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Best way to get different voltages from a tapped transformer
Best way to get different voltages from a tapped transformer
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Old 13th March 2018, 06:19 PM   #31
tizman is offline tizman  Canada
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My power supply will use two seperate power transformers, one each for the preamp and power tubes. I am assuming that I will also require two seperate filament transformers for the two rectifiers if I use the centre tap of one of them to get B+. If I don’t use the centre taps of the rectifier filament transformers to get B+, can I use one 5 Volt filament transformer for both rectifier tubes? This is new territory for me....
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Old 13th March 2018, 08:01 PM   #32
DF96 is offline DF96  England
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Quote:
Originally Posted by tizman
Also, I need to purchase a 5 volt transformer for the rectifier filaments. If I buy a centre tapped 5 volt transformer, can I use its centre tap for the B+ and avoid the two extra diodes?
As the silicon diodes connect to the anodes and the heater connects to the cathode it is unclear what you think you mean by this question.

You can use one 5V heater supply for more than one rectifier (i.e. join the heaters), provided that the rectifier cathodes are also joined. And, of course, the heater wiring is somewhere connected to the rectifier cathodes - many rectifiers do this internally, but if not you have to add an external connection.
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Old 13th March 2018, 08:45 PM   #33
tizman is offline tizman  Canada
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DF96: I tried to draw it out and realized that I was left without a ground that looks workable. I modified an organ amp once where the B+ came from the centre tap of the 5V rectifier filament winding, but in that case, if I remember correctly, the ground wire used was the centre tap for the B+ winding. I have attached a drawing of how I thought it could work with my power transformer, but it doesn’t seem right.
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File Type: jpeg A59A7A47-34DF-40D1-ACE9-CCA5CEC97BE8.jpeg (471.5 KB, 22 views)
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Old 13th March 2018, 09:17 PM   #34
DF96 is offline DF96  England
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It isn't right. Don't try to build a PSU until you know why.
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Old 13th March 2018, 09:47 PM   #35
jhstewart9 is offline jhstewart9  Canada
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There are several general purpose rectifiers with 5V heaters & two anodes. Commonly used in radio, TV & Hifi. Refer to the attached. Anyone of them would do for this cct. Switching HV is not recommended for the inexperienced. No ordinary switch would do well in this application.

If you see in a catalog of tubes a p/n beginning with 5 & ending in P1, P2, P7 & so on, that is a CRT, commonly used in an oscilloscope. The 'P' number indicate the color & attack/decay properties of the phosphor. P4 was used in TV.

The 5R4 series was more commonly found in Military/Industrial equipment. And might be an expensive choice buying new old stock (NOS). Something like the 5U4 series would do just as well.

For the bridge cct as drawn in your post of 5:52 PM today is correct. The diodes are part of the bridge configuration.The +ve lead still comes off the rectifier 5V heater transformer. The -ve lead comes from the center point of the silicon diodes. No CT is required on the HV winding of the B+ supply transformer.

Care is important, these voltages can be & are deadly. Even some of us with considerable experience still get whacked occasionally.

Which Class of operation do you intend to operate your 45s & 300Bs? Both are very expensive now. A simpler cct with common tubes would probably be a better choice if you are just starting out. The fact that you have those transformers should not be a factor while starting out. There are simpler & safer ways to go that deliver very good performance,
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File Type: pdf 5R4GY Tungsol.pdf (152.9 KB, 0 views)
File Type: pdf 5R4GY RCA.pdf (157.9 KB, 0 views)
File Type: pdf 5R4GB.pdf (204.9 KB, 0 views)
File Type: pdf 5RP1 Dumont.pdf (159.2 KB, 1 views)
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Old 13th March 2018, 09:59 PM   #36
tizman is offline tizman  Canada
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DF96: It’s a bit late for that, as I’ve already built a couple dozen power supplies. Usually using a standard transformer that has B+ and all filament windings included. This is much more complicated, as I have two power transformers, one of which is a Hammond 274X for preamp duty, that has all filaments, and the other is a Freed power transformer with no filament windings. This means I will need to add two transformers for the heater filaments of the power tubes, one per Coleman regulator, and one transformer for the rectifier filament. Also, as the topic of this thread indicates, I would like to get two different B+ voltages out of the one Freed power transformer. With your kind assistance and that of the others that have contributed to this thread, I now have a game plan for building the amp and I have learned much. Thanks!
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Old 13th March 2018, 10:13 PM   #37
tizman is offline tizman  Canada
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jhstewart9: I will use the schematic that you referred to as being correct. I have four NOS 5R4s on hand, and will use them for their required combination of voltage drop and high PIV. I am building an amp that will have two different circuits. Both are Class A. At the moment, I am considering Thorsten Loesche’s SV83/300B amp, and the Bugle 45 amp. It will be built with two chassis. One for power supply and one for the circuits. My primary motivation for doing this is to use a single set of output transformers for multiple amplifier circuits.
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Old 13th March 2018, 10:26 PM   #38
tizman is offline tizman  Canada
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jhstewart9: What sort of switch would you recommend? As an alternate to the switch, I could install and wire two seperate rectifier sockets, one for each circuit, and insert the corresponding rectifier tube when I use the circuit. Is this a sensible alternative to a switch?
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Old 13th March 2018, 10:58 PM   #39
JMFahey is online now JMFahey  Argentina
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Quote:
Originally Posted by tizman View Post
JMFahey: Yes, I need 70 mA for the two 45s and there is only 35 mA on the 395 volt tap. What I want to know is if it is possible to use just tap 4 and 6, or 4 and 6 with 5 as their center tap, to get a lower voltage from this transformer. I would use either this combination of taps for 45 tubes, or the normal 3-5-6 (520-0-520 volts) for 300B tubes, but never at the same time. I am using a seperate power supply transformer for the pre amp tubes, and will not use the 395 volt at 35 mA tap at all.
No you canīt , period.

"Center tap" includes the meaning of *center* , that is exact halfway between 2 points.
5 is halfway/cen tap between 3 and 6.
It is NOT half way between 4 and 6 so it is not a Center tap; transformer will behave as if having 520V, only loaded in an inefficient way.
Quote:

I believe if you are NOT sucking 250mA on the 520V legs, you could suck 125mA on the one 395V leg. They are surely all the same winding, gauge selected for like 140mA each side.
*Maybe* , but you actually donīt know.
And wire is rated for full cirrent, not half of it.
Just duty cycle is 50% , but thatīs something else.

Donīt mistakenly believe that since duty cyle is 50%,wire heating is also 50%, so "itīs as if it were 125mA all the time" which you seem to imply by your words above.

125mA 100% of the time is the same as 250mA half the time ... if we are talking average current.

BUT if we talk RMS (which describes its heating power), 250mA half the time is twice 125mA all the time.

Old time designers couldnīt save much copper section, if at all, by making "full wave" rectifiers; they made them just because only 2 plate common cathode tube rectifiers were widely available, and nothing else.
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Last edited by JMFahey; 13th March 2018 at 11:14 PM.
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Old 13th March 2018, 11:19 PM   #40
PRR is offline PRR  United States
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Your "Does This Work?" clearly does NOT work. It gives 2.5V AC where you seem to be expecting large DC.

Quote:
Originally Posted by tizman View Post
...the B+ came from the centre tap of the 5V rectifier filament winding, ....
That IS the "ideal" case for a naked-filament dual rectifier. Taking B+ from one end or the other adds/subtracts 5V on alternate waves, adding some 50/60Hz tone to the big 100/120Hz ripple.

But I once prodded a friend to 'scope it, and neither of us could see the difference (for a 400V supply, 5V rect).

And so many rectifiers are indirect-heat cathode, in which case the "ideal" connection is to that end of the heater.
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