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69.105.1.75
In Reply to: RE: 300B Tube-life expectancy... SURVEY!!! posted by Tone! on November 29, 2007 at 23:41:19
I've been running new production WE300B's in a Welborne Laurel stock stereo amp since 1999. Just this week one blew out.
Earlier this year my amp had a bit of "meltdown". The 2kx2 (1k) cathode resistors in 1 channel got so hot they became unsoldered, blackend the standoff board, and melted some solen caps. I put fresh solens in amp (both channels). I put in new cathode resistors. I also swapped the driver/mu to 300B coupling cap. I had jensens in there, now it has hovlands. I put black gates in the 6SL7 cathode circuit. All the other electrolytics are original. Since that fix everything has been running OK. Until this week...
This week a WE300B blew. Thankfully it wasn't a spectacular flaming blowout that took speakers/xovers/or amp parts with it. But it was a WE300B, and I expected those suckers to last 40 years. I just got new WE300B's and decided it's time to bench test the amp a little closer than last time. I hooked up old speakers and a music source, flipped the amp over, and started checking voltages. Here's what I get:
B+ = 415VDC (spec is 415) OK
Cathode Bias = 75VDC (spec is 75V on the schematic, 65V +/-5V in the troubleshooting/checkout section at the back of the manual) OK?
6.3V fils = 6.3VAC OK
5V 300B fils = 6VDC. NO GOOD!!!
What do I need to do to get that 300B filament voltage back down to 5V?
Someone suggested a 25W 50-ish Ohm pot in the Pi filter of the 300B supply. Who sells such a thing?
I'm also tempted to put some test points on the top plate to make periodic voltage checks a bit easier.
Randy
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