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Old 15th April 2008, 04:21 PM   #1
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Default Problem with Stealth clone (12A6)

After almost 5 years I finally soldered the very last cables in place in my Stealth clone (Mapletree Audio Design).
The amp that is NOT suppose to be visible (metal tubes - 12SJ7 + 12SC17 + 12A6), was slowly "heated up", all voltages checked etc etc and music added to the inputs. However - one channel was hardly audible, so I started swapping all I could swap - tubes, connections to the volume pot, input connectors and loudspeaker connectors. All components rechecked for the fifty-eleventh time (that's how we express it in Sweden), but still one channel was much quiter than the other.

HERE IS THE SCHEMATIC. It has been available on MADs but just in case, I publish the one I downloaded a few years ago.

One note on the schematic: I have built my amp as a stereoamp with a "common" PSU. But I am using ONE transformer feeding a diode bridge and ONE filtercap, which feeds two "ladders" - RC-networks - like the one on the schematic, one for each channel.

I hate faultfinding, well I love finding the faults but hate looking for them, but anyhow, I will try to swap between the two networks in the PSU and next swap OP transformers. I also faintly suspect the caps.

But at this very moment, before I reach for the jackhammer and chainsaw - where could I possible find the fault?
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Old 15th April 2008, 04:59 PM   #2
kevinkr is offline kevinkr  United States
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Do you have any test equipment at all? Sounds like you are using a diagnostic sledge hammer approach to trouble shooting, what you need is a little finesse.

Simple meter measurements will probably tell you quite a lot. Start by measuring and comparing all of the dc voltages between the good and bad channel. (I hope you have a dvm at the least.)

Check to make sure you have no obvious wiring errors. Check that filament voltage is present on all sockets on the affected channel.

Make sure your output binding posts are not shorting to chassis - I've seen a few newbies make this mistake with very amusing results.

Yes output transformers can be bad, but this is the LAST thing you should check by substitution - examine all of the easier things first as this is the least likely.

Finally double and triple check to make sure that you have made no wiring errors. (And yes I am being redundant.)

In a pinch have someone else also look at your work to see if they notice something you didn't.
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Old 15th April 2008, 05:06 PM   #3
kevinkr is offline kevinkr  United States
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That's one very wimpy LTP splitter implemented with the 12SC7, not even 1mA of plate current total.

Tell me what you have for test equipment and I will think about leaving additional clues.
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Old 15th April 2008, 05:20 PM   #4
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I am using a good ole FLUKE 8012 DMM. I am measuring all the voltages as indicated in the schematic and they are all OK within a few %.

I have an oscilloscope but no probes ... , haven't got that far yet.
I have measuered all resistors, making sure they are the same values and in the right positions.

Filament voltages ... sir! yessir!!! Haven't checked that. Normal glass tubes glow orange so lovely but the metal ones doesn't (at least not visible).


(Why did I go for this design? I wanted to - as simple as that. I wanted a 5W PP-amp in a sort of different design. I had just completed my 6550SE, a GU50-SE and a 50CA10-SE.

Click the image to open in full size.
)
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Old 17th April 2008, 01:44 PM   #5
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I have been very methodical when looking for the fault. Disconnecting one channel and step bu step swapped tubes from RH channel to LH. Then swapped the connections to the PSU, which is sort of split for two channels.
Swapped connections to the potentiometer - that's an obvious source for problems of this kind.
Finally I swapped OP transformers and as the faulty channel still was faulty I guessed I had managed to double-swap the transformers and installed them as they were originally, which indicated a faulty transformer, so simply installed another set of transformers and ... tada! ... there was still a silent raspy noise coming from the faulty channel.
At this point it is a good idea to throw the amplifier out through the window (not opening the window of course), but I just wanted to check ONE little detail I hadn't checked and I found the fault ....


.... a small piece of metal splinter made a slight cotact between one speaker connector and chassis ....


Now I have learned NOT to look for the obvious.
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Old 17th April 2008, 01:51 PM   #6
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Quote:
Originally posted by kevinkr
<snip>

Make sure your output binding posts are not shorting to chassis - I've seen a few newbies make this mistake with very amusing results.
Quote:
.... a small piece of metal splinter made a slight cotact between one speaker connector and chassis ....
Glad to hear you did finally find the problem..

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Old 17th April 2008, 02:10 PM   #7
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Me to.

There will be a DIY-meeting in Gothenburg, Sweden on Saturday and I JUST HAVE TO be ready with this amp. Now I wasted 2 days finding the fault so I propably have no time finish the 2A3SE ...
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Old 17th April 2008, 03:23 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by soundbrigade
Me to.

There will be a DIY-meeting in Gothenburg, Sweden on Saturday and I JUST HAVE TO be ready with this amp. Now I wasted 2 days finding the fault so I propably have no time finish the 2A3SE ...
I predict a couple of all-nighters in your immediate future.. We've all been there, last event I hosted one of the fellows brought a pre-amp that he had just finished, it hadn't even been powered up. We fixed various problems during the course of the day, some of which were quite shocking, literally..

Good luck!
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Old 17th April 2008, 03:43 PM   #9
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I will try to fix some kind of report from the meeting. We have 7 rooms to our disposal so there will be quite some material - speakers and amps.

It's easy to look for complicated problems - broken tubes, faulty and shorted transformers, resistors, caps and diodes.

I had just finished a pair of loudspeakers and couldn't get one of them to play. After some thinking and checking I found that I had managed to short the connector. That made my amp (6AQ5-PP) very quiet - but I have revived it but something is still faulty as it started to play out loud, no matter how the volume was set ...

Well that is next thing to do - RIGHT NOW!
Bye ..
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