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Old 24th June 2008, 12:17 PM   #21
oshifis is offline oshifis  Hungary
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If you put the grid resistor before R1, it will be parallel with R7 (and the internal plate resistance) IMHO. It shunts the input of the four-pole so it modifies the already low source impedance.
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Old 24th June 2008, 12:26 PM   #22
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Quote:
Originally posted by oshifis
If you put the grid resistor before R1, it will be parallel with R7

True. But this will lose you gain for no reason. Is it worth it just to simplify the calcs a bit?

Edit: i may be wrong about this; gotta go to gym now, will look into it later
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Old 24th June 2008, 02:07 PM   #23
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Well, this is the original one:
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File Type: pdf curcio sch.pdf (41.1 KB, 157 views)
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Old 24th June 2008, 03:06 PM   #24
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Thank you Carsten!
This clears things up considerably and shows that oshifis was dead on when writing that the caps are to high by a factor 10!
Looks like "somebody" made a conversion error from uF to nF....

This also shows me how to increase the gain, grat!
If I may ask, do you have the mentioned part-list as well?
Can you say what input voltages (before plate resistor) we are talking about?

regards
Dieter
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Old 24th June 2008, 03:28 PM   #25
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I actually remembered the 715k from Daniel but was lazy to look for my Audio Amateur stack. Built it soon after the article came up and didn't like it much, but no doubt the riaa is correct. Apart from the cap values it also has 10M instead of 2.2M (parallel is 667k). Whoever built your pre was quite clueless.

Btw, there are two sets of values for the cascode resistors, so changing the 390R to 100R increases the gain substantially.
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Old 24th June 2008, 04:32 PM   #26
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Do you know of any better design using two 6DJ8 or point me in the right direction.
I have a lot of old CCAs from Siemens (1962) and would like to use a proven high quality circuit for my phono-amplification.

Another amateur question: I made some Simulation with B2 Spice (downloaded yesterday evening) and these values seem to be better suited (attachment). Am I wrong, respectively can I apply them to this circuit or is the one form Audio Amateur just fine?
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File Type: jpg riaa netzwerk.jpg (85.5 KB, 343 views)
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Old 24th June 2008, 05:42 PM   #27
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75k makes it more dificult to ignore the output impedance of the cascode. To get an exact value is not that simple and the best route is to actually measure the particular tubes at the operating point. In any case practical values are usually a compromise as getting precise values with a good sounding dielectic is not trivial.
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Old 24th June 2008, 07:20 PM   #28
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by dikarner
Do you know of any better design using two 6DJ8 or point me in the right direction.
http://vacuumstate.com/various/SP-15_Article.pdf
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Old 30th June 2008, 09:33 AM   #29
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Hi all,

I just wanted to report back the final results. The picture shows the circuit as it is now. Don't get fooled by the curve, the variance is within 0.3db
Now it sounds as it should be, even the gain is sufficient.
Thanks for all your help, I learned a LOT!

BR
Dieter
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File Type: jpg tmt phonosection.jpg (98.7 KB, 303 views)
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Old 30th June 2008, 09:50 AM   #30
oshifis is offline oshifis  Hungary
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I have just a small problem with this. According to the Philips data sheet, the grid resistor should be 1M or less. Also the position and value of C4 seems a bit strange for me. But if it works...
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