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Old 8th April 2005, 04:11 PM   #1
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Default compression?

Hi!

Today i got a visit from a friend who works as a sound engineer.....
I let him listen to my amp....the first thing he said was: the sound is compressed

(like altering it with a compressor which you use for P.A)
The only thing i can think of is....the coupling caps

(0,47uf/450v)

The pre-amp/splitter has been improved to the one in the schematic (5965)
The coupling caps never have been changed....but i have no idea if this is the problem or even wich value they should be

i could really use some suggestions
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Old 8th April 2005, 04:31 PM   #2
svokke is offline svokke  Belgium
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It could also be in the power supply. I model the power supply of my guitar amplifiers to get some compression. Ca you give us a picture of the b+supply?
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Old 8th April 2005, 05:03 PM   #3
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Default maybe under biased preamp tube also?

http://www.artsandmedia.com/5965.html

take alook at the link, those tests were done usually with 8ma per triode. this could be a combination of power supply weakness AND under biased tubes.

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Robert Morin
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Old 8th April 2005, 06:15 PM   #4
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here's the psu....
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Old 8th April 2005, 08:59 PM   #5
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I would suspect the 3k3 resistor. Any increase in current drawn by the output stage (on audio peaks) will increase the voltage drop across the resistor and reduce B+ temporarily .
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Old 9th April 2005, 01:07 PM   #6
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Hi there,

@martinab2 - i looked into that but i don't see the current go down at any volume or music style (the 82uf only has to deliver 10ma)

@robert morin - hmmm, you could have a point there...strange 5ma is way in the curved area of the graph....why could he have chosen this??
I have done a try at lowering the 39k resistors to 22k. (on both sides of the amp....)
Results....well...now there runs 7,9ma per side in the 5965
bias: -1,5
Va=145v

It did change somehow...but i can't decide yet if it's better...it sounds just like if the sound has to go through a 'curtain' before it reaches your ears

Today at a radio fair someone gave me another solution....
Try an ecc83 instead of the 5965

I only had 1 ecc83 but i don't think the results are accurate because the lm317 provides current for right&left channel.
It looked like the ecc83 was 'stealing' from the 5965 because its anode voltage dropped to 77v, the ecc83 rose to 235v...and began to run quite hot. the sound was'nt good either
Hopefully i can try with 2 ecc83's sometimes.

@svokke: could you find anything in the psu???
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Old 9th April 2005, 01:16 PM   #7
SY is offline SY  United States
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Sound engineer? Your problem is probably more related to the volume control knob and its setting. Sound engineers listen to things LOUD. A wimpy little tube amp in a home setting won't satisfy them.
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Old 9th April 2005, 01:45 PM   #8
jeapel is offline jeapel  Canada
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hi

nothing sure but maybe:

1-your friend listen all day very neutral transistor sound

2-change 3.3k by irf820 to create a small 300v regulator
like me easy to put inside your box

3-jump up 300uf to 500 or 600

4-remove c4 c5 = -gain but +linear just for see

5-upgrade 317 ccs to a good transistor ccs like c4s
but the reverse one version for cathode side
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Old 9th April 2005, 04:29 PM   #9
svokke is offline svokke  Belgium
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The powersupply looks fine to me.
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Old 9th April 2005, 05:54 PM   #10
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@robert morin

i tried biassing the tubes to approx 8,2ma (7,9)
I lowered the anode resistances from 39k to 22k
bias is now -1,56v
VA is 155v
7,8mA per triode

I also lowered the 3k3 resistors to 1k5

The sound seems a bit clearer but still has the 'curtain' effect....bass is MUCH less, busy music gets a bit garbled up
Hmmm,after some time these 22k resistors run quite hot

Maybe it's my speakers....

@jeapel

I'm going to do the ccs for sure, just have to save a bit money
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