5e3 Power Tube Socket Wiring Question

[Chris-in-LA]

I have this same topic posted on another forum but I’m not getting any responses, I’m hoping you guys have the answer to my questions.

For my first amp build I put together the Tube Depot 5e3 kit with a PCB. Later, I dropped in a Mojotone eyelet board for a number of reasons. I just noticed a small difference between the two builds. The Tube Depot kit has an extra resistor on the power tube sockets going from pin 1 to pin 4.



The Mojotone kit eliminates that resistor. Can somebody tell me why Tube Depot added it or why Mojotone may have eliminated it? I also just noticed that TB connects the high voltage to pin 1 and MJ hooks it to pin 4. Are there any advantages to one method over the other?



According to the Fender wiring drawing, the MJ way seems to be closer to the original designs.

 

[mickeydg5]

They added screen resistors as done on later versions/revisions of the Deluxe.

 

[Chris-in-LA]

They added screen resistors as done on later versions/revisions of the Deluxe.

I assume that was supposed to be an improvement. What did it improve?

 

[mickeydg5]

Limits screen current, more stable especially with today's power tubes.

 

[DSharp]

My understanding is that at high volume, plate voltage might sag below screen voltage, new production tubes will red plate under such conditions, these screen resistors will help avoiding this.

 

[Mr. Bones]

They are not needed if you run JJ 6V6S tubes, but they might be a good idea if running NOS tubes or other current production tubes. The JJ 6V6S tubes aren't really a true 6V6GT and will tolerate much higher voltages especially on the plates.
Today wall voltage is higher than in the 1950's ( ~117V to today's ~123V) and seems most current production transformers are putting out a higher B+ voltage due to being multi use/application along with the higher wall voltage we have today.
Keep in mind pin 1 is unused, they are just using this as a connection point and a way to support the screen resistor feeding pin 4, same goes for unused pin 6, it's supporting the grid stopper, pin 5 is actually the control grid.

 

[Chris-in-LA]

Thanks for the helpful comments.

 

[ampmadscientist]

I assume that was supposed to be an improvement. What did it improve?

In the early 1970s, power tubes were re-designed.
Presumably to make them more competitive with transistors...
at this time the screen grid current was increased to make the tube more efficient.

EL34 was also redesigned.

This required adding an extra resistor to the old designs, for screen current limiting.

On the old schematic, this resistor was never required because the tubes used were Pre-1970s.

On the new schematic (with the newer power tubes) the resistors were added to limit current.

If you use an older amp with newer tubes, you need to add that resistor to each socket.

Older Amps:
if the resistors are not added, the amp might blow fuses when the volume is cranked up high.