I need help to figure out if power up hum on my recently Built 300B SE amp is normal, or what does really cause it?
Every time when I power amp up, it generates very loud hum for 3-5 sec, than the hum goes away and Amp is working fine.
The other mystery I can't solve, same amp does not like to work with signals generated by phono carriages (of cause coming out of phono preamp , and the preamp is not a reason ), as soon as stylus hit the record and moves little bit (even slight vibration), I can hear foreign notice on background very similar to start up noise, but much lower in the level. Interesting enough that all other sources of input signal like CD player , phone etc. do not have this issue.
Any ideas will be really appreciated.
3.2K should be OK. I chose 5K for two reasons, first I have read 3.5-4K makes more power but has a little more distortion, 5K makes a bit less power but has less distortion. The other reason is, I already had a pair of 5K 25W ones from an earlier KT88 project. :)
I think that too is part of your problem if you only have 6.3V DC feeding the 5V regulator. They need to be a couple of volts higher than they are set to work right and not cycle on and off, which creates humming. And yeah, those amps with the power transformer in the center tend to have the issues you describe.
There is your problem. The 300B tubes can't share a transformer winding, even when using DC and regulators. Power the driver heaters off that one winding and then get a separate dual winding filament transformer to power the 300B heaters. Once the amp is fully warmed up, the hum pots shouldn't make a huge difference with DC heaters unless the regulators are truning on and off from not having enough supply voltage. BTW what is the voltage on the 300B heaters before and after the regulators? And did you use fixed 5V regulators or the adjustable kind? And if I was doing DC with regulators again for a 5V tube, I would use a dual 7V transformer to give some voltage headroom for the regulators,. something like a Hammond 266L14
Could be 5v heater problem . I used XPWR011 power transformer that has only one 6.3v winding , I connected 2 dc 5v regulators to that winding in parallel . Do you see it is a problem ? Drivers heaters are powered from separate transformer.
the hum pots adjusted to the position that gives less hum when amp works . Should It be in the middle? It definitely creates significant hum when in the middle.
That sounds like an issue with the 5V heater circuit and/or the regulators. Also do you have the hum pots adjusted in the middle? Does adjusting them during this startup hum have any impact? That is not normal, mine has a tiny bit of hum on initial turn on, but nothing like what you posted. Also the phono issue is weird, have you tried hooking it up through another input? Also could be a ground issue, turntables are sensitive to that. Have you tried grounding the turntable to the amp rather than the preamp?
First try the power up with the driver tubes removed and see if it is still there. Is it one or both channels? Is this is my schematic you used?