The GZ37 - what's its history and is it a good rectifier ?

[trobbins]

There may be some change in effective loading resistance of the output stage as B+ level falls, depending on the various circuitry time-constants and the operating loadline. A pentode PP output stage with fixed bias may tend to act with a constant current characteristic if the fixed bias level doesn't change much.

I don't think I have seen a test report on the rate of cathode temp drop after heater power is stopped. It's likely quite dependant on the cathode/filament thermal mass and the anode temperature, so would tend to be slower for larger bottles operating up to their design centre rating. Plus there would be some +/- margin on cathode operating temperature and current capability due to actual heater voltage (as per a typical end-of life test by lowering the heater voltage). About the closest test I can recall is when an amp is intentionally turned off with the aim of listening to if hum varies or not.

440uF does sort of indicate the design is intent on quite a low B+ ripple level.

[GrimJosef]

I wouldn't read too much into the 'design' details. The amp(s) started life as a pair of monoblock self-contained units, each with an internal solid-state HT bridge rectifier followed by a 220uF-100R-220uF filter. The owner bought the after-market HT supplies and asked me to fit them, which I did. The standard practice with these supplies is to remove the R from the original filter and parallel the Cs.

The 100Hz HT ripple with the original CRC filter was 155mV p-p, and when I ran the amp at 5W output at 1kHz I could detect 48mV of 1kHz ripple on the HT rail. With the after-market supplies fitted the 100Hz ripple dropped to 56mV and the 1kHz ripple under load was 28mV. So there were clear measurable improvements. Whether they are audible or not is a subject which probably belongs in a different thread. Suffice it to say that the customer was pleased.

Cheers,

GJ