power transformer that may be difficult to replace.
- Gary
http://www.the-planet.org
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Follow Ups
- RE: are ST70 this sensitive? - Gary Kaufman 02/12/0919:42:40 02/12/09 (0)