The sunn Forum
Sunn Musical Equipment => DIY => Topic started by: doc10039 on June 11, 2011, 03:37:34 pm
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Playing a gig with my 200S with a Peavey 115BX with 15'' 4 ohm Black Widow and impedance set at 4 ohms. The last of the second set my amp just quits. All tubes are still lit up. Just no sound. Took the chassis out today and checked for hi voltage and bias voltage and all OK. It either comes down to the OT or the solid state rectifier. All filter caps have been replaced and orange drops for the tone caps. I am looking for suggestions on how to proceed. I can check the voltages at the rectifier but the only that will prove is the correct VDC at the rectifier. Also what are the primary impedance needs for the 200S? Any help will be appreciated.
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Measure the voltage at the output of the rectifier. All the rectifier does is change AC to DC so if you have the correct DC measurement its working. I'd be suprised if that's the problem but its only 4 diodes anyway...maybe $1 to buy a new one.
I'm curious how you diagnosed your problem. Based on what you've said you know your power supply is functioning...you haven't really proven anything else.
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To use the 4 ohm output tap, you should have been plugged into the "Extension Speaker" jack. Try plugging the speaker into the other jack. There is a mechanism on the Extension jack that switches the tap from the 8 ohm tap to the 4 ohm tap when a plug is inserted. It sometimes goes bad.
Don't play long or loud if the amp is making no sound, you could do some extensive damage. Have the amp serviced.
Check that the speaker and cable work when plugged into another amp.
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It has a selector switch to go from 4 to 16 ohms. I am guessing it is the the OT at this point.
GS
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I can't comment on how your amp could have been modified, but it is unusual for an output transformer just to go dead without a loud bang, bad smell or fire. Is high voltage getting to pin 3 of the power tubes?
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Yes to to high voltage at pin three.
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I was under the impression that the 200s only has 8/extra 8 ohm-extension outputs (minimum of 4) on the back? The OT would have to have been replaced in order for there to be 4-16 ohm taps on the tranny, with a selector switch? If it were the rectifier, your amp wouldn't even power. Smell the OT, if there's an obvious baked smell, it's safe to say that's your issue. See if you can find any cold solder joints, they'll look like cracked shitty pot metal. If you're savvy, you can reflow all of the joints (after draining the caps, STILL KEEP ONE HAND OUT OF THE AMP AND IN YOUR BACK POCKET AT ALL TIMES.)
A few more things to check, preamp tube wiggled it's way out of the socket, since it was most likely sitting on top of the cab. You can do the click test, go inside, with the amp hooked up, and on, use a non-conductive stick of some sort (bamboo chop sticks work well,) and poke around the signal path, you should hear little clicks coming out of the cab... once you find a place there isn't a click... that's your issue. Those amps are crazy simple, so tracing the path from the input to the output wouldn't be too hard. Be careful, and report back, buddy.
EDIT: sorry for the old thread revival, forgot how slow this board moves sometimes...
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The stock OT has 4, 8 and 16 ohm taps, they were factory wired for 8 and 4 or 16 and 8. I don't think any old Sunns had impedance switches though, that must be a mod.
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Indeed, Ed, Thanks for clarifying!