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Old 12th July 2009, 07:45 PM   #1
ldflan is offline ldflan  United States
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Default newbie simple SE issue

First build. Channel 1 works great. Channel 2 has barely any signal to speaker. Swapped out KT88s between channels, makes no difference. Swapped the inputs to the PC board, and that makes no difference. Swapped the driver tube with another 12AT7, thinking half of the 12AT7 might be bad - no difference. Checked all resistors, all seems OK. Clue - filament at bottom of KT88 in second (problematic) channel glows much more brightly than the other. Short in the socket, maybe? Any ideas? Thanks very much for any thoughts!!!

Leonard
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Old 12th July 2009, 08:27 PM   #2
Zibi is offline Zibi  Poland
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Bias? Coupling capacitor?
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Old 12th July 2009, 09:32 PM   #3
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I'd get out the schematic and a diagram of the tube sockets. With all the tubes pulled out and the amp unplugged, try ohm'ing out the pins in the power tube sockets.

If everything matches, I might be tempted to very cautiously measure the heater voltages on the two power tube sockets with the amp turned on. Keep the rectifier tube out for this test.

Can you post any hi-res photos of the board and wiring?
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Old 12th July 2009, 09:49 PM   #4
ldflan is offline ldflan  United States
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Correction - on the channel that gives little signal, the KT88 is dimmer than the other side.
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Old 12th July 2009, 09:49 PM   #5
ldflan is offline ldflan  United States
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Swapped the coupling caps. No difference. Not clear on your "bias" question. Currently set up for cathode bias, but makes no difference whether it's on or off... No other adjustment possible in standard setups...
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Old 12th July 2009, 10:27 PM   #6
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Quote:
Originally posted by ldflan
Correction - on the channel that gives little signal, the KT88 is dimmer than the other side.
If you swapped tubes, and the problem stays on the same channel, then you probably have a crummy connection in the socket. Try measuring the heater voltage on both sockets. If the heater voltage is too low, the filament won't glow brightly and the tube won't work correctly. It'll be too quiet, which is consistent with what you are seeing.
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Old 12th July 2009, 11:20 PM   #7
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Quote:
Originally posted by Ty_Bower


If you swapped tubes, and the problem stays on the same channel, then you probably have a crummy connection in the socket. Try measuring the heater voltage on both sockets. If the heater voltage is too low, the filament won't glow brightly and the tube won't work correctly. It'll be too quiet, which is consistent with what you are seeing.
I second the motion... check the solder on the socket and the socket itself. George had a really good suggestion of buying a cheap pair of plus 2 or better glasses that you can get at lots of stores. If you can't see any metal contacts (looking from the top of an empty socket) for either of the two heater pins you may have a bad socket.

What I've done before is to wrap a wire (thin guage) around the two pins of interest on the tube, put it in the socket and measure the resistance of the wire to the pcb pad. It should be nearly zero. If you are careful you can power up the amp this way and measure the voltage. You should see 6.3vac between the two filament wires.
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Old 13th July 2009, 01:56 PM   #8
ldflan is offline ldflan  United States
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Thanks for all the suggestions. I managed to trace it down to a short at the 220K resistor across the inputs. Not sure why, but it disappeared when I tore it all down and rebuilt it. Go figure, I guess.

I think I may also have a funky 50K pot on this thing. Strange readings off it, and one channel still a bit louder than the other. Am thinking of using one pot for each channel instead of a ganged pot. Any suggestions for good ones to use? 50K? 100K?
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Old 13th July 2009, 01:56 PM   #9
ldflan is offline ldflan  United States
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I guess I could be looking at two issues. The one short that I found, and perhaps a problem with the socket / heater voltage on top of that, with a lesser effect. I'll check that out, too. Thanks.
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Old 13th July 2009, 01:56 PM   #10
ldflan is offline ldflan  United States
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Yes, definitely a bad solder joint on the socket, too. Jiggling the tube, you could hear it come in and out. Touched up the solder, all sounds sweet and balanced now. Thanks so much for the guidance.

I bought a pair of Goodwill $5 bookshelf speakers so as to have something sacrificial to test the Simple SE on. (Sony SS-U3030s). I would never have thought such dinky, cheap little speakers could sound so good (when kept to the levels they can handle, of course). Can't wait to close up the chassis and hook it up to the real thing!
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