ubxf

RFT 7151 Bottle microphone
« on: September 12, 2004, 11:14:37 AM »
Hello,
Is anyone familiar with this microphone and what it takes to make the
power supply external
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=64449&item=3744891320&rd=1&ssPageName=WDVW
Best Regards
francois


zebra50

RFT 7151 Bottle microphone
« Reply #1 on: September 12, 2004, 12:11:52 PM »
I don't agree with their expert's story! Sounds like something is faulty in there and it needs fixing rather than a big conversion job.

See if the seller will give you photographs of the capsule condition before you part with money.

Stewart
Ribbon microphone services
http://www.xaudia.com
Microphone blog

gyraf

RFT 7151 Bottle microphone
« Reply #2 on: September 12, 2004, 12:45:00 PM »
I wonder if those are VF14-tubes?


Jakob E.
..note to self: don't let Harman run your company..

RFT 7151 Bottle microphone
« Reply #3 on: September 12, 2004, 01:06:42 PM »
Last time I saw such a mic was with EF12 tube. The other is probably EZ11 or something like that, because PSU is in the microphone.

PRR

RFT 7151 Bottle microphone
« Reply #4 on: September 12, 2004, 11:29:10 PM »
> Sounds like something is faulty in there and it needs fixing rather than a big conversion job.

It looks a little like a 1940s US radio inside. And after 60 years, these all need new power filter caps. And of course it will hum with the covers off: time to figure out what shorts when the cover is on.

While having the AC supply in the mike body sounds like a dumb idea, this was not a cheap mike and I'm sure it didn't hum (much) when working right.

The electronics are likely fixable, and probably simple.

The capsule could be like-new, or useless.

If you think the "looks" are worth $850+, go ahead and get it. At worst, you drill-out a rotten capsule and put in a $2 Panasonic capsule, hide a 9V battery inside that ample case, and run an XLR out. Since nobody you know has another mike like this, they won't know that it is bogus under the skin. (Oh... you might need to leave the power cord on, wired to a 7W light bulb in the case, so it "gets warm" like a tube mike should.)

ubxf

RFT 7151 Bottle microphone
« Reply #5 on: September 13, 2004, 12:39:06 AM »
actualy i already have one like this, i included the link for the pix.
mine works like it should but the fact that the power supply is in the mic
results in a pretty loud 60hz hum there is a pot to try to minimize it
but it's still close to unusable. i guess they were designed as a PA mic
so the line level that comes out of it was more important than the hum.
Anyhow do you think it's worth making the power supply external ?
i have a schematic if anyone is interested

thanks
francois

PRR

RFT 7151 Bottle microphone
« Reply #6 on: September 13, 2004, 01:05:42 AM »
Fix what's there before you go hacking externally.

The large electrolytics are surely decayed. The rectifier should be checked or just replaced.

Marik

RFT 7151 Bottle microphone
« Reply #7 on: September 13, 2004, 02:17:58 AM »
Quote from: "ubxf"
actualy i already have one like this, i included the link for the pix.
mine works like it should but the fact that the power supply is in the mic
results in a pretty loud 60hz hum there is a pot to try to minimize it
but it's still close to unusable. i guess they were designed as a PA mic
so the line level that comes out of it was more important than the hum.
Anyhow do you think it's worth making the power supply external ?




First, I would replace this huge selenium rectifier with something more modern and compact, so you get more room inside. Then make a little board for regulated filament. This, as well as new electrolitics might take care of most of the hum.
Of course, you can make an outboard PSU, but... it wouldn't be RFT 7151 anymore. Also, resale value most likely will drop.
Samar Audio & Microphone Design

www.samaraudiodesign.com

The Art of Ribbon Microphones

soundguy

RFT 7151 Bottle microphone
« Reply #8 on: September 13, 2004, 03:30:44 AM »
I commented to a degree on another thread in regards to this.  Ive updated all my old tube mics with modern power supplies and havent in a single instance looked back.  If you are using that mic to make recordings, if it were mine, I would take the supply out, put it in a box, label it and forget about it.  Build a whole new supply from scratch and put it in an external box.  Whats it gonna be, WORSE than the low freq hum that you have now.  Thats what I would do.  Of course, once you do that, some vintage collector wont give you a "fair" price for it, but those people are running a museum in most cases...  Oh, you recapped it, you ruined the vintage value...

dave

chips are good with dip...

toby

RFT 7151 Bottle microphone
« Reply #9 on: September 14, 2004, 02:15:20 PM »
the guy from www.vintagecity.de has one, with external power supply. says the tubes are ef12.


gyraf

..note to self: don't let Harman run your company..

lolo-m

RFT 7151 Bottle microphone
« Reply #11 on: December 14, 2005, 07:45:43 AM »
Hi, I'm a new member and I'm a proud owner of a CM7151 RFT bottle mic. I want to make a few mods like taking out the power supply but on the mod schematics provided by Francois, I noticed a lot of differences with the original. Can someone help me understand this ? The other point is to read a few resistors values on the mod schematics...
Hard to be a punk... But sometimes you have to...

RFT 7151 Bottle microphone
« Reply #12 on: December 16, 2005, 03:50:01 PM »
My friend/co-worker just bought one of these.  He's sending it to Korby for an overhaul/power supply removal.  I'll have him take pics to document before and after. :thumb:

lolo-m

RFT 7151 Bottle microphone
« Reply #13 on: January 18, 2006, 09:49:39 AM »
I modified my RFT bottle using the original schematics and just building an out board power supply inpired with the gyraf mic one (just a few values mods). It works OK for now but the real tests will occur this week-end. At the moment, I can say there's a pretty low noise floor even if it is really upper than a solid state mic. I will keep you informed...
Hard to be a punk... But sometimes you have to...

lolo-m

RFT 7151 Bottle microphone
« Reply #14 on: March 06, 2006, 02:08:23 PM »
I checked my new baby with a three different singers and I can say now that the RFT bottle with a M7 doesn't suit to every voices (even if it's always good)... The output level is huge with a solid state input mic preamp (focusrite green) but is more classical with a transformer mic input (avalon VT-737). I do really preffer this mic with the avalon even if I preffer to use the focusrite with a 414... So, I'm still in love with my mic, more than ever, because when it suit to a voice, the sound is really really wonderfull. My mod is OK, no hum or noise I can hear (the headphones noise is louder) but I really would like the exact voltage I can apply to the tubes. The caps of my mike was so dry that I'm not sure about the measures I made ... It seamed to be 130 Volts... If someone can measure this on an unmodified mic, I would be glad.
Hard to be a punk... But sometimes you have to...

synthi

RFT 7151 Bottle microphone
« Reply #15 on: March 06, 2006, 07:28:02 PM »
Hi Lolo,

I`m modding my 7151 with the help of Mr. Andreas Grosser. He is one of the most more experts in bottle mics and we had some conversations about the original cm7151. If you are following the original schematic the mic goes to 10k only, not suitable for studio use... I`m following the "neue" schematic with some values changed as Andreas told me. Also the original output transformer is lo quality and you must change it for turning it in a world class mic  :thumb: just email me if you need more info!

About the PSU I`m thinking in reuse one from the "thomman2 T-bobe tube mic, it is 200 and 6.3V output. The Ht is ok but heater voltage must be changed to -6.3V. Also I have a G7 board set here, so maybe I can build the modified G7 too... What values did you change? and also how did you get the negative 6.3 v?

Thanks!

Synthi.

rodabod

RFT 7151 Bottle microphone
« Reply #16 on: March 06, 2006, 07:33:27 PM »
Quote from: "synthi"
The Ht is ok but heater voltage must be changed to -6.3V.


Why is it negative?

I don't understand why it would make any difference if it is only for the heater?
Quote from: tv
punchy fat bastard chip

synthi

RFT 7151 Bottle microphone
« Reply #17 on: March 06, 2006, 07:42:55 PM »
I was thinking the same about the negative voltage but someone told me that "is" required... :shock:

I`ll ask why...

Synthi

Gus

RFT 7151 Bottle microphone
« Reply #18 on: March 06, 2006, 07:44:52 PM »
In Jakob E post

The first schematic is cathode biased

 the 2nd is fixed bias (first stage) using a voltage divider on the -6VDC heater to optain the - bias voltage.  NOW there is a way to do fixed using + heater voltage, look at the fixed bias M49 schematic and other neumann schematics to see.

synthi

RFT 7151 Bottle microphone
« Reply #19 on: March 06, 2006, 07:48:46 PM »
ok,

so if using the "neue" schematic "as is" then you MUST have -6VDC, right?

Synthi


 

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