EF80 vs EF86

[newbadboy]

Hi.

I will build the schematics below

http://diyaudioprojects.com/Schematics/Mullard-SE-EL84-Tube-Amp-Schematic.htm


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

 

[Tubes4e4]

Hi.

I will build the schematics below

http://diyaudioprojects.com/Schematics/Mullard-SE-EL84-Tube-Amp-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]

<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

 

[jbefumo]

Just stumbled across this old thread . . .

Built an amp using an EF86 in the preamp, and since I had an extra hole in the chassis, and an extra EF80 laying around, I wired up an extra socket in parallel, correcting the appropriate pins, and tried comparing the circuit alternating the tubes -- both sound fine -- just did it for a few seconds, and haven't taken any measurements yet.

I'm attaching the relevant. circuit segment.

Does this look as if it will damage either the tube or any other components?

Joe

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.

 

[DF96]

Does this look as if it will damage either the tube or any other components?

In the long run you risk cathode interface problems, as Tubes4e4 said. You are also likely to be getting significantly more noise and distortion than an EF80 is capable of in the correct circuit. EF80 and EF86 are simply very different valves: they are both pentodes, both use B9A base and both have 6.3V heater but apart from that they are very different.

 

[jbefumo]

FWIW -- I built parallel tube sockets in my prototyping chassis so I could interchange EF80 and EF86 tubes, and the EF80 sounds great, despite not having adjusted the cathode and grid resistors to more appropriate values. AND, I have it in a TINY high-power combo - - - 2XEL34 in a box smaller than a Fender Champ, and no issues with microphonics so far. It has a bit more gain than the EF86 (u 50 vs. 38, if memory serves) but also seems to have a more complex tone to my ears.

I will probably put a switch in so I can reconfigure a single socket to work with either one.

Joe

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.