Counterpoint 5.1- Beyond My Comprehension

If that tube is used as a voltage reference or as a voltage regulator and the voltage was low, that could potentially cause a loss in gain. I'm just guessing at this point. The dark band/ring around the center of the plate of the tube usually indicates the tube is worn out or damaged.

The 6CA4 inthe SA5.1 is the rectifier (note: not regulator) for the high voltage B+ of the SA5.1.
It converts AC to some 500V of DC that goes into the regulator.
If the B+ going into the regulator has low voltage, then obviously gain will be diminished.
Glad you didnt damage the regulator by replacing the 6CA4 quick.

Johnsonwu is correct. The Counterpoint regulators seem to be an issue now that the units are getting older. Either Johnson or myself can repair the Counterpoint units and we are both listed on the Alta Vista Website.

Happy Listening.

@ Johnsonwu & Bigkidz

Do either one of you do upgrades like Alta Vista used to do on SA 5.1s?

Thank you

Travis

I have done upgrades to SA1000, 2000, 3000, and limited stuff on the 5000 which is very hard to work on.
The SA5.1 schematic is floating around the Internet and the official one available from Catherine Weber per Altavista. Both Bigkidz and I have similar ideas of what Mike used to do with the mods (chokes, resistors, oil bypass, naked vishays, Non inductive WW plate resistors, etc.) It wont be 100% what the premium / gold level mods Mike used to offer but the ideas would be close.
Bringing a box to spec by replacing noisy resistors and old lytics is easy. Executing the mods take a lot of creative mounting work due to the 1U size of the SA5.1 vs the the HUGE size of the oil caps and the modern better sounding coupling caps (there is a reason why Mike sticks with Dynamicaps)

-Johnson