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In the Forum: Audio Discussions
In the Thread: Initial thoughts about new/old Lamm ML2s

Post Subject: Cycling bias: ImprovementPosted by jessie.dazzle on: 1/10/2010

Romy, thanks, as usual, you seem to have been right.

The system is on and I just checked V2 voltage; its way off, above .4VDC, so I grabbed a hot pad from the kitchen and rocked the V2 tube. The socket made the familiar a dry creaking sound and seems nice and tight, though I wouldn't feel it if there were a single loose pin. The voltage changed with the rocking, which I continued to do, in all directions, for about 30 seconds. I then let the voltage stabilize, adjusted it down to .3VDC, and set the volt meter to take max high and max low readings over the period of one hour (a nice feature of Fluke meters). Results: Voltage now cycling between .252 and .294V, (.042V range). Over a period of 3-5 minutes, there is little change, so the situation has improved, but the reading is not as stable as in the case of the other amp, which cycled between .297 and .317V (.020V range) over one hour.

With the V2 reading brought back to .3VDC, the voltage for the regulator tube (V1) is dead stable.

The reason I renewed all tubes in the amps was that one of the V2 tubes failed, making the horns emit a very loud buzz. Fortunately I was standing near by, and yanked the power cord out of the back of the amp. I no longer let the amps warm up while running to the grocery store. The failure occurred in the amp that has the cycling issue; I suspect the failed tube was abused. I may need to further clean the socket. At worst I will replace all sockets (an operation I'm now doing on the L2 preamp, where a failed socket took out some capacitors... Currently using my old L1). It would be nice to know what, if any, potential dammage to other internal components could result from a failing V2 socket/tube.

Paul, thanks for the tips; I don't currently have a backup pair of V2 tubes on hand (in France, the post office closes before I leave work... I need to get one of those old-school, stay-at-home wives so I can start using mail order!). I would say changing them out once a year seems excessive, but not if you're getting a second year of service in V1 position.

jd*

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