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Old 16th August 2008, 02:00 AM   #1
Zap is offline Zap  United States
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Default Further Troubleshooting Simple SE

Hi all,

The Simple SE has been playing nicely for a while again, so I suspect (and believe my tester shows) a bad 6550 tube. However, earlier today I decided I wanted to listen in UL, and discovered static on one channel of the amp (the one I believe red plated with the bad 6550). The really strange thing I just noticed now that its darker in the room is switching to UL makes the blue glow in the affected channel disappear! (This is running EH KT88's known for the harmless glow effect.)

That can't be a good thing. If anyone might suggest what would cause static and the glow to literally vanish going into UL and return going into Triode, that would be helpful. I am wondering if this is a bad component manifesting itself now or somehow a wiring issue (though UL worked great before the tube issue).

Thanks
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Old 16th August 2008, 02:30 AM   #2
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Check the cathode voltage of the tube in glow mode and non glow mode. Is there a difference? Check the screen grid voltage in the two modes. Check the screw terminal and the switch wiring to the screen grid in the affected channel for loose connections. Those screw terminals can work loose after a while, tighten them all.

It is possible that the screen grid resistor (100 or 150 ohms from the screw terminal to the screen grid pin) got zapped when the tube failed and is just now showing up. It may check OK with an ohm meter but break down under power. I have managed to blow a couple of them in this fashion. You can see them frying in a dark room. There are little sparks on the resistor surface accompanied by a crackling sound in the speaker.
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Old 16th August 2008, 03:08 AM   #3
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Blue glow is weird stuff. Ignoring gassy tubes and all the other bad reasons why it happens, I think blue glow occurs when the electrons go zinging through the vacuum so darn fast they fly right past the plate and bonk into the glass on the other side.

My 6pi14pi tubes glowed blue when they were brand new, but now they don't do it so much. Only when I first turn on the amp with the standby switch open, then flip it closed and hit them with greater than full B+. They flash blue for a second, then the power supply gets pulled down to normal working voltages and the blue fades.

Maybe in triode mode there's a good potential (hah!) for getting the blue glow. When you switch to UL, and the screen is at a different voltage the electrons tend to take a different path through the vacuum and the glow doesn't happen.

Do what George says, and check your screen resistors. If they check out OK, I'd be inclined not to worry about it too much.
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Old 16th August 2008, 03:25 AM   #4
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I have the same set of EHKT88's in the amp now that were there when I took the first glowing pictures. They don't glow like they used to. They are about 2 years old now.
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Old 29th August 2008, 03:16 PM   #5
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Just an update to this thread - I went ahead and tore the amp apart a few days ago. Somehow one of the UL tap wires came undone from the switch!! I hooked it back up and after realizing that I knocked a ground loose on the stepped attenuator, everything started working great again.

I also added an NTC thermistor on the CT to slow the B+ application. So far everything looks and sounds great. I almost wonder if the input ground was sketchy before because the amp almost seems cleaner. There is NO noise whatsoever from low 90's speakers when there is no input.

I'm still impressed with my Simple SE every time I listen to it, and I'm planning on trying the Silver Iris speakers eventually which should only improve things.

Thanks again George, and hope everything gets better for you with all the troubles you've come across lately.
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Old 29th August 2008, 08:01 PM   #6
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I have a question for ya'll. Last night I decided to play with fire and I finally wired up the switches for UL/Triode and the feedback switch. The UL/Triode switch works like a champ, but the feedback switch is like switching between mono/stereo. I've stared at the diagrams for about an hour or so. It would seem that I would have to wire the secondaries backward as explained by the instructions.

Only thing is that I've got the negative terminal on the speaker output connected to earth ground.

How do ya'll have yours wired?

I also installed a CL-90 thermistor and I've put an order on some new FREDS.

On another note my SED EL34's glow blue. The Phillips ECG 6GB6GA's I'm using now have some crazy blue glow! I believe the glow has something to do with electrons bumping into the glass. I would guess impurities in the glass phosphoresce as a result? Anyway, I've not associated it with gassiness, although I suppose if your tube had air in it the air would phosphoresce as a result.
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Old 29th August 2008, 08:32 PM   #7
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Funny coincidence. I did something similar myself last night. I re-wired mine for triode / no CFB. I'm now a big fan of triode mode. If nothing else, it puts the overall gain of the amp right where I want it. UL was too loud, if that makes any sense. I sure George wouldn't approve.

I didn't install switches, so I'm not sure I can help with your dilemma. I hope to get switches in there someday, but I'm not there yet. Regardless, eliminating cathode feedback should be dead simple. Just hook a jumper wire from T-SEC O1 to T-SEC O2. If you want to do both channels on a single switch, you'll need a DPDT switch. Wire the poles to T-SEC O2. Wire one throw to T-SEC O1, and the other throw to the transformer's secondary wire (green in my case). Make sure you don't accidentally cross wire between T2 and T3.

Yes, you probably do need to wire the secondaries "backward" as you say. You don't want whatever wire is carrying the signal to the speaker binding post to get inadvertently shorted to ground. I doubt it'll damage the amp, because tubes tolerate shorts much better than SS amps. But, it won't sound right at all.

CL-90s are good. I have one which I intended to install some day. I also bought a pair of FREDs, but I doubt I'll ever put them in.

Out of my pair EH 6CA7, one has a nice blue glow and the other doesn't. I'd love to order a couple of the 6BG6-GA, but I can't get over the "startup cost" for the required adapters plus shipping. It's only on the glass as you have noticed - there's nothing in the vacuum space. I'm pretty confident the vacuum is tight. My 6p14p tubes do the same thing. It tends to fade after a couple years of usage, or if you run them at lower voltage.
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