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Old 15th September 2012, 06:16 PM   #1
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Default Grommes/PE S-35 questions

I'm new to tube amps, but have a pretty good understanding of physics and electronics. I've been reading quite a bit to get a handle on the basics, but it's sometimes better to throw a question to the maestros! I picked up an on Grommes/PE s-35 PA amp at a garage sale. It seems to be pretty well known model, at least there is some info online, and schematic. I made the mistake of turning it on without using variable voltage. All the tubes glowed and I had a speaker hooked up so it would have a load, but no sound at all comes out. I modified a guitar cable to have RCA jack to use the AUX input because the mic inputs are an old coaxial type( Im going to solder a 1/4" jack, but first just wanted to see if it works). After letting it warm up for a while, everything seems fine, no excessive heat or smells, tubes glowing etc. but the "neon power indicator" as its called in the schem. doesn't come on. There is a bulb that lights up the logo that works, and the power ind. did come on for a bit when I turned it off. What ive concluded is there IS power getting to the tubes, but no sound at all, so maybe a tube is bad? Or maybe the power transformer?
Thanks for any input you all might have...
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Old 16th September 2012, 05:12 PM   #2
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Start by measuring the voltages and comparing to the schematic. Make sure all the heater currents are correct; then follow the B+ to make sure they are correct. Measure the voltages on the output tubes and ensure they are correct; there should be -27V on the output tube grids. Make sure this is the case or the coupling caps to the output tubes will need to be replaced or the bias supply is not working. Basically, you need to go through the amp with your volt meter to ensure all the voltages are correct. If they are not correct you need to find the reason why.
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Old 18th September 2012, 08:28 PM   #3
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Thanks for the response! With the help of a tinkering friend, found that one of the pre-amp tubes, two 6eu7s, was shorting out. I swapped them with a few that came with the amp at the garage sale, and it came to life! It sounds great as is, but ill probably do a few mods, maybe rewire the preamp sockets to take 12ax7s, as ive heard they are pretty close to 6eu7s. Thanks again!
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