Ampex PR10 - DC Filament Issue

[smilan]

Hi,
I've finish recaping an Ampex PR-10 (mono version with a mic preamp).
I replaced the filament selenium rectifier with a couple of 1N5408 diodes.
The original rectifier rated for 2.4A while the 1N5408 is rated for 3A, so it should be a good replacement.
The preamp working but with a very strong 100 Hz noise.
I've tried to replace tubes and the noise remains.
While I was looking at the filament output there is a 500 mV at 100Hz AC between the DC filament leads.
This is a normal behavior or I should expect to see a DC voltage with a little ripple?

 

[abbey road d enfer]

I replaced the filament selenium rectifier with a couple of 1N5408 diodes.

You can't just do that. Selenium rectifiers have a higher loss, so the heater voltage is probably significantly higher than expected, and the the ripple will be higher too. Have you checked the DC voltage?
You would have to add a resistor between the common cathodes and the first cap, that you may have to increase too.

The original rectifier rated for 2.4A while the 1N5408 is rated for 3A, so it should be a good replacement.
The preamp working but with a very strong 100 Hz noise.
I've tried to replace tubes and the noise remains.
While I was looking at the filament output there is a 500 mV at 100Hz AC between the DC filament leads.
This is a normal behavior or I should expect to see a DC voltage with a little ripple?

 

[smilan]

You can't just do that. Selenium rectifiers have a higher loss, so the heater voltage is probably significantly higher than expected, and the the ripple will be higher too. Have you checked the DC voltage?
You would have to add a resistor between the common cathodes and the first cap, that you may have to increase too.

Yes, I've checked the heater DC voltage. With a full load and with no dropping resistor it's 12.3V at 0.945 A , a little bit lower than I expected but within the ±10% tolerance.

I need to think if it's possible to apply star ground at this case since this circuit assembled on a PCB.

I've installed two new 2200 uF / 15V caps instead of the original ones (2000/15).
Adding a 4700/16V in parallel with C29 makes a small improvement, the hum is still very loud.

 

[abbey road d enfer]

I need to think if it's possible to apply star ground at this case since this circuit assembled on a PCB.

Actually, this circuit does not need to be grounded other than by the heaters of the first tube.
Star ground is the not the magic receipe you think it is...

 

[smilan]

I went thru a long journey with this Ampex PR-10 preamp. After finish recapping it there was a terrible 100 Hz hum at the output.
First I suspected the filament DC circuit, but it was fine. Then I was thinking it some kind of ground loop and tested all the ground points on this PCB (I wish it was a point to point circuit) but all seems to be fine.
Until today I thought to check the input transformer and I found that one of the primary winding was burnt...
Now with a new input transformer it works as is should!
Beside that I added a mains power jack and a fuse.
There's an un used 56V tap on the PT, so the next step will be to add a switchable phantom power circuit.