Re: 6N1P Bass DI - Revisited
« Reply #80 on: January 24, 2018, 06:50:21 PM »
Hey Andy, thanks for your suggestions.
I tried 2 power transformers for this. The first had 8v on the secondary but is only rated 600mA so that's not enough after the rectifier. The second transformer I tried is rated as 6,3v (4A) on the secondary but unloaded I get 7,4v. After the rectifier that's 8,9v so that should be enough for the voltage drop of the LM317. But when I insert the tube the voltage after the rectifier drops to 7,8v and after the LM317 it drops to 5,8v.
And this is resulting in a too loud hum. Or maybe that's caused by something else. I'm trying to find out.

I'll try to run the heaters with AC. But I think I better try to drop the 7,4v a bit.

BTW, your plan for adding an EQ to this design sound great. Keep us posted!
Interesting.  I did DC heaters without a Vreg, I imagine you will need to adjust it per data sheet and formula. Just to clarify, if your PT has a heater CT you cannot connect it and also chassis ground post rectifier.
I implemented DC heaters with big resistors and 10k mfd caps, as an alternative.
The hum sounds like you need to chassis ground the DC heater circuit (again, not a with PT heater center tap (CT) simultaneously).
Here is a relevant link:  apologies if you have already read this.  DC heaters are discussed at the bottom.
edit: here is a thread from a while back on my dc heater circuit, I made several changes when finally finished that I didn't add and the device is gone, I think it was just another 10k mfd cap, but it shows another means and has helpful input from the super-users. Just scroll towards the end when I switched to unregulated :-) as an alternative.
« Last Edit: January 24, 2018, 09:43:21 PM by andyfromdenver »
my la2a scratch build:
my orig tube DI w/ EQ build:
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Re: 6N1P Bass DI - Revisited
« Reply #81 on: November 22, 2020, 05:54:42 AM »
Hello guys,

Thank you for this very useful thread. I built 2 units based on the scheme showed in the first message and they sound great. I used Carnhill transormers. Both units are in the same box and are powered by the same PSU.
When i test each unit individually, i get a B+ voltage of about 190 V with about 3 V on the cathode resistor (which is 220 Ohm as stated on the diagram). However, when the two units are feed in parallel from the PSU, the B+ voltage drops to about 150 V with a cathode voltage of 2,2 V approximatively and I can not figure out why ? The low voltage supply for heaters remains quite stable in both cases.

Do you have any idea ?



Re: 6N1P Bass DI - Revisited
« Reply #82 on: November 22, 2020, 12:58:03 PM »
If the HT supply uses simple RC filtering then the additional current draw of two using will drop the HT and alter the operating point which is reflected in the cathode voltage.



'The only people not making mistakes are the people doing nothing'


Re: 6N1P Bass DI - Revisited New
« Reply #83 on: November 22, 2020, 04:43:53 PM »
Since the HT supply in the build I'm testing indeed uses simple RC filtering, it makes sense. Thank you for your answer, I will check this out.
« Last Edit: November 22, 2020, 05:31:03 PM by yvus »