EF86 as triode - internal shield on cathode potential?

[rock soderstrom]

Hello, I have two questions about a EF86 tube that is wired as a triode.

The suppressor grid should be connected to cathode potential (according to the data sheets). But there is also information (e.g. Svetlana app. notes) where G2 and G3 are tied together on anode potential. What is the difference for a small signal pentode that is operated with very little current?

My second question is, is it possible to connect the internal shield (pin 2 or 7) to cathode potential instead of ground? What difference would that make?
By the way, the stated distortion of less than 0.05% in this document is wishful thinking, isn't it?

 

[CJ]

Good question for the mighty PRR, did find some searchables>

https://groupdiy.com/search/268210/?q=EF86&c[users]=PRR&o=relevance

 

[rock soderstrom]

Good question for the mighty PRR, did find some searchables>

https://groupdiy.com/search/268210/?q=EF86&c[users]=PRR&o=relevance

Thanks CJ for the links. I'm just reading through, super interesting no doubt, many threads I already knew, some are new to me. It's a real shame that PRR no longer posts here.

On the topic, I am basically interested in the questions, but I also have a specific application that is affected by this.

When creating a PCB layout, especially with limited space, I find the pinout of the EF86 a bit awkward. In my opinion, the closely related EF804s makes it much easier for the PCB designer.

The data sheets for EF86 tubes are clear about this topic. Strapped as a triode, the G3 should be wired to cathode potential, the internal shielding should be wired to ground. PRR assumes that the G3 could also be wired to ground potential without major differences.


So far so good.


On the other hand, Svetlana's app. notes shows G3 wired to anode potential (like some microphone manufacturers do it as well) and the internal shielding could be wired to cathode potential. Regarding G3, the Tom Schlangen document also mentions that it would be possible to connect G2/G3 to the anode, but he favors the data sheet method. Why is not mentioned.


The "super anode" consisting of the original anode, G2 and G3 would simplify the layout for me if there are no counter-arguments or real disadvantages. The same applies to the wiring of the internal shielding to the cathode.


As an example, here is a simple EF86/EF804s gain stage in comparison.

EF86 strapped as triode with Anode+G2+G3:



EF804s strapped as triode:



EF86 strapped as triode with G3 on cathode potential --> needs a jumper wire



I have to admit that there are bigger problems on this planet. But I'm still interested in the questions, as they could avoid some stress, especially with more complex layouts.

 

[rock soderstrom]

By the way, as member @emrr rightly points out, the electrically very similar 6J7 is used in a large number of American preamps in V1 position with a combined anode + G2 and G3. This was virtually standard for decades.