Help with converting RFT 7151 to use external power supply

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Trent _L

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Dec 20, 2020
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Hello! First post here, but happy to be part of this forum  :)

A while ago I acquired an old RFT 7151 tube bottle microphone in non working condition. It seems to have been attempted to be recapped at some point, but seems incomplete as there are a few loose wires floating around inside it. My plan is to rebuild / rewire it, replace the selenium rectifier with a diode bridge, and remove the power transformer and related circuitry and locate it in an external enclosure to reduce mains hum.

I know vaguely what I need to do, but I have never done any kind of power supply design, so I thought I'd better check with some people who know what they're doing. My guess is that the two mains input pins on the mic will become one high voltage pin ( ~200V DC ) and the other with be low voltage for the heaters ( ~6.3V DC ). The power transformer will be moved into the external supply along with the filtering caps and the bridge rectifier. I'm also guessing that the 6.3V for the heaters will need its own bridge rectifier, and they will be run on DC rather than AC. Also do you think I should I use the original power transformer, or get a modern replacement?

Anyway, if anyone has some resources for this I'm be extremely happy to have them. From what I've read this is a common modification, and I'd rather not reinvent the wheel. I've attached the schematic for anyone who is interested in helping.

Thanks!

Trent
 

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gridcurrent

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Aug 28, 2009
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don't waste your time with the original 220 VAC input power transformer. 
get a 200 VDC linear for the B+,
https://www.mouser.com/ProductDetail/International-Power/IHB200-012/?qs=0xCm9DOQnC7%252BW4SCa00vHA%3D%3D,
and either find a 12 VDC linear,
or build what you need.
re-worked many, with the original EF12k or 6072 tube fitted.
 

Trent _L

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Dec 20, 2020
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Thanks for the replies! Does anyone know why the two heaters are run on different transformer taps? If anyone has a schematic for a regulated 6.3v DC supply that'd be a big help.
 

gridcurrent

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Aug 28, 2009
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Trent _L said:
Thanks for the replies! Does anyone know why the two heaters are run on different transformer taps? If anyone has a schematic for a regulated 6.3v DC supply that'd be a big help.
the 1st tube heater supply has a hum adjust control.
the 2nd tube has one side of the heater grounded.
less voltage drop in the cable if powering the series connected heaters on 12 VDC.
 

Trent _L

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Joined
Dec 20, 2020
Messages
4
gridcurrent said:
don't waste your time with the original 220 VAC input power transformer. 
get a 200 VDC linear for the B+,
https://www.mouser.com/ProductDetail/International-Power/IHB200-012/?qs=0xCm9DOQnC7%252BW4SCa00vHA%3D%3D,
and either find a 12 VDC linear,
or build what you need.
re-worked many, with the original EF12k or 6072 tube fitted.

Do you know how big that linear power supply is? I'm planning on building all of this into an Analog Vibes tube mic psu enclosure, and I'm not sure exactly how much space I have to work with. I'd almost prefer to use a single plate/filament toroidal transformer with 200v and 6.2/12v secondaries, but I'm having a hard time finding an appropriate model other than the original one from the mic. All the ones I'm finding are too large and too many watts.

I'd also prefer a transformer with a center tapped primary so that it will work on both 120v and 240v mains.

I'm super not understanding the heater circuitry. I though they just needed 6.3v ac or dc pushed through them. Why is one side of the 2nd heater grounded?
 

Trent _L

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Dec 20, 2020
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4
I'm happy to report that I have got the power supply build and the mic is working! The last step is for me to machine a replacement plug as the original was missing. Does anyone happen to know the specs of the mic output transformer? I've seen people replacing it with a more modern Sowter xformer, but I am unsure of the correct ratio to use.
 

jhaell

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Mar 26, 2020
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Location
Sweden
Hi! I have found this site in Germany that creates a replica of the connector. I don't know if its still avaliable, but it looks good. I have also purchased one of these mics and I'm about to do the same modification.
 

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