question on differential stage

vega65 · 17th December 2008, 06:34 AM

At the moment I am working on a project with 13E1. Of course I had a closer look on MJ's crystal palace.In the second diff stage there is a 1,8M resistor (attached drawing) from grid to b-.
I do not understand the reason for such a large value here.Can anyone explain?

gingertube · 18th December 2008, 02:07 AM

You want to use as large a resistor as possible so that you can use a smaller coupling capacitor from the previous stage while maintaining the low frequency "corner frequency".

FL = 1/2piRC.

If the R is bigger the C can be smaller for the same corner frequency.

This is mostly a cost saving thing - really good audio caps in smaller values are much cheaper. I say "mostly" as it is also true that smaller caps tend to be better quality and sound better than larger caps anyway.

The maximum value of that resistor can usually be found on the tube data sheet - as the max Rg1 value, which will be determined by the amount of grid current the tube is likely to exhibit.

Cheers,
Ian

vega65 · 18th December 2008, 04:55 AM

Thanks Ian
I should have added som more information in the first post.
I made the simple drawing as I didn't want to violate the copyrights of Mr Jones, by posting the schematics or part of it.with the drawing.It should be possible to find the answer without the schematics at hand.

The drawing is the driver stage, ac coupled from the input/ltp splitter, dc coupled to a cathodefollower.

As you say one wants as large as possible resistor.And the maximum value is usually found in the datasheet.

Point here is that MJ used a resistor 1,8 m or 800K larger than the maximum value given in the datasheet for the actual tube.

I guess he had a reason for superseeding the given max value.And I wonder what that reason was.

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18th December 2008, 02:07 AM	#2
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	You want to use as large a resistor as possible so that you can use a smaller coupling capacitor from the previous stage while maintaining the low frequency "corner frequency". FL = 1/2piRC. If the R is bigger the C can be smaller for the same corner frequency. This is mostly a cost saving thing - really good audio caps in smaller values are much cheaper. I say "mostly" as it is also true that smaller caps tend to be better quality and sound better than larger caps anyway. The maximum value of that resistor can usually be found on the tube data sheet - as the max Rg1 value, which will be determined by the amount of grid current the tube is likely to exhibit. Cheers, Ian

18th December 2008, 04:55 AM	#3
vega65 diyAudio Member Join Date: Nov 2007	Thanks Ian I should have added som more information in the first post. I made the simple drawing as I didn't want to violate the copyrights of Mr Jones, by posting the schematics or part of it.with the drawing.It should be possible to find the answer without the schematics at hand. The drawing is the driver stage, ac coupled from the input/ltp splitter, dc coupled to a cathodefollower. As you say one wants as large as possible resistor.And the maximum value is usually found in the datasheet. Point here is that MJ used a resistor 1,8 m or 800K larger than the maximum value given in the datasheet for the actual tube. I guess he had a reason for superseeding the given max value.And I wonder what that reason was.