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Old 29th March 2008, 09:17 AM   #1
waam68 is offline waam68  United Kingdom
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Default Tubelab simple SE help needed...working but only on one channel

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This is my first attempt at building an amp (Tubelab simple SE). I'm very much a noob & have been working on this on & off when work has permitted over the winter. Got to the stage of firing it up last weekend & I was surprised how great it sounds BUT only on ONE channel. The speaker outs nearest the RCA inputs produce only a very faint tinny bit of music out of the speakers. I have tried touching up the joints, connecting & re connecting wiring.

The screws at the primaryside of the output transformer terminals all read 460v, a bias probe kit I have shows 61.3mv per tube using EL34's & a 560ohm bias resistor. Solder pads next to C12/22 read 75v AC & 34V DC. Resistance measured between the +ve & -'ve of each speaker terminal reads 0.6ohm on the "good" side & 0.2 ohm on the bad. I de soldered & replaced the Coupling caps that are russian surplus PIO.

George has been helping but I thought I would throw it open as he's pretty busy with work etc. I took my time with this but could easily have done something quite dumb! The supplemental cap & mode switch wiring hasn't yet been wired up until I can get it all working.

Does anyone have any ideas of likely areas to focus on? All help greatfully received.
Thanks
Mike
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Old 29th March 2008, 05:39 PM   #2
dubdub is offline dubdub  United Kingdom
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Im a noob myself so I think my input might be limited-have you tried swapping the valves around? the fault could be with the valve.

It sounds like youve gone over the board again-double check the wring from those edcors?

Do you have any dry joints? My guess this could be most likely be the problem


Keep us posted!
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Old 29th March 2008, 07:47 PM   #3
John L is offline John L  United States
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I too am interested.
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Old 29th March 2008, 10:05 PM   #4
waam68 is offline waam68  United Kingdom
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It's not the tubes, I've swapped around & between sets & changed the preamp tube. There is music through the faulty channel but very low & distorted. I've tidied up the joints as best I can. I'm assuming that if I can hear music but very low the input wiring must be ok? Something is out of whack but I need a steer on the likely suspects.
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Old 29th March 2008, 10:37 PM   #5
w5jag is offline w5jag  United States
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Are your inputs wired to a switch? Jump around that and see if that resolves your problem. Also, a poor connection of the driver tube to the socket can cause weak distorted output on one or both channels as you describe.

Pull the 12AT7 and make sure the tube pins are clean and making a good fit into the socket. Rocking the tube around sometimes helps to isolate a tube to socket problem.

Good luck.

Win W5JAG
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Old 30th March 2008, 05:35 AM   #6
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I'd check to see if a whisker of wire or solder has shorted the input to the quiet channel. The faint noise you get could be crosstalk from the working channel.

Been there, done that.

Bill
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Old 1st April 2008, 02:34 AM   #7
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Hmmm. If you have any jumper wires try bypassing the volume control? Your ground it the yellow wire right? Try moving that to the other side of you volume pot. I can't see the input wiring very well.

Wild guesses on my part.
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Old 1st April 2008, 03:32 AM   #8
chrish is offline chrish  Australia
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Did you make sure that your input and output connectors were isolated from ground?
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Old 1st April 2008, 01:56 PM   #9
waam68 is offline waam68  United Kingdom
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Hi All
thanks for the response. I havent had too much time to work on the amp but the other half is out of town for a couple of days so I may get back to it. There are definately no solder whiskers. I have noticed that the chassis is just touching the back of the 12AT7 where it isn't quite clearing the hole I made for the tube. I will file out the hole a little tonight, that said the tube feels secure in the socket but may not be. The ground is the yellow wiring although I have since re wired the whole input & speaker sides & replaced the pot for a spare I had lying around (all to no avail!). Chris..I'm not sure what you mean about grounding, the inputs are on white plastic washers & the speaker connectors on plastic bases so I'm assuming this means they are isolated from ground? . I will try & post pics of the new input wiring soon. I have tried jumpers on the centre pins of the pot & swapping wires but so far nothing has gotten the 2nd channel up & running.
In the meantime a friend who is an audio engineer (but not local) has suggested trying this:
1 Check the voltages on the anode grid and cathode of the output valve compare the working channel with the duff one.
Next step is pull out the driver valve and using a small screwdiver touch the grid connection of the output valve you should hear a buzz on the loudspeaker if so the fault is in the driver !
plug in the driver valve and then check the anode/cathode voltages they should be the same.
It's frustrating as it sounds really good & surprising amounts of Bass from the little Edcor's I went for 5k 15 ohm as my speakers are vintage.
Mike
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Old 1st April 2008, 03:06 PM   #10
chrish is offline chrish  Australia
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G'Day Mike,

Might not be the problem, but when constructing it is a good idea to do a continuity test on things like the input and output sockets before connecting any wiring to them. That is, use your multimeter to make sure that there is no short between the sockets and the chassis. Sometimes those plastic spacers are not that great at preventing contact if things shift around while you are tightening them up.

How have you wired it up? Basic triode mode? I would suggest doing this to eliminate external wiring as the culprit. If you still have problems, take out the board and check for solder errors (bridges etc) - you may have done this. Double check all components are soldered in to the correct place (double check with a meter). Sometimes you can get a resistor value wrong by a factor of 10 or 100 or 1000 etc. Double check that you have installed the constant current devices correctly. Can be confusing with above/below board mounting. If you are new to soldering, you might have damaged the constant current device with too much heat or static electricity. Someone else might be able to give you some advice on measuring if that is the problem.

That's all I can think of at the moment,

Good luck!

Chris
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