Mainline regular maintenance showed unusual readings

[HirkEukvic]

Hi, I've owned a Mainline since early 2023, and I was doing one of my occasional maintenance checks of terminal readings to ensure things weren't acting up. I recently noticed a few rattles/buzzes at certain frequencies. I got the following unusual measurements during the troubleshooting, though all resistance checks matched the safety check to within a few ohms:

Breg on regulator board (both sides) - 217 V
Kreg, A side - 16 V
Kreg, B side - 11 V
Terminal 20 - initially read 135 V upon check. Adjusted trim potentiometer to 145.
Terminal 30 - initially read 129 V upon check. Adjusted trim potentiometer to 145.

These numbers suggest to me that something is off after a few years of use, but I'm not sure what it would be (I'm a writer, not an engineer). Do I need to replace a tube, or is there something else that I'll need to fix? I've made no modifications to the amp and I've been using new stock tubes purchased from a trustworthy supplier.

Thanks!

 

[Paul Birkeland]

These don't look too far off. The 16V on KregA is a little unusual, but that would tend to indicate a 12AU7 that's a little more potent than it should be rather than a worn tube.

 

[HirkEukvic]

These don't look too far off. The 16V on KregA is a little unusual, but that would tend to indicate a 12AU7 that's a little more potent than it should be rather than a worn tube.

Thanks, Paul. It was particularly that Kreg value that I thought was weird since it’s outside the expected range by a few volts (well, +33% over the max of 12 that the specs called for). Can you define what you mean by “more potent” here?

It is a quite nice-sounding 12AU7 actually, the best I’ve found for this amp (Northern Electric was the manufacturer). So long as it’s not likely to cause issues, I’ll keep using it. Even bought a spare of the same brand for a while down the road when the current one gets to end-of-life.

 

[Paul Joppa]

The Breg being at 217v both sides indicates both shunt regulators are working correctly.

The T20/T30 values being a bit low indicates the emission strength of the 6C45 cathodes is a little stronger than the last time it was adjusted. Probably the cathodes are just a bit better broken in; it can take a few hundred hours for the cathodes to fully form on special-quality tubes like the 6C45. I would expect that at your next check, they will need much less adjustment. Eventually, those numbers will go up, which would indicate they are beginning to fade. But that may take many years.

 

[Paul Birkeland]

The Kreg voltage is set by the leftover current from the A side C4S that flows through the shunt regulator. A weak 12AU7 will need lower cathode bias voltage to siphon off that current, while a strong testing tube will show much higher bias voltage under the same operating conditions.