ul - triode coupled screen grid resistor

[blue andry]

Hi all,

it just maybe newbie question.

May gurus here explain what did r62 or r63 in this schematic
did both on triode or ul mode?

Thanks for your help.

An externally hosted image should be here but it was not working when we last tested it.

 

[DF96]

Possibly damping a high Q HF resonance in the transformer? Normally a snubber network would be used for this, as a plain resistor will absorb power throughout the audio spectrum.

 

[hidnplayr]

Possible the author used a break-before-make type op switch, and added the resistors so he can safely switch while the amp is powered on

 

[TheGimp]

Since the screen switching is controlled by a relay it is unlikely that it is expected for one to turn off power to switch modes, else it won't work.

Therefore I concur with hidnplayr. The resistors insure a bias to the screens during the time the contact is switching.

 

[DF96]

Yes, I agree. I hadn't spotted the switching. It would never occur to me to design an amp like that so I suppose I don't notice such things.

 

[blue andry]

Hi ,
thanks all for your opinion

btw is there a way we can determine the correct values for this

resistor?

Thanks

 

[DF96]

It needs to be low enough to provide some screen current and add some load to prevent switching spikes from getting too big, and high enough not to consume too much audio power under normal conditions. In a sense there is no correct value because this is not really the way to do it. Changing from triode-UL-pentode really needs a change to feedback etc. too otherwise for at least one of the modes you are running with the wrong setup.