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What is a good value for B+ ripple on 480VDC
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Planobilly:
I have decided to power the preamp AC for the moment and go on to resolve some other issues.

Thanks for all the feedback so far.

Billy
calexanian:
There appears be a few issues in that schematic. First the secondary of the power transformer is drawn with a tap to ground. Somebody else mentioned this. Depending on how that transformer is made will determine what you need to do with that tap. What is the total AC voltage across that secondary? That will help with that. You may find that that tap needs to go between the two main filter caps such as in later fender amplifiers. As far as the filament supply. DC on filaments are never worth it. The unbalanced DC hum finds it way into the signal more than a balanced AC filament supply does. Just ensure proper lead dress and balance the filament supply with a couple of 100 ohm resistors referenced to ground. This appears to be an approximation of a fender style circuit. The standby switch needs to be after the main filter caps and the B+ supply after that with no capacitor there. It will cause nothing but an annoying noise, pop, or whistle when switched the way you have it there. Apart from that you should measure the ripple with the amp biased properly. I see nothing here that stands out as a potential problem. Under load fender amps were to have a less than 20 volt ripple. That was considered acceptable, under load. The push pull nature of the circuit common mode rejects most of it. The screen supply should be less, and by the time you reach the lower supplies it should be very little if any measurable. I suggest doing all tests at this point with only the power tubes and phase inverter tubes installed. and feed test signal directly to the phase inverter input. Lift the primary side of its input cap as to not introduce power supply noise directly into the phase inverter input. If you still have hum in your output at that point it makes testing that much easier to have eliminated the whole preamp.

 
vk6zgo:

--- Quote from: cvanc on December 03, 2016, 07:58:04 pm ---The output tube B+ supply is before the fuse but the screen grid supply is after the fuse.  What happens in your output stage when the fuse blows?  The screen grids will go to zero volts but the plates will still be on...

--- End quote ---

Not a major problem---the other way round is what kills tubes!
calexanian:
I personally have never really liked the idea of fuses on the B+ supply. To dangerous to change. Make your power transformer right and rate the mains fuse properly.
cvanc:

--- Quote from: calexanian on December 03, 2016, 09:45:46 pm ---There appears be a few issues in that schematic. First the secondary of the power transformer is drawn with a tap to ground.
--- End quote ---

You're right; I hadn't spotted that.  Also notice another ground, below main filter cap C38.  There's no way it can actually be built this way, with both of these grounds - the secondary would be shorted to ground thru the bridge rectifier on both half-cycles.

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