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Old 28th February 2007, 04:42 AM   #31
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hi guys

Just a hint

A variac-type of transformer can make a real good pp or
parafed output transformer for 100Vrms across the usually
0-250V winding.
You have to wind the secondaries by hand on top but it has the advantage that you can "interleave" windings easely. Best suitable size is a variac with a wirethickness below 0.6mm diam.
0.5 will get you beyond the audioband without undue losses.
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Old 4th August 2007, 09:18 AM   #32
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I am looking for a cheaper parafeed OPT and reading through this thread, I might be able to get away with using a toroidal power transformer.

I am doing a parafeed 12b4 as power tube and 6j5 for driver stage. I will be using a 40H 30ma plate choke for the 12b4, B+ of around 250V cathode resistor of around 1.2K.

I am thinking of using a toroidal transformer for the parafeed output. The toroidal's primary is 0-230 and two secondaries of 0-6V. I computed the primary Z at around 5.8K at 4ohm secondary. The toroidal power ratingis 30VA.

What do you think guys?

Thanks.
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Old 5th August 2007, 12:38 AM   #33
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hi alexG,

you can get this 110/220volt pri. to 12volts sec. 500mA transformer, cost less than 100 pesos(about U$2) as parafeed output traffo.

use the 110volt winding and 12volt, it will give you a 9.1 turns ratio, so for a 600ohm sec load, reflected impedance would be around 50k,
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Old 5th August 2007, 02:29 AM   #34
alexg is offline alexg  Philippines
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Quote:
Originally posted by Tony
hi alexG,

you can get this 110/220volt pri. to 12volts sec. 500mA transformer, cost less than 100 pesos(about U$2) as parafeed output traffo.

use the 110volt winding and 12volt, it will give you a 9.1 turns ratio, so for a 600ohm sec load, reflected impedance would be around 50k,
Tony,

I will be loading the secondary with either 4 or 6 ohm load (speaker load), with 600 ohm load, would that be headphones?

Thanks.
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Old 5th August 2007, 08:59 AM   #35
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What about those 100V line distribution transformers. What can be done with them. Can they be used in as parafeed transformers or similar applications?
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Old 5th August 2007, 09:37 AM   #36
AJT is offline AJT  Philippines
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Quote:
Originally posted by alexg


Tony,

I will be loading the secondary with either 4 or 6 ohm load (speaker load), with 600 ohm load, would that be headphones?

Thanks.

in that case you can use the 220v pri and tap the 6volt secondary, this set up gives you an 8 ohm secondary to 10kohm primary.....just about right...
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Old 5th August 2007, 10:01 AM   #37
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Quote:
What about those 100V line distribution transformers. What can be done with them. Can they be used in as parafeed transformers or similar applications?
Yes

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Old 5th August 2007, 12:09 PM   #38
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Shoog: I am listening ..... I guess as they are supposed to have a hi-Z primary and lo-Z secondary that it is possible to find a tube and/or application that works.
Could they work with tubes like 12B4A? I have 20 of those tubes, and 20 transformers ....
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Old 5th August 2007, 12:48 PM   #39
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You need to work out the winding ratio, and then the impedance ratio before you can start to work out which tubes might work for you.
Because you have only got a relatively low ratio, you will have to get imaginative about suitable valves for the application. Some of the hi-current low-voltage valves should be your starting point. Look at the PL36,PL509, 6080, or any of the sweep output tubes. The good news is that all of these are cheap and cheerful. All of the more typical output tubes are well out of the question - which is a good thing to my mind.
If you could get the 6080 to work and you used a dual triode and SS rectification, you could get a two valve stereo amp. This approach also makes the design of CCS plate loads slightly easier. The 6080 works excellently up to 100mA down at 100V.

I saw a design for a PL36 cathode follower which might suit these transformers. In this application you don't need a plate load and the Constant Current Sink at the cathode can be a simple LM317.

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Old 5th August 2007, 01:34 PM   #40
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Thanks for your ideas.
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