Marshall Cathode Follower

[TriodeLuvr]

I was in the middle of a discussion elsewhere regarding the H-K limit of tubes when I suddenly realized someone here might have some helpful empirical data. The issue involves a cathode follower circuit I'm designing that will use a 12AU7. The cathode of the follower will operate at about +100V to +120V DC, and I'm concerned this might be high enough to cause gradual degradation. I realize this tube is specified with a +200V limit, but it's not clear to me that this number is meant to be used to create a permanent operating condition.

My question here is whether Marshall users with the 2204 (JCM800) amp have experienced any unusual or frequent failures of the second 12AX7 in the preamp? This tube has the same specification as the 12AU7, and Marshall runs it with about +170V on the cathode. The Marshall schematic I have indicates the filaments aren't elevated, so the entire +170V appears at the H-K interface.

Any help with this will be appreciated!

Jack

[Bieworm]

I was in the middle of a discussion elsewhere regarding the H-K limit of tubes when I suddenly realized someone here might have some helpful empirical data. The issue involves a cathode follower circuit I'm designing that will use a 12AU7. The cathode of the follower will operate at about +100V to +120V DC, and I'm concerned this might be high enough to cause gradual degradation. I realize this tube is specified with a +200V limit, but it's not clear to me that this number is meant to be used to create a permanent operating condition.

My question here is whether Marshall users with the 2204 (JCM800) amp have experienced any unusual or frequent failures of the second 12AX7 in the preamp? This tube has the same specification as the 12AU7, and Marshall runs it with about +170V on the cathode. The Marshall schematic I have indicates the filaments aren't elevated, so the entire +170V appears at the H-K interface.

Any help with this will be appreciated!

Jack

I haven’t got enough experience with that, but all I know is that a buddy of mine has had a 2204 fior decades, and the only tube that has been replaced is the PI

[JMPGuitars]

I wouldn't worry about it at all. The attached datasheet specifies "Maximum Ratings, Design-Maximum Values:"

If it's specifying that designing your circuit up to that value is good, then you're fine. You're not even close to the safe limits.

[TriodeLuvr]

Yes, I'm probably overthinking this. There doesn't seem to be any "noise" online about failures in the Marshalls. A cathodyne PI using a 7199 or 12AU7 also typically operates at +100V or higher DC on the cathode. I'm going to proceed with the project as-is and not worry about this. Thanks for the help!

Jack