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Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
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Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
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In Reply to: paramours + c4s = outstanding sound posted by berlin on September 22, 2001 at 06:43:05:
I posted a while back on this (see below); basically 180-200v is the best value at the plate of the driver. My previous post probably went into a little too much detail as I was still working it out in my head.My recommendation now is that the driver plate voltage (B6) should be 2/3 of the top of the C4S ("strip"). Design target values are 200v and 300v.
The lower value of 155 is what was measured on the first few prototypes of the C4S upgrade, and is typical for many common 12AT7s. It works well; there is a fairly wide range that is satisfactory. It turns out that the C4S circuit is much more sensitive to tube parameter variations than the original SRPP was, and a few tubes will go outside the acceptable range. Of course, swapping in a different tube is a workable solution in some cases, if you have a few different versions laying around.
The only real problem occurs when the driver plate voltage is too high, in which case there is not enough compliance. Compliance is the difference between the top of the C4S ("strip") and the plate (B6). The compliance should be at least 70v in order to drive the output tube to full power. There are a few 12AT7 types out there which have this problem, and you can generally fix it by either decreasing the cathode resistor, or increasing the C4S current set resistor which will reduce the C4S current.
Several posts have covered the reduced cathode resistor, but I don't remember any on modifying the C4S. There are two 499-ohm resistors in parallel on the C4S board. I recently helped with a Paramour that had this problem, and one of the 499-ohm resistors was replaced with a 1000 ohm resistor which was satisfactory in that case. YMMV!
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Follow Ups
- Don't mess with it... - Paul Joppa 10:22:08 09/22/01 (1)
- Re: Don't mess with it... thanks Paul! <nt> - berlin 17:10:41 09/22/01 (0)
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