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Author Topic: Power transformer for tube mic PSU?  (Read 4020 times)

mike_mccue

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Power transformer for tube mic PSU?
« on: February 22, 2013, 08:31:39 AM »

Hi, I am getting very close to starting a build project using the Tab-Funkenwerk "U-47" alternate tube circuit.

I want to build an unregulated power supply that is based on a transformer with both a B+ and a dedicated filament tap.

The transformers I have found with the suitable voltage taps are all sized for small power amplifiers and have lots of excess current capacity and physical size.

I have corresponded with a transformer winder I work with for guitar amps parts who says they will fabricate a smaller transformer for me, to order, at a reasonable up charge. I know I can keep searching... but I like working with this vendor.

I am estimating that a useful current capacity for use with a tube mic's single tube can be something like

B+         20mAmp (anticipated use 1mAmp)
6.3vDc     1Amp   (anticipated use of 0.200-0.300Amps)

It seems to me that this will provide plenty of extra capacity as a safety margin.

My question: Am I crazy? Or does this seem sensible?


Thank you.


best regards,
mike
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Mike McCue
Tallahassee Florida USA

klaus

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Re: Power transformer for tube mic PSU?
« Reply #1 on: February 22, 2013, 11:18:54 AM »

Unregulated power supplies with excessive current capacities in the heater rail can make precise heater adjustments tricky. Plus, voltage drift can be excessive, sometimes drifting dangerously upwards, into the realm where harm from over-supply to the tube may occur.

My advice: stick with transformer specs of conventional current capacity for a 6.3VDC miniature tube like EF86, 6072, etc.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

mike_mccue

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Re: Power transformer for tube mic PSU?
« Reply #2 on: February 22, 2013, 11:54:28 AM »

Thanks for the info.

My understanding is that the EF86 pulls about 200mA and the EF800 is about 275mA.

The transformers I was finding that put out 200-250vDC for B+ all seemed to have several amps capacity with the 6.3v lead so when I asked for a 1 amp capacity it seemed like a small number.

Those same transformers put out 200-300mA for the B+, so I thought 20mA was also relatively low.

I was worried that I might be asking for too little capacity... and asking for some reassurance that it was adequate. What you say makes sense... Are you suggesting that I should ask for a 6.3vDC supply that has even less capacity than 3-4x extra? I'm eager to learn.


best regards,
mike


   
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Mike McCue
Tallahassee Florida USA

mike_mccue

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Re: Power transformer for tube mic PSU?
« Reply #3 on: February 26, 2013, 07:44:04 AM »


After having some time to reflect on this, I realize that my heater feed output at the transformer needs to be higher than 6.3vAC.

All the guitar amps I have made had AC powered heaters so it didn't occur to me at first.

I intend to use a DC heater supply and so now I realize that I need to start higher than 6.3v if I want to filter the power with an unregulated design.

I am thinking that if I request a 12.6vAC output a 1A that I will get the voltage I need while also addressing Klaus' comment about surplus capacity causing possible issues.

I welcome any comments or observations that you may share.

Thanks.


best regards,
mike

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Mike McCue
Tallahassee Florida USA

Jim Williams

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Re: Power transformer for tube mic PSU?
« Reply #4 on: February 26, 2013, 10:53:34 AM »

I prefer to use a decent regulator and drive heaters in series with 12.3 volts DC. A slight dip in the voltage supplied does extend heater life with no detrimental effects on performance. I use 317 style regulators, the LT1085 is excellent. You will need to use a 12.6v AC tap and run that into a full wave rectifier to obtain 17 volts DC un-regulated. Use a huge heatsink and enjoy the clean DC.

Small tube filiment transformers are available from Hammond Manufacturing.
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