[AlexMUK]

Hi,

I have noticed that one the Electroharmonix KT88s in my Rogue Audio Stereo 90 is slipping from the set 35ma to about 30ma over a period of a couple of weeks. On the Stereo 90, bias current is adjustable for each output valve independently via a trim pot.

Resetting the bias current to 35ma will stay stable for a short while before dropping. Is this one of the valves getting tired, or could it be a problem with the trim pot or something else in the amp?.

Any suggestions as to how to diagnose before ordering a replacement?.

Regards,
Alex

[Ericus Rex]

Sounds like a tube issue.  But just to be safe, I'd give Mark (or Nick) at Rogue a call and discuss the problem with him.  Would hate for you to get a new set (which you don't have to do since each tube is individually biased - just one would do, or a pair if you're anal retentive like me) just to have another tube go bad due to amp problem.

You could use this opportunity to try a quad of KT90s and get back with us about your impressions.  (ULTERIOR MOTIVE ALERT!  I also own a Stereo 90 and often wonder what the KT90 would sound like in mine - currently running Gold Lion KT88s)

Welcome to AC!   

[AlexMUK]

Hi,

Nice try Ericus 

Actually I was thinking about the EH KT90 or the GL KT88s as a possible replacement. I wonder if the 'Magnum' Chronos running the KT90s uses the same B+ as for 6550/KT88s? (530v?).

Fortunately (for me) I have a quad of matched EH 6550s to substitute before I can justify getting another set. It is interesting that there doesn't appear to be any visual differences at all between the EH KT88 and 6550 internally - does anyone know if they are indeed the same device? I wouldn't mix them in any case, but am just interested.

To be sure that it is the tube, I think I may swap the suspect tube into another position and see if it holds bias in that socket over a few weeks - if not it could be the trim pot or bias resistor.

Thanks - I'll let you know what happens.

Regards,
A.

[Ericus Rex]

Moving the tube to another socket would be a good test.  Keep us informed on that one.

I believe the Cronus Mag can still handle 6550/KT88s which would imply that plate voltages are the same.  However, KT88's and 6550s can't fit through the holes on the cover of the Cronus.  Only slim-based octals for that amp I'm afraid.

You want to try the KT90s.  Go ahead and do it! (and be sure to let me know how they sound)

I very much like the GL KT88s in my Stereo 90.  I haven't had it long enough to try some of the other tubes I have in my stash (EL34, 6CA7, KT66, 6L6).  Should get on that just for comparison purposes.  Mark recommended 35mA bias (actually 40mA to tube) but that seems awfully high for these other types; maybe 30 would be better.