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Old 27th August 2008, 12:56 PM   #131
Salas is offline Salas  Greece
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Also if you keep the feedback cap as it is, its good to include the 4.7mF.
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Old 27th August 2008, 05:36 PM   #132
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Hmm..

I'm starting to wonder if its a phase issue in the feedback path due to the 90' phase shift of current due to R and C being in series. I must say tho i've just got home and using a graphic equaliser i've rolled off the frequency responce from 250Hz down and the amp sounds more normal now. Maybe it is the cap thats too small causing an unnatural rise in bass frequencies. I cants see why tho as the scope tests didnt reveal any changes in amplitude from 10Hz to 200Khz and the calculated time to charge the feedback cap is just over half a second.

I still havn't found out is grid current flowing could be a major problem either... I'm busy trawling the net to see if it reveals any clues....


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Old 28th August 2008, 09:37 AM   #133
Salas is offline Salas  Greece
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I am suspect of the 4.7mF output cap being outside the feedback loop, for the subjective ''thickening'' of the sound (if I got right what you experience). Just take the feedback point after that cap and make a before/after listening comparison. At least you can eliminate that scenario if it does not make a sonic difference.
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Old 28th August 2008, 09:44 AM   #134
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Salas,
My thoughts are that i wont hear any difference but i will do it when i get home tonight. There is currently a 100n poly bypass cap accross the 4700u output cap. Your right tho, i cant just rule it out without trying it so i'll do it tonight and see if it makes a difference. I found info about grid currents and tube biasing but its a good 40 pages worth. I'll be absorbing that info over the next few nights as a bedtime reading session. It's proved quite revealing so far..

As for the sound the amp sounds a lot more natural when i implement a low frequency roll off so it does indeed seem like somthing is going on at the bottem end of the spectrum in the amp. Highs are still amazeing me tho.... At least i did a good job of half of the spectrum LOL


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Old 28th August 2008, 10:34 AM   #135
Salas is offline Salas  Greece
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OK, do that tonight. The main idea is to eliminate possible causes one step at a time so to avoid confusion, starting from the easiest ones.
I am sure that you will fix it and learn a bunch in the end. Only be sure that it is not something in the rest of the system or speakers and acoustics. Better listen to the amp in a friend's system if possible before going deep. Just to be sure there is truly something objectionable in the amp itself.
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Old 17th September 2013, 03:02 PM   #136
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Default Update...

Just an update to this ancient thread. A lot of builds come and go and you never get to know how things turned out in the end or if the build in question has survived the test of time, so i thought i'd fill you all in on how things are now. You'll be pleased to know ( I hope ) that all is still well. The amp is now 5 years old and has been in constant use every day since conception. I use it for both my music and telly viewing so it gets about 3 to 4 hours use every single day. As far as performance everything is still operating and sounding like it did 5 years ago. I checked the biasing of the valves recently and found they are all still as first set, no signs of aging or emission fall off at all! Sound is still razor sharp, still consider this amp to be the best sounding i ever built.....

however that title may soon change as i'm currently working on a little brother for this amp, a 100% tube version using EL34 PP as outputs.....

Hope this update is of interest to you all... i'll update again after 10 years

Happy building...

Leigh
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