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Old 25th July 2007, 06:49 PM   #1
mwl6m is offline mwl6m  United States
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Default Low Voltage (+70V) Power Supply Design

I need to design a +70V power supply for use with a low voltage tube preamp. I have 2 25V-0-25V center tapped transformers, and I would like to know if it would be possible to obtain 70 volts with these transformers.

I also need 6.3V @ 350mA for the heater supply (ECC88 tubes). If anyone has any help for such a supply then it would be greatly appreciated.

I am new to amplifier design, and I am having a lot of trouble with the power supply aspect of my design. Thanks ahead of time.
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Old 25th July 2007, 06:58 PM   #2
Nordic is offline Nordic  South Africa
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http://www.kronjaeger.com/hv/hv/src/mul/

A PCB mounted transformer in this range should be pretty cheap..$2 -$3 Combine with LM317 and you are done.
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Old 25th July 2007, 07:39 PM   #3
mwl6m is offline mwl6m  United States
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Do you know any good sites to find such a pcb mount transformer? With 4700uF smoothing caps and a full wave bridge rectifier, I can get an output voltage of +/-35V (no load). Is there any way to obtain 70V from this transformer?

Also, are there any voltage regulators that can be used for 35V and above? I haven't found any on the internet, but some regualtors specify a maximum differential voltage of 35V. Could something like this be used for greater than 35V? Thanks ahead of time for the help.
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Old 25th July 2007, 08:36 PM   #4
es44 is offline es44  Denmark
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Quote:
Originally posted by mwl6m
I can get an output voltage of +/-35V (no load). Is there any way to obtain 70V from this transformer?

Actually you do have 70volts (no load), just "ignore" the center tap, and place your voltage tester at the 2 other terminals for a reading.

best regards
Ebbe
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Old 25th July 2007, 09:10 PM   #5
mwl6m is offline mwl6m  United States
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So I would still use the bridge rectifier and use the negative rail as the ground reference voltage? This does give me an output of 70V, but I want to be sure to use the negative rail as the reference. Thanks.
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Old 25th July 2007, 09:13 PM   #6
mwl6m is offline mwl6m  United States
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I tried to supply the heater from a lab 6V power supply, but it did not draw very much current. Should the tube glow with only this voltage applied, or does the B+ voltage need to be connected in order for the tube to work? Thanks.
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Old 25th July 2007, 09:52 PM   #7
mwl6m is offline mwl6m  United States
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Here is a schematic of my heater setup. It is 6.3V paralled to pin 4 and 5 of a 9-pin tube socket. Would I connect the reference to ground or some other point? I've heard that 20-40V is a better reference with less hum. Any help would be appreciated, as this is one of my last steps to completing my tube preamp. Thanks.
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File Type: gif heater.gif (1.6 KB, 450 views)
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Old 25th July 2007, 10:13 PM   #8
star882 is offline star882  United States
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Just a bridge across the entire secondary (ignoring the center tap) will get you 70v, but without regulation. Adding half of the other transformer will get you 106v, which can then be regulated to 70v. (You can connect two secondary halves in parallel and use the ends to ensure load balancing.) At those voltages, it's easiest to use discrete components although there are regulator chips that can work with those voltages.
For the 6.3v supply, you can use a LM317 with the correct resistors or a 7805 with two 1N4148 diodes in the ground path.

BTW, I think tubes must be warmed up before applying plate voltage and plate voltage removed before filament voltage to ensure optimum tube life. There is a heat pump out there that uses a special oscillator tube to generate very high frequency AC (at about 900MHz) to defrost the coils with. It worked by forming a very lossy capacitor between the coil fins and the feeder wire with the frost as a dielectric. The frost first melts into water, which is even lossier so it heats up very quickly into steam. The steam then builds up pressure and blows the remaining frost and water out of the coil. In order for it to work effectively, the power must be very high for a short time, so very large capacitors were charged from a small transformer and discharged very quickly. Initially, the plate was connected directly to the capacitors and discharge started by applying filament voltage. It saved the cost of a high voltage switching device, but they experienced lots of early tube failures in prototype units. They realized that the cathode heats up unevenly during warmup so the hot spots took most of the load, causing them to wear out faster, thereby making them warm up faster next time, etc. until the tube burned out completely. Now, they use a high voltage SCR that is activated about 20 seconds after turning on the filament supply.
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Old 26th July 2007, 02:01 PM   #9
mwl6m is offline mwl6m  United States
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Would I still connect the center tap to the power star ground, or would I just ignore the center tap altogether? Also how would I connect up the heater so it is referenced to 20-40V instead of ground? Thanks.
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Old 26th July 2007, 03:19 PM   #10
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Ignore the center tap. Also since the 50vrms you have will smooth out to about 70vdc with a bridge rectifier and caps, you may not need regulation. Most tube amps don't use any, but there are a few that do. Depending on your power transformer rating, you may be able to get 6.3v at 350ma, or you may not. A cheap radio shack transformer is also an option for that.

good luck
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