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Angelpat 21st August 2012 05:24 PM

Modification of 300B SE Amplifier
 
Any idea if changed the input stage 6SN7 from SRPP to parrallel connection. Is that help to reduce the over gain of the circuit? Do concert if the current of the input stage will be more than the driver stage (maintain the SRPP).. Should I maintain the 1.5K cathode resistor as is in order to keep the same current flow over the input stage... I have to calculate the new value of the plate resistor afterward.

Please share comment.
thanks!


300BSch (Patrick) | Flickr - Photo Sharing!

Angelpat 21st August 2012 05:26 PM

http://www.flickr.com/photos/8403854...in/photostream
original schematic enclosed

Dimitris AR 21st August 2012 07:49 PM

You can reduce the gain by simply remove the bypass capacitor in the first SRPP stage.

Palustris 21st August 2012 09:32 PM

You can remove the entire first stage by connecting the input to the second SRRP stage.

Angelpat 22nd August 2012 01:37 PM

I did tried to remove the bypass cap fron the input tube and yes, the gain reduction achieved but it lose the "mid" gthe high and the bass are both acceptable) ... May be too much reduction that I'm not sure and thus consider making something in between.

palmas 22nd August 2012 03:54 PM

What is the supply voltage?

Angelpat 22nd August 2012 05:13 PM

the plate of the lower tube : 185Vdc
cathode of lower tube : 4Vdc

palmas 22nd August 2012 05:14 PM

Quote:

Originally Posted by Angelpat (Post 3136068)
the plate of the lower tube : 185Vdc
cathode of lower tube : 4Vdc

the top triode plate voltage?

Angelpat 22nd August 2012 05:28 PM

370Vdc

palmas 23rd August 2012 10:54 AM

Quote:

Originally Posted by Angelpat (Post 3136093)
370Vdc

With that voltage you can use the 6SN7 in a griunded cathode configuration, with both triods in parallel (Rplate=10k to 12k and Rcathode=1K and 8uF).
You'll get about 23dB of gain (14x), but you'll have only about 60kOhm of input inpedance.
I suggest you use one triode only, making the triode 1 for the left channel and the 2 for the right one, so you can trade tubes between channels and have a pair of "new" triods.
For that you can use the same values for resistors and cap and get about 20dB gain (12x), but have an input impedance of more than 160kOhm.

Let me know if you need some recalculations or new specification.


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