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Old 17th October 2010, 08:24 PM   #1
Divad89 is offline Divad89  Denmark
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Default Simple SE - very low volume

I've just assembled the Simple SE in the chassis (and not just on bench test), but I think its output volume is very low, have tried with 6L6GC and 6550 tubes, and with/without UL mode, but it is still very low. I've just tried to measure the resistance between the positive and negative output poles on both channels, and it shows less than 1 ohm, so there has to be a shorting somewhere, couldn't that be the problem, and where should I look? Because it also had low volume when bench testing...

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Old 17th October 2010, 08:50 PM   #2
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Originally Posted by Divad89 View Post
I've just assembled the Simple SE in the chassis, but I think its output volume is very low...
I would consider the Simple SE as a relatively high gain amplifier. Mine tends to get very loud rather quickly on low settings of my preamp volume control. Sometimes I wish it were quieter. I would say your experience is not usual.

If you are trying to measure the DC resistance of the output transformer's secondary winding (i.e., checking between the red and black speaker binding posts) I would expect you should see at least 100 ohms. I wouldn't be surprised to see as much as 500 ohms with some transformers. If you've got less than 1 ohm, I'd agree - you've inadvertently shorted something somewhere.

Check the DC resistance from the red post to chassis ground, and from the black post to DC ground. Tell us what you find.

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Old 17th October 2010, 11:16 PM   #3
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I've just tried to measure the resistance between the positive and negative output poles on both channels, and it shows less than 1 ohm........I would expect you should see at least 100 ohms.
A reading of less than one ohm across the speaker terminals is completely NORMAL. I measure My Simple SE at 0.6 ohms. If you take an OPT apart you will find that the secondary is made of about 25 feet of heavy wire. The primary however should be 100 to 400 ohms.

There have been a few builders who have had problems with low volume. all but one were traced to shorted input or speaker jacks. The lone oddball was caused by the builder drilling out the holes in the PC board to fit larger leads. This destroys the plating inside the holes which makes the connection from one side to the other.

Did the board work OK on a bench test before final assembly?
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Old 18th October 2010, 05:34 AM   #4
Divad89 is offline Divad89  Denmark
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I've just discovered when going through Ty_Bower's checkout again, that the 220K grid resistor on 12AT7 (pin 2 and 7) only is reading out just about 100ohm, could that be the problem?

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Old 18th October 2010, 07:11 AM   #5
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A reading of less than one ohm across the speaker terminals is completely NORMAL. If you take an OPT apart you will find that the secondary is made of about 25 feet of heavy wire. The primary however should be 100 to 400 ohms.
Of course. Sorry for making such an obvious mistake. No idea how I mentally swapped the primary and the secondary.
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Old 18th October 2010, 12:25 PM   #6
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I've just discovered that the 220K grid resistor on 12AT7 (pin 2 and 7) only is reading out just about 100ohm
Did you put a volume control in your amp? That might interfere with your checkout. Otherwise, if you've done something like inadvertently swap R11 and R12 (220K for 100 ohm) then maybe you're loading down your source. That might cause low volume.
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Old 18th October 2010, 05:52 PM   #7
Divad89 is offline Divad89  Denmark
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Yes I did put in a 47k volume pot, so it is probably why it measures 100 ohm...
But I haven't found out the cause of its low volume.

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Old 18th October 2010, 06:02 PM   #8
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Yes I did put in a 47k volume pot
If you look at the schematic, you can see the volume pot should be in parallel with R11. Depending on which way the pot is turned, it can affect your measurement of R11. I assume you are checking from the grid pin of the 12AT7 socket. Turn the pot all the way towards quiet, and you'll probably measure 100 ohms. Turn the pot all the way towards loud, and I'd expect you would measure ~38.8K ohms (47K in parallel with 220K, plus 100 ohms for R12).
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Old 24th October 2010, 08:05 AM   #9
Divad89 is offline Divad89  Denmark
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Could the low volume be caused by the 12AT7 tube?

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Old 25th October 2010, 04:02 PM   #10
rknize is offline rknize  United States
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It could, but if it is a new tube that is less likely. I think we need to look again at your input wiring. Can you re-check the parts in the R11/R12 positions (read the stripes). Also detail how you wired in your POT. Take a close-up picture of that area of the PCB, if possible.
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