Akai / Roberts Tube Amp Modification
« on: July 21, 2018, 02:49:23 PM »
Hi,
I have one Akai 707 tube preamp and I would like to mod it into a two mic preamps.
I found this schematic that I would like to build:


The only thing that is missing here is a phantom power.
So I was thinking to add this phantom circuit, connecting it to the 230V coming out from the rectifire tube and regulate the 48V with this LR8 regulator as shown here:

Since the original phantom circuit working on a 50V and regulated by a LM317 I guess I need to change the value of some of the following components:
C04, C05, C06, D04, R22, R23, R24

How can I calculate values for the components on the phantom circuit?


squarewave

Re: Akai / Roberts Tube Amp Modification
« Reply #1 on: July 21, 2018, 03:54:33 PM »
You don't need a fancy circuit like that. You could just add few dropper resistor / capacitor steps with a ~50V $0.10 zener diode shunt before a final low resistance RC filter. Just fiddle with the values until you get ~5-10mA through the zener and 48V with a 4k7 load (10mA). Caps can probably be quite small. My first guess would be something like 4k7/10u > 4k7/10u > 51V zener -> 220R/220u. That would be very quiet.

Don't try to tap off of the other droppers though because voltage over everything will drop drastically. You can probably tap into the supply after the choke though.

ruffrecords

Re: Akai / Roberts Tube Amp Modification
« Reply #2 on: July 21, 2018, 05:18:20 PM »
You definitely cannot add 470uF across the rectifier tube  output it will blow the tube. As already suggested some more modest RC filers and a final zener is probably the best approach.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

Gene Pink

Re: Akai / Roberts Tube Amp Modification
« Reply #3 on: July 21, 2018, 05:58:40 PM »
You definitely cannot add 470uF across the rectifier tube
Especially one rated at 100V.

Gene

Re: Akai / Roberts Tube Amp Modification
« Reply #4 on: July 21, 2018, 06:42:30 PM »
Thanks guys, I'll change my  circuit design  :)


 

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