Cressy Snr wrote: ↑Mon Feb 22, 2021 6:04 pm
It sounds cleaner overall, with a bit more insight into the background details; probably due to a drop in distortion. Quite a useful little set of mods then...
Cleaner indeed it is but at switch-on and switch-off there was a hiss occurring through the speakers, that sounded like air being let out of a tyre. This faded away at switch on as the 807s came up to voltage, and when the amp was switched off, the hiss appeared as soon as the power was taken off and faded out slowly until everything had ceased amplifying.
I suspect it was the fact that zeners need a minimum amount of standing current through them before their noise will fall to the correct level, and the higher the voltage/wattage spec, the noisier they are, and the higher is the minimum current level requirement, before they will shut up.
Anyway, to cut along story short, for each 807 screen grid, I made up a composite zener out of some 5.6V, 500mW jobbies I had left over from building a couple of MJ statistical regulators. Using these, silenced the start-up and power down hiss-problem pretty handily. I was also able to adjust the screen voltages in 5.6V increments by strapping out zeners until I got what I wanted, ie the ability to set the screen/anode relationship very easily.
I wanted to use expensive rectifiers such as my GEC U52 or Mullard fat bottle CV378 (GZ37) without reducing the standing current in comparison to a GZ34. This worked great. Below is a composite zener, with a couple strapped out to give 290V screens with 317V anode:
Fat bottle GZ37 in use:
I also redid the input stage power supply, rectification: taking out the ridiculously over-specced GZ34 and replacing it with a far more appropriate for the job EZ80. 600mA heater vs 2A for a GZ34 will save a bit little of juice too.