EF80 vs EF86

[newbadboy]

Hi.

I will build the schematics below

http://diyaudioprojects.com/Schemati...-Schematic.htm


I have though a couple of EF80. Is it possible to just swap the EF86 with EF80???

[Tubes4e4]

Originally posted by newbadboy
Hi.

I will build the schematics below

http://diyaudioprojects.com/Schemati...-Schematic.htm


I have though a couple of EF80. Is it possible to just swap the EF86 with EF80???

No, it is definitely not possible.

The EF86 works under extreme current trickler conditions in that Mullard 3-3 circuit, see plot available here. While EF80 / 6BX6 as a typical RF/IF stage pentode is designed for working with much more current going. Running it at such very low cathode currents will harm its cathode and is not recommended: According to my TFK handbook, standard EF80 doesn´t sport a "zwischenschichtfreie Kathodenbeschichtung", which means, an intermediate layer in the cathode coating will build up when the tube is used at very low (or none) cathode current going. This intermediate layer building up will harm emission capability of the cathode coating.

Besides that, characteristics of EF86 and EF80 are so different, that I doubt EF80 would work satisfactory in this circuit anyway - if at all.

Regards, Tom.

P.S.: Besides that, running EF80 at more usual and appropiate currents makes it a very nice, linear, capable and cheap driver tube.

[newbadboy]

<Too bad. do i have to change much in the cirquit to be able to use ef80?

[Tubes4e4]

Originally posted by newbadboy
<Too bad. do i have to change much in the cirquit to be able to use ef80?

Yes, almost everything regarding the driver stage - if you want to run EF80 under decent conditions, the special screen feed method used in the Mullard 3-3 circuit won´t do. In turn, there would be no more need to run the EL84 at such an artificially elevated cathode voltage, since the main reason to do so (screen feed voltage supply for the driver stage) is gone.

Would be simpler and more efficient to design a generic EF80/EL84 based amp just from scratch.

Tom