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Old 25th November 2012, 02:55 AM   #1
arcorob is offline arcorob  United States
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Default Help with a Velleman K4040 ...

Hello, I know this is not a FAV amp for many but putting that aside, I hope someone can help me fix two problems.
BZZZ in the left channel (faint but there) 120hz type ..like standing under a streetlight at night
High gain hiss, crackle in right channel - again low, neither can be heard 2 feet from the speakers but I DO know they affect the overall sound.

Whats been done
Switched tubes - Currently running NOS GE 6L6GC's (nice sound with them)
Its been biased.
The R89-R96 have been changed to 1kohm 5watt as per another thread here. - No change
Channel path that goes from the right side in front of the transformers and such that feeds the left channel has been jumpered so its not running right past. - No change
Power to the unit is running through an ADC power regulator/filter so power is clean.
Occurs with no input connected.
Pulled the 12AU7 tube and its still there.
Pull the 12AX7's and off course it goes away but thats because there is no feed.

Bottom line, whats next ? What can I check/change/replace to keep myself from going nuts ?

Just so you know, I am a fairly new tube/electronics newbie so speak slow...LOL

Serious, I know someone out there must know these things or knows how to read the schematic and find where I should look...

Thanks in advance !
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Old 25th November 2012, 11:31 AM   #2
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Since this is a kit amplifier I have to ask this. Did you build it? If so has it ever run correctly? Did you buy it already running or did you buy it non-working?


Need information here in order to try to help. Might try posting a schematic of it as well as the working voltages. I suggest that you do not start off swapping tubes and modifying a non-working amplifier. Get it working first have proper working voltages and then modify one step at a time.
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Old 25th November 2012, 02:10 PM   #3
arcorob is offline arcorob  United States
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Default Well, guess what..

I fixed it (95% anyway)

One of the other steps was right from the manual - to increase the resistance value on 2 resistors if you hear bzzz/crackling or oscillation..so change those out last night after I wrote this ..

Buzz on left gone...99% dead silent.
Gain hiss crackle on right gone - replaced by very very low hum..think thats chassis/ground hum of some kind but ALOT more pleasant than gain hiss crackle...lol

SO now I will hunt this down..

Oh, by the way, I bought it this way, I didn't build it...
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Old 26th November 2012, 12:09 AM   #4
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Glad you got it running
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Old 5th December 2012, 02:42 AM   #5
arcorob is offline arcorob  United States
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Default Well...almost fixed.....HELP !!

Okay..got rid of 99% of the buzz, 99% of the hiss...awesome and the amp sounds great BUT...here is the issue. When I measure the 6.3vdc taps, I get 3.5vdc on all 4...huh ? whats up with that...since there is nothing else in the circut at that point, can it be a bad transformer ?

If it is, how the heck do I replace it ? I can;t find any specs for the power transformer except the 6.3v taps..is it bad to run like that ?

Thanks in advance...
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Old 5th December 2012, 08:22 AM   #6
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A transformer outputting DC would be strange to say the least.

couldn't it be that your transformer has centertapped 6.3volt windings ?


v4lve.
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Old 6th December 2012, 04:25 PM   #7
arcorob is offline arcorob  United States
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Default I was measuring wrong

Forget what I said...lol
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Old 8th December 2012, 11:37 AM   #8
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Location: Alps:Tube amp designs over 150W, SMPS guru.
Many Hifi'ers in the past have given me 4040's to fix...some easy, some awkward faults..I have to say it, some have taken on far more they can handle, often not understanding.. after inviting many owners into my shed, I've managed to entice them into a complete redesigned better sounding amp..guaranteed happyiness .
It's often better to 'put an old dog down' than spending monies on a half-working limping amp which becomes a constant source of hassle.

What I don't want to see happen is exactly in the pic. Fortunately, tube amp incidents are rare, but it is festive time. TAKE CARE. Fusing is all important in the design.

richy
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Old 8th December 2012, 01:42 PM   #9
arcorob is offline arcorob  United States
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Default Thank you !

I would put this dog down but to be honest, I have too much invested in tubes.... (over a grand). And she is the amp from hell.

I had this puppy sounding awesome. Made a minor tweak. Went to put the covers on, hum. Took them off, no hum, Repeat twice and hum was gone. So was 30% of the sound in the lft channel. I spent 9.5 hours last night till 1:30 am reworking, etc. and FINALLY got it right again, covers on an all.

I DONT want to to touch it anymore ...serious. I am sick of the journey. The sad part ? It sounds so good ! Very hard to let go...If I didi I would have to find an amp that would appreciate my 6L6GC's...

Thanks again...
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Old 7th February 2013, 05:14 PM   #10
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Richwalters,
A few friends and I are looking at tube amp to build and saw the Velleman K4040. Which "complete redesigned better sounding amp" are you talking about in your reply above?
Regards,
LG
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