moamps

Re: Ampex 600/601 Tube mic & Line amp
« Reply #40 on: July 15, 2020, 06:11:04 PM »
6.9V is on high limit, you can insert a resistor in series to drop it, or two anti-parallel diodes.
Using AC for tube heating in a mic preamp is sub-optimal IMO,  you can get better results using a good filtered DC voltage.
Have you installed hum-balancing resistors or pot in heaters AC line?


extempo

Re: Ampex 600/601 Tube mic & Line amp
« Reply #41 on: July 15, 2020, 06:28:47 PM »
Thanks!

I thought about doing DC for the heaters, but wanted to try AC first and see how bad it actually was.

I have a 100 ohm pot for hum balance of the AC heaters, just like in the stock design (schematic attached).

moamps

Re: Ampex 600/601 Tube mic & Line amp
« Reply #42 on: July 16, 2020, 08:39:11 AM »
Is it 60Hz hum for sure, or maybe 120Hz?
Can you post high res picture of the inside of the preamp?


extempo

Re: Ampex 600/601 Tube mic & Line amp
« Reply #43 on: July 16, 2020, 06:50:17 PM »
I found and fixed one issue, which was that I forgot to ground the wiper of the  hum balance pot. That explains why the pot wasn't doing anything!

I also used the anti-parallel diode trick for the AC heater and it brought the  voltage down to just under 6.3 VAC. Thanks for the tip moamps.

I still have the issue with loud hum at the output unless the 250k pot feeding the 12au7 is set just so. This is actually 120Hz, not 60 Hz.

I'll upload some high res pics ASAP.

moamps

Re: Ampex 600/601 Tube mic & Line amp
« Reply #44 on: July 17, 2020, 05:30:30 PM »
I still have the issue with loud hum at the output unless the 250k pot feeding the 12au7 is set just so. This is actually 120Hz, not 60 Hz.
This hum comes most probably from not so good filtered power supply high voltage B+, and maybe wrong grounding scheme. You can check the ripple on B+ with DMM set to AC or scope. The minimum of hum you get at one position of the pot is because of summing hum from first stage with hum in second. They are flipped in polarity so this summing became subtraction.

extempo

Re: Ampex 600/601 Tube mic & Line amp
« Reply #45 on: July 18, 2020, 12:23:27 AM »
OK! Fixed the issue. Had a funky solder joint on the ground lug for the multicap can housing the B+ filter caps. All good now. So often it's the simple stuff. Many thanks  :)