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Old 21st February 2005, 07:48 AM   #1
sajti is offline sajti  Hungary
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Default DC coupled long tailed pairs

Hi,

is there anybody who used DC coupled longtailed pairs to drive PP output?
I want to use E88CC+ECC99 to drive two pairs of 6550s, triode connected. My problem is the DC coupling. This configuration will be stable for long time? Large offset in the first stage results distortion, and unbalanced bias in the second stage.
Any schematic would be great too.

Thanks!
sajti
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Old 21st February 2005, 08:15 PM   #2
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Yep, so you need DC feedback to stabilize it.

Better to use coupling caps instead. No odd voltage supplies too.

Tim
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Old 21st February 2005, 08:18 PM   #3
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Maybe these schematics can help you.

http://www.wdehaan.demon.nl/el84dc.html

Regards stigaard
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Old 21st February 2005, 08:51 PM   #4
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Quote:
Originally posted by stigaard123
Maybe these schematics can help you.

http://www.wdehaan.demon.nl/el84dc.html


That EL84 circuit....the front end directly coupled to El84's looks if it is running in starvation mode......omo.. I'm against these circuits as they drift with time. I find thd rather high for hifi.....who would agree ? UL with feedback would prune that down quite nicely. The designs look eaSY to build.

richj
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Old 22nd February 2005, 02:41 AM   #5
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Quote:
Originally posted by richwalters



That EL84 circuit....the front end directly coupled to El84's looks if it is running in starvation mode......omo..
richj

I just realised my machine didn't download all the schematics....infact jammed up on the 1st one so I never saw the neg rail....that changes all....can happen to anyone.
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Old 22nd February 2005, 03:06 AM   #6
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Quote:
Originally posted by sajti
is there anybody who used DC coupled longtailed pairs to drive PP output?
Take a look at this one, which I found on the web:
http://www.dogstar.dantimax.dk/tubestuf/driverpa.htm
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Old 22nd February 2005, 04:01 AM   #7
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6L6's at 600V........ 5881's would sweat also in UL...no screen resistors ?..rest circuit looks similiar to my trumpet amp except using 807's in true tetrode..but 75W...these can take 600V with a breeze. Only personal choice as sound is far sharper and one hears more wind harmonics with these beautiful RF tubes.

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Old 22nd February 2005, 04:34 AM   #8
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Yep-- Fred himself (RIP) said it worked great - just don't cheap out on the 6L6s.

Go figure, he was always more a fan of 807's

Tim
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Old 22nd February 2005, 08:29 AM   #9
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Default dc coupled

Think Sajti means a dc coupled driver stage.

ARC is doing so in their CA 50, two diffamps (6922) working on current sources. Not a good idea, tube matching is critical and with aging the driver is running out of symmetry, more distortion.

Reinhard
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Old 22nd February 2005, 09:23 AM   #10
sajti is offline sajti  Hungary
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Default Re: dc coupled

Quote:
Originally posted by reinhard
Think Sajti means a dc coupled driver stage.

ARC is doing so in their CA 50, two diffamps (6922) working on current sources. Not a good idea, tube matching is critical and with aging the driver is running out of symmetry, more distortion.

Reinhard
Yes. I think to use DC coupled driver stage. And yes, ARC use it. Somewhere with servo (in VT150), somewhere without servo (VT100). I will put the schematic I planned, to see it!

sajti
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