What Did I Do Wrong?

And you know this how??

Have you been provided with a definitive indication, from the manufacturer or elsewhere, that EL34's/EL34B's can be used in your amp? And by someone who seemed to know what he or she is talking about? The EL34's/EL34B's differ dramatically from KT120's in several respects, although I am aware that some other amps, such as some of the PrimaLuna models, are designed such that they can accept either tube.

In looking at this datasheet for the KT120, I see that the suppressor grid is connected internally to the cathode, with both being connected to pin 8. And pin 1 is connected to nothing internally. While on the EL34/EL34B the suppressor grid is just connected to pin 1. So if the design does not specifically address that difference, probably by externally connecting pins 1 and 8 to each other, the tube won't operate properly.

Also, the EL34 has a plate resistance of 15,000 ohms, and based on the info Bill provided above the EL34B is apparently a bit higher than that. While the KT120 is indicated in the datasheet I linked to as having a plate resistance of only 3000 ohms.

Also, the amp is rated to provide 100 watts/channel, which is far more than can be expected from a push-pull pair of EL34's, and I believe also from a push-pull pair of EL34B's. Which would seem to reinforce doubt that the amp has been designed such that it can accommodate those tubes.

There are several other differences that could also be significant, but you get the idea.

Good luck. Regards,
-- Al

Rogue initially had EL34 tubes in the amps and eventually they changed to the KT120's. Nick at Rogue Audio told me they will work fine and to set the bias at 30ma. That's all I know.

09-06-15: Stereo5
Rogue initially had EL34 tubes in the amps and eventually they changed to the KT120's.

I believe the EL34 was used in the Cronus, rated at 55W/channel, but as far as I know never in the Cronus Magnum or Cronus Magnum II, which were/are rated at considerably higher power levels.

If you happen to have a multimeter, after the amp has been off for a good while, say an hour or more, remove one of the power tubes and see if there is continuity (0 ohms resistance, or very close to it) between pins 1 and 8 of the socket. Those are the two pins closest to the orientation notch on the socket, one of them on one side of the notch, and the other on the other side of the notch.

Regards,
-- Al

Almarg

Are you talking the KT120's that are in there now or the EL34B tubes which I removed already? Do you think the EL34's are toast? I should have listened to Uncle Kevin at Upscale, he advised against it.