Need some help adding +48 to Ampex 601

[substitute]

Hello, I'm refurbishing an ampex 601 for stand alone use as a mic preamp.  So far I have it recapped and running nicely, I'd like to add phantom power.  I'm tapping off the B+ high voltage rail just after the 4.7k resistor after the choke.  So far I've tried two approaches...

1) From the above mentioned node, to a 1m trimmer wired as a voltage divider.  I dialed down the voltage to around 60v, then connected it to a TL783 high voltage regulator.  The first thing I noticed was that once the VR circuit was connected the voltage coming off the wiper of my 1m trimmer dropped down to just a few volts.  By going back and forth between the 1m trimmer and the 10k trimmer in the VR circuit I was able to get to 48v, but it's pretty unstable.  I let it sit for about a half hour, checked it again and it had dropped down to about 4 volts.  I did check the voltage at the node on the power supply and it was still at 340v

2) Strapped a 24v zener from that same node to ground.  4.7k resistor started to burn up. 

Does any one have any tried and true methods for safely pulling 48v out of a tube amp power supply?

http://www.ti.com/lit/ds/symlink/tl783.pdf

 

[moamps]

Does any one have any tried and true methods for safely pulling 48v out of a tube amp power supply?

One solution is using Maida regulator.

 

[substitute]

Very cool, that's a new one to me.  Thanks!

 

[Brian Roth]

It worries me to drop a HT voltage down to 48 VDC.  Seems that a lot can go wrong, and then fry the mic.

Bri

 

[substitute]

Yeah, that's what I meant by safely.  Even if the VR circuit puts out 0v if it fails it still seems like too much of a risk.  I'm thinking maybe the easiest, safest, approach is using a small dedicated transformer.

 

[gridcurrent]

I'm thinking maybe the easiest, safest, approach is using a small dedicated transformer.

A small,  inexpensive 120V:12V power transformer, wired backwards, on the 6.3VAC heaters (low magnetic field).
Apply your favorite rectification/regulation scheme.

 

[substitute]

Now we're talking.  Thanks!

 

[Audio1Man]

Using the HV supply is not the best as the hum and unregulated voltages with changing loads (amplifier output levels) could be poor. The system may not be grounded correctly for this application. I have attached a schematic that may work in this system.

Good hunting. Duke

 

[substitute]

Thanks Duke, that looks like it tightens up the screws on what I've been messing with.

I hooked up the 12v transformer tonight, works pretty good.  After the rectifier I've got 47v.

 

[PRR]

> After the rectifier I've got 47v.

Do you have a cap on the rectifier??

6.3V into 12V:120V transformer should give 63V AC. Rectified with no cap you get a lump-wave which some DC meters will read as 56V, and some will read other ways. With a capacitor to catch the peaks (1uFd is more than ample for meter-test, though you want 100uFd+ for Phantom) you should get very steady DC which will read near 79V.

 

[substitute]

Do you have a cap on the rectifier??

I did not.  Now I do, and I've got my 48v.  Thanks everyone!

 

[Whoops]

> After the rectifier I've got 47v.

Do you have a cap on the rectifier??

6.3V into 12V:120V transformer should give 63V AC. Rectified with no cap you get a lump-wave which some DC meters will read as 56V, and some will read other ways. With a capacitor to catch the peaks (1uFd is more than ample for meter-test, though you want 100uFd+ for Phantom) you should get very steady DC which will read near 79V.

Hey Substitue have you read what PRR wrote?
He was basically explaining that something is wrong.
With that transformer you should get 79V DC after the rectification and never 47V DC,
so 2 options or you are doing something wrong or you are not telling us all the info about the circuit that is after the transformer secondary.

Anyway you should always listen to PRR

 

[substitute]

Yes, indeed, if I may quote myself...

Quote
Do you have a cap on the rectifier??

I did not.  Now I do, and I've got my 48v.  Thanks everyone!

PRR has gotten me out of so many jams!