Pentode/triode switch question

Stormo · 5th April 2007, 08:19 AM

Hi everybody !

I found this schematic of a el84 output stage with a switch used for toggling between triode/pentode mode, wich i like to build.

Could this switch be a potentiometer so that you could switch gradually - and while the amp is working ?

Kind regards,
Stormo

ray_moth · 5th April 2007, 08:59 AM

No, it won't work with a simple pot because the screen needs a low impedance feed and you won't get that from a pot on its own. In any case, taking a significant current through the wiper of a pot is doomed to failure. However, you might like to look at the Adjustable distributed load discussion, which is about that very subject.

Stormo · 5th April 2007, 01:45 PM

Thanks for the link to the other thread - lots of information there :-)

But - i did belive that there was no current flowing in to the second grid ?

Kind regards,
Stormo

SY · 5th April 2007, 01:58 PM

Quote:

But - i did belive that there was no current flowing in to the second grid ?

Unfortunately, the second grid does not share your belief.

Stormo · 5th April 2007, 05:32 PM

--> SY, good answer

blindsjc · 12th October 2007, 08:11 AM

Hi friends,
How to add a triode/pentode switch in a PP amp?
After disconnect 470/470 resistors from B+, what
else I need to do? Where I need to connect them?

Project fragment:
http://server6.pictiger.com/img/7257...pp-el-84--.gif

Thanks
Blindsjc

ray_moth · 12th October 2007, 08:05 PM

Try connecting them individually to each plate, so that the screen of each EL84 is connected to the plate of the same tube. The optimum plate loading for EL84 in triode PP is 25% higher than for pentode mode, so you might get more distortion than you like.

If you like what you hear and decide to install a switch to change mode, beware that the contacts will be at B+ voltage. Also, turn the amp off before you switch modes.

5th April 2007, 01:45 PM	#3
Stormo diyAudio Member Join Date: Sep 2002 Location: Denmark	Ok - but Thanks for the link to the other thread - lots of information there :-) But - i did belive that there was no current flowing in to the second grid ? Kind regards, Stormo

12th October 2007, 08:11 AM	#6
blindsjc diyAudio Member Join Date: Nov 2006	How to do in PP? Hi friends, How to add a triode/pentode switch in a PP amp? After disconnect 470/470 resistors from B+, what else I need to do? Where I need to connect them? Project fragment: http://server6.pictiger.com/img/7257...pp-el-84--.gif Thanks Blindsjc

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5th April 2007, 08:59 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	No, it won't work with a simple pot because the screen needs a low impedance feed and you won't get that from a pot on its own. In any case, taking a significant current through the wiper of a pot is doomed to failure. However, you might like to look at the Adjustable distributed load discussion, which is about that very subject.

5th April 2007, 05:32 PM	#5
Stormo diyAudio Member Join Date: Sep 2002 Location: Denmark	--> SY, good answer

12th October 2007, 08:05 PM	#7
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Try connecting them individually to each plate, so that the screen of each EL84 is connected to the plate of the same tube. The optimum plate loading for EL84 in triode PP is 25% higher than for pentode mode, so you might get more distortion than you like. If you like what you hear and decide to install a switch to change mode, beware that the contacts will be at B+ voltage. Also, turn the amp off before you switch modes.

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