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Old 18th March 2013, 11:41 AM   #11
MartinR is online now MartinR  Germany
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Join Date: Apr 2005
Location: Frankfurt
Hello Alberto,

you should also consider the usage of a anode stopper (22-47R) with such long leads to the anode resistor and coupling cap. Also the usage of ferrite beads on g1, g2, anode and cathode is highly recommeded with a tube like the D3a. To make things even better you can connect a 1nF ceramic cap from the each heater pin to the chassis.

Best regards,
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Old 18th March 2013, 12:38 PM   #12
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Join Date: Mar 2003
Location: Zürich
Hi Alberto!

great that you can move ahead now. I would really consider the recommendations given by the other members, like the precautions against oscillation and reducing the heater voltage on the D3a.

saludos Brasileños de Moçambique! Erik
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
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Old 18th March 2013, 03:41 PM   #13
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Join Date: Aug 2008
Location: Bulgaria
Ceramic resistors in cathode of 300B is not good idea. You can try to change to Mills resistors- 12 W and sound will be really great. Mills resistors page
Better You can use shunt el. caps Black Gate , Elna,
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Old 19th March 2013, 10:22 AM   #14
MartinR is online now MartinR  Germany
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Join Date: Apr 2005
Location: Frankfurt

Originally Posted by azazello View Post
Ceramic resistors in cathode of 300B is not good idea.
Of course, you shouldn´t use them if the heaters between 300B and D3a are shared. Only if the D3a has its own heater.

By the way, the best way to get a solid rock operating point of the D3a is to follow the datasheet by using a positive grid voltage (about 10V) and a higher value cathode resistor. But this requires a coupling cap between input and grid.

Best regards,

Last edited by MartinR; 19th March 2013 at 10:24 AM.
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