Soft Start for Mic PSU's?

Hey gang,

I'm building a Poctop M49 and I'm springing for a real AC701(k!). I'd like to make something fairly awesome.

My concern with such a valuable tube is accidentally destroying it's delicate innards on inrush currents. I read some things Oliver mentioned about using an RC delay for it, but couldn't seem to find how it might be implemented or what he would have suggested.

I've been doing a helluva lot of reading about it, but as I'm not an EE all I can come up with is using some kind of soft start for the B+ such as this one: http://www.kandkaudio.com/otherkits.html

I spoke with the creator Kevin and he seems to think his time delay circuit for B+ only will be sufficient since I'm using Poctop's PSU which he said had massive filtering and resistance, so soft-starting the heater isn't necessary. What's kinda cool is his little circuit will delay B+ for 30 seconds. He also has a soft-start version that additionally adds a 30-second ramp up on B+ voltage, but it uses a MOSFET.

I also considered using an NTC thermistor or a MOSFET across the mains, but these seem to come with the possibility of fire or explosion if they fail or are implemented improperly (yikes!).

What say you guys? Worth using? Will it change sonics? Decent protection or unnecessary overkill?

Thanks in advance your thoughts,

Mike

Yeah, those don't really like shock therapy. (Nor does you wallet, presumably.)
And I've heard they like to see pretty much spot on 4V. Not much more, but not much less, either.

If you're really scared by now, no worries. Just send it to me and I'll take good care of it.

Hahahaha Henk you crack me up!

What's your address?

After talking to Kevin from K&K audio - great guy - he said Dany's PSU is massive and beefy and has enough filtering to protect the tube on start. (Yay Dany!). He said it actually had soft start built in due to the RC legs. I've tried to unnerstan all this, but it's kinna beyond me at the moment after many hours googling/studying, so if anyone wants to chime in about how it works I would be grateful.

Here's the link to the PSU: http://dl.dropbox.com/u/43869772/PSU249P.pdf

Kevin did suggest a delay on the B+, so I got a little kit from him to do that. Could be "overkill" but hey, anything to help the tube live longer will be a good thing. CJ mentioned in another thread he likes PSU's to be simple and bulletproof, and he said things like relays etc break - let's hope not!

Anyway I'll keep you guys abreast of how it progresses; could take me some time.

Best to you all, and any other thoughts/suggestions are welcome!

Oh by the way I actually had a cool idea for initial voltage setups. Throw a DPDT switch just before outputs, and put dummy loads on one side of the switch that will mimic the load the tube will have. Not so necessary for 5840's but definitely important for an expensive tube!.

I could open the PSU, flip it to "calibrate" mode and make sure the PSU has the right voltages under load. Then flip switch back for "normal" operation and know it's calibrated correctly.

Overcomplicated, or a good idea?

Mike

I built Danny's  m49B build about 5 months ago. From my observations while setting the power supply up it is a passive supply and gradually ramps up is current and voltage upon startup. So I think you'll be fine. Bear in mind though as the PSU is a passive design you'll need to compromise slightly on your heater voltage as it will vary ever so slightly due to mains voltage variations. I'd go slightly under 4v to be safe 3.8-4.0v to be safe.

Hi Nyquist, thanks for the info!

To anyone who knows, what would be the downside to adding a regulator just to make sure voltage stays constant?

Thanks,

Mike

The issue I see is a regulated supply will hit the tube with full voltage a current from the get go which tubes hate. You'd have to design a soft start into the build which means more stuff in the mix. Just so you know the original M49 power supply is a passive design and the power supply also effects the sonics of the mic. I'd stick with the passive PSU and take extra care when setting the PSU up. Maybe use a 5840 tube for your tweaking and then install the AC701. Note before you buy the AC701 make sure it is tested for low noise and microphonics. Most you see on Ebay will be duds. Expect to pay upwards of $800 for a quite tube that meets spec. Most suppliers will also not sell the tube separately. They want to ensure the tube is going into an original Neumann and the only way it will happen is if you bring your Mic to them.  To be honest I tried to secure a few good AC701's for my build but, no one would sell one to me with out sending my Neumann M49 in so it could be serviced and the tube fitted.

I'd stick the the 5840 tubes. Buy a bunch of them and you'll find a quite example. I found the RCA 5840 were the best for noise and microphonics.

I used these tubes in the end. Bought 10 of them. Got to number 3 and that was more than acceptable for acoustic recordings.

http://www.ebay.com.au/itm/5x-RCA-CV-3929-EF732-EF-732-Subminiatur-Rohre-NOS-/321105571871?pt=DE_TV_Video_Audio_Elektronenr%C3%B6hren_Valves&hash=item4ac362301f

After 2 days burn in they came up very nice. 

Thanks for sharing Nyqvist! I appreciate your insights.

This AC701k is from a reliable source and is Neumann tested NOS and in box with wrapper. Seller offers 1-yr guarantee! I will take your thoughts to heart and I have learned much from you, so thank-you!!!

Based on what you shared I will avoid the regulator.

Best,

Mike