Well done John, not so lonely now...
Fixed my hum too. Just an earth loop between my amp and the pre.
I have left just the mains earth connected to the chassis and temporarily lifted the 'star ground' off the chassis. I'll add a ground lift resistor/cap later.
No hum even with the TX closer. (Good, as mine has to be a one box unit.)
I've mitigated the power up rumble so far by switching the power amp on after the pre has warmed up, but am wondering about fitting a simple mute switch across the output for now.
Ideally the output cap would be >10uF but the issue is the switch on thump and the possibility of it becoming large enough to potentially damage the input of a SS power amp. It will always be fine with a valve power amp.
The two 47uF caps at the anodes charge up relatively slowly via the 15K resistors, and this, combined with the current limiting of the 30K anode load resistors and the Zeners, means that even with 10uF or more
you should be fine... I left the output cap a relatively small 2.2uF in the original schematic just to be able to guarantee that it would be safe with all power amps. Sorry I can't answer questions on the lines of "I have an Acme MkIV power amp, will it be safe with 15uF output capacitors?"... there's just to many variables... I guess power amps with a FET input would be the most easily damaged...
In mine I have 8uF at the moment from 4 x 2uF in parallel but going up to as much as 22uF
could be advantageous for sound quality, especially the bass. If keeping it polypropylene then the Zeners help in allowing lower voltage rating caps to be used. 200V rating is fine and 150 V no doubt fine but no wriggle room left (it only gets to 150V max for the time it takes the valve to warm up and then drops to 70V). 350V+ polyprops @22uF get rather large and expensive!
A simple mute switch at the output, shorting the output end of the caps to ground until it's warmed up, will do fine... providing you remember to use it!
A valve rectifier can be used and would solve the problem by warming up slowly with the ECC88.
If there is a demand for it I will publish the automatic switch on mute circuitry but it does mean considerable extra complexity, including having to rectify and smooth the heater supply so it can also power the SS delay circuitry and relay.
A pair of 12V Zeners with a pair of 1N4148's or similar could be arranged to make an output limiter but there is a
chance of very slightly reduced sound quality due to modulation of the non linear reverse capacitance of the diodes. There could well be no sound quality reduction but I'm just mentioning the possibility anyway..
Another alternative, but not exactly user friendly, is to not connect it to the power amp until it has warmed up for a few minutes...